Advanced Magnetoresistive Sensors for IndustrialApplications

Tiago Afonso Carocho de Sousa Costa

Thesis to obtain the Master of Science Degree in

Engineering Physics

Supervisor: Prof. Susana Isabel Pinheiro Cardoso de Freitas

Examination Committee

Chairperson: Prof. Pedro Miguel Félix BrogueiraSupervisor: Prof. Susana Isabel Pinheiro Cardoso de FreitasMember of the Committee: Dr. Karla Marina Jaimes Merazzo

October 2017

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Acknowledgments

First of all I would like to thank Prof. Susana Freitas for the opportunity to work in INESC-MN for the last

year and for all the guidance and support, for also allowing me to develop as a researcher, a professional

and a person.

I would like to especially thank Karla for her unconditional support and help in this project. Without

her, the challenges would have been much harder to overcome. I would also like to acknowledge all

the senior researchers and PhDs for always being available for any questions or doubts, and also the

technical staff for teaching me the processes and for their availability.

I’m very grateful to all my friends and colleagues at INESC-MN for the great team-work environment,

and thank you for the lunch hours where my mind could relax for a while.

Thank you to all my friends at Tecnico for making the last five years not only about studying and

working and for providing some of the best moments in my life. To my big friends Marisa and Ana for

keeping me sane throughout the years. I’m especially thankful to Basti for supporting me emotionally,

especially during the last few months.

Last but not least, I acknowledge my family: my father Pedro, my mother Gui and my brother Diogo

and sister Marta for always being present no matter what, and for supporting me unconditionally. Thank

you for everything!

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Resumo

Metrologia e posicionamento preciso sao de extrema importancia para a industria. Hoje em dia, ex-

iste uma procura competitiva para aparelhos diminutos, de menor custo e capazes de cumprir mesmo

em ambientes adversos. A codificacao precisa pode, por isso, beneficiar de tecnologia magnetica.

Sensores magnetoresistivos (MR) sao de tamanho diminuto, de baixo custo e oferecem grande sen-

sibilidade. Como portador de informacao, o uso de tinta magnetica (TM) e vantajosa em relacao a

tecnologia magnetica existente.

Sensores MR avancados - as juncoes de efeito de tunel - sao implementadas num sistema, em

conjugacao com ımanes permanentes, com o objetivo de magnetizar e ler padroes de TM. O de-

senvolvimento deste sistema e feito em dois passos essenciais. Primeiro a determinacao da melhor

configuracao ıman-sensor atraves de medicoes e de simulacao por modulacao de elementos finitos.

Posteriormente e feita a validacao de duas configuracoes de sensores diferentes. Validacao feita

medindo estruturas bem definidas, microfabricadas em ambiente de sala limpa, de uma liga ferro-

magnetica, e posteriror simulacao 2D das mesmas, usando a aproximacao de Coulomb para o calculo

do campo magnetico.

De seis configuracoes de ıman-sensor diferentes, duas foram escolhidas e testadas. A configuracao

resultante e capaz tanto de eficientemente magnetizar as estruturas de TM, como de nao influenciar a

resposta magnetica dos sensores. A simulacao das estruturas ferromagneticas permitiu a validacao das

duas configuracoes de sensores consideradas, resultando na escolha do sensor com maior resolucao

espacial e sensibilidade alta, providenciando assim uma base para um futuro desenvolvimento e optimizacao

do sistema.

Palavras-chave: Juncoes de efeito de tunel, Tinta magnetica, Codificadores magneticos,

Medicoes de campo magnetico disperso, Simulacao de campo magnetico

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Abstract

In Industry, metrology and accurate positioning is of major relevance. Nowadays there is competitive

demand for small size devices at lowest cost possible and capable of performing even in harsh environ-

ments. Precision encoding can therefore benefit from magnetic technology. As a reading technology,

magnetoresistance (MR) devices offer low sizes, high sensitivity and low costs. For information storage,

patterns of magnetic ink (MI) as an alternative to the existing technology is also of advantage.

Advanced MR sensors - the magnetic tunnel junctions - are therefore implemented in a system along-

side magnets aimed to magnetize and read MI patterns. The development of this system is done in two

main stages. First the search for the best magnet-sensor configuration using both simulation with finite

element modeling (FEM) and measurements. Then the validation of different sensor array configura-

tions. This validation is done through measurements of well-defined structures of a hard ferromagnetic

alloy, micro-fabricated in a clean-room environment, and comparison with a 2D simulation, which is

based on a Coulomb approach to the calculation of the magnetic field.

From six different magnet configurations, two were chosen and tested. The resulting configuration is

able to efficiently magnetized the MI structures as well as not influence the sensor’s magnetic response.

The simulation with the hard magnetic micro-fabricated structures allowed for the validation of both

sensor configurations considered in this work - resulting in the choice of a high spatial resolution and

high sensitivity sensor, providing a basis for further system development and optimization.

Keywords: Magnetic tunnel junctions, Magnetic ink, Magnetic encoder, Stray field measure-

ment, Magnetic field simulation

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Contents

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

Resumo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi

List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii

Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv

1 Introduction 1

1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 State of the Art . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2.1 Positioning Systems - Encoders . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2.2 Magnetic Sensors for Scanning Applications . . . . . . . . . . . . . . . . . . . . . 2

1.2.3 Magnetoresistive Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2.4 Magnetic Ink . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.3 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 Theoretical Background 7

2.1 Magnetism and Magnetic Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.1.1 Magnetic Dipole Moment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.1.2 Topics of Magnetostatics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.1.3 Classification of Magnetic Materials . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.1.4 Magnetic Ink . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.2 Magnetic Tunnel Junctions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.2.1 Tunnel Magnetoresistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.2.2 Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.2.3 MTJ Linearization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.2.4 Sensitivity, Voltage Bias and the Output Signal . . . . . . . . . . . . . . . . . . . . 18

3 System Description and Characterization 21

3.1 System Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.1.1 Magnetic Ink . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.1.2 Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

ix

3.1.3 Simulation of the Magnets Configurations . . . . . . . . . . . . . . . . . . . . . . . 24

3.2 Sensing Head Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.2.1 Magnetoresistive Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.2.2 Magneto-transport Characterization Tool . . . . . . . . . . . . . . . . . . . . . . . . 30

3.2.3 Magnetotrasport Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4 Hard Magnetic CoCrPt Structures 37

4.1 Micro-Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

4.1.1 Deposition by Magnetron Sputtering . . . . . . . . . . . . . . . . . . . . . . . . . . 38

4.1.2 Pole Definition by Litography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

4.1.3 Etching by Ion-Milling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

4.1.4 Magnetization Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

4.1.5 Magnetic Properties Measurement - Vibrating Sample Magnetometer . . . . . . . 41

4.2 Simulation Model of the CoCrPt Structures . . . . . . . . . . . . . . . . . . . . . . . . . . 42

4.2.1 Magnetic Field from a Flat Surface - 2D Model . . . . . . . . . . . . . . . . . . . . 43

4.2.2 Two-Dimensional Field from the CoCrPt Sample . . . . . . . . . . . . . . . . . . . 44

5 Measurements 47

5.1 Measurement System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

5.2 Measurements on Magnetic Ink Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

5.3 Sensor Validation with CoCrPt Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

5.3.1 Sensor S1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

5.3.2 Sensor S2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

6 Conclusions 55

Bibliography 57

A CoCrPt Structures Micro-Fabrication Process Runsheet A.1

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List of Tables

3.1 Physical parameters of the magnets used. . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.2 Physical parameters derived from the transfer curves of sensor S1, without magnets and

with configuration Std 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.3 Physical parameters derived from the transfer curves of sensors S1 and S2, without mag-

nets and with configuration Std 3 and a = 0.5 mm. . . . . . . . . . . . . . . . . . . . . . . 35

4.1 Parameters for sputter deposition of CoCrPt. . . . . . . . . . . . . . . . . . . . . . . . . . 38

4.2 Parameters for etching of CoCrPt. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

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List of Figures

1.1 A scheme of a magnetic rotary encoder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 Details of the different Magnetoresistive sensor technologies. . . . . . . . . . . . . . . . . 4

1.3 Applications of magnetic ink. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.1 Calculation of the magnetic field outside a magnetized cylinder. . . . . . . . . . . . . . . . 10

2.2 Magnetic response of a diamagnetic and paramagnetic material in the presence of an

external magnetic field. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.3 Magnetic response of a ferromagnetic material in the presence of an external magnetic

field. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.4 Magnetic response of a superparamagnetic material in the presence of an external mag-

netic field. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.5 Schematic of the spin-dependent tunneling effect. . . . . . . . . . . . . . . . . . . . . . . 14

2.6 Basic structure of a MTJ sensor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.7 Ideal transfer curve of a MTJ sensor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.8 Typical structure of a junction with both sensing and reference layer pinned. . . . . . . . . 18

3.1 Configuration standards considered for the project. . . . . . . . . . . . . . . . . . . . . . . 23

3.2 Proposed concept for the system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.3 3D geometry used on the simulation software. Geometry of the magnets: 10×4×1 mm3;

b = 6 mm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.4 Magnetic field vectors for the configuration with magnets with OOP magnetization . . . . 26

3.5 Simulation of the different components of the field in given regions of the simulated space

with Std 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.6 Magnetic field vectors for the configuration with magnets with OOP magnetization . . . . 28

3.7 Simulation of the different components of the field in given regions of the simulated space

with IP magnetization magnets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.8 Plot of Bx, By and Bz on line 4. Sensor at a = 0.5 mm. . . . . . . . . . . . . . . . . . . . 29

3.9 MTJ pillar structure and scheme of the junction array of sensors S1 and S2. . . . . . . . . 31

3.10 Reference transfer curves of sensors S1 and S2, without magnets present on the system. 32

3.11 Set of transfer curves obtained for sensor S1 in a Std 1 configuration, alongside the ref-

erence curve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

xiii

3.12 Study on the transfer curves of sensor 1, with configuration standard Std 3. . . . . . . . . 34

3.13 Transfer curves for the sensor S1 and S2, with configuration Std 3 and a = 0.5 mm. . . . 35

4.1 Structure of the micro-fabricated CoCrPt structures. . . . . . . . . . . . . . . . . . . . . . 37

4.2 Schematic view of the deposition chamber of Alcatel SMC450. . . . . . . . . . . . . . . . 39

4.3 Hysteresis loop of magnetization vs. applied field for the deposited magnetic alloy CoCrPt. 42

4.4 Scheme of the simulation geometry for the CoCrPt structures. . . . . . . . . . . . . . . . . 42

4.5 Ilustration of the derivaton of the magnetic field from a 2D line of constant charge. . . . . 43

4.6 Scheme of the contributions to the calculation of the field of just two PM CoCrPt structures. 45

5.1 The 2D scanning setup used to measure the stray fields from the magnetic ink and CoCrPt

samples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

5.2 A scheme of the scanning measurements done on the different CoCrPt samples. . . . . . 48

5.3 Simulation done for the CoCrPt sample with PP=1mm and at a RD=500±14µm, with N=8. 50

5.4 Comparison of the simulations and measurements done on CoCrPt samples with different

Pole-Pitches, using sensor S1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

5.5 Average amplitudes measured both in the measurements and simulations for sensor S1

for each sample at a RD of half the PP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

5.6 Comparison of the simulations and measurements done on CoCrPt samples with different

Pole-Pitches, using sensor S2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

5.7 Amplitudes measured both in the measurements and simulations for sensor S2 for each

sample at a RD of half the PP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

xiv

Nomenclature

AFM Anti-ferromagnetic

AMR Anisotropic Magnetoresistance

CPP Current Perpendicular to the Plane

FM Ferromagnetic

GMI Giant Magneto Impedance

GMR Giant Magnetoresistance

IP In-Plane

LIH Linear, Isotropic and Homogeneous

MI Magnetic Ink

MICR Magnetic Ink Character Recognition

MR Magnetoresistance

MTJ Magnetic Tunnel Junction

NM Nonmagnetic

OOP Out-Of-Plane

PM Permanent Magnet

PP Pole-Pitch

PR Photo-Resist

RD Reading Distance

SAF Synthetic Anti-ferromagnetic

SNR Signal-to-Noise-Ratio

SQUID Scanning Superconducting Quantum Interference Device

SyF Synthetic Ferrimagnetic

TMR Tunnel Magnetoresistance

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Chapter 1

Introduction

1.1 Motivation

Metrology and accurate positioning is of major relevance for industry. Nowadays there is competitive

demand for small size devices with the lowest cost possible and capable of performing even in harsh

environments. Currently, the most widespread used technology is optical detection, which allow high

accuracy, resolution and reliability. However, in harsh environments such systems become bigger, more

expensive and more fragile. Precision encoding can therefore benefit from the use of magnetic technol-

ogy and in particular from magnetoresistance (MR), which offers clear advantages when compared to

optical detection. MR sensors - in particular state of the art Magnetic Tunnel Junctions (MTJs) - can be

produced in micrometic dimensions at massive scale, at low cost and capable of high sensitivity to weak

magnetic fields [1, 2]. Encoder systems can also take advantage on new, low cost magnetic encoding

tracks. The use of Magnetic Ink (MI), already present in many security applications, allows for the low

cost replacement of the existing technology [3, 4].

1.2 State of the Art

1.2.1 Positioning Systems - Encoders

Encoders are devices capable of converting mechanical rotational or linear movement into an analog or

digital signal. The typical encoder has just two components: a reading head and a track, which encodes

the information. Both components depend on the technology used.

Current industrial applications rely on encoders to determine positions, angles, rotational speeds,

and other related variables. Currently the most widespread measuring principle on encoders is optical

[5]. This method is the first choice due to its high accuracy, resolution and reliability. However, sensors for

detecting such variables need to have a stable operation in harsh environments as well as provide high

resolution and high accuracy. Generally, optical encoders can have high resolution but may not be ideal

for some typical industrial environments such as those with oil, dirt, dust or fluctuating temperature.

1

When prepared for such environments through protection with encasement or other methods, optical

encoders tend to not meet the reduced prices or small sizes that are in increasing demand for industrial

applications [5]. Magnetic technology can overcome these problems and replace optical encoders in

many applications.

