Challenges in using spin torque oscillator (STO) for ...nanospin.agh.edu.pl/wp-content/uploads/AGH-seminar-slide_Tamaru.pdf · Seminar at AGH University of Science and Technology

1

Shingo Tamaru

Spintronics Research Center (SRC)

National Institute of Advanced Industrial Science and Technology (AIST)

Seminar at AGH University of Science and Technology

May. 10th, 2016

Challenges in using spin torque oscillator

(STO) for practical microwave applications

2

Outline

1. Brief introduction of AIST and SRC

2. Challenges in using STOs for practical

microwave applications

1. STO based Phase locked loop (STO-PLL)

2. Ultrahigh sensitivity FMR (I-FMR)

3. Spatially resolved FMR (SRFMR)

3. Summary

Outline

3

1. Brief introduction of AIST and SRC

Brief introduction of AIST

4

AIST =Advanced Industrial Science and Technology


Our workplace

(~50 km NE from

central Tokyo)

5

Organization of Spintronics Research Center (SRC)


Director: Shinji Yuasa

Deputy director: Akio Fukusima

Metal

Spintronics team

Semiconductor

Spintronics team

Theory

team

STT team

Leader: Hitoshi Kubota

Elec. field effect team

I belong to

this team.

Outside collaborators

• Osaka Univ.

• Tohoku Univ.

• CNRS Thales (France)

• Toshiba Corp.

• Tokyo Electron Corp.

• Denso Corp.

• Many other…..

6

Member photo of Spintronics Research Center


Yuasa-san

Fukushima-san

Kubota-san

Me

Anna-san

7

AIST’s mission statement


Greeting from President of AIST

(Mr. Chubachi)

AIST has been pursuing research

under the slogan of bringing

“technology to society” ….

US Department of Commerce, “ The advanced technology program: Reform with a purse” (2002.2)

Valley

of deathFindings in

basic

research

Technologies

useful for

society

AIST is here!

8



1. STO based phase locked loop (STO-PLL)

Challenges in STOs

Rader

Google soli

9

Challenges in STOs

STO’s potential

applications

Consumer wireless

communications

Satellite

communications

Data storage

MAMR STO reader(Zhu, 2008) (Sato, 2012)

Microwave imaging

These apps require

highly stable yet

flexible microwave

source!

10

Challenges in STOs

The main topic of this presentation

• Wireless communication is the most demanding

microwave application in terms of oscillator

performance!

– Why? Because users have insatiable appetite for data

bandwidth.

– Phase noise is the dominant cause of spectrum

broadening.

– Narrower spectrum (equivalently, lower phase noise)

gives wider data bandwidth.

Let’s think about what needs to be done to use

STOs in wireless communication systems.

11

Widely used in many practical microwave applications.

No report of working STO based PLL as of 2014.

Frequency stability of an oscillator is critically important

for wireless communication (and other apps, of course)

Voltage Controlled Oscillator (VCO)

fVCO

VPES

frefPhase locked loop

(PLL)

Why so difficult?

STOs’ frequency stabilities are so much poorer than typical VCOs.

Ex) linewidth of STO > MHz,

VCO < 100 Hz

HB VB

STO

fSTO

VPES+

fref

Research effort 1 - PLL

12

Why STO based PLL?

Ref. in

10-100 MHz

Phase

Frequency

Detector

Voltage

Controlled

Oscillator

1/N

Down counter

Microwave

frequencyVar. in

•Current PLLs use either LC tank or dielectric

resonator based oscillator as a VCO.

•LC tank and dielectric resonators are quite large

(.1 ~ 1 mm).

•STO is much smaller than such VCOs, leading

to significant size and cost reduction!

Phase Error Signal

(VPES)

VCTR

in

RF

out

Loop

Filter

Block diagram of standard PLL

C.-W. Hsu et al., CICC, Sep. 2011


13

What about performance requirement for

wireless communication?


Wide band: up to 8.4 GHz

< 180 fs RMS jitter

SSB phase noise

-90 dBc/Hz @ 50 kHz offset

-128 dBc/Hz @ 1 MHz offset

-150 dBc/Hz @ 10 MHz offset

We have to compete

with this chip!

14

Structure of STO used in this work

MgO(1)FeB(2)

MgO(1)

CoFeB(3)Ru(0.85)

CoFe(2.5)PtMn(15)

Ta(5)

Mfree

Mref

d=400 nm

H. Kubota et. al, APEX 6, 103003 (2013)

VB (mV)

STO having a perpendicularly magnetized free layer, (PMF-STO)


15

-60

-50

-40

-30

-20

-10

0

7.244 7.294 7.344 7.394 7.444

Spectrum of free running PMF-STO

output under different VB

114.9

120.2125.6

128.2130.9

136.2

141.5

VB

(mV)

Frequency (GHz)

Pow

er (

dB

m)

7.31

7.32

7.33

7.34

7.35

7.36

7.37

113 118 123 128 133 138 143

Pea

k f

req.

