Skyrmion Cyclotron Resonance in Ferrimagnets

Abstract

We show that a resonance due to gyroscopic motion of skyrmions, conceptually similar to the electron cyclotron resonance in metals, can be excited in a ferrimagnetic film by a spin current or microwaves. It must permit unambiguous measurement of the skyrmion mass for which a universal expression depending solely on the exchange interaction between spins belonging to two different ferrimagnetic sublattices is derived. The dependence of the skyrmion cyclotron frequency on parameters is computed for a TM/RE ferrimagnet, using CoGd as an example. The cyclotron frequency exhibits a dip near the angular momentum compensation point, where it hybridizes with the ferromagnetic resonance. The skyrmion cyclotron mode is studied for individual skyrmions and for skyrmion lattices, where the effect must be strong enough to be observed in microwave and spin-current experiments.

Figures (4)

• Figure 1: Two-sublattice skyrmion in a TM/RE ferrimagnet. Far from the skyrmion's core, TM spins are directed down (green) and RE spins are directed up (orange).
• Figure 2: The system with a skyrmion lattice (TM spins are shown) for the chosen model parameters and $c=0.7$. The skyrmion size $\lambda_{\mathrm{eff}}/a\approx8.9$ is defined according to Ref. caichugar2012.
• Figure 3: The dependence of the hybridized SCR and LF ferrimagnetic modes on the concentration of Gd in a model of CoGd ferrimagnet. (Single skyrmion in a $116\times132$ system, spin-current excitation, and the TM skyrmion position, $X_{S}$, as the observed quantity. The results for the SkL are similar).
• Figure 4: The dependence of the lower mode on the concentration of Gd for a single skyrmion (SS) and the skyrmion lattice in a model of CoGd ferrimagnet.