perpendicular recording media

Center for Materials for Information TechnologyA NSF Materials Research Science and Engineering Center

Spring Review 2003

PERPENDICULAR RECORDING MEDIA Relaxation of Remanent Magnetization in Perpendicular Media

J.W. Harrell, Shoutao Wang, Scott Brown and Roy Chantrell**Seagate, Pittsburg, PA

Soft UnderlayersSoon-Cheon Byeon and Bill Doyle

High speed switching in Perpendicular mediaExperimental

V. G. Voznyuk and W. D. Doyle

TheoreticalArko Misra and Pieter Visscher


Spring Review 2003

Relaxation of Remanent Magnetization in Perpendicular Media

J.W. Harrell, Shoutao Wang, Scott Brown

Dept. of Physics & Astronomy

Center for Materials for Information Technology University of Alabama, Tuscaloosa, AL

Roy Chantrell

Seagate, Pittsburgh, PA

Support: NSF-MRSEC, MINT Center


Spring Review 2003

Magnetization decay• The thermal decay of the magnetization is a critical issue in

ultra-high density magnetic recording.

• The study of the relaxation of the remanent magnetization in zero applied field after partial demagnetization gives insight into the effect of interactions on the thermal stability.

E

G (E)

EC E

G (E)

EC

DC demagnetized

(DCD type)

AC demagnetized

(IRM type)


Spring Review 2003

Initial ZF viscosity in CoPtCrB film with perpendicular anisotropy

• Zero-field viscosity depends strongly on demagnetization state. Maximum S at ±Mrs.

-8

-4

0

4

8

-1 -0.5 0 0.5 1

IRM-typeDCD-type

S 0 (%/d

ec)

Mr

-12 -8 -4 0 4 8 12

perppara

M

H (kOe)


Spring Review 2003

Calculated ZF Viscosity for CoCrPtB ⊥ Media

Monte-Carlo method(no exchange)

Mean-field calculation: Nd = -0.47

(KV/kT = 55, H0 = 5 kOe, σV = 0.4)

-2

0

2

4

-1 -0.5 0 0.5 1

DCD type (C* = 0)IRM type (C* = 0)

S 0 (%/d

ec)

Mr0

-6-4-20246

-1 -0.5 0 0.5 1

DCD type IRM type

S0 (%

/dec

)M

r0

Mean field method


Spring Review 2003

Quasistatic calculation of demagnetization factor

Choose demag factor to construct ‘best’ log-normal switching field distribution from recoil curves: Nd = -0.40 (Nd = -0.47 from relaxation meas)

van de Veerdonk et al, IEEE Trans. Magn. 39, 590 (2003)

-600-400-200

0200400600

-1 104 -5000 0 5000 1 104

Nd = -0.40

Nd = 0

M (e

mu/

cc)

H (Oe)

-400-200

0200400

-1 104 -5000 0 5000 1 104

M (e

mu/

cc)

H (Oe)

Nd = -0.2


Spring Review 2003

Exchange stabilizes saturation remanence in perpendicular media

Monte-Carlo calculations

-6

-4

-2

0

2

4

6

-1 -0.5 0 0.5 1

S(%

/dec

)

Mr0

C* = 0

C* = 0.1

C* = 0.22


Spring Review 2003

Exchange interaction can dominate shape of relaxation curve in both perpendicular and longitudinal media

Longitudinal CoPtCrBPerpendicular Co/Pd

-2

-1

0

1

2

3

-1 -0.5 0 0.5 1

IRM-typeDCD-type

S 0 (%/d

ec)

Mr

-0.4-0.2

00.20.40.60.8

-1 -0.5 0 0.5 1

IRM-typeDCD-type

S0 (%

/dec

)

Mr


Spring Review 2003

Summary of effect of interactions on remanent relaxation

• No interactions – viscosity independent of MR over wide range.

-1 -0.5 0 0.5 1Ze

ro-F

ield

Vis

cosi

ty, S

0

Remanence Moment, Mr

• Exchange interactions –stabilizes saturation remanence, enhances decay of reduced remanence.

• Demagnetization field –destabilizes saturation remanence.


Spring Review 2003

Synthetic Antiferromagnetic Coupled Fe65Co35/Ru/Fe65Co35 Soft Underlayers

for Perpendicular Media

S. C. Byeon and W. D. Doyle

MINT CenterThe University of Alabama

This project was funded by INSIC.


