Structural and magnetic properties of FePt-Tb alloy thin films

We have studied structural and magnetic properties of ${(\mathrm{FePt})}_{1\ensuremath{-}x}\mathrm{T}{\mathrm{b}}_{x}$ thin films grown at elevated temperatures on MgO(100) substrates. The incorporation of Tb into the chemically ordered $\mathrm{L}{1}_{0}$ FePt lattice gives rise to considerable changes in the structural and magnetic properties, depending on the Tb content and the deposition temperature. When grown at $770{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, pronounced $L{1}_{0}$ ordering of the initial FePt phase is present. It vanishes gradually with Tb addition, thereby forming additional crystalline Tb-Pt phases at higher Tb content. These structural changes are accompanied by a strong reduction of the perpendicular magnetic anisotropy with increasing Tb content, finally resulting in a soft magnetic material with in-plane easy-axis magnetization. Extended x-ray absorption fine-structure measurements were performed to examine the possible incorporation of Tb into the FePt lattice. Based on the number of Pt first neighbors, the transformation from the initial tetragonal $L{1}_{0}$ structure to the chemically disordered fcc ($A$1) phase with increasing Tb content could be firmly concluded. In a further sample series, FePt was grown at a lower temperature of 530 \ifmmode^\circ\else\textdegree\fi{}C, which leads to reduced initial $L{1}_{0}$ chemical ordering. Adding Tb results in strong elastic stress in the lattice, eventually causing full amorphization of the film. Interestingly, in this case, a spin-reorientation transition from in-plane to out-of-plane magnetic easy axis is found at low temperatures, which is associated with an anisotropic chemical short-range order present in the amorphous phase. Furthermore, x-ray magnetic circular dichroism studies at the $\mathrm{Fe}\phantom{\rule{0.28em}{0ex}}{L}_{3,2}$ and Tb ${M}_{5,4}$ edges for all samples reveal strong antiferromagnetic coupling between Fe and Tb, resulting in a reduction of the net magnetization of the film.