Calorimetric Studies of Fe/Pt Multilayer Thin Films

Ysela L. ChiariProf. K. BarmakDavid C. Berry

September 16, 2005

Background Hard disk drives are made of bit cells. For greater

storage capacity: Reducing the amount of crystal grains inside one bit

Maintain good Signal to Noise Ratio for reliable data storage and retrieval.

Reducing the size of crystal grains inside one bit. Superparamagnetic Effect: is the limit for

grains size reduction without having them lose the ability to hold their magnetic orientation at any given temperature.

Represented with the following expression:

Materials with higher Ku have higher thermal stability.

40TkVK

b

u

Introduction Currently used magnetic media material: Hexagonal Co-based alloys Tetragonal L10 alloys (FePt) have higher Ku

ASM Alloy Phase Diagrams, 1996

About the Research Goal: Gain a detailed understanding of the kinetic

and thermodynamic properties of Fe/Pt through the use of multilayer thin films.

Sample: Fe/Pt multilayer thin films of 1m thickness, .

The use of multilayers allow the determination of the enthalpy of formation of the L10 phase from pure Fe and Pt.

The symbol, , represents the periodicity of the multilayer.

C. Michaelsen, K. Barmak, and T. P. Weihs, J. Phys. D , 30, 3167 (1997)

Experiment

50.0 : 50.0 20055.0 : 45.0 20045.0 : 55.0 20050.0 : 50.0 50

at% Fe : Pt (nm)

Film Preparation: Fe and Pt targets were sputtered onto a silicon wafer surface for a calculated time with fixed power.

Four Fe/Pt multilayer films were prepared with nominal compositions ranging from 45 to 55 at.% Fe.

Experiment

C. Michaelsen, K. Barmak, and T. P. Weihs, J. Phys. D , 30, 3167 (1997)

Instrument: Perkin Elmer DSC 7.

Approximately 6.0 mg of free standing sample were used for DSC measurements.

I(CM)UV.<http://www.uv.es/icmuv/c/inve/tec2_1.htm>

Sample and Reference are thermally isolated from one another and each is provided with its own heater. Power Compensation

The DSC consists of two pans.

Experiment

Annealing was done with the DSC 7.

The size of the XRD samples were squares of 5x5 mm.

Phase Identification: X-Ray diffraction (XRD) of 200 nm Fe/Pt multilayer at different stages of the reaction:

As deposited state Annealed at T = 472.7 oC Annealed at T = 700.0 oC

Results – DSC Traces

Results – Experimental Data

(nm) (kJ/g-atom)20 453.540 472.740 470.580 489.580 487.1

160 509.320 450.740 468.640 466.580 487.1160 507.2160 503.4

20 458.540 477.440 474.780 494.4160 513.520 395.920 393.840 408.940 407.480 422.6160 437.8

Tp (°C)

nQ (eV)DH Composition

Fe : Pt

b (K/min)

200

200 50.0 : 50.01.72

± 0.06

25.1 ±

0.9

24.9 ±

0.6

1.11E+10

200 55.0 : 45.01.70

± 0.06

25.4 ±

2.18.84E+09

1.21E+10

50 50.0 : 50.01.86

± 0.05

23.1 ±

2.31.68E+12

45.0 : 55.01.74

± 0.05

Results = 50 nm Sample

Results = 50 nm Sample

(nm) (kJ/g-atom)

20 573.1

20 566.1

40 585.8

40 587.7

80 606.4

160 621.8

Tp (°C)

nQ (eV)DH Composition

Fe : Pt

b (K/min)

50 50.0 : 50.02.36

± 0.16

2.7 ±

1.41.58E+12

Dependence of DH on Composition

20.0

21.0

22.0

23.0

24.0

25.0

26.0

27.0

28.0

44 46 48 50 52 54 56Composition Fe (at. %)

Tra

ns

form

ati

on

En

tha

lpy

, D

H (

kJ

/g-

ato

m)

= 200

= 50

Dependence of DH on Composition

20.0

21.0

22.0

23.0

24.0

25.0

26.0

27.0

28.0

29.0

44 46 48 50 52 54 56Composition Fe (at. %)

Tra

ns

form

ati

on

En

tha

lpy

, D

H (

kJ

/g-

ato

m)

= 200 = 50

1D Diffusion and Interface Controlled Growth Models

D

=

Tk

Q

k

Qk

T

H

t

H

BB

od

2exp

2x,

max bb

D

=

Tk

QHk

t

H

Boi exp

xmax,

b

Diffusion Controlled Growth:

Interface Controlled Growth:

In spite of the good fittings the values for activation energy are inconsistent with the experimental activation energies.

Neither model describes the transformation of FePt multilayers.

Dependence of Q on Composition - Comparison

1

1.5

2

2.5

3

3.5

44 46 48 50 52 54 56

Composition Fe (at. %)

Ac

tiv

ati

on

En

erg

y, Q

(e

V)

exp 200 exp 50

I 200 I 50

D 200 D 50

JMAK – Michaelsen–Dahms Fits Equation:

D=

n

B

B

n

B

B Q

Tk

Tk

Qnv

Q

Tk

Tk

QnvnH

t

H 212

exp60

expexp60

60 bbb

JMAK – Michaelsen–Dahms Fits DH

(nm) (kJ/g-atom) (kJ/g-atom)nn DH Composition

Fe : Pt

200

200

200 8.84E+09

45.0 : 55.0

25.4 ±

2.1

24.9 ±

0.61.21E+10

55.0 : 45.0

50.0 : 50.025.6

± 1.0

25.1 ±

0.9

50 50.0 : 50.0 1.68E+1223.1

± 2.3

22.6 ±

1.8

Experimental Michaelsen - Dahms

1.12E+10

8.36E+09

1.49E+10

1.64E+12

25.0 ±

0.4

1.11E+10

25.9 ±

1.4

Results - XRD

FePt3 was observed after annealing at 472.7 oC. Various Phases are present at the peak transformation

temperature for samples with = 200 nm.

FePt fully ordered was observed after annealing at 700oC.

Conclusions The peak transformation temperature was higher for

= 200 nm films than for = 50 nm films and it increases with heating rate.

The Enthalpy of the transformation from pure Fe and Pt for the = 200 nm and = 50 nm films was approximately the same with an average of 25.3±3.6 kJ/(g-atom).

Fits with 1D Diffusion and Interface controlled growth models were good, but they yielded activation energies that were higher and lower than the experimental values, thus suggesting that the growth is not fully dependent on diffusion processes or on the interface.

Michaelsen – Dahms fits were good, but the existence of FePt3 and other phases aside from the fully ordered FePt at the peak transformation temperature invalidates this model.

Acknowledgements & References Acknowledgements

Professor K. Barmak David Berry Ben Nowak Material Research Science and Engineering Centers (MRSEC)

References C. Michaelsen, K. Barmak, T.P. Weihs. J. Phys. D 30, 1 (1997). K. Barmak, J. Kim, D.C. Berry and W. N. Hanani. Journal of

Applied Physics 97, 024902-1, 2005. C. Michaelsen, M. Dahms. Thermochimica Acta 288 (1996)

9-27. Pool, Robert. “Exploring Frontier Materials”. Think Research.

<http://domino.research.ibm. com/comm/wwwr_thinkresearch.nsf/pages/frontier399.html>.

E. Grochowski and R. D. Halem. “Technological impact of magnetic hard disk drives on storage systems”. IBM Systems Journal Vol. 42, 2, 2003. IBM Corporation. 5 Aug. 2005.<http://www.research.ibm.com/journal/sj/422/grochowski.html>