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Nano Res
1
Tunnel Magnetoresistance with Atomically Thin
Two-Dimensional Hexagonal Boron Nitride Barriers
Andre Dankert1*, M. Venkata Kamalakar1, Abdul Wajid1,
R. S. Patel2#, Saroj P. Dash1†
Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0627-4
http://www.thenanoresearch.com on November 7 2014
© Tsinghua University Press 2014
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Nano Research
DOI 10.1007/s12274-014-0627-4
1
Graphical Table of Contents -
Spin-polarized electron tunneling through a monolayer h-BN barrier in magnetic junction and
observation of tunnel magneto resistance at room temperature
2
Tunnel Magnetoresistance with Atomically Thin
Two-Dimensional Hexagonal Boron Nitride Barriers
Andre Dankert1*, M. Venkata Kamalakar1, Abdul Wajid1,
R. S. Patel2#, Saroj P. Dash1†
1Department of Microtechnology and Nanoscience, Chalmers University of Technology,
SE-41296, Göteborg, Sweden
2Department of Physics, Birla Institute of Technology and Science Pilani – K K Birla
Goa Campus, Zuarinagar– 403726, Goa, India
*[email protected]; #[email protected], †[email protected]
ABSTRACT
The two-dimensional atomically thin insulator hexagonal boron nitride (h-BN) constitutes a
new paradigm in tunnel based devices. A large band gap along with its atomically flat nature
without dangling bonds or interface trap states makes it an ideal candidate for tunnel spin
transport in spintronic devices. Here, we demonstrate the tunneling of spin-polarized
electrons through large area monolayer h-BN prepared by chemical vapor deposition in
magnetic tunnel junctions. In ferromagnet/h-BN/ferromagnet heterostructures fabricated over
a chip scale, we show tunnel magneto resistance at room temperature. Measurements at
different bias voltages and on multiple devices with different ferromagnetic electrodes
establish the spin polarized tunneling using h-BN barriers. These results open the way for
integration of 2D monolayer insulating barriers in active spintronic devices and circuits
operating at ambient temperature, and for further exploration of their properties and prospects.
Keywords: Hexagonal Boron Nitride, 2D layered materials, CVD, Spintronics, Magnetic
tunnel junction, Tunnel magneto resistance, Tunnel barrier.
1. Introduction
3
The quantum phenomenon of electron tunneling enables novel spintronic, electronic,
optoelectronic and superconducting nanodevices with enhanced efficiency and low power
consumption [1]. Such tunnel devices typically require growth of insulating materials of few
atomic layers thin, which is a major challenge in materials science. Mostly used conventional
tunnel barrier materials are made up of metal oxides, which have non-uniform thicknesses,
pinholes, defects and trapped charges. These issues compromise the performance and
reliability of the devices. Tunnel barriers are main building blocks of magnetic tunnel
junctions (MTJs), where two ferromagnetic (FM) contacts are separated by ultra-thin oxide
barriers [2-6]. Such MTJs are currently used in read-heads of hard drives and new emerging
technologies including magnetic random access memory and spin-transfer torque
devices [3-5]. For many of these applications, it is crucial to achieve tunnel
magnetoresistance (TMR) signals with optimal junction resistances [3], where the demand for
controlling thickness of metal oxide tunnel barriers with atomic level precession pose a
serious challenge.
The discovery of the two-dimensional (2D) atomic crystals has opened up the possibility of
exploring their fascinating properties [7, 8]. Recently, graphene has been explored as a barrier
for spin transport, where TMR and spin filtering effects have been observed [9-19], including
the bias dependence of the TMR [20]. However, the absence of a bandgap in semi-metallic
and low resistive graphene, leads to an increasing demand for an insulating 2D crystal [7-9].
Hexagonal boron nitride (h-BN) is an insulating isomorph of graphene with a large bandgap
of ~ 6 eV, making it an ideal candidate for dielectric substrates and tunnel barriers [8, 21-25].
The atomically thin and inert nature of h-BN can provide the ultimate control over the
morphology and can minimize defects related to interface states and interfacial alloy
formation. It has been theoretically proposed to use h-BN as tunnel barrier in MTJs to
4
achieve large magnetoresistance signals [26-28]. The spin filtering nature of h-
BN/ferromagnet interfaces and tailoring of TMR by uniaxial strain has been proposed [26-28].
