ECE 5320 Lecture #9ecnfg/W5L1.pdfMaterial Science of Thin Films, ECE 6348ECE 5320 Stanko R....

Material Science of Thin Films, ECE 6348 Stanko R. Brankovic ECE 5320

ECE 5320 Lecture #9


Nanoparticle vs. Magnetic Properties

Material Science of Thin Films, ECE 6348 Stanko R. Brankovic

Magnetic Anisotropy ∑∑ −⋅−=

< iziji

jiijex SKSSJH 2)(2

Uniaxial Magnetic Anisotropy

Minimization of magnetostatic energy

Moment rotation at a Bloch Wall

AKBW πσ =

san M

KH0

2µ

=

τµ

dBUallspace

B ∫= 2

021

Bulk Co in its demagnetized, multi-domain state

↘

Anisotropy Field

aSJA ex

22=Exchange energy per unit area of Bloch wall where

for a simple cubic lattice with lattice constant a.

leads to domain wall formation


Hysteresis Loop

The hysteresis loop defines the technological properties of the magnetic material

=sM=rM=cH

Saturation Magnetization

Remnant Magnetization

Coercivity

Hard process in a single domain system

Easy process in a multi-domain system


Critical Size for SMD Particles

20

6

sSMDC M

AKRR

µ==

← 3r

←

2r

Magnetostatic vs. wall energy as a function of particle size for a spherical particle of radius r

Below Rc the particle is a Single Magnetic Domain, and thus permanently magnetized. The demagnetized state cannot be formed.

Rc ~ 100 nm


Coercivity as a function of particle size

F. E. Luborsky J. Appl. Phys.32 (1961) S171


Dp Ds Particle Diameter D

Unstable

SP

S-D

Nanomagnetism: Coercivities and Spin Reversal Mechanisms

M-D

Single-magnetic domain particle Coherent spin rotation

Hc

0

Multi-magnetic domain structure Magnetic wall movement

Bulk

K ~103 J/m3

Nanoparticle

K ~ 105 J/m3

sc M

KH0

2µ

=

Maximum coercivity


F. Bødker, S. Mørup, S. Lideroth, Phys. Rev. Lett. 72 (1994) 282

DKKK s

ceff

'6+=

shsceff KKKKK +++= σ

Origin of magnetic anisotropy enhancement in nanoparticles

c = core s = surface σ = stress sh = shape


Nanoparticle coercivity for coherent spin rotation (Stoner and Wohlfarth model)

s

uc M

KH0

2µ

=

Maximum coercivity for coherent spin rotation of a single magnetic domain particle with uniaxial total effective anisotropy

E.C. Stoner, E.P. Wohlfarth, Trans. Roy. Soc. Lond. A 240 (1948) 599

coherent moment rotation


Spin Dynamics in Magnetic Nanoparticles θθ 2sin)( VKE ua =

−=

21

0

2512VKkT

MKH

us

uc µ

=

kTVKuexp0ττ

Temperature dependence of coercivity Superparamagnetic relaxation time

Due to fast moment reversals at elevated temperatures the internal magnetic order of the particle escapes detection. You must either lower the temperature or use ultrafast measuring techniques that can record the moment before it flips.

(thermally assisted spin reversals)

Easy axis

Superparamagnetism of Small Magnetic Particles

Relaxation Time tRELAX = t0 exp (KuV/kΤ)

Energy barrier Δ E = KuV where Ku is the effective uniaxial magnetic anisotropy Energy density and V is the particle volume

Observe net magnetic moment when tMEAS < tRELAX

Magnetocrystalline Anisotropy

Shape Anisotropy

Surface effects


Micro-magnetics and Spin Dynamics

-Mössbauer spectroscopic measurements

Probe local magnetic moments and internal magnetic fields, with a response time of

τm = τMöss = 10 ns

-DC Magnetization measurements

Probe global magnetic properties in an applied field, with a response time of

τm = τSQUID = 10 s


Hysteresis Loops for CoFe2O4 Block Copolymers

The temperature at which the coercivity vanishes defines the blocking temperature TB for SQUID magnetometry.