When compared to optical encoders, magnetic technology based encoders can take the form of very

small, inexpensive devices, in high volume applications, such as industrial automation systems, anti-

block braking systems or medical equipment [6]. Magnetic encoders, like any encoder, have a reading

head using some sort of magnetic sensor technology, and a magnetic encoding track comprised of a

magnetic media with a given magnetic pattern, which encodes the position. It is the magnetic sensor on

the reading head that by measuring the stray magnetic field produced between the poles in the magnetic

media that positioning information can be extracted [4].

Figure 1.1: A scheme of a magnetic rotary encoder. The track is represented with red and green bitsrepresenting the magnetic poles. Taken from [4].

The company Bogen Electronic GmbH produces and commercializes magnetic encoders [4]. This

company utilizes mainly reading-heads with Hall-effect based sensors and is already implementing MR

based sensors, placing itself in the front of the development of MR based measuring tools. The tracks

produced by this company are state-of-the-art hard magnetic, which are made of a continuous magnetic

media: an elastomer filled with ferrite. The magnetic poles are then engraved in the track using a process

patented by this company. On Figure 1.1, a scheme of a rotary magnetic encoder is presented. The

magnetic track is located around the axis and in this case, the position is encoded as a periodically

alternating north and south pole.

1.2.2 Magnetic Sensors for Scanning Applications

Some sensing technologies have been used for magnetic scanning of a surface, such as Hall-Effect,

SQUID (Scanning Superconducting Quantum Interference Device) and GMI (Giant Magneto Impedance)

[7]. For scanning applications two very important properties of the sensors are evaluated: the Spatial

Resolution, which is normally related to the size of the sensor and it’s the minimum size that the sensor

2

can measure (the smallest structure it can measure); and also the Field Detection or Sensitivity of the

sensor, which determines the lowest magnetic field and field variation it can measure.

Due to its high field detection and somewhat smaller sizes, SQUIDs are mainly used for NDT testing

such as buried defects, however, since it operates at cryogenic temperatures, they are not suitable for

most industrial applications.

GMI sensors have very high magnetic field sensitivity of as high as 500 %/Oe, but have very low

spatial resolution due to its required large dimensions, being mainly used for NDT such as surface

cracks detection.

Hall-Effect sensors are transducers, on which a change of the external magnetic field translates into

a change in the output voltage of the sensor [8]. Although these sensors offer lower field detection than

other magnetic sensing technologies [9], they have the advantage of reaching lower spatial resolution

(in the nanometer scale).

For positioning applications, the most widely used magnetic sensor is the based on Hall-Effect sen-

sor. It has thus far allowed for good magnetic field sensitivity and also good spatial resolution and

adaptable for industrial environment. However, industry is demanding for even higher resolution and

better performance. That’s why MR based sensor are being developed and implemented on positioning

devices for industrial applications [6, 4].

1.2.3 Magnetoresistive Sensors

The use of magnetic sensors for scanning applications is not new and already widely spread in today’s

technology, such as in Non-Destructive Testing (NDT) using eddy current inspection, document valida-

tion with magnetic ink, hard disk drives and even biological applications [1, 7, 10].

The MR sensor is a solid state transducer, which converts directly a magnetic field into a resistance.

Magnetoresistance is defined as the change in the electrical resistance of a material as a response to an

externally applied magnetic field. The idea of using MR sensors as elements to detect magnetic stray

fields arises from the widely implemented hard-disk-heads [7]. The spatial resolution of a MR device

depends directly on the dimension of the sensors used. Unlike the other previously discussed magnetic

technologies, MR sensors are easily scalable using micro- and nanofabrication techniques, which allows

the implementation of very small devices with very low spatial resolution [1, 2].

Depending on the physical principle, the MR sensors can be divided into three different types: the

Anisotropic Magnetoresistance (AMR), the Giant Magnetoresistance (GMR) and the Tunnel Magnetore-

sistance (TMR). The table on Figure 1.2 resumes the main characteristics and differences between the

three kinds of devices.

When compared to Hall-Effect sensors, AMR sensor have higher sensitivities and comparable spatial

resolutions. However, nowadays due to the clear advantages regarding field detection, GMR and TMR

sensors prove much more advantageous for magnetic sensing than AMR sensors, thus replacing AMR

in most applications.

MR devices and in specificic state-of-the-art Magnetic Tunnel Junctions (MTJs) have high Magne-

3

Figure 1.2: Details of the different Magnetoresistive sensor technologies. Taken from [2].

toresistance ratios (on the order of 300% [2]) which lead to high sensitivity and thus low field detection.

They can also be fabricated in large scale and at low costs, being also very easily implemented. They

are therefore a very good solution for positioning measurements, and has the sensing technology for

magnetic encoders.

1.2.4 Magnetic Ink

For magnetic measuring purposes, magnetic ink (MI) can be very versatile. As a liquid, this ink is com-

monly composed of four types of ingredients: the colorants or magnetic pigments, where the magnetic

behavior is introduced and have sizes mostly of nanometer scale (70 nm for Fe3O4 nanoparticles [11]);

the vehicles or binders, which disperse and bind the pigments modifying the rheological and mechanical

properties; the solvents which dissolve the other components and adjusts its viscosity; and additives

which depend on the properties to be enhanced [12].

(a) A typical bank check with a MICR security line on thebottom part, taken from [13].

(b) The magnetic ink profile of the serial number of a 20Cbank note, taken from [14]

Figure 1.3: Applications of magnetic ink.

Traditionally magnetic ink is used as one of the most important counterfeit materials. It has been

4

drawing wide attention and applied to vital fields for checking authenticity, such as for identity cards,

checks, paper currency, tickets, and others, which is do to their efficient information storage, legibility,

efficient anti-counterfeit and low cost [3]. The main use of magnetic ink is, however, on MICR (Magnetic

Ink Character Recognition), which is used on checks to add an extra layer of security to these documents

[13]. Another wide use passes by impregnating this ink on banknotes, such as in the 20C bill, which

provide a forgery control test using an appropriate magnetic reading head [14]. These two applications

are exemplified on Figure 1.3.

1.3 Objectives

This work is developed in the scope of the project GePos. It is a collaborative project between INESC-

MN and Bogen Electronic GmbH. This project has the main objective of developing a positioning system

based on encoding using magnetic ink, read by state-of-the-art TMR sensors. Thus developing a com-

petitive positioning solution, which is lower in cost production and as reliable as the existing technologies.

The work developed for this master’s thesis makes the first approach and takes the first steps on

the development of such a system. As the main goal comes the development of a reading head based

on state-of-the-art TMR sensors for a magnetic encoder, on which the information is produced using

magnetic ink.

5

6

Chapter 2

Theoretical Background

2.1 Magnetism and Magnetic Materials

2.1.1 Magnetic Dipole Moment

In solid state magnetism, the elementary quantity is the magnetic moment m [15]. In the atomic level, the

magnetic moment arises from two main sources: the orbital movement of electrons around the nucleus,

and the electron spin. Except for some transition metal atoms or ions, which retain a resultant moment

on the atomic scale of the solids, most of the atomic magnetic moments tend to cancel out. Of course,

in a paramagnetic state of a solid, the moments cancel out - but due to thermal fluctuations. They arise

spontaneously in a ferromagnetic ordered state.

Fundamental in magnetostactics is the continuous media approximation , on which the magnetization

of a solid is represented by the quantity M. This quantity is no more than a time averaged local magnetic

moment density, or, given a mesoscopic volume δV :

δm = MδV. (2.1)

In a ferromagnetic domain it is the spontaneous magnetization MS, or in the case of a paramagnet or

diamagnet induced by an applied field, the arose uniform magnetization.

The magnetization can also usually be extended to a macroscopic average over a sample as a sum

of all domains i, with volume Vi:

M =∑i MiVi∑i Vi

. (2.2)

According to Ampere, a magnet is equivalent to a circulating electric current, and therefore, the

elementary magnetic moment can be represented by a tiny current loop, which is generally calculated

with:

m = 12

∫r× j (r) d3r, (2.3)

7

where j (r) is the current density at point r.

The units of the magnetic moment are Am2 and of the magnetization A/m (as defined in equation

2.1).

2.1.2 Topics of Magnetostatics

The B-field

The field created by the magnetic moment has the same form of an electric dipole p = qδl formed of

positive and negative charges ±q separated by a distance δl, being the vector p directed from −q to +q.

Hence regarding the magnetic moment as a magnetic dipole [15].

The magnetic field δB created by a small current element jδV at any point is given by the Biot-Savart

law:

δB = −µ0

4πr× jr3 δV, (2.4)

where µ0 is the magnetic permeability in free space. With integration of this equation arises the magnetic

moment m, and in terms of the position vector r:

B = µ0

4π

[3(m · r)r

r5 − mr3

]. (2.5)

The field falls off as the cube of the distance from the magnet. The field thus defined has certain

properties, which are related to the Maxwell’s equations. First it is divergenceless:

∇ ·B = 0, (2.6)

which means it has no sources or sinks. This field is said to be solenoidal - the lines of force form

continuous loops. It is also called the magnetic flux density, since from Gauss’s theorem the element

magnetic flux dΦ = B · dA can be defined, being dA the area through which the magnetic flux flows.

The B-field can be created by a) moving charges, including electric current; and b) magnetic moments,

which are equivalent to current loops. In a steady state, the relation between the magnetic flux density

B and the current density j is given by the Maxwell’s equation:

∇×B = µ0j. (2.7)

This relation is specially used when deriving Ampere’s law and calculating field due to highly symmetric

current distributions. The B-field has units the Tesla (T).

The H-field

The magnetic H-field is an indispensible auxiliary field when dealing with magnetic materials. The

magnetization of a solid reflects the local value of H. In free space, both the B and H are related

by the magnetic permeability µ0 as B = µ0H. Therefore, in free space, the derivation of both fields

8

is interchangeable and immediate. The difference arises in a material medium, where equation 2.7 is

related, instead, to the total current density [15, 16]:

∇×B = µ0 (jc + jm) , (2.8)

where jc and jm are respectively the conduction current (such as in electrical circuits) and the magne-

tization current (associated with the magnetized medium). As opposed to jc, jm cannot be measured.

However it is related to the magnetization through the relation jm = ∇×M.

So that the relation from equation 2.7 holds for these materials, with quantifiable physical parameters,

a new auxiliary field (the H-field) is defined such that ∇ ×H = jc. This field relates to the B-field and

the magnetization M through the relation:

H = Bµ0−M. (2.9)

This field is, however, no longer divergenceless. It has sources and sinks associated with nonuniformity

of the magnetization:

∇ ·H = −∇ ·M. (2.10)

This definition, when one compares to the electric field, implies the existence of fictitious charges, usually

called North and South Poles. The field of H appear to originate on these horizontal surfaces of the

magnet, where a magnetic charge density σm = M · en exists, being en the unit vector normal to the

surface. So we can imagine that H, like the electric field, is created by a distribution of positive and

negative magnetic charges qm, which would be given by the relation:

H = qm4π

rr3 . (2.11)

The magnitude of the field falling with the square of the distance. This is called the Coulombian approach

to the magnetic field.

H is not only created by conduction currents. Any magnet will produce this field both in space and

in its own volume. Therefore H can also be written as the sum of the contributions from the conduction

currents Hc and the magnetization distributions Hm:

H = Hc + Hm. (2.12)

The second contribution is known as the stray field when outside a magnet or as the demagnetizing field

(Hd) within it, since therein it is oppositely directed to M. The units of H are A/m, the same as for M.

Magnetic susceptibility and permeability

For some materials, the B-field and the H-field can be further related to each other using the perme-

ability [15]. The simplest materials are linear, isotropic and homogeneous (LIH). This means that the

9

susceptibility (χ) is small, and a small uniform magnetization is induced in the same direction as an

external field. The magnetization is related to the internal H-field by a dimensionless scalar known as

the internal susceptibility χ: M = χH. The Permeability is related to the susceptibility, defined in the

internal field. In LIH media the permeability µ is given by

B = µH, (2.13)

where µ can be related with the permeability in free space µ0 through the relative permeability µr:

µ = µrµ0. From the relation B = µ0(H + M) comes that the relative permeability is µr = 1 + χ. χ is

generally a tensor, which for ferromagnetic results in M different from zero even when H is zero. But for

diamagnetic and paramagnetic materials χ is a scalar.

H-field calculation from a magnetized medium

Figure 2.1: Calculation of the magnetic field outside a magnetized cylinder by summing the fields pro-duced by the distribution of magnetic charge. Adapted from [15].

The calculation of the magnetic field created by a magnetized material, with no flowing currents,

can be approached by using the equivalent distributions of magnetic charge in the bulk ρm and on the

surfaces σm [15, 16]. Defining these quantities as:

ρm = −∇ ·M,

σm = M · en.(2.14)

So, from the coulombian approach from equation 2.11, the field due to a small volume element δV and

a charge qm is:

δH = qm4π

rr3 δV, (2.15)

which, integrating in the bulk material and introducing the charges from equation 2.14, results in a stray

field at point P:

10

H(r) = − 14π

∫V

d3r′ (∇ ·M(r′)) r− r′

|r− r′|3 + 14π

∫S

d2r′ (M(r′) · en) r− r′

|r− r′|3 , (2.16)

where the integration for the bulk charges is made on the bulk volume V , and the surface charges on

the charged surface S, where r′ is the position vector of the charge and r the position vector where the

field is calculated. Figure 2.1 shows the geometry and the concept used for this calculation approach.

2.1.3 Classification of Magnetic Materials

Diamagnetism and Paramagnetism

In most materials electrons in an atom move in away that they cancel each other’s magnetic moments.

Nonetheless, when a magnetic field is applied the orbital motion of the electrons is affected and a very

small magnetic moment is induced, being opposite in direction to the applied field. This phenomenon

is called diamagnetism. For these materials the susceptibility χ is negative and very small, being a

typical value of -10−5. Figure 2.2 shows the magnetic response in blue of a diamagnetic material in the

presence of the applied external field.

Figure 2.2: Magnetic response of a diamagnetic (in blue) and paramagnetic material (in red) in thepresence of an external magnetic field.

