Modification of Damon-Eshbach magnetostatic mode spectra in ferromagnet/paramagnet bilayer

Using the magnetostatic approximation, we calculate the spectra of bulk and surface spin waves in an in-plane magnetized ferromagnet/paramagnet bilayer. Due to the dipolar coupling between the layers, the paramagnet becomes polarized, which in turn modifies the spectrum of the Damon-Eshbach magnetostatic modes. We assume that the paramagnet is characterized by a magnetic susceptibility, $\chi \propto 1/(T-T_C)$, which reaches large values when the system temperature $T$ is close to the Curie temperature $T_C$. We find the conditions under which surface spin waves become unidirectional, i.e., capable of carrying energy in only one direction, and determine the magnitude of their frequency nonreciprocity. We demonstrate the possibility of switching the unidirectional wave regime on and off by varying the external magnetic field or temperature, making the ferromagnet/paramagnet system an attractive platform for tunable magnonic logic devices.