Experimentally, h-BN has been used as tunnel barrier for spin injection in lateral graphene
spin transport devices [29-31]. The chemical vapor deposited (CVD) h-BN is found to show
reproducible tunneling behavior circumventing the conductivity mismatch problem between
the ferromagnet and graphene for efficient spin injection [30]. In order to further establish the
potential of atomically thin h-BN tunnel barriers, it is important to demonstrate TMR in
technologically relevant MTJs, which has not been realized so far.
Here, we employ large area CVD grown monolayer h-BN as a tunnel barrier in MTJs having
ferromagnetic Ni80Fe20 and Co contacts in a vertical structure. We show that atomically thin
CVD h-BN exhibits quantum tunneling of spin polarized electrons for the demonstration of
TMR at room temperature. Measurements on multiple devices with different ferromagnetic
electrodes and bias dependence of the TMR signal establish the spin polarized tunneling
effects using h-BN barriers. Our results demonstrate the integration of atomically thin 2D
insulating tunnel barriers in magnetic tunnel devices and the observation of tunnel
magnetoresistance at ambient temperatures.
2. Experimental
Magnetic tunnel junctions incorporating a h-BN tunnel barrier between two FM metal
electrodes are schematically presented in Fig 1a. When the magnetizations of the FMs are in
a parallel orientation it is preferable that spin polarized electrons will tunnel through the h-
BN than if they are antiparallel, giving rise to a change in the resistance across the junction
[32]. We fabricated magnetic tunnel devices using a h-BN barrier and ferromagnetic Ni80Fe20
(Py) and Co electrodes (Fig. 1b). The main fabrication steps of our devices are presented in
5
Fig. 1c. The bottom Co or Py electrodes are prepared on Si/SiO2 substrate by photo
lithography, electron beam evaporation and lift off methods. Before deposition of FM
electrodes, we deposited a thin layer of Ti in order to increase adhesion of FM to the SiO2
substrate. Subsequently we transferred the CVD grown layer of h-BN on bottom FM
electrodes. We have been able to achieve a ripple free transfer of CVD h-BN over large areas
on our chips by using a frame assisted process. The CVD h-BN layer used in our experiment
was grown on copper substrate (purchased from Graphene supermarket). The h-BN surface is
first covered with PMMA, and then isolated from the Cu substrate by etching in a H2O2-HCl
solution. This Cu etchant produces very clean h-BN in comparison to other available
chemical etching procedures. The h-BN/PMMA layer is washed with deionized water, and
subsequently transferred onto the chip containing bottom FM electrodes in isopropyl alcohol
medium. We avoided the use of DI water on FM contacts to minimize the oxidation.
However, oxidation and contamination of bottom FM electrode due to air exposure could not
be avoided in the present process. After drying the chip in an ambient environment, we
annealed it at 150 C for 10min. It has been observed that this annealing step improved
adhesion of h-BN with bottom FM electrode. The chip was then washed with acetone to
remove the PMMA resulting in chip containing h-BN/FM heterostructures. The top
electrodes of ferromagnetic Co and capping Au layer were prepared on the h-BN/FM
heterostructures by photo lithography, e-beam evaporation and lift off techniques. The atomic
force microscopy was performed with a Bruker Dimension 3100 SPM using tapping mode.
The Raman spectrum was measured with a Horiba XploRA system using a 638 nm laser and
a grating of 1800 lines/mm. TMR measurements were performed at room temperature with
application of an in-plane magnetic field Bin. We sweep the Bin while applying a constant bias
current and measure the voltage change with a Keithley 2400 Sourcemeter. We fabricated
6
various devices consisting of Py/h-BN/Co and Co/h-BN/Co magnetic tunnel structures on
chip scale suing such CVD h-BN.
3. Results and Discussion
Figure 2a shows the optical micrograph of large area CVD h-BN transferred on the SiO2/Si
substrate. Atomic force microscope (AFM) measurements reveal an effective thickness of ~5
Å for the h-BN layer on the SiO2/Si substrate as shown in Fig 2 (b), which corresponds to
single layer h-BN [30]. The Raman shift for the CVD h-BN has been found to be at ~1369
cm-1 (Fig. 2(c)), which matches with the results of exfoliated h-BN [32]. We find the tunnel
resistances of the h-BN contacts are in the range of 0.5 – 1 kΩ.µm2 in our magnetic tunnel
structures. The detailed tunneling characteristics of CVD h-BN barriers have been presented
in our previous work, where an effective barrier height of φ ~ 1.49 eV was extracted [30].