Other materials have unpaired electron spins, which result in a permanent dipole moment. These

dipole moments tend to cancel each other out in the bulk material due to thermal fluctuations. However,

in the presence of an external applied field, the dipole moments align themselves, enhancing the field.

This phenomenon is called paramagnetism. In a paramagnetic material the susceptibility χ is positive

and typical values are in the range between 10−5 and 10−3. Figure 2.2 shows the magnetic response in

red of a diamagnetic material in the presence of the applied external field.

On both these phenomena, no magnetization remains when no magnetic field is applied to the ma-

terial.

Ferromagnetism and Anti-Ferromagnetism

Although in a paramagnetic material some of the magnetic moments align, most of them don’t align at all.

The dipoles are independent of each other. In a ferromagnetic material, however, the magnetic moments

interact so strongly that almost 100% in the material align [17, 18]. These materials are formed by

magnetic domains, which are volumes in the bulk where the atomic magnetic dipoles are spontaneously

aligned. In each domain the atomic magnetic moments are all parallel, but in an initial state, in order

11

to minimize the net free energy of the system, the different domains have different directions. If placed

in a magnetic field, the domains will start to align with its direction, resulting in a magnetic moment

of the material. For a sufficiently large magnetic field, all the domains are aligned, and the material

is said to be in a saturated state (point S or S′ in Figure 2.3). After turning off the strong field, some

of the domains return to a random orientation. However, some will not, with a ratio depending on the

material and geometry of the object. Resulting in a magnetization at zero field, which is called the

remanent magnetization MR or, correspondingly, the remanence BR. The field necessary to nullify the

magnetization of the material is called the coercive field, or coercivity Hc. On Figure 2.3, the magnetic

response of a ferromagnetic material is presented. The curve here shown is called an hysteresis loop.

The magnetization is not linear with the fiels and the susceptibility χ has now a dependence on the

applied field and on the history of the material. Above the Curie temperature, the ferromagnetic material

starts behaving as a paramagnetic material.

Ferromagnetic materials can further be differentiated as soft and hard. The difference lies on the

saturation magnetization. Hard magnetic materials have a magnetization at zero field close to the satu-

ration magnetization. Soft magnetic materials have only a remanenscent magentization, which is usually

lower than the saturation magnetization. The coercivity for soft magnetic materials is also lower than for

an hard magnetic.

Figure 2.3: Magnetic response of a ferromagnetic material in the presence of an external magnetic field.Taken from [18].

An anti-ferromagnetic material is very similar to a ferromagnetic material. However the exchange

interaction between domains create anti-parallel alignment of the magnetic moments. As ferromagnetic

materials, anti-ferromagnetic material behave like paramagnetic materials above the Neel temperature.

Superparamagnetism

By reducing the size of a ferromagnetic material a critical size may be reached, on which only one

domain can be sustained.

This phenomenon is called superparamagnetism [19]. The magnetization curve of a material with

this phenomenon resembles the ferromagnetic curve in a sense that it has a saturation magnetization,

12

however, both the coercivity and remanence are zero, i.e., at zero applied field, the material is perfectly

non-magnetic. When compared to paramagnetism, the magnetic moment that arises from an applied

magnetic field is much higher, since the entire domain aligns with the field instead of just the ions (or

unpaired electrons), hence the prefix ”super” in the name. On Figure 2.4 the magnetic response from a

superparamagnetic material is presented.

Figure 2.4: Magnetic response of a superparamagnetic material in the presence of an external magneticfield.

2.1.4 Magnetic Ink

Magnetic ink is no more than a fluid media with magnetic properties. As it was already said in chapter

1, the ink is usually made of mainly four types of ingredients [12]:

• the colorants, or magnetic pigments, which present the ink with color and the magnetic properties.

They are usually magnetic nanoparticles of some ferromagnetic alloy;

• the vehicles or binders, which have multiple functions in the ink such as dispersing and binding the

particles modifying the mechanical properties, and also presenting some other special properties;

• the solvent, which dissolves all the other components and adjusts the viscosity of the ink;

• and other additives which are specific for each ink and are designed to enhance properties of the

ink.

The magnetic pigments are generally magnetic nanoparticles of sizes comprised between 1 nm di-

mension, and no more than 100 nm [11, 3]. The distance between the nanoparticles in these dispersions

can vary broad limits from tens of nanometers to fractions of a nanometer. As a rule, nanoparticles are

shaped like spheroids. The magnetic properties of nanoparticles are determined by many factors, includ-

ing the chemical composition, the type of the crystal lattice, the particle size and shape, the morphology

(in case of inhomogeneous particles), the interation of the particle with the surrounding matrix (or the

solvent and vehicles) and the neighboring particles. Normally, when addressing magnetic nanoparti-

cles one considers the so called single-domain particles with typical size in the range from 15 to 150

nm. However, particles, whose sizes are smaller than the domain size range are of extreme interest. A

single particle of size comparable to the minimum domain size would not break into smaller domains.

When in this limit, the particles show superparamagnetic behavior.

Although real nanoparticles can have a complex magnetic structure, an assembly of noninteracting

single-domain isotropic nanoparticles behaves like the above described superparamagnetism. At a

temperature T in an applied field H, the average magnetization of the assembly is given by [3]:

13

M = MS

[coth

(µ0mpH

kT

)− kT

µ0mpH

], (2.17)

where mp is the individual magnetic moments of the particles and k the Boltzmann’s constant. This

equation is a Langevin-like function.

2.2 Magnetic Tunnel Junctions

2.2.1 Tunnel Magnetoresistance

Magnetoresistance is defined as the change in electrical resistance as a function of an external applied

magnetic field, for a given structure of materials. Defining the range of resistances between a minimum

and a maximum value, Rmin to Rmax respectively. The magnetoresistive (MR) effect is quantifiable by

defining its value to be the resistance variation (∆R) relative to its minimum resistance value (Rmin):

MR = Rmax −RminRmin

= ∆RRmin

(2.18)

The tunnel magnetoresistance (TMR) effect was first qualitatively interpreted by Julliere in 1975

[20] and it is present in magnetic tunnel junctions (MTJ). MTJs are structures based on a multilayer

structure with two ferromagnetic (FM) layers separated by an insulator (I), typically aluminum oxide

(Al2O3) or magnesium oxide (MgO) [21], in a configuration FM-I-FM. Its electrical resistance, in a current

perpendicular to the plane (CPP) configuration, depends on the relative orientation of its ferromagnetic

layers, which is determined by the spin-dependent tunneling of electrons across the insulating spacer

[22].

FM materials have high electron spin polarization, i.e., they have a great asymmetry of spin states at

the Fermi level. We can analyze the tunneling effect across the insulating barrier as occurring through

two independent spin channels, one channel for spin-up and one channel for spin-down. The electrons

that are present at the Fermi level of the first FM layer tunnel into free equivalent spin states at the Fermi

level of the second FM layer (Fig. 2.5).

Figure 2.5: Schematic of the spin-dependent tunneling effect. Left: The FM layers have a parallel con-figuration, so spin-up and spin-down DOS are similar for both layers and electrons tunnel from majorityspin states to majority spin states and from minority spin states to minority spin states. Right: The FMlayers have an anti-parallel configuration, so spin-up and spin-down DOS are symmetric between layers,so electrons tunnel from majority spin states to minority spin states and vice versa. From reference [1].

The resistance of the junction is related with the conductance (G), which in turn is related with the

14

flow of electrons across the junction. The conductance can be described as the product of the density

of states at the Fermi level in both FM layers for each spin configuration [22]. Defining D↑i and D↓i as

the density of states for electron spin up and down, and for each electrode. The conductance can be

described by the equations:

Parallel configuration: Gp ∝ D↑LD↑R +D↓LD

↓R

Anti-parallel configuration: Gap ∝ D↑LD↓R +D↓LD

↑R

(2.19)

When the layers have opposite magnetization direction, tunneling occurs between spin bands with

higher DOS and spin bands with lower DOS, leading to a lower conductance than for parallel configu-

ration, on which tunneling occurs between spin bands with similar DOS (Fig. 2.5), yielding Gp > Gap.

Since the resistance is inversely proportional to the conductance, a parallel configuration leads to a

lower resistance than an anti-parallel configuration: Rp < Rap.

2.2.2 Structure

The basic MTJ is structured in layers: a thin insulating barrier separates two FM layers. One of which, the

pinned layer, is pinned by an adjacent anti-ferromagnetic (AFM) layer (pinning layer), while the other FM

layer (free layer) is free to rotate, acting as the sensing layer of a sensor (Fig. 2.6). The magnetization of

the pinned layer is fixed to a certain direction due to an exchange bias coupling at the AFM/FM interface

[23].

Buffer or seed layers are employed in order to influence the crystal texture and the smoothness

of the interface, allowing the enhancement of the tunnel junction properties [24]. A thin cap layer or

cover layer is often used in order to prevent corrosion and oxidation of the MTJ stack [25]. Synthetic

Anti-ferromagnetic (SAF) and Synthetic Ferrimagnetic (SyF) layers are also usually employed in the

structure instead of just AFM layers [26].

Figure 2.6: Basic structure of a MTJ sensor.

15

Choice of Insulator

The most commonly used insulators on magnetic tunnel junctions is aluminum oxide (Al2O3). However,

magnesium oxide (MgO) based MTJs, due to its giant TMR values, are starting to be implemented in

practical applications and novel spintronic devices. The difference between these insulators lies on the

physical structure: Al2O3 is amorphous and MgO is crystalline [21]. The difference in the structure

results in different TMR mechanisms. In Al-O amorphous barriers, the tunneling occurs incoherently,

which results in a reduction of tunneling spin polarization and thus in lower MR ratios. On the other

hand, in MgO crystalline barriers the tunneling occurs coherently, allowing much higher TMR values

[21].

Synthetic Anti-Ferromagnetic Pinned Layer

The SAF is a multilayer structure, which consists on two FM layers separated by a thin nonmagnetic

(NM) layer. The FM layer adjacent to the pinning layer (AFM) is pinned by exchange coupling, while the

other FM layer, the reference layer, is antiferromagnetically coupled to the pinned FM.

The SAF pinned layer allows the increase of the exchange bias field between pinned and pinning

layer, improving the magnetic stability. It also improves the thermal stability, which means that the

pinning field is weakly influenced by the raising temperature [27]. It also decreases the magneto-static

coupling between the free and the pinned layer [26].

Synthetic Ferrimagnetic Free Layer

The Synthetic Ferrimagnetic (SyF) free layer is a structure similar to the SAF, which consists of two FM

layers antiferromagnetically coupled through a thin nonmagnetic (NM) layer. The difference lies on the

freedom for the magnetization to rotate, since on the free layer there is no exchange coupling with an

antiferromagnetic layer. Due to the reduction of the effective thickness of the free layer, the SyF layer

structure shows higher sensor sensitivity but increased offset field Hf , which is the displacement of

the sensor transfer curve center from H = 0, by increasing the Neel inter-layer coupling field, which is

indirectly proportional to the effective thickness [26].

2.2.3 MTJ Linearization

A sensor behavior is characterized by its transfer curve. For an ideal magnetic sensor the curve is linear

and hysteresis-free within the desired operating range (Figure 2.7). The curve has two stable plateaus

that correspond to to the anti-parallel and parallel states and a linear reversible path between them.

Magnetoresistive devices will have linear response to an external field if the sensing layer magnetiza-

tion changes its direction, in relation to the pinned layer magnetization between parallel and anti-parallel

states, through coherent rotation. Therefore, in the absence of an external magnetic field, the pinned

layer magnetization is set to a given direction, the easy axis (e.a.), by exchange coupling with the pin-

ning layer, while the sensing layer magnetization is oriented in the orthogonal direction. The coherent

16

Figure 2.7: Ideal transfer curve of a MTJ sensor.

rotation is achieved with magnetic fields applied perpendicular to the sensing layer magnetization but

parallel to the easy axis. However, as deposited, MTJs display a squared output signal, since the refer-

ence and sensing layers show a parallel configuration. So, the reference magnetization direction is set

by setting the exchange coupling direction through annealing in a uniformly strong magnetic field. The

magnetization of the sensing layer is set orthogonally using different strategies [10]:

1. Using deposition systems that allow a crossed configuration to be set during deposition, by apply-

ing orthogonal magnetic fields during film deposition;

2. To tailor the self-demagnetizing field of the sensing layer, by shape anisotropy (the material pat-

terned shape and dimension) and layer thickness engineering.

3. By applying an external field bias transverse to the sensing direction, promoting a hysteresis-free

curve, using permanent magnets (PMs) or a current line loop.

4. Using a CoFeB sensing layer thin enough to have granular film structures that in the limit present

a superparamagnetic like behavior providing linear hysteresis-free responses with simple designs

and low power consumption, without the requirement of large aspect ratios.

5. Introducing a soft pinned sensing layer, which sets the desired orthogonality to the pinned layer at

zero applied field.

Soft Pinned Sensing Layer

The sensors used in this work have a stack, which has incorporated a soft pinned sensing layer.

This strategy requires two AFM layers to be incorporated: next to the fixed magnetization electrode,

and next to the sensing layer. The exchange field of the sensing layer must be carefully thought, by

choosing the adequate AFM material and adjacent FM, as it will define the sensor saturation field, and

thus, its sensibility. For linear behavior, the magnetization on the electrodes must be set orthogonal, so it

is required that both the exchange coupled interfaces have different temperature stability. The exchange

bias vanishes above the blocking temperature Tb. The Tb value is not characteristic of the material, but

17

Figure 2.8: Typical structure of a junction with both sensing and reference layer pinned. The arrows referto the magnetization orientation of the layers at low field. Taken from [10].

depends on the AFM thickness. Usually one chooses the blocking temperature of the reference layer to

be higher than the one of the sensing layer, by using different materials or using the same material but

with different thicknesses. The orthogonal magnetization is set through consecutive annealing steps.

First performed at higher temperature (T ) to set both layers to the desired reference magnetization

direction (T > Tb of the reference layer). The second step is to set the magnetization direction of the

sensing layer to be orthogonal to the reference layer (T > Tb of sensing layer but T < Tb of reference

layer) [10, 28]. Figure 2.8 presents the typical stack with both pinned layers.

