We have investigated the Gilbert damping, α, of a tetragonally distorted, perpendicular magnetic anisotropic (PMA) ultrathin Fe0.5Co0.5 film grown on a Rh-buffered MgO(100) substrate fabricated by magnetron sputtering at room temperature by means of the time-resolved magneto-optical Kerr effect. We obtained the highest PMA value of 0.573 MJ/m3 ever reported for the Fe0.5Co0.5/Rh film. The PMA strongly depends on the lattice distortion which originates from the epitaxial growth in the large lattice misfit system of Fe0.5Co0.5 and Rh. We have estimated an unusually high value of α = 0.041 ± 0.002 for a 1 nm thick Fe0.5Co0.5 film. Based on the microstructural observation and the first-principles calculation, we conclude that the large α in the ultrathin Fe0.5Co0.5 film comes from the minority-spin electron transition around the Fermi level mediated by the spin-orbit interaction, which is caused by the large lattice distortion.
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