Effect of initial stress/strain state on the L10 phase formation of FePt in FePt/Au/FePt trilayers

The influence of the initial stress/strain state in FePt/Au/FePt thin films on the chemical L10-FePt formation after post-annealing is studied. It is shown that the level of stress/strain depends on the Au interlayer thickness revealing a correspondence in L10 ordering-onset temperature. Compressive strain present in the as-deposited films promotes lowering of the L10-FePt ordering temperature. A reduction of the onset temperature for ordering by 100 °C is observed in films having a 30-nm-thick intermediate Au layer compared to samples revealing lower compressive strain values. During annealing, the strain state caused by the difference in thermal expansion coefficients of FePt, Au, and substrate is relaxed during A1 → L10-FePt phase transformation. It is well known that the coercivity of FePt films rises with increasing fraction of the ordered L10-FePt phase. Furthermore, the insolubility of Au in the L10-FePt lattice results in Au diffusion predominantly along grain boundaries leading to magnetic FePt grain isolation, which in turn gives rise to enhanced coercivity. At an annealing temperature of 900 °C, coercivities as high as 27.5 kOe are achieved for FePt samples with an initial 30-nm-thick Au interlayer.