Spin wave damping in the ferromagnetic semiconductor CdCr2Se4

Abstract

The relaxation rate of magnons has been measured in nominally pure CdCr2Se4 with the parallel pumping technique at 9.4 GHz, as a function of temperature T and wave vector k, for 4.2<T<40 K and 0<k<106 cm−1. Three relaxation mechanisms have been identified as responsible for the measured rates. Impurity ions provide relaxation processes which are a strong function of the direction of the applied magnetic field with respect to the crystallographic axes. The damping rate due to this process is maximum along the 〈111〉 axis and minimum along 〈100〉. The measured rates at all temperatures show two distinct k dependences: at higher k values a linear dependence with k and at smaller values a nearly constant behavior. The linear dependence is due to a three-magnon relaxation process, as shown by detailed numerical calculations. The constant behavior at lower k results from the decay of the pumped π/2 mode into degenerate magnons and their subsequent relaxation to the thermal bath via three-magnon processes. A simple two coupled mode calculation explains qualitatively both the T and the k dependence of the data.