Most of the times, however, the transfer curve is not ideal like the one presented on Figure 2.7,

presenting coercive field and offset. Taking the above mentioned strategies will tend to lower the coercive

field and the offset field. From the obtained curve several properties of the sensor can be extracted.

In specific, in the linear region where the ∆R ∝ ∆H, one can define the sensitivity. The saturation

magnetization is also important since it restricts the linear range. The TMR ratio is also taken from the

curve, as well as the offset and coercivity values, which should be as low as possible.

2.2.4 Sensitivity, Voltage Bias and the Output Signal

Sensitivity

Using the physical factors obtained from the transfer curve, one can define the sensor field sensitivity,

which is defined as the variation of the resistance (output) with respect to the magnetic field variation

(input), or how reactive the sensor is to a field variation. For the ideal linear response, the MTJ sensor

sensitivity S can be expressed by the slope o the linear region, normalized to its minimum resistance,

taking into account the MR calculation given by equation 2.18:

S = 1Rmin

(∆R∆H

)linear

= TMR(∆H)linear

[%/Oe] . (2.20)

A sensor can achieve a high sensitivity by reducing the saturation fields and increasing the TMR

ratio.

18

Bias Voltage Dependence

The TMR ratio of a MTJ depends on the bias voltage that is applied, being approximately constant for

bias voltages under 30mV, and approximately linearly decreasing until it reaches a value which repre-

sents half of the maximum TMR ratio (TMR0), with a correspondent voltage represented by V1/2. As a

result, the TMR ratio depends on the bias voltage Vb as follows [1]:

TMR(Vb) = TMR0

(1− Vb

2V1/2

). (2.21)

The decrease of the TMR depends on the barrier type, interfaces and the ferromagnetic materi-

als. In particular, the defects in the insulating barrier allow conduction to happen across the barrier for

higher voltages, decreasing the tunneling effect and thus the TMR ratio [29]. Therefore, to minimize the

dependence, a high quality barrier is required.

The sensitivity will also be affected by the voltage bias dependence. Defining S0 as the maximum

sensitivity value obtained for low bias voltages:

S(Vb) = S0

(1− Vb

2V1/2

)(2.22)

If the bias voltage across the barrier is increased to a certain value, the insulator barrier will be

destroyed, with the creation of pinholes, leading to a sudden decrease in the resistance. The breakdown

of the barrier is directly related to the dielectric strength of the insulator, which is the electric field value

E that results in the breakdown of the barrier. This phenomenon occurs for a breakdown voltage Vbreak

whose value is related to E and the thickness of the barrier t. For the typical insulating barriers the

breakdown voltage is on the order 0.5 − 2V . Usually, in practical sensor applications, the breakdown

voltage can be overcome by incorporating an array of MTJ sensors in series. By doing so, a higher

voltage can be applied to the system and it doesn’t compromise the individual sensors.

Output Signal Variation

In the linear region of the sensor’s response, the sensor resistance can be described as the sum of

a nominal resistance R0, at zero magnetic field, and a variation ∆R that, looking at equation 2.20 is

directly related to the variation of the applied magnetic field (with zero magnetic field) H and the sensor

sensitivity S(Vb):

R(H) = R0 + ∆R = R0 + S(Vb)RminH (2.23)

Consequently, the voltage output variation ∆V due to an external magnetic field variation ∆H =

H2−H1, and the current bias I that flows through the MTJ due to the bias voltage applied can be written

as:

∆V = (R (H2)−R (H1)) I = S(Vb)RminI∆H (2.24)

19

A single sensor output can also be given, taking into account its geometry (width W and height h),

its resistance area product (RA) and the angle α between reference and sensing layer magnetization

directions:

∆V = 12TMR(Vb)I

RA

Wh〈cosα〉 (2.25)

Intuitively, a larger area and a thinner barrier thickness lead to a higher number of electrons tunneling

across the barrier, increasing the electrical current that flows in a CPP configuration. So, the resistance

of the sensor will decrease for higher sensor area. Therefore, the resistance area product RA is a

constant parameter and independent of the sensor area. However, this parameter is an intrinsic property

of the tunneling barrier, being highly dependent on the barrier thickness.

20

Chapter 3

System Description and

Characterization

This chapter is focused on a detailed description of the positioning system in development, which has

the objective of reading structures printed with Magnetic Ink. The two main configurations of the sensing

head will be described and simulated. The two considered sensors are characterized for each configu-

ration and the magnetic behavior studied. The second component of the system, which comprises the

magnetic ink structures, is also characterized and present in Appendix ?? (confidential).

3.1 System Description

The aim of this work is to develop a magnetic ink surface scanning device, therefore it has two main

different units: the sensing head with the magnetic sensor and the integrated electronics; and a sample,

where the magnetic structures are printed.

As an interdisciplinary project, the development has input from the different parts: the magnetic

ink is provided by BOGEN Electronics GmbH1, who is also responsible for printing the magnetic ink

structures and developing the integrated electronics as well as the future casing and packaging; and

INESC-MN is responsible for both the characterization of the ink and the development of the sensing

head configuration for the system.

3.1.1 Magnetic Ink

The magnetic ink for this work was available in powder form and printed on various substrates in the

form of periodic structures. It is a ink specifically designed for the MICR application and has both soft

and hard magnetic properties. Further characterization is present in Appendix ?? (confidential).

1http://www.bogen-electronic.com/en/

21

3.1.2 Configurations

To read MI, a constant magnetization of the sample is needed to magnetize or fully saturate the ink.

The magnetization of the ink creates stray fields between the poles created on the structures, which the

magnetic sensor is then capable of measuring. There are mainly three strategies to magnetize the ink

during measurements:

• using a current line, which creates a field that can be used to magnetize the ink;

• using fixed permanent magnets on the substrates; or

• using permanent magnets on the reading head.

According to the objectives of the project the first two strategies are not considered as the solution.

Using a current line requires an extra power source and thus extra electronics on the system. Using

static permanent magnets would increase the complexity of the information storage, and ultimately the

cost. Therefore, the third strategy is considered for this system.

In total six different configurations using magnets were considered for the project. They were named

Standard 1 through 6, or short, Std i (with i=1,...,6). These are represented on Figure 3.1. The con-

figurations were designed taking into account different kinds of magnets (In-Plane (IP) or Out-Of-Plane

(OP) magnetization) and also different ways to magnetize the magnetic bits. Notice that the direction of

movement is always x.

The project in hands has, however, some requirements that exclude a priori some of the configuration

standards. The main requirements are that:

a) it must be able to measure in both directions of movement (−x and +x);

b) the sensor must be placed so that it can be as close as possible to the magnetic ink (bottom of the

sensing head); and

c) for simplified electronics, the magnetic sensor should be able to create alternating North and South

poles.

As a result, Std 2, Std 4, Std 5 and Std 6 are excluded. To measure a saturated magnetized bit the

magnet should magnetize during or shortly prior to the sensor passage, but Std 5 and Std 6 only allow

the magnetization of the ink in one direction of movement (+x), therefore not complying with requirement

a). The other two configurations magnetize the ink in a perpendicular direction to the movement of the

reading head. By doing so, no alternating poles are created. It comes that only Std 1 and Std 3 (Figures

3.1a and 3.1c respectively) are considered in this work for the development of a reading head for the

proposed system.

A set-up scheme of the system is presented on Figure 3.2a, on which two magnets are placed on

each side of the sensor and the scan is done in the x direction. The sensor is sensitive in the y0z plane

and the field created by the magnets in the region of the sensor is perpendicular (in the x direction) to

the sensitive plane in order to avoid influence on the sensor’s magnetic behavior. On the region of the

MI structures, the magnetic field from the magnets (in blue) create a magnetization on the ink line in the

x direction. This magnetization is responsible for the creation of the stray field which is measured by the

22

(a) Ink magnetized on the x direction. 2 magnets withOOP magnetization. Sensor’s sensitive plane parallel tomagnets.

(b) Ink magnetized on the y direction. 2 magnets withOOP magnetization. Sensor’s sensitive plane parallel tomagnets.

(c) Ink magnetized on the x direction. 2 magnets with IPmagnetization. Sensor’s sensitive plane parallel to mag-nets.

(d) Ink magnetized on the y direction. 2 magnets with IPmagnetization. Sensor’s sensitive plane parallel to mag-nets.

(e) Ink magnetized on the x direction. Magnet with OOPmagnetization. Sensor’s sensitive plane perpendicular tomagnets.

(f) Ink magnetized on the x direction. Magnet with OOPmagnetization. Sensor’s sensitive plane parallel to mag-nets.

Figure 3.1: Configuration standards considered for the project.

sensor. Notice that by measuring the z component of the ink line’s field it is possible to measure north

and south poles in consecutive borders of the structures. The presence of two magnets (one passing

before and another after the point of interest) allow the measurement of both directions of movement

(−x and x). It is important to define some geometrical variables, which is done on Figure 3.2b. The

23

(a) Conceptual scheme of the set-up for the system.(b) Definition of the geo-metrical variables.

Figure 3.2: Proposed concept for the system.

variable a indicates the shift between the sensor and the magnets’ bottom line; the variable b indicates

the distance between the magnets, with the sensor always placed at b/2. The Reading Distance (RD) is

introduced, which is the distance in z from the top of the magnetic ink bit and the sensor.

In this work two kinds of permanent magnets (PMs) with the same dimensions but with different

magnetization direction were used, in accordance with the configuration standards Std 1 and Std 3

from Figure 3.1. The PMs are characterized by three main parameters: the dimension, the relative

permeability of the material µr and the remanence (or remanent flux density) Br. The latter is related

to the magnetization of the magnets at zero applied external magnetic field, and is commonly described

as the maximum magnetic flux created by the PM. Taking into account the geometry and coordinate

system on Figure 3.1 and the physical values of the magnets given by the manufacturer (supermagnete

[30]), the parameters considered for these magnets are:

Magnet Geometry (mm3) µr Br (T) Direction ofMagnetization

OOP Magnetization 1×10×4 1.05 1.43 xIP Magnetization 1×10×4 1.05 1.43 z

Table 3.1: Physical parameters of the magnets used.

The holders for the magnets are fabricated using a conventional 3D printer. The holders are designed

using a technical drawing tool, in specific AutoCAD. Due to restrictions in the assembly of PCB plus

magnet holders, on both configurations the separation between the magnets is b = 6 mm.

3.1.3 Simulation of the Magnets Configurations

Both Std 1 and Std 3 presented on Figures 3.1a and 3.1c were simulated using the software COMSOL

Multiphysics, which is a finite element modeling (FEM) tool. In this software the 3D geometry of the po-

sition of the magnets is designed and the module Magnetic Fields, No Currents (mfnc) is used. The field

created by the magnets is computed by using the remanent flux density Br and defining the magnetic

24

flux conservation on all space. Inside the magnets the magnetic flux density B and the magnetic field H

are related by the constitutive relation

B = µ0µrH + Br,

being µr and Br the parameters of the magnets, whose values are resumed in Table 3.1. Even though

only the magnetic field created by the magnets is simulated, it allows the evaluation of the field created at

any given point of the space defined, therefore allowing the prediction of the effectiveness of magnetizing

the ink as well as of the influence on the sensor.

On Figure 3.3a the 3D geometry is shown with the magnet dimensions, for both configuration sim-

ulations (both kinds of magnets have the same dimensions). The simulation results are shown in a 2D

plane, which is a slice from the 3D geometry simulation. This slice is the plane x0z at y = 0 mm, which

corresponds to the center symmetry plane of the geometry, as seen in Figure 3.3b.

(a) 3D geometry of the simulated configurations. (b) 2D plane slice, where the results are shown.

Figure 3.3: 3D geometry used on the simulation software. Geometry of the magnets: 10×4×1 mm3;b = 6 mm.

OOP Magnetization - Std 1

The first simulation was done for the configuration of Figure 3.1a, on which the magnets are magnetized

in the x direction. A vector field plot was first done to evaluate the direction of the magnetic field in space.

Figure 3.4 shows the resulting plot. The vectors are plotted in a logarithmic scale, as the interest lies in

determining which components are present and not what the strength of the field is.

There are mainly two areas of interest: around the sensor, which is ideally placed at the point (x, z) =

(0,-2) mm; and on the area where the magnetic ink structure is being magnetized for measuring, which

is at any z below z = -2 mm and between x = -5 mm and x = 5 mm.

Immediately it is noticed that on the region where the MI structures is measured (x=0) the field is

very weak and the x component, which is the most important for magnetizing the ink is almost non-

existent. The only regions where the ink is magnetized in the x direction are right below each magnet.

25

Figure 3.4: Magnetic field vectors (in logarithmic scale) for the magnets with OOP magnetization. Thenumbered lines represent the regions where further studies are done.

If soft magnetic ink is used for the structures, the magnetization of the ink could not hold time enough

to be measured at x=0. Futhermore, in the area where the sensor is placed, the field is not purely

and uniformely perpendicular to the sensing plane y0z. A component on the z direction, which is the

sensitive direction of the sensor, is easily seen, thus predicting some influence on the sensor magnetic

behavior. On the same plot three lines are drawn, this lines indicate three regions where the magnetic

field components are studied. These lines are:

1. from the center point between the magnets to 6 mm below this point: from z = 0 mm to z = -6 mm

with fixed x = 0 mm;

2. from the inner pole of the left magnet to the inner pole of the right magnet at the height where the

sensor is placed: from x = -3 mm to x = 3 mm with fixed z = -2 mm;

3. a span of 10 mm where the MI structures are located (if at 2 mm Reading Distance): from x = -5

mm to x = 5 mm with fixed z = -4 mm.

On each line, from one tip to the other a line plot is done of the magnetic field components. The plots

are presented on Figure 3.5.

On Figure 3.5a the three components of the field are plotted along Line 1. Along this axis the y and

z components of the field remain constant and null whilst the x component suffers variation. Since the

latter component of the field is the one that has no effect on the magnetic behavior of the sensor, the

sensor should be placed on this axis, so as expected the sensor must be placed at b/2 - half the distance

between the magnets.