Spring Review 2003

Historical Development of Soft UnderlayersTarget: 200 nm thick ferromagnetic layer and µ ~ 100

FeTaN/IrMn- G/FeTaN(20)/[IrMn(10)/FeTaN(20)]9

19 layers- High 4πMs ~ 20 kG- Thermal instability of FeTaN Fe10Co90/IrMn

- G/Cu/IrMn/[FeCo(50)/IrMn(10)]4/FeCoN(20)11 layers

- Low 4πMs 15 ~ 16 kG

Fe65Co35/IrMn- G/Cu/IrMn/[FeCo(50)/IrMn(10)]4/FeCo(25)

11 layers Not enough thermal stability

Increased Hk

Enhanced thermal stability

Increased 4πMs

Optimize seed layer

Fe65Co35/IrMn- G/Ta/Cu/IrMn/[FeCo(50)/IrMn(10)]4/FeCo(25)

12 layers Enough thermal stability


Spring Review 2003

Outstanding Hysteretic Properties in FeCo/IrMnMultilayer with Ta

G/Ta(20 nm)/Cu(20 nm)/IrMn(10 nm)/[FeCo(50 nm)/IrMn(10 nm)]4/FeCo(25 nm)

-100 -50 0 50 100

-1.0

-0.5

0.0

0.5

1.0

Hpo = 66 OeHeb = 38 OeHc, EA = 11 OeHc, HA = 1.7 Oe

m /

ms

H (Oe)

EA HA

Annealed at 225 oC in H = 500 Oe

-100 -50 0 50 100

-1.0

-0.5

0.0

0.5

1.0

TopFeCo

Hpo = 79 OeHeb = 49 OeHc,EA = 6 OeHc,HA = 1 Oe

m /

ms

H (Oe)

EA HA

As-deposited

The Ta underlayer maintained outstanding hysteretic properties.Annealing enhanced the single domain condition of Heb > Hc.


Spring Review 2003

Synthetic Antiferromagnetic Soft Underlayers

• Advantages

– Fe65Co35/Ru/Fe65Co35 with thick ferromagnetic layer– Better thermal stability than IrMn-based films– Ideal soft underlayers

• No edge demagnetization• Reduced number of layers• Improved efficiency for magnetic flux return

– Thinner spacer layer(~1 nm) than the 10 nm thick IrMn


Spring Review 2003

Expected film parameters• Fe65Co35/Ru/Fe65Co35 trilayer film parameters

– Saturation field (the field at which the moment has attained 85% of the saturation moment) Hs = 2JAF / MFtF (sandwich structure)

– Permeability µ=1+4πMs / Hs• JAF=0.82 erg/cm2, 4πMs=23 kG; Hs=230 Oe is required for µ =100• Fe65Co35 thickness = 47 nm (sandwich structure)

– (reference for JAF value: Huai et al., JAP, 85, 5528, 1999)

-1.0 -0.5 0.0 0.5 1.0

-1.0

-0.5

0.0

0.5

1.0

Mag

netiz

atio

n (A

rb. U

nit)

Applied Field (Arb. Unit)

EA HA

Glass/Ru 2.5 nm/FeCo (tF)/Ru (t)/FeCo (tF)/Ru 10 nm Ideal hysteresis loop

Easy axisZero remanenceGood separation of hysteresis

Hard axisClosed hysteresis loopInsensitivity of hysteresis loop to angle around hard axis


Spring Review 2003

Hysteresis of FeCo 50 nm/Ru 1 nm/FeCo 55 nm

100 -50 0 50 100H (Oe)

FeCo 55nm EA HA

-1.0 -0.5 0.0 0.5 1.0

-1.0

-0.5

0.0

0.5

1.0

Mag

netiz

atio

n (A

rb. U

nit)

Applied Field (Arb. Unit)

EA HA

– Experimental Ideal

Easy axis hysteresis loopGood separation of hysteresis

Hard axis hysteresis loopClosed hysteresis loop


Spring Review 2003

Angular Dependence of Hysteresis– FeCo 50 nm/Ru 1 nm/FeCo 55 nm

-100 -50 0 50 100

H (Oe)

-25o

-100 -50 0 50 100

H (Oe)

-15o

100 -50 0 50 100

H (Oe)

35o

100 -50 0 50 100

H (Oe)

-65o

-100 -50 0 50 100

H (Oe)

0o

-100 -50 0 50 100

H (Oe)

45o

-100 -50 0 50 100

H (Oe)

90o

-100 -50 0 50 100

H (Oe)

105o

-100 -50 0 50 100H (Oe)

115o

Well separated hysteresis loop around easy axisVery narrow hysteresis loop around hard axis (Hc = 3.5 Oe)Very large angular reversibility of magnetization around hard axis ( 60o )

-100 -50 0 50 100H (Oe)

15o

EA

HA


Spring Review 2003

Saturation Field and Coupling Coefficient

0 50 100 150 200

0

2000

4000

6000

8000

Satu

ratio

n fie

ld H

s (O

e)

FeCo thickness (nm)0 50 100 150 200

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Inte

rlaye

r cou

plin

g co

effic

ient

JAF

(erg

/cm

2 )FeCo thickness (nm)

Hs = 2JAF / MFtF

Hs decreases faster than 1/tF.JAF is constant above 10 nm FeCo thickness.


