Magnetic anisotropy and microstructure in molecular beam epitaxial FePt (110)/MgO (110)

We compare the growth, structural, and magnetic properties of Fe_xPt_1-x(x≃0.6) films grown by molecular beam epitaxy on MgO (110) substrates held at 300 and 500 °C. Growth at both temperatures yields nearly single orientation epitaxial films with FePt, Pt [001]//MgO[001] and FePt, Pt (110)//MgO (110). For growth at 300 °C, chemical ordering to the L1₀ (CuAu I) tetragonal phase is incomplete (long-range order parameter, S=0.38) in contrast with growth at 500 °C for which ordering is nearly complete (S=0.81). Despite incomplete chemical ordering, a large two-fold, in-plane magnetic anisotropy (K_u=1.5×10 erg/cm) is present for the film grown at 300 °C. Transmission electron microscopy for the 300 °C film confirms epitaxial growth with few extended defects but also reveals that chemical ordering is distributed inhomogeneously within the film. The direction of ordering is generally uniform (c axis along MgO [001]) but variations in the degree of ordering are evident on a length scale of 1-10 nm.