Anomalous behavior of linear and nonlinear longitudinal susceptibilities below $T_c$ in a weakly anisotropic CdCr$_2$Se$_4$ ferromagnet

Frequency dependences of the linear and nonlinear longitudinal dynamic susceptibilities of an almost isotropic cubic CdCr$_2$Se$_4$ ferromagnet were studied experimentally in the ordered phase. It was found that at frequencies above the two-magnon creation threshold the linear susceptibility decreases as $\chi_1\propto \omega^{-0.28}$ with increasing frequency and the nonlinear susceptibility decreases as $\chi_n\propto \omega^{-0.73}$, irrespective of $n$, where $n$ is an odd number. The observed susceptibility anomalies are due to the dipolar forces that violate the conservation of total spin.