microwave assisted magnetic recordingmicrowave assisted ... · microwave assisted magnetic...

Microwave Assisted Magnetic RecordingMicrowave Assisted Magnetic Recording

Jimmy Zhu, Xiaochun Zhu, and Yuhui Tang

IDEMA Dec. 6, 2007

Data Storage Systems CenterDept. of Electrical and Computer EngineeringCarnegie Mellon University

IDEMA, Dec. 6th, 2007 Jimmy Zhu

OutlineOutline

Microwave assisted magnetic recording

Generation of localized microwave field: spin torque

Analysis of AC field generation

Recording simulation

Summary


Recording Field AvailabilityRecording Field Availability

Fe65Co35

MsH ∼ 0.7 Ms

Ms = 2.5 T


SubSub--coercivity Recordingcoercivity Recording

( )tHH acAC ωcosrr

=

kac HH 1.0

Hr= 0.5Hk

=

-1-0.5

00.5

1

-1-0.5

0

0.51

-1

-0.5

0

0.5

1

rHr

Mr


Switching Dynamics

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

-1.0

-0.5

0.0

0.5

1.0

ω=0.69γHk

Hr=0.17Hk

Nor

mal

ized

Mag

netiz

atio

n M

/Ms

Time t (ns)

Hac=0.1Hk

θ =30o

ACHr

rHr

Mr

θ


dtMdM

MHM

dtMd

rrrr

r

×α

+×γ−=Damping:

10.0=α

-1-0.5

00.5

1

-1-0.5

0

0.51

-1

-0.5

0

0.5

1

02.0=α 20.0=α

-1-0.5

00.5

1

-1-0.5

0

0.51

-1

-0.5

0

0.5

1

-1-0.5

00.5

1

-1-0.5

0

0.51

-1

-0.5

0

0.5

1

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1


Effect of Reversal Field Angle

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

θ=30o

θ=10o

θ=2oHac=0.1Hk

Nor

mal

ized

Sw

itchi

ng F

ield

H/H

k

Normalized Frequency ω (γHk)

ACHr

Hr

Mr

θ

( )tHH acAC ωcosrr

=


Reversal Field Angular Dependence

0 5 10 15 20 25 30 35 40 45 500.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Hac

=0.1Hk

With AC Field (Broadband)

Stoner-Wohlfarth Model

Nor

mal

ized

Sw

itchi

ng F

ield

H/H

k

Reversal Field Angle θ (degree)

ACHr

Hr

Mr

θ


OutlineOutline


Generating localized microwave field: spin torque



Summary


Basic Oscillator StructureBasic Oscillator StructureBasic Oscillator Structure

X. Zhu & J. Zhu, Intermag 2006, Paper EFX. Zhu & J. Zhu, Intermag 2006, Paper EF--0909

Field Generation Layer Field Generation Layer (FGL)(FGL)


About Oscillation FrequencyAbout Oscillation FrequencyAbout Oscillation Frequency

Field Generating Field Generating LayerLayer θπ4exchange cosMHH S−=

rr

mrθθ

θsinα effH

θsinδ

εeM

Jh

⎭⎬⎫

⎩⎨⎧

⎟⎟⎠

⎞⎜⎜⎝

⎛γα

+××α−⎟⎟⎠

⎞⎜⎜⎝

⎛γα

−×−α+

γ= 0021

m̂cHm̂m̂m̂cHm̂dtm̂d rr

Hm̂dtm̂d r

×γ=π

γ=

20HfHm̂c r

−=γα 0

If


Spin Torque Driven OscillationSpin Torque Driven OscillationSpin Torque Driven Oscillation

20.0 20.2 20.4 20.6 20.8 21.0-1.0

-0.5

0.0

0.5

1.0

In-P

lane

Mag

netiz

atio

n M

x/Ms

Time (ns)

JPJ 0=↓

Magnetic easy axis Oscillator size: 35 nm x 35 nmKu = 5 x 106 erg/c.c.

Field generating layer/ spin torque driven layer

Single Domain


Frequency Tuning by CurrentFrequency Tuning by CurrentFrequency Tuning by Current

0.0 5.0x107 1.0x108 1.5x108 2.0x108 2.5x1080

5

10

15

20

25

30

Injected Current Density (A/cm2)

Exc

ited

AC

Fre

quen

cy (G

Hz)

0

30

60

90

120

150

180

Tilting Angle θ

1θ

2θ

Frequency Tuning WindowKu = 5 x 106 erg/cc

J

1θ

2θ

X. Zhu & J. Zhu, Intermag 2006, Paper EFX. Zhu & J. Zhu, Intermag 2006, Paper EF--0909


Novel Write Head DesignNovel Write Head Design


OutlineOutline





Summary


Generating AC FieldGenerating AC Field

Field Generating Layer (FGL) Calculated Magnetization PrecessionFe65Co35 4πMs=2.5T δ =10 nm