On Figure 3.5b, where the three components of the flux are plotted along Line 2, this placement is

once more seen as the best. The y component along this line suffers no change and remains null. The

x component is constant and different from 0 but since it is perpendicular to the sensor sensitive plane,

no influence from this component is expected. However, around the point where the sensor should be

placed, the z component of the field has some variation. Since the z component lies on the sensitive

26

(a) Plot of Bx, By and Bz on line 1 (x=0;y =0). (b) Plot of Bx, By and Bz on line 2 (z=-2mm;y =0).

(c) Plot of Bx on line 3 (RD = 2mm - z=-4mm;y =0).

Figure 3.5: Simulation of the different components of the field in given regions of the simulated spacewith Std 1.

direction of the sensor, this variation shows how precise the position of the sensor must be.

Finally, Figure 3.5c shows the areas where the MI structures will be magnetized by the magnets (if

at a RD=2mm), along Line 3. As it was already observed, the magnetization on the desired direction (x)

is only expected to happen below the magnets, where this component is different from zero. Notice that

right below the magnet, at x =0, the ink is no longer being magnetized (Bx =0).

IP Magnetization - Std 3

The next simulation was done for the configuration of Figure 3.1c, with magnets with magnetization on

the z direction. As before a vector field plot was done to evaluate the direction of the magnetic field in

the space. The resulting plot is represented on Figure 3.6, and once again the vectors are plotted in a

logarithmic scale.

Overall the behavior of the field in this configuration is close to the desired for the sensing head.

Where the MI structures are placed, the x component of the field is visible and present in the region

below the sensor, around x =0. On the area where the sensor is placed, some component of the

undesired z component is visible. However, some regions around this place show low variation and low

presence in the z component. That’s why the field components of the field are also evaluated for a 4th

line, apart from the 3 lines already plotted for the previous simulation. So on the same vector field plot,

27

Figure 3.6: Magnetic field vectors (in logarithmic scale) for the magnets with IP magnetization. Thenumbered lines represent the regions where further studies are done.

four lines are represented, showing where the evaluation of the different components were done. The

lines are:

1. from the center point between the magnets to 6 mm below this point: from z = 0 mm to z = -6 mm

with fixed x = 0 mm;

2. from the inner pole of the left magnet to the inner pole of the right magnet at the height where the

sensor is placed: from x = -3 mm to x = 3 mm with fixed z = -2 mm;

3. a span of 10 mm where the MI structures are located (if at (2-a) mm Reading Distance): from x =

-5 mm to x = 5 mm with fixed z = -4 mm.

4. from the inner pole of the left magnet to the inner pole of the right magnet at the height where the

sensor is placed in a configuration with a = 0.5 mm: from x = -3 mm to x = 3 mm with fixed z =

-2.5 mm;

Figure 3.7 shows the components of the magnetic field plotted on the defined Lines 1, 2 and 3.

On Figure 3.7a the three components of the field are plotted along Line 1. Once more, along this

axis the only non-zero component of the field is Bx, while both By and Bz, which are in the sensitive

plane of the sensor are null. Therefore, the sensor should be placed on this axis.

On Figure 3.7b, for Line 2 the same behavior is again observed, the x component is non-zero, but

it is not expected to influence the sensor. Both By and Bz are zero half way between the magnets.

However, once more, the z component has some variation around this point, requiring thus very precise

positioning at this point.

Finally, on Figure 3.7c Bx is plotted on Line 3 and it is clearly seen that the ink is magnetized in the

x direction, right below the sensor. Therefore expecting a more efficient magnetization of the ink during

measurement.

Since it was observed on Figure 3.6 that by increasing the distance between the sensor and the

bottom line of the magnets, a, the magnetic field seems to be more uniform and purely on the x direction,

28

(a) Plot of Bx, By and Bz on line 1 (x=0;y =0). (b) Plot of Bx, By and Bz on line 2 (z=-2mm;y =0).

(c) Plot of Bx on line 3 (RD = 2-a mm; z=-4mm;y =0).

Figure 3.7: Simulation of the different components of the field in given regions of the simulated spacewith IP magnetization magnets.

Figure 3.8: Plot of Bx, By and Bz on line 4. Sensor at a = 0.5 mm.

the three components of the field are plotted on this Line 4 and presented on Figure 3.8. As expected

from the vector plot, the z component of the field has very low variation around the position of the sensor

(in the middle), being zero both ways for 1 mm (between x =-1mm and x =1mm). So, by lowering

the sensor by 0.5 mm - a = 0.5 mm - on the configuration, the position needs not be so precise and

therefore less influence from the magnets is expected.

29

3.2 Sensing Head Characterization

3.2.1 Magnetoresistive Sensors

In this work, two TMR sensors with different geometries were considered. Both consisting of an array of

MTJs in series. The stack of each MTJ has a soft pinned sensing layer, which is a linearization strategy

described on section 2.2.3. The stack is as follows and graphically described on Figure 3.9a:

• [Ta 5/CuN 25] × 6/ Ta 5/ Ru 5/ Ir20Mn80 20/ Co70Fe30 2/ Ru 0.85/ Co40Fe40B20 2.6/ MgO ≈ 1/

Co40Fe40B20 2/ Ta 0.21/ Ni80Fe20 4/ Ru 0.20/ Ir20Mn80 6/ Ru 2/ Ta 5/ Ru 10,

being the thickness represented in nm and the alloy compositions in %. The barrier thickness was

optimized for a resistance-area product of 20 kΩµm2, deposited in a Singulus Timaris sputtering tool at

INL, on a thick silicon wafer covered with an SiO2 layer of 1 µm. The sensors were then microfabricated

at INESC-MN by a combination of lithography (DWL) and etching (Nordiko 3600) patterning techniques

[31]. Notice that the reference and the free layer are not pinned by a simple AFM layer. In this stack

a SAF and a SF are used as the pinning layers, both described in section 2.2.2. The annealing steps

required during the fabrication process are the same used as described in section 2.2.3, where two

different annealing at different temperatures must be done to set the perpendicular directions on the

different pinning layers. The sensors chosen vary in number of elements and on the junction area,

therefore having different spatial resolutions:

• Sensor S1 is an array of 72 sensing elements (MTJs) with an individual area of 50×50 µm2

distributed along 8 columns and 9 rows occupying a total area of 525×570 µm2 to maximize the

SNR [32];

• Sensor S2 is an array of 4 sensing elements (MTJs) with an individual area of 10×4 µm2 dis-

tributed along 4 columns and 1 row occupying a total area of 58.5×4 µm2.

Each array is represented in a schematic way in Figure 3.9b. The sensor chips were diced and wire-

bonded onto a rigid PCB (printed circuit board) designed for the measuring system (see chapter 5).

Consequently the magneto-transport curves (R(H)) were measured.

3.2.2 Magneto-transport Characterization Tool

As it was seen in Chapter 2, a MTJ sensor is characterized by its magnetic transfer curve. The same

goes for an array of MTJs as in the present work. By measuring the resistance of the sensor as a

function of the applied magnetic field (R(H)), the transfer curve is obtained and the relevant physical

values taken.

The 140 Oe setup is a magneto-transport measuring tool. It is composed of a pair of Helmoltz coils

powered by a current source; four micro-positioning Tungsten needle probes with spatial resolution of

about 10 µm, which can be connected to a current source (bias current) and a voltmeter; a microscope;

and a controlling/acquisition software. The Helmoltz coils are separated by an optimized distance taking

30

(a) Thin film structure of the used MTJ pillar. (b) Scheme of the arrays of sensors S1 and S2.

Figure 3.9: MTJ pillar structure and scheme of the junction array of sensors S1 and S2.

into account the current supplied so that in the center region between the coils an uniform magnetic

field (H) is created in a range between -140 Oe and 140 Oe, or considering B = µ0H (valid in air),

between -14 mT and 14 mT. The controlling of the setup is done using a computer software and the

communication through GPIB communication. The user can control the current bias for the sensors;

the magnetic field sweep range, limited to the setup’s range; the steps during the sweep; the number of

voltage points averaged per H applied; and the stabilization time at each measurement. The magnetic

field sweep is done from minimum to maximum and then back to minimum so that a complete magnetic

transfer curve is obtained. This setup allows also for a bias current sweep is also possible.

The measurement in this work is done by using just two of the four probes, with fixed current bias.

The current is applied in a CPP configuration and a voltage is taken from the terminals. The current

is applied in series and the voltage measured in parallel. By connecting the voltmeter to the terminals

where the current is injected, the voltage is collected and the resistance computed through Ohm’s Law.

The computer software acquires directly the voltage values and makes an initial data treatment allowing

the user to see the transfer curve in real time while the sweep is done.

3.2.3 Magnetotrasport Characterization

System with no magnets

First a reference transfer curve for both sensors S1 and S2 is measured. Since the considered system

has magnets to magnetize the MI, it is expected that cross fields from the magnets influence the sensor,

in particular components of the field laying IP with the sensor’s sensing plane. So as a comparison

and a reference, the transfer curves of both sensors S1 and S2 with no magnets present on the system

were obtained and presented in Figures 3.10a and 3.10b. The current bias used was of 100 µA for

31

S1 and 50 µA for S2. In the linear range 20 points per mT were taken. From the transfer curves, the

important characteristic values are taken, in specific the MR ratio, the minimum resistance (Rmin), the

sensitivity around zero applied magnetic field (S0), the shift of the center of the linear region (µ0Hf) and

the coercivity (µ0Hc).

(a) Sensor S1. (b) Sensor S2.

Figure 3.10: Reference transfer curves of sensors S1 and S2, without magnets present on the system.

Sensor S1 has a 542 Ω minimum resistance, with a TMR of 129%, which is limited by contact

resistance due to the high number of MTJs in series, and a linear linear response within a ±2 mT range

(around zero applied field) with a non-hysteric behavior (low coercivity). A sensitivity of 15.1 V/T is

achieved for a bias current of 100 µA.

Sensor S2 has has a 1187 Ω minimum resistance, with a TMR of 156%, and a linear linear response

within a ±2 mT range with a non-hysteric behavior (low coercivity). A similar sensitivity of -14.2 V/T is

achieved for a bias current of 50 µA.

System with OOP magnetization magnets (Std 1)

The first transfer curves measurements were done using the magnets with OOP magnetization, corre-

sponding to Std 1 configuration of Figure 3.1a. From the simulations it is easily seen a big variation

on the z component of the field around the area where the sensor is placed (Figure 3.5). Therefore,

it is expected that if the sensor is not exactly centered between the magnets, a big influence from the

magnetic field will be evidenced in the transfer curve of the sensor.

First, sensor S1 was measured in this configuration. It was observed that the transfer curve would

be different between measurements, being almost non-reproducible. Any change in the relative position

between both the magnets and the sensor changes in some way the transfer curve. And such a change

can easily happen with time, mounting or usage. Figure 3.11 shows the transfer curves obtained for this

sensor using this configuration Std 1. All transfer curves were performed with a bias current of 100 µA

and in the linear region 20 points per mT were taken.

Between different measurements always some properties changed in particular the sensitivity value.

It is seen on Figure 3.11 four different curves used for measurements, which although with similar in-

32

Figure 3.11: Set of transfer curves obtained for sensor S1 in a Std 1 configuration, alongside the refer-ence curve.

MR Rmin S0 µ0Hf µ0Hc

Ref 129 % 542 Ω 27.7 %/mT 0.4 mT 0.1 mT1 129 % 546 Ω 19.2 %/mT 1.3 mT 0.1 mT2 129 % 546 Ω 16.6 %/mT 1.8 mT 0.1 mT3 128 % 551 Ω 13.9 %/mT 2.1 mT 0.1 mT4 127 % 552 Ω 11.9 %/mT 2.6 mT 0.1 mT

Table 3.2: Physical parameters derived from the transfer curves of sensor S1, without magnets and withconfiguration Std 1.

fluences from the magnets, the resulting sensitivity was somehow different. Therefore, prior to any

measurement with this sensor on a Std 1 configuration, the transfer curve of the sensor was measured

and its values used on the data treatment of those specific measurements. Each one of the curves

presented in this figure have the properties resumed on Table 3.2. Neither the coercivity, nor the TMR

ratio, nor the resistance change significantly with the presence of the magnets. However, notice that the

linear regions of the curves measured with Std 1 are shifted from the reference curve by a minimum of 1

mT, this means that this region lays somewhere between -4 mT and 1 mT. This can be a problem if one

measures fields higher than 1 mT. These measurements would saturate the sensor, and no variation

would be obtained. As a result, the sensitivity is also affected by the presence of the magnets, and

in this case, it is lowered. Since th effect on the transfer curve is a shift (offset), it indicates that the

influence from the magnet is done on the sensitive direction. This is the component of the field in the z

direction, as it was already expected to happen.

Sensor S2 was also measured in this configuration. However, it was observed that the transfer curve

is always shifted and around zero-field, the sensor is always saturated. This means that sensor S2

cannot be used in a Std 1 configuration. This may happen due to the smaller sensing area of sensor S2

(4×59 µm2). Since sensor S1 has a larger sensing area (525×570 µm2), it makes an average of all the

magnetic field that cross it. If the z component of the field diminishes across the array of sensors, then

the influence on the transfer curve of the sensor as a whole is lower than for a much smaller area.

33

System with IP magnetization magnets (Std 3)

Using the magnets with IP magnetization (in the z direction) corresponding to the configuration Std 3

represented in Figure 3.1c, a further study was done on the transfer curves, also taking into account the

results obtained in the simulations on section 3.1.3.

First, using sensor S1 and with a = 0 mm a series of three transfer curves were measured (Figure

3.12b). Making a = 0.5 mm the same number of transfer curves were also measured (Figure 3.12a).

All measurements were performed with a bias current of 100 µA and with a step of 20 points per mT

on the linear range of the curve. By doing several measurements, with some time between them, the

consistency of the configuration can be tested.

(a) a = 0 mm. (b) a = 0.5 mm.

Figure 3.12: Study on the transfer curves of sensor 1, with configuration standard Std 3.

