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 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-wavelength 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 2 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.

Figure 1

(a) Imaginary part of the unrenormalized Kohn-Sham spin susceptibility of the tetragonal FeCo compounds grown on Pd ($c/a=1.13$) for selected wave vectors. The inset shows the spin-resolved total density of states; (b) the same for the renormalized spin susceptibility. Note the different scales of the axes in the two figures.

Figure 2

Magnon dispersion of the tetragonal ordered bulk FeCo alloy as grown on (a) Pd ($c/a=1.13$), (b) Ir ($c/a=1.18$), and (c) Rh ($c/a=1.24$), along the high-symmetry lines in the Brillouin zone.