Study of solid-state reactions and order-disorder transitions in Pd/б-Fe(001) thin films

The formation of the hard-magnetic ordered $L1_0$-FePd phase in thin bilayer Pd/$\alpha$-Fe(001) films has been experimentally studied. Solid-state reactions initiated by thermal heating in bilayer Pd/$\alpha$-Fe(001) films with a thickness of $50$–$60$ nm (the atomic ratio Pd: Fe $\approx$ 50:50) separated from the substrate have been studied using the in situ electron diffraction methods. It has been shown that the solid-state reaction between the palladium and iron layers in Pd/$\alpha$-Fe(001) starts at $400\,^{\circ}$C with the formation of the disordered Fe-Pd phase. At $400\,^{\circ}$C, the ordered $L1_0$-FePd phase is formed. The order-disorder phase transition has been studied. It has been established that the transition of the ordered $L1_0$-FePd phase to the disordered FePd phase starts at $725\,^{\circ}$C. At $740\,^{\circ}$C, only the disordered FePd phase is present over the whole volume of the film. The observed temperature of the order-disorder phase transition is shifted from the equilibrium value by $35\,^{\circ}$C to higher temperatures. This effect is assumingly associated with the higher concentration of palladium atoms at the boundaries of the Fe-Pd crystal grains owing to the grain-boundary adsorption.