Effect of stress on L10 phase formation and magnetic properties of nanoscale Fe50Pt50/Au/Fe50Pt50 films
P.V. Makushko, M.Yu. Verbytska, M.N. Shamis, T.I. Verbytska, Iu. M. Makogon
1Metal Physics Department, National Technical University of Ukraine «Igor Sikorsky Kyiv Polytechnic Institute», Prospect Peremogy 37, 03056 Kyiv, Ukraine
E-mail: makushko@outlook.com

The ordered L10 FePt phase is promising material for HAMR due to high magnetocrystalline anisotropy (7·106 J/m3) and magnetic properties [1]. Industrial application of thin FePt films requires ordering at low temperatures to control of grain size and orientation of easy magnetization axis. 
The aim of this work was to study the influence of stress in Fe50Pt50(15 nm) film with intermediate Au layer on L10-FePt phase formation and the magnetic properties of nanoscale Fe50Pt50(15 nm)/Au(x nm)/Fe50Pt50(15 nm) films, where x = 7.5; 15, 20, 30 nm.
Film compositions were obtained by magnetron sputtering on a SiO2(100 nm)/Si(001) substrate. Thermal treatment carried out in vacuum of 1.310-3 Pa in the temperature range of 300–900 °С for 30 seconds. The phase composition and film structure were determined by X-ray analysis. In-plane residual stress/strain was obtained by sin2-method using (111) reflection. The magnetic properties were determined by superconductive quantum interference device (SQUID) – magnetometry.
The stresses in the as-deposited films considerably vary in a curve with a maximum depending on the thickness of the intermediate Au layer. Ordered L10 phase forms during annealing in vacuum at 650 C in film with intermediate Au(7.5 nm) layer. In film with intermediate Au(15 nm) layer the L10 ordering temperature increases to 700 C due to change the stress from compressive to tensile. 
Increasing of intermediate Au layer thickness to 30 nm lead to increase of compressive stress level and ordering temperature of the film reduction to 600 ºС (on 100 °C) in comparison with 15 nm-thick Au interlayer. During annealing the stresses caused by the difference in thermal expansion coefficients of Fe50Pt50, Au and substrate are relaxed under A1-FePt→L10-FePt transformation, which is accompanied by decrease in volume of the unit cell. At increasing of annealing temperature up to 900°C the amount of ordered L10-FePt phase rises. This is reflected on increasing of the coercivity that is proportional to the amount of L10-FePt phase and thickness of paramagnetic Au walls at the grain boundaries. 
It was established that initial stress affect on ordering process, grain growth in [001] direction and magnetic properties in thin FePt films. The compressive stresses in the as-deposited Fe50Pt50 layers promote reduction in temperature of the ordered L10-FePt phase formation. There is a correspondence in change of mechanical stresses and ordering start temperature in Fe50Pt50(15 nm)/Au(х nm)/Fe50Pt50(15 nm) film compositions.
It was shown that at increasing of the intermediate Au layer thickness up to 30 nm the oriented grain growth of L10 FePt phase in the [001] direction with c-axis of easy magnetization perpendicular to the film plane is suppressed during annealing in vacuum. The initial stress in the films with intermediate Au layer correlates with the change of coercivity.
The authors would like to thank Prof. Dr. M. Albrecht from Augsburg University (Germany) and workers for sample preparation, assistance in conduction of investigations and discussion of results. This work was supported by DAAD Leonard Euler Scholarship Program 2012 – 2013 (DAAD Grant № 55576194), 2013-2014 (DAAD Grant № 57042790). 
1.	D. Weller,G. Parker, O. Mosendz, A. Lyberatos, D. Mitin, N.Y. Safonova, M. Albrecht Review Article: FePt heat assisted magnetic recording media // J. Vac. Sci. Technol. B- 2016.-  34.- P. 060801.