(G

Hz)

VB (mV)

𝑑𝑓

𝑑𝑉𝐵= −2.1

Agility

MHz/mV


16

Ref. in

153 MHz

ref. clock

Phase Freq.

DetectorVar. in

VPES

Loop

filter

Bias-Tee

& HPF

HB

STOLow noise

amp. 1

1/8

Counter

1/6

Counter

fSTO=

7.344GHz

VB

Power

splitter

Spectrum

Analyzer

Low noise

amp. 2

Oscillo-

scope

Trigger

Block diagram of STO based PLL (1st version)


17


STO Based PLL 1st version

18

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

7.319 7.344 7.369

Spectrum of STO stabilized by PLL (1st version)

Frequency (GHz)

Pow

er (

dB

m)

fspan = 50 MHz

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

7.319 7.344 7.369

Phase locked

-100

-80

-60

-40

-20

0

Frequency (GHz)7.344

Po

wer

(d

Bm

)

RBW = 1 Hz

fspan = 1 kHz

Δf<1 Hz

Free running

Δf~4.1 MHz

(Q factor ~ 1800)

RBW = 300 kHz

-72dBm @ 1MHz

-95dBm @ 10MHz

Hittite Microwave

HMC834LP6GE


S. Tamaru et al., Sci. Rep. 5, 18134 (2015)

•Loop bandwidth

~ 8 MHz

•Somewhat

underdamped

•Suppressing phase

noise below 5 MHz

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Real time waveform of STO-PLL (1st version)

Timing jitter

σ ~ 20 ps

Reference

(Trigger)

LNA1

output

Commercial

PLLs

σ ~ 0.2 ps


20


STO Based PLL 2nd version

21


-45

-40

-35

-30

-25

-20

-15

-10

-5

0

5

6.971 6.996 7.021

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

5

6.971 6.996 7.021

Phase locked

Frequency (GHz)

Pow

er (

dB

m)

fspan = 50 MHz

Free running

Δf~4.7 MHz

(Q factor ~ 1500)

RBW = 300 kHz

Spectrum of STO stabilized by PLL (2nd version)

S. Tamaru et al., APEX, 9, 053005 (2016)

•Loop bandwidth

~ 16 MHz

(x2 wider than 1st ver.)

•Somewhat

underdamped

•Suppressing phase

noise below 10 MHz

(x2 wider than 1st ver.)

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Reference

(Trigger)

251.5 MHz

LNA1

output

6.996GHｚ

Real time waveform of STO-PLL (2nd version)

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Reference

(Trigger)

153 MHz

LNA1

output

7.344GHｚ

Real time waveform of STO-PLL (1st version)

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Timing jitter of STO-PLL (2nd version)

σ = 9.4 ps

•~half of 1st ver.

•still much larger

than commercial

PLL chips

25


Phase error of STO-PLL

in time domain

(2nd version)

Noise free signal STO signal

Phase error

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in frequency domain

(2nd version)

(top)

Power spectrum

of phase error

(bot.)

Power spectrum

of freq. fluctuation

(Calculated from

Sν = Sϕ / f 2)

Thermal

agitation

1/f fluctuation

(source

unknown)Relaxation

rate of

precession

trajectory

PLL’s

bandwidth

Unclear why it dosen’t

drop monotonically.

Free running

Phase locked

Free running

Phase locked

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• Very difficult to build such a PLL circuit (i.e. a bandwidth >> 50 MHz).

• Even if we can, STO may not follow.


in frequency domain

(2nd version)

Magnified view

of phase error

• Up to 100 MHz

• Linear scale for

both x and y axes

Free running

Phase locked

How can we further reduce timing jitter? Simple scaling:

1/10 of timing jitter → 1/100 of total noise power → 100 higher Q factor

• Thermal agitation is the dominant noise source (1/f is negligible).

• 92.5% of total noise power is contained in this region.

• If this part were completely eliminated, total timing jitter would be 2.6ps.

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Ref. in

291.5 MHz

ref. clock

Phase Freq.

DetectorVar. in

VPES

Loop

filter

Bias-Tee

& HPF

HB

STOLow noise

amp. 1

1/24

Counter

fSTO=

6.996GHz

VB

Power

splitter

Block diagram of STO-PLL

Requirements for frequencies in STO-PLL

6.996 GHz 291.5 MHz ~16 MHz 4.7 MHz

STO

frequency

Reference

frequency

Loop

bandwidth

STO

linewidth>> >> >>

This seems the only value that may be improved.

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What about power?

Noise causes timing

error at down counter

Larger noise cause

counting error

S. Tamaru et al., submitted to JJAP (2016)

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What about power?

