anisotropy distributions in patterned magnetic media · distributions • continuous media: –...

Anisotropy Distributions in Patterned Magnetic Media

Tom ThomsonHitachi San Jose Research Center

MINT Review & Workshop24-25 Oct. 2006

Acknowledgements

– Manfred Albrecht (Post-doc)– Tom Albrecht– Maggie Best– Liz Dobisz – Guohan Hu (Post doc)– Charlie Rettner (IBM)– Bruce Terris– Henry Yang/Dan Kercher

• Summer students– Katie Humphry– James Williams– Laura Hirsch

Patterned Media Overview

• Patterned media will extend magnetic recording to > 1 Tb/in2

1Tb/in2 = 25 nm period5 TB 3.5-inch drive

1.2 TB 2.5-inch drive80 GB 1-inch drive

Patterned Media -- Very Small BitsIndividual magnetic islands can be created on the diskEach island would represent a single bit of information

-0.2

0.0

0.2

x-position [nm]

Sign

al [a

.u.]

0 200 400 600 800

200 Gb/in2 = 56 nm period

Critical role of anisotropy distributions

• Continuous media:– small change in width

of anisotropy distribution has a large impact 1dB ~ 102 BER

σHA/HA

ΔSN

R(d

B )

σHA/HA

ΔSN

R(d

B )

H.J. Richter

• MRAM: – must be able to

reliably address only the target cell (Oe field or spin torque)

• Patterned media: – Head field must not

switch neighbouring islands

IBM-Almaden

Manfred Schabes

Wide Hk distributionUnintentional writing of adjacent islands

by stray fieldsfrom the head

Addressing the correct island in patterned media

Narrow Hk distributionWrite only the intended track

Pole headMap of field strengthat top of islands

Manfred Schabes

-400

-200

0

200

400

Mag

netis

atio

n (e

mu/

cm3 )

-1500 -1000 -500 0 500 1000 1500

Applied field (Oe)

Co/Pd multilayer (E4085)

Anisotropy distributions are always present!

• Nucleation and domain wall motion masks distribution

• Nucleation of only a few very low anisotropy sites

• Nanofabrication allows full distribution to be probed

Nucleation Domain formation Domain growth

Co/Pd islands for this study

• Motivated by patterned magnetic storage media– Require smooth, clean, surface capable of supporting flying

head– Method of choice: Pattern substrate first, then deposit film

• Process Steps– SiO2/Si patterned by e-beam lithography and RIE etch– Perpendicular Co/Pd multilayers deposited onto islands– Island sizes down to 30 nm (300 Gbit/in2)

• req. > 500 Gbit/in2 for applications

1μm

MFMSEM

SiO2/Si

Pd seed layer [30Å]Co [3.3Å]Pd [9.6Å]CoPd

Pd cap layer [12Å]

CoPd

CoPd

8 bilayers

Magnetic measurements on islands• DCD Remanence curves:

– Saturate with an applied field– Apply an incremental reverse field– Return to zero applied field and

measure Kerr rotation

• Two distinct reversals– Trenches– Islands

• Measure as a function of1,2:– Island size– Applied field angle– Initial state of island

-0.3

-0.15

0

0.15

0.3

-6000 -4000 -2000 0Magnetic Field [Oe]

Trench Material

IslandsKer

r Rot

atio

n [a

.u.]

The MOKE measurements (20μm spot) have been verified by VSM and MFM measurements

SEM

SFD

• Kerr rotation measurements– Median reversal field Hr– Width of switching field distribution (SFD)

1G. Hu, T.Thomson et al. J. Appl. Phys. 97 10J702 (2005)2G. Hu, T.Thomson et al. IEEE Trans. Magn. 41 3589 (2005)

Kerr rotation remanence curves

• Restrict Kerr DCD curves to island reversal region• Systematic increase in reversal field with decrease in

island size• Increase in SFD with decrease in island size• Angle dependence?

-1.0

-0.5

0.0

0.5

1.0

Ker

r Rot

atio

n (n

orm

alis

ed)

80006000400020000Applied Field (Oe)

50 nm100 nm

200 nm500 nm

1 μm5 μm

film

Domain structure in islands• SEM images of 50nm -> 5μm islands

5μm 50nm1μm 200nm

5um 1um 200nm 50nm

• MFM images of islands in a.c. demagnetized states– Single domain behavior is found in islands with a diameter < 100nm.