Tunnel magnetoresistance measurements were performed in a four probe cross bar geometry
(Fig. 3(a)), which enables the measurement of the tunnel junction resistance, while avoiding
other effects such as anisotropic magnetoresistance of the FMs. We applied a fixed bias
current between the top and bottom ferromagnetic electrodes of the junction while the voltage
drop across the junction was measured as a function of the external in-plane magnetic field
(B). A magnetization reversal of the FM electrodes occurs at fields corresponding to their
respective coercivities. The switching field is achieved by using different materials and also
through different geometries of the FM electrodes. Hence, their magnetizations can be
aligned either parallel or antiparallel and the system enables us to observe two well-defined
different resistance states due to difference between the threshold switching fields of the two
FM electrodes.
7
The TMR measurement in Fig. 3b clearly shows that the tunnel resistance abruptly increases
upon switching the magnetization of the FM electrodes from a parallel to antiparallel
magnetization configuration. The sharp switching of the FM electrodes and the flat region for
the anti-parallel magnetization configuration is a strong indication of TMR [33-37]. The
TMR ~0.15 % was calculated by using the relation TMR = 𝑅𝐴𝑃−𝑅𝑃
𝑅𝑃 ×100 %, where 𝑅𝐴𝑃 and
𝑅𝑃 are the tunnel resistances measured at antiparallel and parallel magnetization
configuration of the two FM electrodes respectively [33]. As expected, the TMR signal also
changes sign with reversing bias polarity (shown in Fig. 3c) [33-37]. The observation of
TMR with h-BN barriers were reproduced on different devices and also in junction with
different FM contacts. Figure 4 shows a TMR signal of 0.5% for a Co/h-BN/Co MTJ at 300
K, where different widths of Co electrodes were chosen to achieve different switching fields.
These results suggest that one atomic layer of h-BN is sufficient enough to decouple the top
and bottom FM layers, which lead to observation of TMR signal at room temperature [33-37].
The observed TMR is a consequence of spin-dependent tunneling from a FM through the h-
BN barrier. The relationship between the observed TMR and tunnel spin polarizations (TSP)
of the FM/h-BN interface can be explained by the Julliere model [33]. The spin polarization
of the FM/h-BN contacts are estimated to be P1 ∼ P2 = P ∼ 0.05 – 0.25 % considering the
relation TMR = 2P1P2/(1 − P1P2), where P1 and P2 are the tunnel spin polarizations of the
Py/h-BN and Co/h-BN interfaces are assumed to be similar, respectively [33]. These values
are less than previously reported TSP of P ~ 14 % in Co/h-BN contacts for spin injection into
graphene in lateral spin transport devices, where FM electrodes are placed on the top of the h-
BN/graphene channel [30]. Such spin polarization obtained in graphene spin transport
devices are comparable to that obtained using Al2O3 and TiO2 tunnel barriers [43]. Much
higher values for spin polarization were predicted theoretically considering lattice matched
FM/h-BN/FM vertical MTJ structures [26 -28]. We attribute the lower value obtained for our
8
vertical MTJ devices to the possible contamination of the bottom Py or Co surface during the
air exposure and h-BN transfer process [38-41]. The oxidation or contamination of the
interface is known to produce strong spin-scattering that reduces the tunneling spin
polarization even with conventional metal oxide tunnel barriers such as Al2O3 and MgO [38-
41]. Higher spin polarization can be expected in structures grown in-situ without any air
exposure. Therefore, the direct growth of h-BN on FMs and in-situ preparation of vertical
MTJ layer stack is required to demonstrate the true spintronics potential.
Figure 5 shows the bias dependence of TMR for two devices with Py/h-BN/Co MTJs at room
temperature. The TMR data of the Dev1 at -10 mV and -50 mV are shown in Fig. 5a and the
complete bias dependence of TMR signal for both devices are plotted with bias voltage in Fig
5b. To be noted we restrict our measurements to low bias voltages < 50 mV, because of
atomically thin h-BN tunnel barriers. The low voltage study is also appropriate considering
the fact that tunneling dominates the conduction mechanism [35-37, 40]. The negative and
positive bias range corresponds to spins tunneling through h-BN barrier from Py → Co and
Co → Py respectively. We observe a decreasing trend of TMR at higher bias voltages with a
maximum around zero voltage, which is consistent with behavior generally observed in spin
polarized tunneling [35-41]. Our results reflect the typical behavior for MTJs, where the bias
dependence of TMR relies upon the quality and height of the h-BN barrier, electronic
properties and magnon excitations at the FM/h-BN interfaces [35, 40]. The observations of
such characteristics also exclude any artifacts such as anisotropic magnetoresistance. The
observed asymmetry in TMR with bias can be due to interfaces oxidation, different work
functions, and densities of states for the two different FM electrodes [35-42]. The difference
in the observed TMR values for two different devices can be due to slightly different
interface conditions, which is not known exactly. The CVD grown h-BN is having mostly
9
monolayer thickness, with few patches of bi-layer and thicker layers [30, 31]. Using these
different thicknesses in lateral graphene spin transport devices, an enhancement in tunnel spin
polarization for higher contact resistances corresponding to the thickness of h-BN barrier has
been observed previously [30]. It would be interesting to investigate such thickness
dependence of TMR in h-BN based MTJ structures and its proposed spin filtering attributes
[26 -28].
