Structural and magnetic properties of epitaxial thin films of the equiatomic quaternary CoFeMnSi Heusler alloy

We report the structural and magnetic properties of CoFeMnSi equiatomic quaternary Heusler alloy thin films. The epitaxial growth of the single-crystalline films with full $B2$ and partial $L{2}_{1}$ ordering on the Cr-buffered MgO(001) substrate was achieved using the in situ postannealing at temperature (${T}_{\mathrm{a}}$) of $500\ensuremath{-}600{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. A saturation magnetization value of about $3.5\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$ (where f.u. represents formula unit) was obtained at room temperature for the sample with ${T}_{\mathrm{a}}=600{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, which is very close to the value of $3.7\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$ reported previously for a bulk sample. Ferromagnetic resonance unveiled that negligible extrinsic relaxation due to magnetic softness as well as small intrinsic spin relaxation, i.e., the Gilbert damping constant of 0.005, which is smaller than that of permalloy. The spin magnetic moments for Co, Fe, and Mn atoms for the sample with ${T}_{\mathrm{a}}=600{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ deduced from the x-ray magnetic circular dichroism were in agreement with those previously reported in the bulk sample, the effects of Fe-Mn swapping and Mn antisite disorder were discussed.