It is observed that compared with Std 1 measured previously, a much greater consistency is obtained:

the coercivity, the offset of the curve and the TMR ration suffer almost no changes between measured

transfer curves, nonetheless they differ a bit from the reference curve. Furthermore and in accordance

with simulations it is also noticeable much better transfer curves with a = 0.5 mm, i.e., curves with

a linear behavior around zero applied field and with low coercivity. Therefore for the measurements

done using this configuration the values used for the sensor are the ones with a = 0.5 mm. The same

conclusions on the geometry were made for sensor S2 as well.

On Figure 3.13a the transfer curve is presented for sensor S1 in a Std 3 configuration with a =

0.5 mm. Figure 3.13b shows the resulted curve for sensor S2. The values taken from the curves are

presented and resumed in table 3.3.

Looking at the obtained transfer curve for sensor S1, notice that comparing the obtained curve and

the reference curve, still some influence from the magnets’ magnetic field is observed. The sensitivity

around zero applied field is the most influenced, being decreased by almost half. The offset of the curve

suffers almost no change from the reference curve, so the component of the field that influences the

sensitive direction (z component) is not the one that has the most influence, but the sensitivity is still

altered. This is due to cross fields in the other direction of the sensitive plane (y direction), which will

affect it by increasing the linear range (and not the MR) and so, decreasing the overall sensitivity.

34

(a) Sensor S1. (b) Sensor S2.

Figure 3.13: Transfer curves for the sensor S1 and S2, with configuration Std 3 and a = 0.5 mm.

When it comes to sensor S2, The major influence on the curve seems to be a bigger shift in the

curve. Again, since sensor S2 has a smaller area, it is more prone to be affected by cross fields present

in the region of the sensor. The average on sensor S1 seems to nullify the presence of the cross field in

z, which doesn’t happen for S2. The resulting sensitivity is however higher in the presence of magnets

than it is for sensor S1.

MR Rmin S0 µ0Hf µ0Hc

Sensor S1No Magnets 129 % 542 Ω 27.7 %/mT 0.4 mT 0.1 mT

With Magnets 131 % 542 Ω 14.3 %/mT 0.8 mT 0.1 mT

Sensor S2No Magnets 156 % 1187 Ω -22.2 %/mT 0.4 mT 0.2 mT

With Magnets 159 % 1161 Ω -24.4 %/mT 1.3 mT 0.3 mT

Table 3.3: Physical parameters derived from the transfer curves of sensors S1 and S2, without magnetsand with configuration Std 3 and a = 0.5 mm.

35

36

Chapter 4

Hard Magnetic CoCrPt Structures

In this chapter a strategy to validate the used sensors is developed, by measuring well defined and

well-known structures and comparing with simulation. First the fabrication process of these structures

is explained in detail, along with the characterization of the fabricated samples. Afterwards a simulation

model of the structures is proposed.

4.1 Micro-Fabrication

The samples fabricated in this work were designed to have a periodic signal with alternating north

and south pole. The structure can be seen in Figure 4.1, where the bits have a length named pole-

pitch (PP) and are equally spaced with this same length. The material used for the structures is a

hard ferromagnetic alloy CoCrPt. The micro-fabrication process of the structures is fairly simple. It

has just four steps: alloy deposition, pole definition, excess material etching and finally setting of the

magnetization direction. Each one of the steps of this process are described with some detail in the

following sections. The run-sheet of the process can be found in Annex A, where also the AutoCAD

masks and pictures of the samples throughout the fabrication can be found.

Figure 4.1: Structure of the micro-fabricated CoCrPt structures.

Three different CoCrPt structures were designed for this project differing in the pole-pitch and the

number of consecutive structures: 240 µm, 320 µm and 1000 µm with 35, 26 and 8 structures respec-

tively. The width W of all the structures is of 3 mm and the thickness of 1000 A.

37

4.1.1 Deposition by Magnetron Sputtering

The first step of the structures fabrication is the deposition on glass substrates of CoCrPt alloy, a mag-

netic alloy. This is done using sputter deposition, and in specific DC magnetron deposition [33].

DC planar magnetron discharges are widely used for sputter deposition of metallic thin films such

as aluminum, copper, gold and various alloys. When powered by an RF source it is also used for

sputter deposition of insulating materials. In this deposition technique the target, where the material to

be deposited is located, has a negative bias voltage while the chamber and shield are grounded. In a

given threshold (some hundred Volts) this bias voltage creates the discharge in the inert gas injected

in the chamber creating positive ions (such as Ar+) that are accelerated towards the target, being the

remaining electrons used for further discharge. In magnetron sputtering a concentric DC magnetic

field is created at the target using permanent magnets. This leads to a confinement of the secondary

electrons increasing the ionization probability, therefore more positive ions are accelerated resulting thus

in higher sputtering rates. This technique has nonetheless a side effect on which the the target wear

is more pronounced in a concentric ring (where the magnetic field lines close), forming a groove and

resulting in deposition rate changes through time. The difference between using DC or RF power supply

lies in the type of material. To deposit insulating materials with this technique, an RF source must be

used since the insulating material does not allow neutralization of the incident positive ions.

Sputtering tool - Alcatel SCM450

In this work a DC magnetron sputtering technique is used to deposit the CoCrPt alloy. The tool used is

the Alcatel SCM450, which has a deposition chamber represented in Figure 4.2. This chamber has three

magnetrons where 4 inch targets can be placed. Three independent shutters can be used to cover the

targets. This machine has no load-lock, so the loading and unloading of the samples requires venting

the chamber. The chamber has also four stations for 3 inch wafer holders, three of which are water-

cooled. A base pressure of 10−6 or less is achievable 24 hours after pump down with a turbomolecular

pump. The chamber pressure during the process is uncontrollable by the user and directly dependent

on the gas (Ar) flow.

In this work two depositions of CoCrPt were done. First, a calibration sample to confirm the deposition

rate of 6.48 A/min, determined in a different process, described in [35]. After the deposition rate was

confirmed by using a profilometer technique (see the run-sheet in Annex A), the final sample deposition

of 1000 A of the alloy was done. On Table 4.1 the parameters used and observed on both the depositions

are resumed.

SampleBase

Drive BiasChamber

Ar flowDeposition Total

Pressure Pressure Rate Time

Calibration 1.62×10−6 Torr 40 WRF 820 V 2.14 mTorr 20 sccm 6.48 A/min 154 minFinal Sample 3.60×10−6 Torr 40 WRF 800 V 2.12 mTorr 20 sccm 6.48 A/min 154 min

Table 4.1: Parameters for sputter deposition of CoCrPt using Alcatel SCM450.

38

Figure 4.2: Schematic view of the deposition chamber of Alcatel SMC450. Taken from [34].

4.1.2 Pole Definition by Litography

Lithography is usually the first step of pattern transfer in micro-fabrication. A photo-sensitive polymer

(photoresist) is coated on the sample, which is then exposed to light with a certain wavelength. It

comprises three main steps: photoresist (PR) coating; lithography; and finally development.

PR Coating

First of all the sample is subjected to a pre-treatment in the Vapor Prime oven, where a surface purge

is performed and an organic compound, HMDS, is sparayed over it at 130°C in vacuum. This additional

initial step improves the adhesion of the PR on the material’s surface due to the affinity of the PR to

certain chemical groups of the HMDS.

A positive PR is then coated on the sample. A 1.5 µm thickness is aimed for by defining the proper

spinning velocity during coating, which for this specific PR is of 2500 rpm. Afterwards it is baked at

100°C for 1 min. This allows the evaporation of the solvents and further improves the uniformity of the

coated PR.

Photo-Lithography

In this step a laser is swept over the area to be exposed, turning on and off as defined by the mask,

whose design is to be transferred to the material. The laser has a specific wavelength, which breaks

the chemical bonds of a positive polymer increasing its solubility, and strengthens the chemical bonds

of a negative PR. The areas exposed are chosen depending on the type of PR, whether it is positive or

negative, and on what is the posterior use, whether the next process steps comprise etch or lift-off.

The design transfer is performed by a Direct Write Laser system - Heidelberg DWL 2.0. It uses a

diode laser with wavelength of 405 nm, and the pattern is designed by stripes of 20 µm. This system has

39

a maximum feature resolution of 0.8 µm, which corresponds to the laser spot size, and has an alignment

precision of 0.1 µm, which corresponds to the resolution of the x-y stage.

In this work, a positive PR is used and the next step on the process is an etching step, on which

the unwanted deposited alloy is removed. Therefore the mask to be transferred is given to the control

software of the DWL as an Inverted mask, which means that the only unexposed areas are the ones

that comprise the poles of the designed track.

PR Development

After the Lithography step, the exposed PR is removed in a development step. The sample is first baked

at 110°C for 1 min, so that the chemical reactions started by the laser exposure stop. After cooling down,

the sample is subject to a certain developer solution designed to dissolve the areas that were exposed

by the laser, while the unexposed regions remain intact.

After development, the PR on top of the deposited CoCrPt alloy has the shape of the desired struc-

tures, protecting the material which is not to be removed.

4.1.3 Etching by Ion-Milling

The last step for the structure definition is the etching of the unwanted material - the material that is not

protected by the remaining PR from the last step.

In this work an etching by an Ion-Milling technique is used. It is a strongly directional and non-

selective etching process. It is based on the bombardment of high energy ions (in this case Ar+), which

remove the unprotected material solely by physical impact.

Ion-Milling tool - Nordiko 3000

The machine used for the etch is Nordiko 3000, which is an automated ion beam deposition and milling

system for up to 6 inch wafers. For this work only the milling system is of interest, which uses only the

assist/etch gun, which is the source of the high energy ions beam, and the sample holder. The holder is

water-cooled and can be positioned with an angle between 0° and 80° in relation to the etch gun source.

This gun has 25 cm diameter and is directed to the sample holder. A plasma is created inside the gun

and a voltage difference is applied on the beam exit grids making it responsible for accelerating the

ions. The removal of the material is done with a controlled etch rate and a well defined incident angle,

which allow the control of the profile on the borders of the etched material. The sample holder is also in

constant rotation, thus ensuring uniformity of the etching.

To define the poles on the sample, the material that is not protected by the PR is completely removed

down to the substrate. The etch is performed at an angle of 70° and with a rotation speed of 40 rpm.

Before the process the chamber reaches a base pressure of 7.8×10−7 Torr. An etching rate of 0.8 A/s

was considered. On table 4.2, the conditions of the performed etching are resumed.

40

Power Voltage + Current + Voltage - Current - Ar Flux Pan Rotation Time

53 W 488.3 V 28.7 mA 194.3 V 1.7 mA 7.9 sccm 70° 40 rpm 1250 s

Table 4.2: Parameters for etching of CoCrPt using Nordiko 3000.

4.1.4 Magnetization Definition

The last step on the micro-fabrication process of these structures comprises the definition of the direction

of the magnetization on the structures, so that the desired alternating poles are created.

Being a magnetic alloy, there is no need to anneal the sample, since it already shows magnetic

behavior as deposited. Therefore, in principle, only a strong magnetic field in the desired direction

needs to be applied. However, in this work an anneal was done in the presence of a strong magnetic

field. The high temperature, together with the strong magnetic field organizes the structure of the film

and maintains the magnetization. Notice that thermal annealing is not usually done on this alloy, and as

such some study on the effect of this step needs to be done. Since, in this work not such a study was

done, the tracks produced and used ended up being the ones annealed.

Annealing Setup

The annealing of the samples was done in a setup, on which the magnetic field is applied during a

complete thermal cycle. This setup comprises of a sample holder, for up to 1×1 inch2 square samples;

and a pair of Helmoltz coils, whose current and distance between the cores define the intensity of the

applied field, with a maximum value of 0.5 T. The heating of the sample is done using a halogen lamp,

also calibrated so that the user knows which temperature to set. The annealing is performed in vacuum,

with a pressure of below 10−6 Torr.

For this sample the annealing was done with a temperature of T =350°C for 2 hours with an applied

field of 0.5 T.

4.1.5 Magnetic Properties Measurement - Vibrating Sample Magnetometer

The Vibrating Sample Magnetometer (VSM) is a characterization technique of the magnetic properties

of magnetic materials. The sample is mounted on the tip of a rod, which vibrates around the vertical

axis. It is located and displaced in an uniform field region, created by Helmoltz coils. The flux changes

created by this movement produces in a second set of coils - the pick-up coils - a signal (V ) proportional

to the magnetic flux (φ) rate variation V ∝ dφ/dt. The voltage signal is then fed to a lock-in amplifier,

which uses the vibrating frequency of the rod as reference.

The VSM tool allows the measurement of the hysteresis loops of the material, which are measure-

ments of the magnetic moment as a function of the applied magnetic field.

After annealing the magnetic properties of the sample were obtained using VSM. The field on the

VSM was applied parallel to the direction defined at the annealing and the hysteresis loop of magnetic

moment vs. applied field was obtained. By knowing the volume of the sample, the magnetization was

41

calculated from the magnetic moment and the resulting plot presented in Figure 4.3, along with the

most important values of the film thickness, the coercivity µ0Hc, and both the saturation and remanent

magnetization, MS and MR respectively.

Figure 4.3: Hysteresis loop of magnetization vs. applied field for the deposited magnetic alloy CoCrPt.

4.2 Simulation Model of the CoCrPt Structures

The simulation of the structures preformed in this work are based in a model described in [16]. It takes

into account the definition of the static magnetic field created by a magnetized media, with no currents

(∇ × H = 0). As a first approach, a 2D geometry will be considered. The plane where the simulation

is performed is the cross section of the structures at its center, or the x0z plane at y = W/2, as it is

sketched in Figure 4.4.

Figure 4.4: Scheme of the simulation geometry for the CoCrPt structures.

42

4.2.1 Magnetic Field from a Flat Surface - 2D Model

In 2D, assuming no variation in any properties in the track direction, the magnetic field defined in equation

2.16 can be integrated as:

H(r) = − 12π

∫S

d2r′ (∇ ·M(r′)) r− r′

|r− r′|2 + 12π

∫P

dr′ (M(r′) · en) r− r′

|r− r′|2 , (4.1)

where as before r− r′ is the vector between a point in the magnetized material (r′) and the point where

the field is calculated (r). The integration over the volume poles becomes an area integration (S) over

the (x, z) plane projection of the sample volume. The surface integral becomes a line integral (P ) around

the projected area. The position vector is thus now r = (x, z).