Ahmed, Ogal, Papaefthymiou, Ramesh and Kofinas, Appl. Phys. Letts 80 (2002) 1616

Hysteresis due to particle moment rotation away from the particle’s easy axis to the direction of the applied magnetic field.

Material Science of Thin Films, ECE 6348 Stanko R. Brankovic ECE 5320 Velocity (mm/s)

Mössbauer spectra of lyophilized, in vitro reconstituted HoSF ferritin.

80 K

40 K

30 K

25 K

4.2 K

Modeling Dynamical Spin Fluctuations in Isolated

Nanostructures

G. C. Papaefthymiou, et. al. MRS Symp. Proc. Fall 2007

=

B

um kT

VKexp0ττ

Experimentally the temperature at which the Mössbauer spectra pass from magnetic, six-line spectra to paramagnetic or quadrupolar, two-line spectra defines TB for Mössbauer

Theoretically TB is defined by:

)/( 0ττ m

uB nk

VKT

=

Spectrum Key Magenta: spectral signature of magnetic particle core (internal iron sites) Green: spectral signature of surface layers (surface iron sites)

→

G. C. Papaefthymiou, Biochim. Biophys. Acta 1800 (2010) 886

Determination of Blocking Temperature

TB = 40 K


Zero-field cooled and field-cooled magnetization of lyophilized HoSF

ferritin

Typical ZFC/FC behavior of an ensemble of magnetically isolated superparamagnetic particles

25-nm thick protein shell FC

ZFC

Note: Saturation magnetization is ~ 0.05 emu/g, weakly magnetic.

Determination of Ku for an ensemble of superparamagnetic nanoparticles

=

B

um kT

VKexp0ττ

=

kTVKuexp0ττ

1. Determine average particle volume <V> by TEM

2. Determine TB with two different techniques, whose measuring response times lie in different time windows

3. Use the Arrhenius equation above to determine τ0 and Ku


−=

VKkTHTH

effhfhf 2

1)( 0

Collective magnetic excitations below TB

Surface Effects:Temperature Dependence of Mössbauer Magnetic Hyperfine Fields

S. Mørup and H. Topsøe, Appl. Phys. 11 (1976) 63

CME model, double potential well

complex potential energy landscape at the surface

Velocity (mm/s)

4.2 K

25 K

30 K

40 K

80 K


949596979899

100101

96

97

98

99

100

96

97

98

99

100

-10 -8 -6 -4 -2 0 2 4 6 8 10

98.5

99.0

99.5

100.0

4.2 K

78 K

Tr

ansm

issi

on (%

)

150 K

Velocity (mm/s)

300 K

-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 1299.75

99.90

100.05

300 K

Velocity (mm/s)

99.84

99.96150 K

Tra

nsm

issi

on (%

)

99.75

99.90

100.05

78 K

99.60

99.75

99.90

100.05

4.2 K

Mössbauer Spectra of γ-Fe2O3/Solid Silica Nanoarchitectures

Bare 12 nm particles 12 nm particles with 25 nm SiO2 shell

G.C. Papaefthymiou et. al. Phys. Rev. B 80 (2009) 024406

Spectral Key: Blue A-sites, Green B-sites of spinel structure


-10 -8 -6 -4 -2 0 2 4 6 8 10

99.8

99.9

100.0

Velocity (mm/s)

-10 -8 -6 -4 -2 0 2 4 6 8 10

99.499.699.8

100.0

Tra

nsm

issi

on (%

)

99

100

Effect of silica shell on the RT Mössbauer Spectra

Bare γ-Fe2O3 nanoparticles

γ-Fe2O3 nanoparticles with 4 nm silica shell

γ-Fe2O3 nanoparticles with 25 nm silica shell

Behavior typical of strongly interacting particles

G.C. Papaefthymiou et. al. Phys. Rev. B 80 (2009) 024406


Magnetization of γ-Fe2O3/Solid Silica/Mesoporous Silica Nanoarchitectures

0 50 100 150 200 250 3000

1

2

3

4

5

6

7

8

9

M (e

mu/

g)

Temparature (Kelvin)

A B

0 50 100 150 200 250 3000.0

0.2

0.4

0.6

0.8

1.0

M (e

mu/

g)

Temparature (Kelvin)

C D E

A-bare * B-4 nm (S)

C-25 nm (S)

D-25 nm (S) + 10 nm (MS)

E-25 nm (S) +21 nm (MS)

Typical behavior of strongly interacting magnetic nanoparticles, spin-glass-like systems.

