Dependence of the structural and magnetic properties of FePt foil magnets on annealing ramp rate

Bulk Fe/Pt multilayers were annealed to form hard magnetic L10 FePt-based permanent magnet foils. The phase composition of the foils, and thus their magnetic properties, were found to depend not only on the temperature and time of annealing, but also on the heating ramp rate used. For short annealing times (p1 h) and low annealing temperatures (p450 C), a slow ramp rate (900 K/h) resulted in the formation of an FePt diffusion barrier at the multilayer interfaces, while a fast ramp rate (142 103 K/h) promoted a more complete interdiffusion of the Fe and Pt atoms. The relatively low value of coercivity of samples annealed for short times (p1 h) following a fast ramp, is attributed to the presence of the soft-magnetic disordered FCC FePt phase, exchange-coupled to the dominant hard magnetic L10 FePt phase.