Figure 4.5: Ilustration of the derivaton of the magnetic field from a 2D line of constant charge. Adaptedfrom [16].

If one considers plane surfaces, or lines in 2D, with constant pole density (∇ ·M(r′) = 0), a sim-

plification can be made. The line, corresponding to the surface, lies on the z axis (Figure 4.5), and let

the vector r′ − r be described by the complex variable ω = x + iz. Denoting σm as the surface charge

density, and using the second term of 4.1, the field can be written from

H∗ (ω) = iσm2π

∫ ω2

ω1

dω′ · (ω − ω′)|ω − ω′|2

= iω

2π log(ω − ω2

ω − ω1

), (4.2)

where ω1 and ω2 are the tip coordinates of the 2D surface as can be seen in the scheme of Figure 4.5,

and H∗ (ω) is the complex conjugate of the field

H (ω) = Hx (x, z) + iHz (x, z) . (4.3)

By writing the complex variable in polar coordinates ω = r expiθ, the obtained field in terms of these

coordinates is

H (r, θ) = −σm2π

(∆θ + i log

(r2

r1

))= Hn + iHt, (4.4)

yielding a normal (Hn) and tangential (Ht) field to the magnetized surface. And being r1 and r2 the

distances to the corners of the surface line, and ∆θ the subtended angle.

43

4.2.2 Two-Dimensional Field from the CoCrPt Sample

In this work, the alloy structures are magnetized in the x direction, therefore both the surfaces on the y0z

plane of each line contribute to the calculation of the field. Since the surfaces are of opposing poles, the

surface charge density has opposing signs. In this simulation the value for the surface charge density

will be taken as the remanent magnetization MR measured with VSM, presented on Figure 4.3, since it

is given by σm = MR · en, and an uniform charge distribution is considered. In this approximation, the

lines corresponding to these surfaces lie on the z axis, therefore the x and z components of the field will

correspond to the normal and tangential components respectively:

Hx = Hn = −MR∆θ2π ,

Hz = Ht = −MR

2π log(r2

r1

).

(4.5)

The sensors considered in this work are both sensitive to the vertical component of the field, which

is the one on the z axis. Therefore, for the calculation of the field only the second equation in 4.5 is

considered. The samples to be simulated are, however, made of multiple structures, each magnetized

in the x direction, and each with two ”charged” surfaces. At the point where the field is calculated, the

contributions from all this surfaces must be accounted for. Let N denote the number of structures on the

sample (and k = 0, ..., (N − 1) the index of the structure), t the thickness of the structures (in z) and pp

the Pole Pitch length (in x) as previously defined (Figure 4.4). The magnetic field at any point r = (x, z)

is calculated by summing the contributions from all structures, each having two charged surfaces with

opposing charges:

Hz (x, z) = MR

2π

N−1∑k=0

(− log r

k2rk4

+ log rk1rk3

), (4.6)

being rk2 and rk4 the magnitudes of the vectors between the point of the calculated field and the corners

of the positively charged surface; and rk1 and rk3 the magnitudes of the vectors between the point of the

calculated field and the corners of the negatively charged surface. The corresponding vectors are given

for any (x, z) by:

rk1 =(x− 2k ∗ pp, z − t

2

),

rk2 =(x− 2k ∗ pp, z + t

2

),

rk3 =(x− pp− 2k ∗ pp, z − t

2

),

rk4 =(x− pp− 2k ∗ pp, z + t

2

).

(4.7)

These equations form the general case. Figure 4.6 illustrates the different contributions from different

structures to the calculation of the field. In this case, only two structures are represented. For more

structures, the same type of vectors are computed and the field calculated using 4.6.

To perform the simulation, the software MATLAB is used. After the parameters such as the PP, RD, t

44

Figure 4.6: Scheme of the contributions to the calculation of the field of just two PM CoCrPt structures.The red arrows indicate the direction of the magnetization of the structures.

and MR are defined, the program computes the vectors as given in 4.7 and applies the equation for the

z component of the field 4.6.

45

46

Chapter 5

Measurements

In this chapter the measurements using the developed system are presented and discussed. Sensors

S1 and S2 are validated using the micro-fabricated CoCrPt samples. The simulation is performed and

compared to each measurement done. Measurements on Magnetic Ink were also performed in order

to characterize it and conclude on the best configuration for the system in development. The latter

measurements can be found in Appendix ?? (confidential).

5.1 Measurement System

The setup used in this work was originally designed for Eddy Current Testing, a non-destructive testing

(NDT) technique [31]. The setup is composed of an XYZ automated scanner with a micrometric resolu-

tion. Some electronic instruments are also present to generate the required signals and to demodulate

the sensor’s output. The system has three different source instruments. The sensor is polarized by an

AC voltage source Agilent 33210A conneted to a voltage divider (with a resistance much bigger than

the sensor) to set a constant bias current throughout the measurement. The sensors used in this work

have a maximum resistance of 3kΩ, so the voltage divider has a resistance of 10kΩ. Also present on

the system is a second voltage source instrument (Agilent 33220A) with the aim to generate the eddy

current for the NDT technique, but it was disconnected from the system throughout this work. The sen-

sor’s voltage output is demodulated by a 200 kHz bandwidth lock-in amplifier (Signal Recovery 7265),

with a reference frequency generated by a third voltage source instrument (Agilent 33250A). Figure 5.1

presents the system used for the measurements. For more information about the measurement system

the reader can consult reference [31].

Each measurement performed in this work consisted on a magnetic scan over a a sample with

magnetic structures, whether made with MI or CoCrPt, using the above described system. In specific

for CoCrPt samples, the raw measurements were taken so that some values could be more accurately

calculated. First, each scanning is done over an area where no magnetic signal is expected (where

no magnetic bits are present), so to provide an accurate offset value (voltage at zero field), this area is

taken 2 mm before the first structure on the x direction. The number of structures to be scanned was

47

(a) Detailed view of the XY table and sen-sor setup.

(b) Overall view of the scanning system and electronic instruments.

Figure 5.1: The 2D scanning setup used to measure the stray fields from the magnetic ink and CoCrPtsamples.

always chosen as to incorporate at least 6 consecutive poles. When only a curve is represented (and

not the entire 2D scan), it is chosen as the center line of the scan, at W/2 where the field is expected

to be stronger and more uniform. Figure 5.2 shows a scheme of the scanned areas and also the center

line. Each measurement has also a specific resolution in x and y, which correspond the size of the step

between consecutive measurements. The step in x is always smaller than the step in y, since the sought

after behavior happens on the movement on x.

Figure 5.2: A scheme of the scanning measurements done on the different CoCrPt samples.

The z axis motor on the scanner has a minimum resolution of 4µm. This is the resolution used to find

the origin in z of the measurement, from which the reading distance is added. The measurement setup

has no automatic way to find the position zero, which happens when the sensor is in contact with the

sample. Therefore, the set of z=0 is made by eye and using the allowed resolution of the motor. During

measurements this position was found using a motor step of 10µm, which is a movement distinguishable

by eye. Therefore, the position is set with an uncertainty of ±10µm. When the RD is set, the uncertainty

of this movement is of 4µm. Therefore, each set reading distance has a systematic error of ±14µm.

48

Which is accounted for in the analysis of the performed measurements.

The data taken from the setup is of the voltage output of the sensor. In this stage of the work, we

are interested in evaluating the field created by the structures and also the capability of the sensors

to measure that field. Therefore, the voltage information is converted into field using the magnetic

properties from the sensors taken from the transfer curves in chapter 3 and the bias current fed to the

sensor. With these parameters, the conversion is simply done using the following equation:

H = V − VoffRS0Ib

, (5.1)

where H is the magnetic field magnitude in mT, V the output voltage from the sensor in V, Voff the offset

voltage of the measurement in V, Rmin the minimum resistance in Ω, S0 the sensitivity of the sensor in

%/mT and Ib the bias current fed to the sensor in A.

5.2 Measurements on Magnetic Ink Samples

All measurements of the magnetic ink samples are confidential and presented in Appendix ??.

Measurements on MI samples with sensor S1 using both Std 1 and Std 3 allowed the evaluation

of the best solution for the system. The use of Std 3 resulted in a higher effective magnetization of

the ink during measurements, which was already expected with the simulations of section 3.1.3, due to

the presence of the x component of the field in the region right below where the sensor is measuring.

Using both configurations resulted also in similar spatial-resolutions, since the same sensor was used.

However, the higher magnetization of the ink yields higher magnetic signal, thus more distinguishable

measuremets using Std 3.

Sensor S2, which has a greater spatial resolution than sensor S1, was tested on the same samples

and doing the same studies as with sensor S1. Sensor S2 resulted in higher spatial resolution, allowing

the measurement of smaller structures. It measures also higher magnetic signals from the samples,

due to its lower effective reading distance. However, this sensor has a lower SNR, resulting in worst

performance reading smaller magnetic signals.

5.3 Sensor Validation with CoCrPt Samples

The simulation of the CoCrPt samples using the model described in chapter 4 is done and compared

with measurements. This is done using both sensors S1 and S2 considered for this work, mounted in a

configuration Std 3, which has been proven to be the most advantageous in the previous section. This

configuration magnetizes in a more effective way the ink. It was seen however that the field created by

the magnets is not strong enough too change significantly the magnetization of the structures, therefore,

in simulations the values used for the magnetization is the remanent magnetization MR derived from the

plot on Figure 4.3. The sensitivities for both sensors in Std 3 configuration were measured in chapter 3,

yielding 14.3 %/mT for a Rmin =542 Ω for sensor S1, and -24.4 %/mT for a Rmin =1161Ω for sensor

49

S2. Sensor S1 is fed with a current bias of 100µA and S2 with 50µA.

5.3.1 Sensor S1

First, a validation of sensor S1 is done. This sensor, described in section 3.2 is made of 72 MTJs

connected in series in an array spanning in a area of 525×570µm2 in the y0z plane. So the simulation

needs to take into account that the field measured by the sensor isn’t just in one point, but an average

over all this area. Since the simulation is done using a 2D model in the x0z plane (see section 4.2), the

field resulting from the simulation is an average of the field calculated for 10 points over 570µm above

the value of the reading distance (RD) in the z direction. In this simulation we take also into account a

correction to the RD. In the measurements, the set RD is always the one pretended. However, the set

RD has an estimated error of 14µm, and the sensor does not start exactly at the bottom of the PCB,

starting around 26µm above it. So in the simulations the reading distance used, or the initial zinit for

the field calculation takes into account the nominal RD rd and an added distance zover that takes into

account the above discussed parameters and given a value of 40µm: zinit = rd+ zover. Each simulation

was done setting the desired pole-pitch, corrected reading distance and the number of structures N,

which is the same as the number of structures on the samples.

The measurements performed were done above the CoCrPt using the setup described previously.

The three CoCrPt samples are used for the validation of the sensor. These have pole-pitches of 1000µm,

320µm, and 240µm. The measurement resolution in x is of 0.05 mm for the first two samples and of 0.01

mm for the 240µm PP sample. Each sample was measured with half the pole-pitch reading distance,

i.e., 120µm, 160µm and 500µm respectively for the 240µm, 320µm and 1000µm pole-pitch samples.

Figure 5.3: Simulation done for the CoCrPt sample with PP=1mm and at a RD=500±14µm, with N=8.

Figure 5.3 shows an example of a simulation performed for sensor S1 for the 1mm pole-pitch sample

with N=8 structures. The resulted curve shows higher peaks for the first and the last poles, whose

amplitude we call Large Amplitude. These highest peaks are due to the absence of other magnetic

50

structures before and after the respectively first and last structure, resulting in a non-closure of the stray

fields, and thus a higher z component signal. On both simulations and measurements the values for the

large amplitude and for the small amplitude, which is taken in the center region, are calculated.

(a) PP = 1000 µm. (b) PP = 320 µm.

(c) PP = 240 µm.

Figure 5.4: Comparison of the simulations and measurements done on CoCrPt samples with differentPole-Pitches, using sensor S1.

Figure 5.4 shows the comparison between the simulation performed for each sample with sensor

S1 and the measurement for the same sample using the sensor in a Std 3 configuration. Although the

full samples were simulated and measured, only the first part of the plot is showed, for better com-

parison. For all samples the behavior of each real measurement is very close to the behavior of the

simulation. The high peak for the first structure is always present and the other peaks slightly lower,

with the periodicity corresponding to the PP of the structures. The measurements, however tend to have

variation between peaks, which appear to be offset variations, which are uniform on the simulations.

This variations can be due to differences between the structures and also due to external noise of the

measurement. For instance, the 240µm and 320µm PP samples show the same type of ”offset” variation

between x=4mm and x=8mm, which indicates that some variation in the background magnetic field is

sensed in this region. This also may explain why the last poles measured on Figure 5.4a are somewhat

lower than the simulation. To conclude the closeness between the measurement and the simulation the

51

two amplitudes (the large and the small) are calculated. Calculating these amplitudes along the sample

eliminates the difference that comes from the background measurement.

The plot on Figure 5.5 shows the comparison between simulation and measurements. Overall the

tendency of the measured amplitudes corresponds to the predicted by the simulations, although neither

the values nor the error bars make it a complete match. The large amplitude increases for smaller pole-

pitch structures since we are putting the sensor gradually closer to the sample as the PP decreases,

the small amplitude on the other end tends to decrease, this is due to the fact that the poles are getting

closer together, closing the stray field force lines at a closer scale resulting in a gradually weaker z

component of the field.

Figure 5.5: Average amplitudes measured both in the measurements and simulations for sensor S1 foreach sample at a RD of half the PP.

The fact that the amplitudes don’t match may come from different factors. First, an underestimation of

the uncertainty in the RD of the measurements. 14 µm was considered as the uncertainty, however, this

values may be higher, and if so, the difference between simulation and measurement could be explained.