Angle Dependence of Reversal

• Coherent rotation (Stoner-Wohlfarth reversal)

( ) ( )

( ) ( )[ ]θθθ

sin 3/2cos 3/2 2/30

+= H swH sw

0.5

0.75

1

0 15 30 45 60 75 90Field Angle θ [°]

Hsw

(θ)

/Hsw

(0)

• Domain wall motion (Kondorsky reversal)

1

1.5

2

2.5

3

0 20 40 60 80

Field Angle θ [°]

Hsw

(θ)

/Hsw

(0)

M

M

( ) ( )( )θθ

cos0HH sw

sw =

Ha

θ

Ku

MHa

θ

Ku

Vwall

0.5

0.6

0.7

0.8

0.9

1

0 20 40 60 80

5um1um500nm200nm100nm50nm30nm

Hc/

Hc(

0)

Angle [degree]

Islands: Angle dependent switching

Island arrays with sizes ranging from 5μm to 30nm exhibit Stoner-Wohlfarth like angle dependent switching with a minimum at 45°.

easy axisHappl.θ

1

1.2

1.4

1.6

1.8

2

2.2

0 20 40 60 80

Hc/

Hc(

0)

Angle [degree]

100μm

0.8

1

1.2

1.4

1.6

1.8

0 20 40 60 80

Hc/

Hc(

0)

Angle [Degree]

20μm

0.7

0.8

0.9

1

1.1

0 20 40 60 80

Hc/

Hc(

0)

Angle [Degree]

10μm

0.8

1

1.2

0 20 40 60 80

Hc/

Hc(

0)

Angle [degree]

1μm

With the decrease of island size, the island switching behavior changes from a wall motion dominant process to a nucleation controlled one.

The reversal characteristics of continuous film are only recovered in islands with a diameter of 50μm to 100μm.

Size dependent reversal mechanism

1

1.5

2

2.5

3

3.5

4

0 20 40 60 80

Hc/

Hc(

0)

Angle [degree]1

1.5

2

2.5

3

3.5

0 20 40 60 80

Hc/

Hc(

0)

Angle [degree]

50μmContinuous film

××

××

H=Hsatperp H=0

H×

•

•

•

•

•

•H

H=Hin-plane H=0

Hin-plane=-2.16kOe Hin-plane=-2.41kOe Hin-plane=-2.54kOe

H

H=Hsatin-plane H=0

M

Introduction of nucleation sites – the experiments

1um island arraysSaturate

perpendicular

Nucleate with inplane field

-2.16kOe

-2.54kOe

-2.41kOe

Reversal changes with nucleation sites

-0.5

-0.4

-0.3

-0.2

-0.1

0

-3600 -2400 -1200 0

Norm-1.91kOe-2.16kOe-2.41kOe-2.54kOe

Kerr

Rot

atio

n [a

rb.]

Magnetic Field [Oe]

1 3

2

• 1--- Islands without nucleation sites; 2--- Trench; 3 --- Islands with nucleation sites.

• The field required for wall propagation is much lower than for nucleation.

0kOe

0.5

0.6

0.7

0.8

0.9

1

0 20 40 60 80

Hc/

Hc(

0)

Angle [degree]

1

1.5

2

2.5

3

0 20 40 60 80

1um Round

Hc/

Hc(

0)

Angle [degree]

• Nucleation controlled reversal processes follows the Stoner-Wohlfarth angle dependent switching model .

• After introducing nucleation sites, the angle dependent switching behaviour follows the Kondorsky model.

Reversal mechanism comparison

Model of reversal in Co/Pd islands

• Model of island reversal from full remanence:– A small volume reverses by rotation – Rapid domain wall motion then ensures the entire island reverses from the

nucleation site– Initial reversal depends on local anisotropy and total effective field– Nucleation field (Hn) >> pinning field (Hp)

• Consequences– Angle dependence has Stoner-Wohlfarth form– Distribution of nucleation (anisotropy) fields can be determined by varying island

size– Reversal field depends on initial state i.e. on the presence of reversed regions

• Tests– Compare experimental and simulated results for Hr and σHr– Functional form of SFD– Time dependence of coercivity

Length scale for nucleation of reversal

• Length scales in magnetism (Co/Pd)– Exchange length – sets limits on coherence

– Activation volume (length)

– Domain wall width (180º Block wall)

1.25μm

nmMA

ls

ex 24172 2 −==π

assuming A ~3-6 x 106 erg/cm

SMkTVtl irr

sact

χπ ==2 lact ~ 30nm

nmKADWW

u30~π=

400

300

200

100

0

10-9

Pro

babi

lity

14121086Anisotropy Constant K1 (106 erg/cm3)

Gaussian distribution of intrinsic anisotropy

• Assume intrinsic anisotropy is spatially varying

• Variation described by a Gaussian function

• No correlation between regions

• Length scale is set by nucleation (activation) volume

Gaussian anisotropy distribution

+

Distribution of nucleation fields

• Assume: – Nucleation field (Hn) >> pinning field (Hp)– Nucleation volume 25 x 25 x t nm