Timing jitter caused by Johnson / shot noises

This result indicates that STO’s output power

has to be larger than -20 dBm (10 μW)!

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Challenges in STOs

In order to use a STO for wireless communication,

We have to achieve,

• Output power on the order of 10 μW,

(Current world record: 2.5 μW @ 6.75GHz)Wang et al., JSAP Autumn Meeting, Nagoya, Japan (2015)

Quantitative analysis submitted to Jap. J. App. Phys.

• Q factor on the order of (perhaps) million,

(Current world record: ~3200 @ 15GHz for

MTJ-STO)Maehara et al., APEX 7.2, 023003 (2014)

Quantitative analysis now underway

These values look really formidable, don’t they?

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Challenges in STOs



2. Ultrahigh sensitivity FMR (I-FMR)

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Challenges in STOs

Efforts to improve STO performance within AIST

• Better MTJ for higher MR ratio and lower RA

(Yakushiji et al.)

• Different STO structures (Kubota et al.)

• Phase locking of multiple STOs through

electrical connection (Tsunegi)

• Larger STO size (Tamaru)

• Search for new ideas (Everyone)

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Why larger STO size? > For higher thermostability

But if we simply make a STO larger, what happens?

• Higher order modes excited

• Mode hopping

Challenges in STOs

Power spectral density of

STO as a function of VB

HB=257mT

Real time waveform

of STO signal

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What we want are,

• As large STO size as possible,

• Coherent rotation of Mf as a single spin,

• As stiff Mf as possible.

Challenges in STOs

How can we estimate exchange stiffness? of Mf ?

->Let us take a look at mag-noise.

VB

-

Bias tee

DC

Voltage

source

V

LNA

BW:16GHz

Real-time

Oscilloscope

HB

STO

Fre

quency [

GH

z]

Bias voltage [mV]

All data are

taken under

Vb=40mV

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Challenges in STOs

300nm

200nm

Log(W

) 100nm

Mag-noise spectra of three PMF-STOs

Wider mode intervals for smaller

STOs.

Mode frequencies shift linearly as

HB increases.

Spin wave resonances excited on

perpendicularly saturated free layer

AEX can be estimated from mode

intervalS. Tamaru et al., MMM Denver (2013)

37

Challenges in STOs

Analysis of SWR modes (d=100 [nm], HB=375 [mT])

gL 2.132 Slope of main peak

BS 1.83 [T] VSM

HK -236 [mT] Fitted to main peak

AEX 6.5x10-13 [J/m] Fitted to mode intervals

d 100 [nm] Target in microfabrication

0 1 2 3 4

0 6.62 8.04 9.33 10.57 11.77

1 9.71 11.20 12.61 13.97 15.30

2 12.85 14.39 15.88 17.33 18.76

PSD of mag-noise

Peak freq. [GHz]

(0,0) (0,1) (0,2)(1,0) (0,3) (1,1) (0,4) (1,3)(1,2)(2,0)

Mref

Parameter list

• 1 order of magnitude smaller than bulk value(~2x10-11 [J/m])!

(Note: Prof. Suzuki’s estimate is about 40% of bulk)

• Large device-to-device variation

rc

Frequency [GHz]

38

Challenges in STOs

Some researchers I met in MMM2013 or in Japan

• The estimated AEX may be correct, judging from the good

agreement.

• Then what about other parameters affected by exchange

coupling, such as TC or domain wall thickness?

• Why does AEX vary for difference devices?

• How and why does Aex vary as a function of film thickness?

Prof. Slavin

• Shingo’s AEX should be wrong.

• Interlayer coupling should be taken into account.(J. Phys. Condens. Matter 22 (2010) 136001)

My decision:

I have to take out the free layer from STO and measure FMR!

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Challenges in STOs

Conventional VNA-FMR setup

VNA

P1

S21

CPW

Magnetic dot

HRF

HB

P2

If the dot is small,• an array of dots is needed to compensate for low sensitivity.

• Q: Is the signal from an array the same as from one dot?

FMR on single nanodot is desirable!

But how can I achieve such a high sensitivity?

A: Not always.

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Challenges in STOs

Ultrahigh sensitivity FMR based on microwave

interferometer (I-FMR)

VNA

P1

S21

CPW

Magnetic element

HRF

HB+HLF

P2 Rec.

Phase shifter Var. attenuator

Power

divider

Power

combiner

LNA

Tamaru et al. IEEE Magn. Lett., 5, 3700304 (2014)

41

Measurement results on 5 nm CoFeB

FMR spectra on 400/100 nm

single dot

SNR = 59.2 dB

SNR = 17.2 dB

Comparison between I-FMR

and conventional FMR

(800 nm diameter single dot)

Challenges in STOs

42

Accomplishments

• Ultrahigh sensitivity FMR (I-FMR) was built, which successfully

resolved,

– Kittel mode on f=100nm, t=5nm CoFeB single nanodot.