* Bare particles are covered with a very thin layer (~1 nm) of oleic acid. Saturation magnetization of the order of ~ 8 emu/g, strongly magnetic


Conclusion

Ferrihydrite is an antiferro-magnet. Magnetization of ferritin is due to uncompensated spins at the surface → Weak magnetism. Protein coat of only 2.5 nm thickness sufficient to magnetically isolate the ferritin iron cores

Maghemite is a ferri -magnet due to uncompensated spin sublattices in its spinel structure. In small particles uncompensated spins at the surface also contribute → Strong magnetism. Silica coat of 23 nm thickness insufficient to isolate the γ-Fe2O3 cores

Dipole-dipole interaction 3

21

rµµ

⋅~

© 2012 HGST, a Western Digital company 2

Heat Assisted Magnetic Recording Media - Topics

IntroductionMagnetic Recording Media background and areal density projections

Chemically ordered L10 FePt mediaKey media parameters and requirements

Microstructure

Magnetics

Status and ongoing efforts

Summary

Heatsink/Plasmonic Underlayer (20-200 nm)

Hi T Glass

Adhesion layer

Seed layer (5-20 nm)

Carbon Overcoat /Hi T Lube

FePt

Reader

TFC: thermalfluctuation control

Heater

LD: Laser Diode Heat dissipation

Write coil

Joule heating Magnetic pole

Media Hot spot

NFT: near field Transducer

Scattered light

10/06/2012 CAISS9

Seagate HAMR Demo

1007 Gbpsi (1975 kbpi x 510 ktpi)

Demo Criteria

• Adjacent tracks written both sides at track pitch with the same laser power and pattern as data track

• On-track bit error rate = 10-2.0 with no correction/iterations

Limiting factors• Head Media Spacing

• much larger than state-of-the-art PMR• media roughness, coating thickness,

head thermo-mechanical, and clearance management

• Media Distributions• distributions much larger than PMR• large effective gradient helps

• Electronic NoiseLower Mrt and high HMS

Dr. Steve Hwang, Vice President Media R&D,

Seagate Technology, RMO Fremont

] A. Q. Wu, Y. Kubota, T. Klemmer, T. Rausch, C. Peng , Y. Peng, D. Karns, X. Zhu, Y. Ding, E. KC Chang, Y.

Zhao, H. Zhou, K. Gao, J.-U. Thiele, M. Seigler, G. Ju, and E. Gage,” HAMR Areal Density Demonstration of 1+

Tbpsi on Spinstand”, IEEE Trans Mag. 49, 779 (2013)

© 2012 HGST, a Western Digital company 11IEEE SCV, May 25, 2010

Ultimate Areal Density – HAMR + BPM

Mark H Kryder IEEE Houston, 03-08 updated by Steve Hwang 2012

5? 10?

Nanostructured Disks Suppress NoiseIssue: Smaller grains require higher fields to write & maintain thermal stability

Track Sector

Physical

Grains <D>

Magnetic

Clusters <D*>

“Disk”

D = 7.2 1.4 nm

Smaller grains

“Lower” exchange

Tighter Distributions

10 nm resolution MFM

Seagate 2004 J. Ahner, D. Weller

Cross track correlation ~ 25 nm

10/06/2012 CAISS14

Key Elements of HAMR Media Design

Good Microstructure Well Defined Thermal Profile

Good Texture and Ordering Magnetics & Distributionscompared to CoCrPt

alloys used in PMR

FePt L10 materials used

for HAMR media offer

• higher anisotropy

larger stability

• lower TC

• larger dHK/dT

• tunable by doping, e.g,

with Ni or Cu

substrate

COC

FePtX-Y

heat sink

-60 -40 -20 0 20 40 60-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

HAMR vs PMR Media Loops

Mperp/M

S

Field (kOe)