On the other hand, the model for the simulation is a 2D approximation, where an average is performed

over the span of the sensor. This approximation may, however be faulty, and is not considering the full

extent of the field variation along the sensor. Other factor may come from the model itself, since we are

considering perfect magnetization uniformity within the material.

Nevertheless, It is safe to say that measurements done with sensor S1 are reliable. The measured

signal is very close to the predicted, and therefore one can say that the signal measured on the magnetic

ink samples with this sensor are correct and can be taken as a reference for future measurements.

5.3.2 Sensor S2

For sensor S2 the simulation simplifies since the sensing region in the x0z plane is much smaller, being

of just 4µm. Therefore, the field calculated in the simulation for this sensor is done on just one point at

52

the corrected RD, which is calculated as it was done in the previous section. Again the CoCrPt samples

with pole pitch of 240µm, 320µm and 1000µm are used. The measurements for each sample were done

for a reading distance of half the pole pitch. The measurement resolution in x was defined as being of

0.05 mm for the 1000µm and 320µm PP samples and of 0.01 mm for the 240µm. The amplitudes, as

defined in Figure 5.3, are also calculated for every simulation and measurement.

(a) PP = 1000 µm. (b) PP = 320 µm.

(c) PP = 240 µm.

Figure 5.6: Comparison of the simulations and measurements done on CoCrPt samples with differentPole-Pitches, using sensor S2.

On Figure 5.6a the different measurements alongside the corresponding simulations are shown.

Once again the behavior on the measurements is very close to the one predicted by the simulations.

However, when compared to sensor S1, the measurements here obtained are much more noisier, well

evidenced by the big variation in the field in the first couple mm of the measurements. This was already

expected, since S1 is averaging over 72 junctions, lowering the measurement noise. Between different

peaks, some variation is noticed, which can be originated from noise or background measurement.

A comparison is then performed between the different amplitudes measured on the samples, and

simulated. On Figure 5.7 this comparison is shown. Overall the behavior predicted by the simulation is

followed by the measurements, noticing in specific that the decrease in the small amplitude between the

320µm sample and 240µm also happens in the measurement, even if at a bigger scale. As already seen

53

for sensor S1, the increase of the large amplitude as the pole-pitch decreases has to do with getting

gradually closer to the sample. However, the predicted behavior for the small amplitude is different from

the predicted behavior for sensor S1. It must be present here that sensor S1 is averaging over an area

that spans 570µm over z, this results in an effective reading distance of the half pole-pitch plus around

200µm which is of 700µm for 1000µm PP, 360µm for 320µm PP and so on. While 200µm is just a

relatively small fraction of 1000µm, it makes already a big portion of 320µm adn 240µm. Therefore, with

sensor S2 the sensor is significantly closer to the sample, and so, the magnetic field from the smaller

pole pitches is higher than for 1000µm sample. Then, comparing the lower PP sizes, the explanation

given for sensor S1 holds, since the structures are much closer to each other, the stray field closes in a

smaller scale, decreasing the signal.

Figure 5.7: Amplitudes measured both in the measurements and simulations for sensor S2 for eachsample at a RD of half the PP.

Sensor S2, even though having a lower SNR, in a Std 3 configuration its sensitivity and spatial

resolution are higher than for sensor S1. Resulting in a much better expected performance in terms of

resolution for this sensor. This comes as a great advantage for reading magnetic ink. The closeness

of the measurements to the simulations validates also this sensor, and the procedure used, making it a

reliable sensor for reliable measurements.

54

Chapter 6

Conclusions

The aim of this work has been to develop a reading head based on the TMR sensor technology for

reading magnetic ink patterns, in particular for positioning application. This system should be able to

magnetize the ink as well as measure it with high accuracy and sensitivity.

As a first stage, the strategy for magnetizing the ink was developed. From the multiple choices to

create a field in the track region, the implementation of magnets alongside the sensor was chosen,

due to its simplicity and non-requirement for further electronics. The configuration of the magnets in

relation to the sensor had to be carefully thought, taking into account the desired direction for the ink

magnetization (x) and other specifications of the project. Resulting in the placement of two magnets

at the same distance from the sensor, which is placed halfway between them. The simple simulations

performed for the configurations resulted on a reliable prediction on the efficiency of magnetization of

the magnetic ink, as well as providing a good prediction on the influence on the TMR sensors mounted

on the system. The two resulting configurations differ in the direction of magnetization of the magnets

and on the relative position of the sensor (Std 1 and Std 3).

The influence of these configurations on the two considered sensor configurations was tested. Sen-

sor S1, comprised of 72 MTJs in series occupying an area of 525×570 µm2 with a large SNR, a minimum

resistance of 542Ω and a sensitivity of 27.7%/mT; and sensor S2, comprised of 4 MTJS in series oc-

cupying an area of 58.5×4 µm2 with a large SNR, a minimum resistance of 1187Ω and a sensitivity of

-22.2%/mT. The influence of the first considered configuration Std 1 on sensor S1 was large, resulting in

non-consistent responses on different measurements, resulting always in a decrease on the sensitivity

due to influence of the external field on the sensitive direction of the sensor. On sensor S2, the influ-

ence of this configuration was too big, and no linear region was obtained. The influence of the second

configuration Std 3 was mainly on the sensitivity value due to the influence of cross fields in the sensor

sensitive plane. Decreasing the sensitivity of sensor S1 to 14.3%/mT and increasing the sensitivity of

S2 to -24.4%/mT by shifting the curve.

A strategy to validate the sensors was also developed, comprising on the micro-fabrication in a

clean-room environment of well-defined structures of a CoCrPt hard ferromagnetic alloy. The simulation

of this structures compared with measurements showed that the measurements for both sensors are

55

reliable, and that their response is very close to the simulations, showing the same tendency on both

sensors. Tendency apart, the values of the amplitude still didn’t exactly correspond, showing the results

of the approximations taken in the simulation performed. It was a 2D model, therefore leaving out some

3D geometrical features that would influence the signal. These measurements allowed as well a good

comparison between the two sensors used. Even though having a lower SNR, sensor S2 is capable of

measuring higher amplitudes and has a better spatial resolution than sensor S1.

Measurements performed on the Magnetic Ink samples, led to the conclusion on the best performing

configuration for this kind of system, revealing that one of the configurations, Std 3, is more efficient in

magnetizing the magnetic media upon measurement.

With this work, we were therefore able to find a configuration with a very good performance in mag-

netizing the magnetic ink, as well as validating the state-of-the-art TMR sensors for this application.

Furthermore, the foundations for a future development of the system has been set.

Future work in this project should include the optimization of the sensors for this configuration. Even

though a high spatial resolution sensor was found, the SNR is still low, requiring some future improve-

ment. The CoCrPT tracks, which proved to be a good validation tool for the sensors can also be opti-

mized by fabricating higher thicknesses of alloy and therefore measuring higher signals, making a more

versatile tool for different kind of studies.

56

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60

Appendix A

CoCrPt Structures Micro-Fabrication

Process Runsheet

Runsheet of the micro-fabrication process of the CoCrPt structures described in section 4.1.

A.1

Run Sheet – GePos |Magnetic Scales

Responsible: Tiago Costa Process Start: 24 / 07 / 2017 Process Finish: ____ / ____ / _______

STEP 1: CoCrPt (1000 Å) Deposition 24-25/07/2017

Machine: Alcatel SCM450 – Radio Frequency (RF) Sputtering Deposition Operator: Tiago Monteiro

Tiago Costa

Sample (2 phases):

1. Calibration sample: 1 glass substrate (25x25mm2)

2. Working samples: 3x glass substrates (25x25 mm) + silicon substrate for composition test

Material: Co66Cr16Pt18 Thickness: 1000Å

Conditions:

1. Calibration Sample

Base Pressure: 1.62x10-6 Torr, RF Power: 40 W, Ar flux: 20 sccm, Vbias : 820 V

Chamber Pressure: 2.14 mTorr, Deposition rate: 6.48 Å/min, Total time: 154 min

2. Working samples + silicon substrate

Base Pressure: 3.60x10-6 Torr, RF Power: 40 W, Ar flux: 20 sccm, Vbias :800 V

Chamber Pressure: 2.12 mTorr, Deposition rate: 6.48 Å/min, Total time: 154 min

Observations

Glass Substrates cleaned : Alconox + Ultrasounds / DI water rinse / Dry with compressed air gun

Silicon substrate cleaned: Acetone / DI water rinse / Dry with compressed air gun

For calibration sample, vacuum done from Friday 21/07 to Monday 24/07. (~55h)

Before calibration sample, target was cleaned for 3hours:

o 40WRF/20sccm/800Vbias/2.10mTorr/3h

For working samples, vacuum done from Monday 24/07 to Tuesday 25/07. (~20h)

Before working sample, target was cleaned for 1hour:

o 40WRF/20sccm/800Vbias/2.09mTorr/1h

Analysis :

1. Profilometer with calibration sample (all in Å) – Dekdak Operator: Tiago Costa

Scan length: 300μm / Speed: Low / Resolution: High. Av. High and Av. Low taken from the program.

1 2 3 4 5 6 7 8 9 10 11 12 13

Av. Low -69 178 86 2 64 80 161 119 160 -244 158 -5 87

Av. High 1110 1128 1031 956 1128 1179 887 1149 1134 990 1103 1087 1042 Overall Av.

Av. Thick. 1179 950 945 954 1064 1099 726 1030 974 1234 945 1092 955 1011±123

CoCrPt (1000 Å)

Glass Substrate

STEP 2: Lithography – Magnetic Poles definition /2xsamples 16-18/ 08 /2017

Machine: DWL - Direct Write Laser Litography (diode laser) Operator: Karla / Tiago Costa

1. Sample no. 1 – Full Scales

1.1 Vapor Prime 30 min (Recipe – 0)

(25 min N2 atmosphere + 5 min HDMS) at 130ºC and P=5Torr

1.2 Coat 1.5 µm Photoresit (Recipe 6/2)

Photoresist

Spinning for 40s at 2500 rpm (1.5 µm thickness);

Baking for 60s at 100ºC 1.3 Lithography (Single Exposure)

Map: AMSION Mask: cocrpt_scales_full_INV (INVERTED)

Energy: 55 Focus: 0

Die dimensions: [X: 19400; Y: 19400] m

# Dies: 1 Total Time:

[X0: 3000Y0: 3000 ] m (From bottom left corner)

1.4 Development (Recipe 6/2)

Time: 3min 10s

Sample heating: 60s at 110ºC; Cool down: 30s; Developing: 100s

2. Sample no. 2 – VSM and Profile Test structures

2.1. Vapor Prime 30 min (Recipe – 0)

(25 min N2 atmosphere + 5 min HDMS) at 130ºC and P=5Torr

2.2. Coat 1.5 µm Photoresit (Recipe 6/2)

Photoresist

Spinning for 40s at 2500 rpm (1.5 µm thickness);

Baking for 60s at 100ºC

2.3. Lithography (Single Exposure)

Map: AMSION Mask: cocrpt_scales_tests_INV (INVERTED)

Energy: 55 Focus: 0

Die dimensions: [X: 19400; Y: 19400] m

# Dies: 1 Total Time:

[X0: 3000Y0: 3000 ] m (From bottom left corner)

2.4. Development (Recipe 6/2)

Time: 3min 10s

Sample heating: 60s at 110ºC; Cool down: 30s; Developing: 100s

Observations:

Lithography performed 1.5 days after photo-resist coating.

Scheme :

Resist Coating : After Development ;

(x0,y0)

25.4mm

25.4mm

CoCrPt (1000 Å)

Glass Substrate

Photoresist

Sampe no. 1 AutoCAD mask:

o Pole-pitch (width of the stripes) from bottom to top (m): 80, 160, 240, 320, 1000.

o All scales have same length (3mm)

Optical Inspection

The sizes of the structures are good. The borders are also well defined. Apart from some non-uniformity on the PR

due to scratches observed on the surface of the metal, there is nothing else to account for Some pictures were

taken:

~

19.4mm

19.4mm 3mm 3.88mm

Marks for

dicing

200μm 200μm

200μm 200μm 200μm

200μm

200μm

200μm 200μm

STEP 3: CoCrPt etching by Ion Milling 19/08/2017

Machine: Nordiko 3000 Operator: Karla

Total thickness to etch: 1000 Å (etch rate: 0.8 Å/s); time: 1250 sec @ 70º

Base Pressure (Torr): 7.8x10-7 Torr Batch: Junction Etch

Recipe: Etch Junction Pillar 4

Steps: Vacuum wait / etch gun stab 70º pan / junction etch 70º pan / end etch

Observations:

After Etching:

Optical inspection :

It is visible that the etch is done completely. Borders are well defined. Just some sparse residues of metal where it

should have been etched.

Note: No pictures were taken after this step.

STEP 4: Resist Strip at Wet Bench 19/08/2017

Operator: Karla

Conditions: Microstrip + Temperature: 60ºC + Ultrasounds

Cleaning: IPA rinse / DI water rinse / Dry with compressed air gun

Observations:

Optical Inspection: 24/08/2017

Overall the structures are well defined and almost no anomalies on the structures.

Some pictures were taken: On the second, seventh and eigth pictures one can see the pieces of metal that were not

etched. Apart from these ones, no others were evidenced. On the eighth picture the anomalies appear to be due to

scratches on the substrates.

Assist Gun Power (W) V+ (V) I+ (mA) V- (V) I- (mA) Ar Flux (sccm) Pan (deg) Rotation (rpm)

Read Values 53 488.3 28.7 194.3 1.7 7.9 70 40

CoCrPt (1000 Å)

Glass Substrate

Photoresist

CoCrPt (1000 Å)

Glass Substrate

STEP 5: Annealing 25/08/2017

Machine: Old Annealing Setup Operator: Tiago Costa

Conditions:

Observations : A current I=182000ppm to the Helmoltz Coils is applied.

Note : Tset = 375 ºC --> Treal = 350 ºC

Heating Rate = 5.9 ºC/min

200μm 200μm

200μm 200μm

200μm

200μm

200μm 200μm

60 min

22.4

2 h

22.4

375

2 h

22.4

0.5 T

2 h

22.4

Applied Field