50 nm islands 500 nm islandHn = 1000 Oeσ = 100 Oe

• Island reversal occurs when the lowest nucleation site switches

Nucleation site

Simulation

• Stoner Wohlfarth reversal of nucleation volume– Input measured Ms and Kint (mean)

• Shape anisotropy calculated using method of Aharoni1– Input measured shape of islands

• Sharrock2 extrapolation for finite temperatures and times to compare with experiment– Output nucleation (volume) constrained by length scales

• Distribution of intrinsic anisotropy is independent of island size– Output σKint

1Aharoni J. Appl. Phys. 83 3432 (1998)2Sharrock and McKinney IEEE Trans. Magn. 17 3020 (1981)

Inputs

Outputs

• Optimise simulation to match both measured Hcr and SFD with a single set of parameters

• Find value of σKintacross all island sizes and obtain Stoner Wohlfarth equivalent nucleation volume

Simulation and experiment

Co/Pd multilayersMs = 400 emu/cm3

Kint = 2.20 x 106 erg/cm3

σKint = 7.6%lnuc = 38 nm

-1.0

-0.5

0.0

0.5

1.0

Rem

anen

t Ker

r rot

atio

n (n

orm

.)

80006000400020000

Applied field (Oe)

50nm500nm5 μmfilm

Remanence curves (islands only)

10000

8000

6000

4000

2000

0

Hcr

(Oe)

5 6 7 8 9100

2 3 4 5 6 7 8 91000

2 3 4 5

Island size (nm)

800

600

400

200

0

SFD

[Std. D

ev.] (Oe)

Hcr SFD

SFD Neighbours

T.Thomson, G. Hu and B.D. Terris, Phys. Rev. Lett. 96 257204 (2006)

Functional form of SFD

• Simulation correctly reproduces changes in the functional form of the SFD as a function of island size

2.0

1.5

1.0

0.5

0.0

10-3

Sw

itchi

ng P

roba

bilit

y

8000600040002000Applied field (Oe)

50 nm100 nm

200 nm

500 nm

1 μm

5 μm 1.5

1.0

0.5

0.0

10-3

Sw

itchi

ng P

roba

bilit

y

320030002800260024002200200018001600

Applied field (Oe)

1 μm islands

800

600

400

200

0

10-6

Sw

itchi

ng P

roba

bilit

y

80007000600050004000

Applied field (Oe)

50 nm islands

Time dependence of coercivity

• Good agreement with experimental time dependence of coercivity (VSM)

• Compare H0 with hard axis (inplane) field (Hn-inplane) needed to create nucleation sites

• Reasonable agreement between H0 and Hn-inplane (Sanity check)

8000

7000

6000

5000

4000

3000

2000

1000

Hcr

(Oe)

10-10 10-8 10-6 10-4 10-2 100 102

Time (s)

50 nm

500 nm

5 μm

Inplane (hard axis) field required for nucleation

Summary• Controlling the distribution of anisotropy is

critical for nanomagnetic technologies

• Patterning magnetic arrays is a novel approach for determining intrinsic distributions of magnetic properties

• SFD is due to intrinsic properties of the magnetic film and is not a patterning induced artifact

• Significant impact on the direction of patterned media development towards the goal of 1 Tbit/in2

Topical Review"Nanofabricated and Self-Assembled Magnetic Structures as Data Storage Media"B.D. Terris and T.Thomson J. Phys. D: Applied Physics 38 (2005) R199

-1.0

-0.5

0.0

0.5

1.0

Kerr

Rot

atio

n (n

orm

alis

ed)

80006000400020000Applied Field (Oe)

50 nm100 nm

200 nm500 nm

1 μm5 μm

film

T.Thomson, G. Hu and B.D. Terris, Phys. Rev. Lett. 96 257204 (2006)

Patterned Media:Status and Outlook

• Patterned magnetic nanostructures:– Topographically patterned island arrays have

been fabricated by e-beam lithography and nano-imprinting at 300Gbit/in2

– Challenge is to reliable fabricate at low cost and scale towards 1 Tbit/in2

– Most recently we have developed a new understanding of the SFD width in terms of a distribution of intrinsic anisotropy

Topical Review"Nanofabricated and Self-Assembled Magnetic Structures as Data Storage Media"B.D. Terris and T.Thomson J. Phys. D: Applied Physics 38 (2005) R199

• Recording physics– Read/Write experiments demonstrated at 200Gbit/in2 - 1Tbit/in2 possible?

– Noise is ultimately lithography limited, and in test structures is superior to similar continuous media

– Model of the write process developed to understand synchronisation

End of Presentation

anisotropy distributions in patterned magnetic media · distributions • continuous media: –...

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