Spring Review 2003

Conclusions

• Synthetic antiferromagnetic soft underlayer looks promising.• Antiferromagnetic coupling

– Glass/Ru 2.5 nm/FeCo (tF)/Ru (t)/FeCo (tF)/Ru 10 nm• Ru (t) = 0.6 - 1.0 nm for FeCo (tF) = 5 - 200 nm• Saturation field = 25 – 6000 Oe• JAF is constant above 10 nm FeCo thickness.

– Glass/Ru 2.5 nm/FeCo 50 nm/Ru 1 nm/FeCo 55 nm/Ru 10 nm• Well separated hysteresis loop around easy axis• Very narrow hysteresis loop around hard axis ( Hc = 3.5 Oe )• Very large angular reversibility of magnetization around hard axis (

60o )• Permeability of µ = 200 is obtained.

– Can be decreased ( µ = 100 ) using multilayer or thinner FeCo layers


Spring Review 2003

Future Work

• Identify optimum configuration.• Measure permeability directly.• Model angular dependent hysteresis process.

– Why negative remanence?– Why perpendicular remanence?

• Test thermal stability.


Spring Review 2003

Simulation of artificial antiferromagnetsArkajyoti Misra and P. B. Visscher, Department of Physics and Astronomy

M2FeCoExperimentally, one sees negative

remanence at certain angles. We have examined a possible mechanism for negative remanence, involving a slight misalignment of the easy axes in the two ferromagnetic layers.

Ru

M1 FeCo


Spring Review 2003

Negative-remanence mechanismWhen H is decreased after saturation, the thin (green) layer moves toward its easy axis, which is closest to H. This forces (via the AF interaction) the thick layer to move the other way. At H=0, thecomponent of M1 along H exceeds that of M2.

EAthick

H

x

φ

θ

M1M2

EAthin

M-H Loopy


Spring Review 2003

High speed switching in perpendicular media

V. G. Voznyuk and W. D. Doyle

Center for Materials for Information Technologyand Department of Physics,

University of Alabama

Supported by the NSF Grant No. ECS-0085340 and made use of the NSF MRSEC Shared Facilities Grant No. DMR-0213985


Spring Review 2003

Experimental Setup. Pulse Generation and Monitoring.

High Voltage DC Power Supply

Coaxial Cable RG-213

Spark Gap

Transient DigitizerSCD 1000

Microstrip Line AttenuatorsR

Trigger Unit

all interconnections made with coaxial RG213 type cables

Microstripline with perpendicular media cross-sectionMicrostripline interconnect design

Ground Plane

x

y

z

Kapton insulator

Sample under conductor


Spring Review 2003

Sample Structure and Magnetic Properties

5 nm20 nm1 nm4 nm400 nm7 nmCCo Pt12 Cr18 /CrTaNiAl/ /CoNb8Zr5 /NiGlass / Al /

Sample is provided through INSIC–EHDRM by Yoshihiro Ikeda, IBM Almaden Research CenterRecording layer (RL)

Hcr [100 s] = 4350 OeMst = 0.75 memu/cm2

Mr/Ms ~ 1

Soft Underlayer (SUL)

Mst = 34 memu/cm2

Hc < 0.1 Oe (10 Hz)


Spring Review 2003

What is the actual field generated by the microstripline?Sample: Glass / NiAl [7nm] / CoNb8Zr5 [400nm] / NiAl [4nm] / CrTa [1nm] / Co Pt12 Cr18 [20nm] / C [5nm]

CZVH

0

=

10-11 10-9 10-7 10-5 10-3 10-1 101 1030

1000

2000

3000

4000

5000

6000

n=2/3 Sharrock fitH

0 = 6000 ± 200 Oe

KV/kT = 170 ± 20

HPS

HTD

MOKE long time data Sharrocks fit

Hcr

(Oe)

pulse width (s)

Z0- microstrip line characteristic impedance, C – calibration constant of themicrostip

HPS – field calculated from current distribution using a power supply voltage (VPS):

