Spin waves in alloys at finite temperatures: Application to the FeCo magnonic crystal

We study theoretically the influence of temperature and disorder on the spin-wave spectrum of the magnonic crystal Fe1-cCoc. Our formalism is based on the analysis of a Heisenberg Hamiltonian by means of the wave vector and frequency-dependent transverse magnetic susceptibility. The exchange integrals entering the model are obtained from the ab initio magnetic force theorem. The coherent potential approximation is employed to treat the disorder and random phase approximation in order to account for the softening of the magnon spectrum at finite temperatures. The alloy turns out to exhibit many advantageous properties for spintronic applications. Apart from a high Curie temperature, its magnonic band gap remains stable at elevated temperatures and is largely unaffected by the disorder. We pay particular attention to the attenuation of magnons introduced by the alloying. The damping turns out to be a nonmonotonic function of the impurity concentration due to the nontrivial evolution of the value of exchange integrals with the Co concentration. The disorder-induced damping of magnons is estimated to be much smaller than their Landau damping.