HAMR Media

PMR Media

Steve Hwang Seagate 2012

Media SNR

SNR~log10(N)

Small Grains (V)

Thermal Stability

KuV = 40-80 kBT

1

2/3

0

H

HVKE

d

uB

Writability

H0 < Head Field

2

0 Seff

S

u MNM

KH

2

3

0

1

H

HVKE

d

uB

SNR~log10(N)

seff

s

u MNM

KH

20

Media Design Constraints – “Trilemma”

D. Weller and A. Moser, “Thermal Stability Limits in Magnetic Recording” IEEE Trans. Mag. 35 4423 (1999) IBM

We are now down to 6-10

grains per bit !

100

12


Smallest thermally stable grain size - details

n

K

SSK

SBKp

HM

MH

TkrkD

2)4

1(

2

d n=1/2, k=4/d for cylinders

n=1/3, k=1 for cubes

n=1/3, k=6/ for spheres

d/D

=4

n=1/3, k=1/4 for prisms

1.2 eV

f0: attempt frequency gHK 109 1012Hz

SBB TkEef

/1

0

EB/kBTS =ln (f0)=rK~50 for =10 years

2

41

K

SuB

H

MVKE

D. Weller and A. Moser, “Thermal Effect Limits in Ultrahigh Density Magnetic Recording”, IEEE Trans. Magn.., 35, 4423 (1999);

D. Weller and T. McDaniel in Springer 2006 Advanced Magnetic Nanostructures, eds. D. Sellmyer and R. Skomski, chapter 11


HAMR media: high anisotropy, low Curie temp, small grains

~10x higher Ku “low” Tc 2x smaller grain dia

D. Weller et al., Phys. Status Solidi A 210, 1245 (2013)

PMR

HAMR


Composition and growth temperature of Co1-xPtx alloys

Y. Yamada, W. P. van Drent, E. N. Abarra, and T. Suzuki, “High perpendicular anisotropy and

magneto-optical Activities in ordered Co3Pt alloy films” JAP 83, 6527 (1998)

25 at% Pt 400oC S=0.5

10 Gbit/in2

product media

Nanoparticle arrays

12 nm grains

sarea 0.9

4 nm particles

sarea 0.05

35 Gb/in2

prototype media

8.5 nm grains

sarea 0.6J. Li, et al.,

J. Appl. Phys. 85, 4286 (1999) M. Doerner et al.,

IEEE Trans. Mag. 37 (2001) 1052

S. Sun et al.,

Science 287, 1989 (2000)

Narrowing Grain Size and Distribution

600 Gb/in2

prototype media

8.5 nm grains

sarea 0.36

Tanahashi et al.

TMRC 2008

1990 LMR

2000 LMR

2008 PMR

CoCrPt FePt

HAMR, SOMA

Current PMR product densities of ~ 750 Gb/in2 are extendable to ~1-1.3 Tb/in2

Future HAMR technology may start around 1-1.5 Tb/in2

Key: the number of grains per bit went down from 1000 to < 10note: sarea = 2x sdiameter

CURRENT FUTURE


• Annealing leads to formation of ordered, high-KU ferromagnetic phase

• It also leads to particle agglomeration & disorder in the array

chemically disordered

fcc structure

superparamagnetic

chemically ordered

fct structure

ferromagnetic

Annealing at 600oC

As-Deposited Annealed@530oC

S. Sun, C.B. Murray, D. Weller, L. Folks, A. Moser, “Monodisperse FePt Nanoparticles”, Science 287, 1989 (2000)

TJ Klemmer, C Liu, N Shukla, XW Wu, D Weller, “Combined reactions & L10 ordering”, JMMM 266, 79 (2003)

