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Strong magnon softening in tetragonal FeCo compounds

Magnons play an important role in fast precessional magnetization reversal processes serving as a heat bath for dissipation of the Zeeman energy and thus being responsible for the relaxation of magnetization. Employing \emph{ab initio} many-body perturbation theory we studied the magnon spectra of the tetragonal FeCo compounds considering three different experimental $c/a$ ratios, $c/a=$1.13, 1.18, and 1.24 corresponding to FeCo grown on Pd, Ir, and Rh, respectively. We find that for all three cases the short-wave-length magnons are strongly damped and tetragonal distortion gives rise to a significant magnon softening. The magnon stiffness constant $D$ decreases almost by a factor of two from FeCo/Pd to FeCo/Rh. The combination of soft magnons together with the giant magnetic anisotropy energy suggests FeCo/Rh to be a promising material for perpendicular magnetic recording applications.