4. Conclusions
In conclusion, we have demonstrated the feasibility of spin-dependent tunneling employing
single-layer insulating h-BN barriers in MTJs. We measured a TMR effect of 0.3 – 0.5 % at
room temperature corresponding to tunnel spin polarization P of 0.05 – 0.25 % for FM/h-BN
junctions. The lower values of TMR and P are attributed to oxidation and contamination of
the bottom FM electrode during h-BN transfer process. It is expected that much higher TMR
ratios can be obtained through improvements in fabrication process with cleaner interfaces,
incorporating multi-layer h-BN barriers [26-28], and use of h-BN/graphene heterostructures
to achieve spin filtering [26]. These results demonstrate that uses of atomically thin large area
CVD h-BN tunnel barrier are generic for a large class of devices requiring tunneling of spin
polarized carriers and particularly interesting for technologically important magnetic tunnel
junctions. Such h-BN tunnel barriers can also be employed for efficient spin injection into
silicon and other semiconductor materials [44, 45]. Opportunities are growing with
improvements in the growth process of large area CVD h-BN and h-BN/graphene van der
Waals heterostructures on ferromagnets, which can be used in future spintronic devices to
achieve desired contact resistances and large magnetoresistance signals [46-49].
Acknowledgement
10
The authors acknowledge the support from colleagues of Quantum Device Physics
Laboratory and Nanofabrication Laboratory at Chalmers University of Technology. The
authors would like to thank Jie Sun and Niclas Lindvall for sharing the recipe for 2D layer
transfer process. This project is financially supported by the Nano Area of the Advance
program at Chalmers University of Technology, EU FP7 Marie Curie Career Integration
grant and the Swedish Research Council (VR) Young Researchers Grant. RSP acknowledges
the financial support from the Department of Science and Technology, Government of India
through nanomisson project (grant No. SR/NM/NS-1002/2010).
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Figure 1| Hexagonal boron nitride magnetic tunnel junction. (a) Schematics of spin
polarized tunneling in a FM/h-BN/FM tunnel junction. (b) Optical micrograph of a fabricated
magnetic tunnel junction with Py/h-BN/Co contact. (c) Fabrication steps of magnetic tunnel
junction with CVD h-BN and FM electrodes.
14
Figure 2| CVD Hexagonal boron nitride: (a) Optical micrograph of CVD h-BN on a
SiO2/Si substrate. (b) Atomic force microscopy image of a CVD h-BN layer on a SiO2/Si
substrate. The red line is the step scan showing an effective h-BN height of ~5 Å. (c) The
Raman peak of CVD h-BN on a SiO2/Si substrate.
Figure 3| TMR with Py/h-BN/Co MTJ at room temperature. (a) Schematics of a Py/h-
BN/Co MTJ in a four probe cross bar geometry. (b, c) TMR measurements with in-plane
magnetic field B in Py/h-BN/Co MTJ for applied bias voltages of +/- 5 mV at 300 K. The
arrows indicate the up and down B field sweep directions.
.
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Figure 4| TMR with Co/h-BN/Co MTJ at room temperature. (a) Schematics of a Co/h-
BN/Co MTJ in a four probe cross bar geometry. (b) TMR measurements with in-plane
magnetic field in Co/h-BN/Co MTJ for applied bias voltages of 125µV at 300 K. The arrows
indicate the up and down B field sweep directions.
Figure 5| Bias dependence of TMR with h-BN barrier at room temperature. (a) TMR
plots for Dev1 at bias voltage of -10 and -50 mV at 300 K. The arrows indicate the up and
down B field sweep directions. (b) Full bias dependence of TMR signal for two different
Py/h-BN/Co devices measured at 300 K (bule circles - Dev 1, and red circles - Dev2). The
lines are guide to the eye.