SOMA FePt Nanoparticles – fcc-fct phase transformation

oleic acid and oleyl amine stabilizers

Annealed@650oC

A1

a/c=1

L10

a/c~0.96

Fe

Pt

c-axis


HAMR

HGST Confidential

Seed layer for L10 order for FePt

MgO: FCC rocksalt, a = 0.421nm

<001> orientation, 9% mismatch

Others: CrRu, CrMo, TiN, TiC, Cr, Ag, Pt

Heat Sink / Plasmonic Underlayer: smooth

Examples: Ag, Al, Cu, Cr, Au, NiAl, NiTa

FePt + segregant X

Segregants promote grain isolation and

define grain shape

Carbon, SiO2, SiNx, B2O3

other nitrides, oxides, carbides

Heatsink/Plasmonic Underlayer (20-200 nm)

Hi T Glassup to 650oC

Adhesion layer

MgO seed layer (5-20 nm)

Carbon Overcoat /Hi T Lube

400-650oCHeating

A1 – L10 chemical ordering transition

Adhesion layer

Example: NiTa

Sputtered HAMR Media Stack

“Early” FePt HAMR media microstructure – spherical grains

0 2 4 6 8 10 12 140

20

40

60

80

100

Co

un

ts

Diameter (nm)

Counts

Lognormal Fit<D> = 7.20 nm

s = 16 %

O. Mosendz, et al., J.Appl. Phys. 111, 07B729 (2012)

Granular FePtAg-C media grown at ~550oC 2011

Used a new Lean 200 sputter tool w/ 20 chambers

Low thickness d ~ 7 nm and relatively high roughness

Average grain size <D>~7.2 nm, grain pitch ~9 nm

grain aspect ratio d/D~1

many small grains D<3 nm (thermally unstable)

Graphitic Sheets

16%


Importance of Columnar Grains

Advantages of columnar grain growth:

Decouple grain diameter from grain thickness.

Thicker media will increase readback signal.

Smoother surfaces and better flyability.

Get laterally smaller, thermally-stable grains.

Narrow distribution in optical absorption and

consistent vertical heat flow from grain to grain.

Enable functional layered structures.

ColumnsSpheres vs.

Graphitic Sheets

FePt-CDSI Image

DSI: Data Storage Institute, Singapore


K. Hono - Nat’l Institute of Materials Science Japan (NIMS)

C segregant (40 vol%) SiO2 or TiO2 segregants (50 vol%)

1. Good particle separation 1. Poor particle separation

2. High degree of L10 ordering 2. Poor degree of L10 ordering

3. Spherical type grains 3. Cylindrical type grains

4. Rough surface 4. Excellent surface smoothness

B. Varaprasad,…K. Hono, IEEE Trans Mag 49, 718 (2013)

Currently working on C and Y2O3 or Cr2O3 segregants to combine these 2 effects

K. Hono

2013 ASTC

presentation

dual mag layers

higher thickness d ~ 10 nm improved read back signal

average grain size <D>~6.3 nm, grain pitch~ ~7.3 nm

grain aspect ratio d/D~1.6

less grains with D < 3.5 nm

smoother surface

BUT: “worse” grain size distribution

FePt dual layer media with more cylindrical grains 2012

50 nm

(b)0 2 4 6 8 10 12 14

0

20

40

60

80

100

Co

un

ts

Diameter (nm)

<D> = 6.28 nm

sD = 26%

50 nm

Granular FePtX-C/Y media grown at ~620oC


26%

0 2 4 6 8 10 12 14 16 18 200

10

20

30

40

50

60

70

80

90

Co

un

t

Pitch Diameter (nm)

Counts

LogNormFit

 = 8.55

Width = 0.28

≈ 11 nm

≈ 5 nm

FePt triple layers w/ less smaller grains 2013

DP ~8.5nm

sP=28%

Cluster size – 16.9nm

-80 -60 -40 -20 0 20 40 60 80

-1.0

-0.5

0.0

0.5

1.0 HC = 3.6 T

No

rma

lize

d M

ag

ne

tiza

tio

n

Field (kOe)

iSFD = 12.6 kOe

Hexternal

= 4.1 kOe

Granular FePtX-Y media grown at ~640oC

sP=28%

• reduced amount of tiny grains

• significantly improved size distribution

Improved grain size distribution 2014

<D>=8.4 nm, sD=0.18 =10.0 nm, sP=0.20

0 2 4 6 8 10 12 14 16 18 200

20

40

Co

un

ts

Grain Diameter (nm)

