Possible nearly loss-free ferrimagnetic resonance in small samples

Abstract

Relaxation occurs in ferrimagnetic resonance by processes which transfer energy from the uniform precession magnon mode, which is excited in the resonance process, to other magnons and to phonons. The relaxation due to these processes is usually calculated using Fermi Golden Rule time dependent perturbation theory, whose application depends on the modes involved in the relaxation processes forming a continuum. Since for a finite isolated solid this is not generally true, the possibility exists that such relaxation processes might not occur for sufficiently small samples. Because it is reasonable to consider the phonons as belonging to both the sample and sample holder, it is reasonable to assume that they form a continuum. The intrinsic linewidth (i.e., inverse lifetime for a defect-free single crystal), which is due to phonons excited by the Kasuya–Le Craw mechanism, is already comparable to the magnon mode spacing for samples of linear dimensions of the order of 10 μm, indicating that finite sample effects could potentially become important for samples of fairly large size. Previous work by the present author on the one-dimensional Heisenberg model has shown that nonlinearity in the magnons can lead to a transition from lossy to loss-free behavior as the sample size decreases, if the temperature is sufficiently low. Here, model calculations of this effect in a two-dimensional Heisenberg model magnet are presented in order to show that loss-free behavior can occur for sufficiently low temperature and sufficiently small sample size. These results open up the interesting possibility of producing high anisotropy magnetic materials as a collection of very small crystals with extremely small linewidths.