Frequency nonreciprocity of surface spin wave in permalloy thin films

Abstract

One of the most peculiar properties of spin waves in magnetic thin films is nonreciprocal wave propagation, i.e., a dependence of the wave properties on the propagation direction. The authors study this effect in permalloy films of thickness varying from 6 to 40 nm and find that the difference in the frequencies can reach several tens of MHz. The authors show that this frequency nonreciprocity is mainly caused by asymmetric magnetic anisotropies at the two surfaces of the ferromagnetic film. This result is important to understand recent works on the interfacial Dzyaloshinskii-Moriya interaction, an unconventional chiral magnetic interaction that generates a similar frequency nonreciprocity. The authors demonstrate that while these contributions usually occur at the same time, the two can be distinguished via the difference in their dependence on the film thickness.