Calorimetric Studies of Fe/Pt Multilayer Thin Films Ysela L. Chiari Prof. K. Barmak David C. Berry August 11, 2005

Background Hard disk drives are made of bit cells. For greater storage capacity: Reduce the amount of crystal grains inside one bit Maintain good Signal to Noise Ratio for reliable data storage and retrieval. Reduce 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.

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. The use of multilayers allow the determination of the enthalpy of formation of the L10 phase from pure Fe and Pt. The symbol, L, represents the thickness of the multilayer. C. Michaelsen, K. Barmak, and T. P. Weihs, J. Phys. D , 30, 3167 (1997)

Experiment 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 range of 45 to 55 at.% Fe.

Experiment 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> The DSC consists of two pans. Sample and Reference are thermally isolated from one another and each is provided with its own heater. Power Compensation C. Michaelsen, K. Barmak, and T. P. Weihs, J. Phys. D , 30, 3167 (1997)

Experiment Phase Identification: X-Ray diffraction (XRD) of 200 nm Fe/Pt multilayer at different stages of the reaction: As deposited Annealed at T = 472.7 oC Annealed at T = 700.0 oC Annealing was done with the DSC 7. The size of the XRD samples were squares of 5x5 mm. XRD measurements were done with RIGAKU.

Results – DSC Traces

Results – Experimental Data

Results L = 50 nm Sample

Results L = 50 nm Sample

1D Diffusion and Interface Controlled Growth Models 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.

JMAK – Michaelsen–Dahms Fits Equation:

JMAK – Michaelsen–Dahms Fits

Results - XRD FePt3 was observed after annealing at 472.7 oC. Various Phases are present at the peak transformation temperature for samples with L = 200 nm. FePt fully ordered was observed after annealing at 700oC.

Conclusions The peak transformation temperature was higher for L = 200 nm films than for L = 50 nm films and it increases with heating rate.. The Activation Energy of L = 200 nm films were Q = 1.72 ± 0.06, 1.70 ± 0.06 and 1.74 ± 0.05 eV while it was Q= 1.86 ± 0.05 eV for L = 50 nm films. The Enthalpy of the transformation from pure Fe and Pt was 25.1 ± 0.9, 25.4 ± 2.1, 24.9± 0.6 and 25.8 ± 2.7 kJ/(g-atom) for L = 200 nm and L = 50 nm films. Fittings 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 opening the possibility for an in between growth model. Michaelsen – Dahms fittings were good, but the existence of FePt3 and other phases aside from fully ordered FePt at the peak transformation temperature invalidates this model.

Acknowledgements & References 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>