Calorimetric Studies of Fe/Pt Multilayer Thin Films Ysela L. Chiari Prof. K. Barmak David 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 K u have higher thermal stability.
Introduction Currently used magnetic media material: Hexagonal Co-based alloys Tetragonal L1 0 alloys (FePt) have higher K u 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 L1 0 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 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. 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 = o C Annealed at T = o C
Results – DSC Traces
Results – Experimental Data
Results = 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 FePt 3 was observed after annealing at o C. Various Phases are present at the peak transformation temperature for samples with = 200 nm. FePt fully ordered was observed after annealing at 700 o C.
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 FePt 3 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, , C. Michaelsen, M. Dahms. Thermochimica Acta 288 (1996) Pool, Robert. “Exploring Frontier Materials”. Think Research.. E. Grochowski and R. D. Halem. “Technological impact of magnetic hard disk drives on storage systems”. IBM Systems Journal Vol. 42, 2, IBM Corporation. 5 Aug wski.html