Chemical ordering in magnetic FePd/Pd(001) epitaxial thin films induced by annealing

Chemically disordered FePd epitaxial layers are grown at room temperature by molecular beam epitaxy on a Pd(001) buffer layer and then annealed in order to induce the chemically ordered L1(0) (AuCu I) structure. Contrary to what is observed in the case of ordering during growth above room temperature, the ordered structure appears here with the three possible variants of the L1(0) phase. The ratio of the three different variant volumes is set by the residual epitaxial strain in the layer before annealing. It thus explains that for long annealing times, the long-range order parameter associated with the L1(0) variant with c along the (100) growth direction saturates at a value close to 0.65, and never reaches unity. Magnetic consequences of the ordering are studied.