Ι

H

KU=1.5x108 erg/cm3


Precession in FGLPrecession in FGL

I = 0.55 mA

I = 1.95 mA

I = 1.7 mA

I = 1.4 mA

I = 1.1 mA

I = 0.85 mA

MxMy

Mz


Power Spectral DensityPower Spectral Density

0 5 10 15 20 25 30 35 40 45

0.01

0.1

1

Oscillating Layerδ=10nm4πMs=2.5TArea=35x35nm2

0.55

0.851.1

1.41.7

I=1.95 mAP

ower

Spe

ctra

l Den

sity

Frequency f (GHz)


Interlayer Exchange CouplingInterlayer Exchange Coupling

AK2wall =γ

KA

=wallδ

Perpendicular layer

The effective interlayer exchange coupling on FGL is:

M

HeffectiveAK2=σ

FGLS tMHH

⋅=≈

σexchangeeffective


Field and Injected Current Field and Injected Current

0.0 1.0 2.0 3.0 4.00.0

0.4

0.7

1.1

1.4

1.8

2.2

2.5

2.9

3.2 42GHz35GHz20GHzδ = 15 nm4πMs=2.5T

K=3x108 erg/cm3

K=2x108 erg/cm3

K=1x108 erg/cm3

AC F

ield

Am

plitu

de H

(x10

3 Oe)

Current Density J (x108 A/cm2)

AK2wall =γ

M

Heffective

FGLS tMAKH⋅

≈effective


Higher Modes ExcitationHigher Modes Excitation

0 5 10 15 20 25 30 35 40 45 50

0.02

0.04

0.06

0.08

0.10

0.12

I = 4.5 mA

Pow

er S

pect

ral D

ensi

ty

Frequency f (GHz)

Oscillating Layerδ=10nm4πMs=2.5TArea=35x35nm2

When the current is too large, the magnetization precession becomes spatially non-uniform.


Frequency and CurrentFrequency and Current

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.00

10

20

30

40

50

60

δ=15nm4πMs=2.5T

K=1x108 erg/cm3

K=2x108 erg/cm3

K=3x108 erg/cm3

Injected Current I (mA)

Fre

quen

cy f

(GH

z)


AC Field StrengthAC Field Strength

0.5 1.0 1.5 2.0 2.5 3.0 3.5

1.0

1.5

2.0

2.5

3.0

3.5 f = 35 GHz δ = 20 nm

δ = 15 nm

δ = 10 nm

δ = 5 nm

AC F

ield

Am

plitu

de H

ac (x

103 O

e)

Current Density J (x108 A/cm2)

-50 -40 -30 -20 -10 0 10 20 30 40 50-3000

-2000

-1000

0

1000

2000

3000


OutlineOutline





Summary


Recording FieldsRecording Fields

Physical write head track width: 120 nm

Trailing Shield present


Recording FootprintRecording Footprint

Write head field: Rise time 0.2 ns, Duration 1 ns, Magnitude 12 kOe

Ms = 300 emu/ccHk = 8 kOeHAC = 0.0

Conventional:

Ms = 300 emu/ccHk = 30 kOeHAC = 3 kOef = 32 GHz No dispersion

MAMR:


AC Field FrequencyAC Field Frequency

Ms = 300 emu/cc , Hk = 30 kOe HAC = 3 kOe Hhead = 12 kOe

f = 24 GHz f = 32 GHz f = 60 GHz


Comparison at 1 MFCIComparison at 1 MFCI

Conventional Recording MAMR: Hhead = 12 kOe

Hk = 10 kOeHhead = 12 kOe

Hk = 30 kOe

GHzf 32= OeHac 3000=


MAMR @ 1.6 MFCIMAMR @ 1.6 MFCI

GHzf 32=Head: kOeH 12head = kOeHac 3=

nm 10=δMedium: Oe 30k kH = nm 5grain =D 05.0* =C

0°easy axis and 0 % Hk dispersion 3°easy axis and 3% Hk dispersion


Exchange CouplingExchange Coupling

GHzf 32=Head: kOeH 12head = kOeHac 3=

nm 10=δMedium: Oe 30k kH = nm 5grain =D

05.0* =C 0.0* =CMAMR @ 1.6 MFCI


Random BitsRandom Bits

All "1"s pattern

Random bits

No nonlinear transition shift is observed.


Summary

An in-plane ac field at the FMR frequencies results significant reduction of perpendicular switching field.

The scheme enables recording at a write field that is significantly below the medium coercivity.

Spin momentum transfer can be used to generate localized ac field.

Micromagnetic simulation of recording shows very promising results.

microwave assisted magnetic recordingmicrowave assisted ... · microwave assisted magnetic...

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