Rotational magnetic anisotropy in polycrystalline FePt films fabricated by solid-state synthesis

It is shown that annealing 550$^\circ$C of the Fe/Pt bilayer films fabricated by layer-by-layer deposition onto polycrystalline Al$_2$O$_3$ substrates in an atomic ratio of Fe: Pt = 71: 29 leads to the formation of the inplane rotational magnetic anisotropy with the easy axis that can be aligned by magnetic field in any direction in the film plane. The increase in the Pt content in the investigated film system to 50 at % leads to an increase in the anisotropy constant to 6.5 $\times$ 106 erg/cm$^3$, which exceeds the value in the Fe$_{71}$Pt$_{29}$ sample by a factor of 6. In addition to the rotational anisotropy, the В Fe$_{50}$Pt$_{50}$ films are characterized by the perpendicular rotational magnetic anisotropy, which is higher than the film shape anisotropy. Therefore, the easy axis in these samples can be aligned by magnetic field in any spatial direction. According to the X-ray and magnetic measurement data, the synthesized samples have a polycrystalline structure consisting of two ordered phases: magnetically hard L1$_{0}$–FePt and magnetically soft L1$_{2}$–Fe$_{3}$Pt. This gives us grounds to suggest that the rotational magnetic anisotropy originates from the epitaxial intergrowth and exchange coupling of these two phases.