Counts

LogNormFit

<D> = 8.4 nm

w = 0.18

Packing Fraction = 68%

0 2 4 6 8 10 12 14 16 18 200

20

40

60

80

100

120

 = 10.0 nm

w = 0.20

eliminating D<4nm Grains

Co

un

ts

Pitch Size (nm)

Counts

LogNormFit

Packing fraction = 68%

<D>=8.4 nm, sD=0.18

=10.0 nm, sP=0.20

d/D~1.5

~1

2 n

m

tiny

gra

ins

media 2011

Current HAMR media microstructure

media Nov 2013

- small grains

- tight size distribution

- spherical grains

- low signal

- rough media

- larger grains


- more columnar grains

- increased signal

- reduced roughness

- smaller grains


- columnar grains

- further increased signal

- smoother surface

<D>=8.4 nm

w=0.18

0 2 4 6 8 10 12 14 16 18 200

10

20

30

40

50

Co

un

ts

Grain Diameter (nm)

Counts

LogNormFit

<D> = 8.4 nm

w = 0.18

media May 2014

HAMR media microstructure evolution

<D>=8nm, sD=18%<D>=8.4 nm, sD=18%

<D>=7.2 nm, sD=16%

Thickness d=7 nm Thickness d=10 nm Thickness d=12 nm


-9 -6 -3 0 3 6 9

-1.0

-0.5

0.0

0.5

1.0

3.0 kOe

Hext

M (

au

)

H (T)

Hint

20 kOe

room temp

Minor Loop Analysis: Switching Field Distribution (300K) 2011

sint = 15 kOe (VSM)

Grain volume distribution: svol = 3.7kOe

- from TEM grain size analysis

Grain texture distribution: saxis = 6.6kOe

- from rocking curve width, XRD

Anisotropy distribution: sHK=12.9kOe

- from VSM, may arise from

variations in L10 order, lattice

strain & defects

Micromagnetic model needed to go

beyond these estimates

M

22

axis

22

int Hkvol ssss Large iSFD:

Small eSFD small cluster size (14nm)

low exchange and magnetostatic

interactions

What is iSFD at the recording

temperature, near Tc ?

S. Pisana et al., Effects of grain microstructure on magnetic properties in FePtAg-C JAP 113, 043910 (2013)

Magnetic properties of “early” FePt-C samples 2011

J. Becker, O. Mosendz, D. Weller, A. Kirilyuk, J.C. Maan,

P.C.MM. Christianon, Th. Rasing and A. Kimel, “Laser

Induced Spin Precession in Highly Anisotropic Granular

L10 FePt”, Appl. Phys. Lett. 104, 069416 (2014)

S. Wicht, V. Neu, L. Schultz, B. Rellinghaus, D. Weller, O.

Mosendz,, G. Parker, and S.Pisana, “Atomic resolution

structure–property relation in highly anisotropic

granular FePt-C films with near-Stoner-Wohlfarth

behaviour” J. Appl. Phys. 114, 063906 (2013)

HC ~ 5T HK ~10T MS ~ 1040 emu/cm3 at 290K

KU ~ 5.2 x 107 erg/cm3 <D>=nm grain diameter

HC~5T HK ~10T MS ~ 950 emu/cm3 at 290K

KU ~ 4.8 x 107 erg/cm3

HC = 8.2T at 4.2K

~ 0.1 damping parameter in FMR

IFW Dresden

Radboud University, Nijmegen


XRD and MOKE hysteresis of improved media 2013

-8 -6 -4 -2 0 2 4 6 8

M (

a.u

.)H (T)

reference

improved magnetics

further improved magnetics

FePtAg-C

HC=5.1 4.7T

Modified deposition parameters result in suppression of very small grains and

reduced noise in recorded media

20 25 30 35 40 45 50

500

1000

1500

2000

2500

Fe

Pt(

20

0)

Fe

Pt(

11

1)

Mg

O(0

02

)

Fe

Pt(

00

2)

Co

un

ts (

arb

. u

nits)

2 (deg)

Further improved FePt

Improved FePt

Reference FePtF

eP

t(0

01

)


(001)/(002) XRD ratio 1.9 – 2 chemical ordering S~0.90

reduced M


20 25 30 45 50 55

45 46 47 48 49 50 51

FePt

200

FePt

002

Inte

nsity (

a.u

.)

