Evolution of stress with L1 ordering in FePt and FeCuPt thin films

The evolution of stress in 50 nm FexPt1−x (x=0.41, 0.43, 0.49, 0.52, and 0.56) sputtered films was monitored in situ as a function of the temperature. In the as-deposited state, films have a disordered face-centered-cubic (fcc) structure with a strong (111) fiber texture and 685–978 MPa compressive stress. Below 200 °C, thermal expansion of the FexPt1−x fcc lattice produced an initial increase in residual in-plane compressive stress. Above 250 °C, a transition to a low stress state was observed, with the relaxation in the Fe0.52Pt0.48 film showing stronger temperature dependence. Comparisons of stress to a sputtered film of Fe0.35Cu0.15Pt0.5 indicate a faster transition rate to the low stress state can be achieved with the addition of Cu. In situ high temperature x-ray diffraction measurements show a decrease in (111) d spacing with an increase in temperature, and are consistent with the changes observed in the residual stress measurements. The large stress transition appears to arise from changes in the fcc phase prior to L10 phase transformation, because a substantial increase in film coercivity appears only after the transition to the low stress state has been completed.