HTD – field calculated from amplitudes of pulses recorded on Transient Digitizer (VTD)

( )

−=

n

cr lntfln

KVkTHH

21 0

0f0 - thermal attempt frequency ~ 109 Hz, H0 - intrinsic switching field Sharrocks fit1:

1 P. J. Flanders, M. P. Sharrock, J. Appl. Phys. 62 (7), 2918, (1987)


Spring Review 2003

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.740

45

50

55

60

outin

Z (O

hm)

time delay (ns)

microstripline

sample

Time domain reflectometry data

Modeling the Measured Impedance Characteristics of the Pulse System

( )1

10

−+

=ρ

ρZZ ρ - reflection coeffisient., Z0=50 Ohm

0 1 2 3 4 5 6

0.0

0.2

0.4

0.6

0.8

1.0

Tran

smis

sion

, ref

lect

ion

coef

ficie

nts

frequency (GHz)

Coaxial Cable Microstripline with cables measured measured simulation simulation

Frequency domain data

1/G (f)

Model

R (f) L

CZ0=56 Ohm Z0=55 Ohm Z0=56 Ohm

Z0=44 Ohm Z0=44 Ohm


Spring Review 2003

Simulation results.Voltage and current at different points. 2.5 ns pulse.

Transient Digitizer

SCD 1000

Microstrip Line AttenuatorsCoaxial Cable RG-213 RG-213

Pulse Generator

V0 VMS I0 IMSVTD ITD

0 5 1 0 1 5 2 0 2 5

02468

1 0

volta

ge (k

V)

t im e (n s )0 5 1 0 1 5 2 0 2 5

0

5 0

1 0 0

1 5 0

2 0 0

curr

ent (

A)

t im e (n s )

X 3626 X 3626

%.520

=IIMS%.88

0

=VVTD


Spring Review 2003

The field on the microstripline is very close to HPS

10-10 10-8 10-6 10-4 10-2 100 102 1040

1000

2000

3000

4000

5000

6000

n=2/3 Sharrock fitH0 = 6000 ± 200 Oe KV/kT = 170 ± 20

HPS HTD corrected based on simulation MOKE long time data Sharrock fit

Hcr

(Oe)

pulse width (s)

10-9 10-75000

6000


Spring Review 2003

ConclusionsThe electrical behavior of the pulse system was simulated with

PSPICE using the measured impedance characteristics

For this particular microstripline power supply voltage provides more accurate field values than those determined from the Transient Digitizer

Hcr[t] data with the corrected field values exhibit an increase at short times as expected

Future workIncorporate the spark-gap into the model

Measure Hcr[t] vs. the initial remanence


Spring Review 2003

Simulation of fast switching in perpendicular media

Arkajyoti Misra, P. B. Visscherand D. M. Apalkov

Department of Physics and AstronomyThe University of Alabama

Supported by NSF grants # ECS-008534 and DMR-0213985, and DOE grant # DE-FG02-98ER45714


Spring Review 2003

Objective

• Simulate experiments under way on fast switching in perpendicular recording media

• Initial simulations are on CoCrPt system with no soft underlayer; simulations with underlayer are under way

• Coercivity: 2.5 kOe

• Material parameters from experiment


Spring Review 2003

160 nm

Simulated System

160 nmy

z

x

20 nm

Landau-Lifshitz simulation using periodic BC’s in x and y, random thermal field (T = 300K);

cell size in simulation ≈ grain size


Spring Review 2003

02.0 kOe 2.9

)exp(0

0

==

=

k

k

Hk

Hkk

H

xHH

σ

σ

5.0 J/m 107

)exp(120

0

=×=

=−

A

A

A

xAA

σ

σ

07.02/12 =∆θ

kOe 02.84 =sMπ

Material parameters from hard axis loop

Log-normal distribution of anisotropy field:

(x is normally distributed with variance 1)

Similarly, exchange constant distribution:

Hard axis loop


Spring Review 2003

Pulsed Field and Magnetization Response

Experimental pulse profile

Trapezoidal fitRise Time = 0.71 ns = Fall Time

Rise Time = 0.71 ns = Fall Time

= 2.6 ns

)( ∞== tMM r

)0( == tMM rs


Spring Review 2003

Simulated Remanent Magnetization

ns 6.2=∆t


Spring Review 2003

Time dependent coercivity


Spring Review 2003

Counter-intuitive effect of the initial remanenceExpect high initial M (relatively unswitched) to be harder to switch:

In fact, however, it switches more easily:

Possible explanation: High-M state has more demag energy, which when released into spin waves, heats the system and accelerates thermal switching.

perpendicular recording media

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