2 Theta (deg)

FePt

200

FePt

002

Inte

nsity (

a.u

.)

2 Theta (deg)

FePt

001

46 48 50 52

200 / (002+200)

~ 5.7%

Inte

nsit

y (

cp

s)

2 (degrees)

older media

Fit Peak 1

Fit Peak 2

Total Fit

46 48 50 52

Inte

nsit

y (

cp

s)

2 (degrees)

improved media

Fit Peak 1

Fit Peak 2

Total Fit

200 / (002+200)

~ 1.9%

-80 -60 -40 -20 0 20 40 60 80

-1

0

1

Mr/Ms = 9%

Mr/Ms = 13%

No

rm.

Ma

gn

.

Field (kOe)

older media

improved media

Mr/Ms = 93%

Mr/Ms = 89%

25 30 35

FePt 001

FePt 110

IP XRD

Inte

nsity (

a.u

.)

2 (degrees)

older media

improved media

OOP XRD

Quantifying amount of in-plane easy axis grains with XRD and VSM


C.-B. Rong, et al. 2006 Arlington,

Texas, Adv Materials, vol 18, 2984

H. M. Lu, et al. 2008: Nanjing,

China JAP103,123526

Recent Modeling by Seagate and HGST

O. Hovorka, ….., G. Ju, R. W. Chantrell, 2012: “The Curie

temperature distribution of FePt granular magnetic

recording media”, APL 101, 052406 York U. – Seagate

A. Lyberatos, D. Weller, G. Parker, 2012: “Size

dependence of the TC of L10-FePt nanoparticles” JAP

112,113915 Crete U. – HGST

Experiments

Experiments and Modeling

Chemical ordering S and Curie temperature TC vs grain size

8 nm FePt nanoparticles

Perf

ect

L1

0o

rder

see: D. Weller et al, “The HAMR Media Technology Roadmap to an Areal

Density of 4 Tb/in2” IEEE Trans Mag 50, 3100108 (2014)


Effect of grain size and aspect ratio on TC - Modeling

/11 DxTDT occ

• TC smaller than 750 K due to exchange truncation/abandonment in single particle modeling

• Cylindrical grains with an aspect ratio of ~2 reduce x0 by ~20%, i.e. “minimize” the grain

size induced reduction of TC

• 0.7/0.09 is compatible with 3D Ising/Heisenberg models

• Tc determined from peak susceptibility χ(Τ) using Monte Carlo method

A. Lyberatos, D. Weller, G. Parker, “Finite size effects in L10-FePt nanoparticles” J. Appl. Phys. 114, 233904 (2013)

2 3 4 5 6 7 8 9 10500

520

540

560

580

600

620

640

Dz/D

x=1

Dz/D

x=2

Tc (

K)

D (nm)

5-20% drop of TC

~0.7/.09Finite size scaling theory

1 2 3 4 5 6500

520

540

560

580

600

620

640

Tc (

K)

Grain aspect ratio Dz/D

x

Dx = 3 nm

Modeling


Atomistic Calculations

Variations in Tc arise from the dispersion in grain size and chemical order.

Recording performance is highly sensitive to Tc and HK distributions.

Reducing D increases sTc /TC from ~1% (D=8nm) to ~2.5% (D=4nm).