– higher order modes on larger CoFeB single nanodots.

Next actions

• Implement I-FMR on a larger electromagnet capable of generating

up to 2.2 T.

• Automatically change frequency and bias field magnitude and

angle, and adjust interferometer.

• Further improve the sensitivity.

• Measure magnetic nanodots to answer the questions about

spin dynamics in the STO’s free layer.

Challenges in STOs

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Challenges in STOs



3. Spatially resolved FMR (SRFMR)

44

V. Demidov et al., Nat. Mater., 9, 984 (2010) M. Madami et al., Nat. Nanotechnol., 6, 635 (2011)

Direct observation of a propagating spin wave

induced by spin-transfer torque

Direct observation and mapping of spin waves

emitted by spin-torque nano-oscillators

AIST wants to see spin dynamics excited by STO oscillation

Challenges in STOs

45

Spatially resolved ferromagnetic resonance (SRFMR)

Is Microfocuse BLS the only solution to do this measurement? NO!

Challenges in STOs

Incident light

Bias field

Reflected light

Excitation field

Block diagram of SRFMR

Tamaru et al., JAP, 91, 8034 (2002)

1D profile of decaying plane MSSW

propagating on Py film

Tamaru et al., RPB, 70, 104416 (2004)

Spatial mode profiles of spin wave

eigenmodes on square Py film

Am

p.

Ph

ase

50

mm

Tamaru et al., JAP, 91, 8034 (2002)

46

Limitation

in SRFMR

Excitation frequency must be an integer multiple of the laser

repetition rate, which is fixed at 80 MHz in Ti:S mode-locked laser

New SRFMR

design goals

• Work at arbitrary frequencies between 20MHz-20GHz

• Detect in-plane dynamic magnetization

• Maintain high spatial resolution

Block diagram of new SRFMR

Semicon.

laser

PLL Freq.

synthesizer

Microwave

synthesizer

10MHz

Master clock

-

4.6KHz

Modulation

signal

Lock-in

amp.

Two-split

photo

detector

NPBS

Obj. lens

Sample

XY motorized

state

Trig. in

Ref. in.

Mod. in.

Ref. in.

Sig. inRef. in.

PPulse width: 25ps

Host PC

Challenges in STOs

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Measurement of magnetostatic surface wave (MSSW)

Py Coupon

(100x100 mm2)

Coplanar waveguide

8 GHz

Drive Current

340 Oe

Bias field

Spin

Waves

Sample geometry

0-50 50-25 25

Position [mm]

Phase [

rad]

p

-p

0

-12

-16

-20

-24

-28Log(a

mplit

ude)

[arb

.]

Laser pulse rate: 40 MHz

Challenges in STOs

48

What are good about SRFMR?

• Much cheaper than MF-BLS,

• Comparable or higher sensitivity,

• Same spatial resolution (optical diffraction limited),

• Vectorial component detection,

• Good at relatively low frequencies (currently targeting

20MHz ~ 20 GHz), so that it can capture vortex dynamics,

• Easily converted to TR-MOKE,

• Phase sensitive.

Should be a powerful tool to study spin dynamics!

Challenges in STOs

49

Accomplishments

• While working in CMU, I built SRFMR and demonstrated that it

can capture spin wave propagation with high spatial and temporal

resolutions.

• A new SRFMR system is now being built in AIST. The first

demonstration confirmed successful operation.

Next actions

• Better sample stage and electromagnet will be built for SRFMR.

• We plan to use it to study spin dynamics excited by sombrero

STOs (MTJ based point contact STO).

• It is planned to be combined with SNOM for higher spatial

resolution than optical diffraction limit.

Challenges in STOs

50

3. Summary

Summary

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Summary

In order to use STOs in wireless communication systems,

1. STO performance has to be largely improved, i.e.

• x100 ~ x1000 higher Q factor (maybe even higher),

• ~ x4 higher power.

2. To achieve such large improvements, AIST is working on,

• Better MTJ (higher MR, lower RA) – Yakushiji

• Different STO structures – Kubota

• Synchronization of multiple STOs – Tsunegi

• Larger STO – Tamaru

• New ideas – Everyone

3. To have a larger STO, we need,

• Larger exchange stiffness constant,

4. To estimate stiffness constant of Mf,

• I-FMR system now under development.

5. To study spin dynamics excited by STO,

• SRFMR system now under development.

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Summary

I think it is time for every one of

us to get together and propose

new ideas to overcome these

difficulties in order to make

STOs useful for human society.

Any comment on this talk is

highly appreciated!

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Challenges in using spin torque oscillator (STO) for ...nanospin.agh.edu.pl/wp-content/uploads/AGH-seminar-slide_Tamaru.pdf · Seminar at AGH University of Science and Technology