3 4 5 6 7 8 9

5

10

15

20

25

30

Dz/D

x=1

Dz/D

x=2

sT

c (

K)

Dx (nm)

scx

o/L

x/a)^(-1/)*s

L

Lz/L

x=1, x

o=2.37, 0.7

Lz/L

x=2, x

o=2, 0.7

c=750K,s

L=0.2

sD=0.2

~2.5%

~1%

A. Lyberatos, et al, “Size dependence of TC of L10-FePt nanoparticles” J. Appl. Phys.. 112, 113915 (2012)

A. Lyberatos, et al, “Memory erasure and write field requirements in HAMR using L10-FePt nanoparticles” (2014)

sTC vs grain diameter Dx


HAMR Media Design Challenges

HGST Confidential


atom num bers size(nm )

201 1.77

459 2.31

1289 3.27

2075 3.85

4033 4.77

5635 5.35

9201 6.31

11907 6.87

Starting to count individual atoms …

At ~3 nm diameter particles have 25-

30% atoms on surface! Properties

change as a result of that!

2 nm

Oleg Mryasov, 2003 (Seagate)


Particle Size Effects: 3d(Fe,Co)-5d/4d(Pt/Pd) High Anisotropy Alloys

Curie Temperature Reduction Anisotropy Energy Reduction

0

0.02

0.04

0.06

0.08

0.1

2 4 6 8 10 12

[Tc (

inf)

- T

c (

d)]

/Tc (

inf)

Diameter (nm)

0

1

2

3

4

5

6

7

8

0 2 4 6 8 10 12 14

particle size (nm)

T = 0 K

"two-io n"

"s ingle-ion"

PtFe

j

PtFe

iPt

ij JJJ

kd

20

)0()2(

][

Finite size effects due to interactions mediated

by induced Pt magnetic moment

Particle

Surface to volume fraction increases to 20-

40% for 3 nm FePt particles (1000 atoms)

O. N. Mryasov - U. Nowak - K. Y. Guslienko - R. W. Chantrell, Europhy. Lett. 69, 805 (2005)

0.1

0


Buschow "Handbook of Magnetic Materials”, Elsevier 2011, J. Lyubina, B. Rellinghaus, G. Gutfleisch, M. Albrecht

“Structure and Magnetic Properties of L10-Ordered Fe-Pt Alloys and Nanoparticles”

Kussmann, A, von Rittberg, G.Grfn., “Study of conversions in the Platinum –Iron System “, Z. Metallkd. 11, 470 (1950);

A. Z. Menshikov, Yu. A. Dorofeev, V. A. Kazanzev,. S. K. Sidorov, Fiz. metal. metalloved. 38, 505 (1974).

Strong dependence of TC on chemical ordering A1 L1O (TC =165K)

TC =585 K

Full chemically ordered TC =750 K


Dustin Gilbert IEEE SCV 11/19/13

UC Davis – Seagate


0.40 0.45 0.50 0.55 0.60

10

20

30

60

70

80

90

100

110

Hc, H

k (

kO

e)

Fraction of Fe Content

Hc

Hk

Ms

600

700

800

900

1000

1100

Co

re M

s (

em

u/c

c)

Optimal values of coercivity and

anisotropy at x=50%

Composition dependence in FexPt1-x–C and FeXCuYPtZ


300 400 500 600 700 800 900

0.0

0.2

0.4

0.6

0.8

1.0

Fe55

Cu0Pt

45

Fe39

Cu7Pt

54

Fe39

Cu9Pt

52

Fe33

Cu13

Pt54

Fe23

Cu23

Pt54

M/M

30

0K

T (K)

granular continuous

TC~730K

Curie temperature reduction to 600-

650K by adding 9-13at% Cu


FeCuPt IEEE SCV 11/19/13

UC-Davis - Seagate

D.A. Gilbert, et al., Appl. Phys. Lett. 102, 1324006 (2013)


Grain Size and Microstructure from CoCrPt PMR to FePt HAMR 2013

HAMR Media HAMR: more Voronoi and columnar

Improved Grain Size (Pitch) & Distributions

Typical PMR

ECE 5320 Lecture #9ecnfg/W5L1.pdfMaterial Science of Thin Films, ECE 6348ECE 5320 Stanko R....

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