Instability in domain wall dynamics in almost compensated ferrimagnets

TL;DR

This paper investigates the unique and unstable dynamics of domain walls in nearly compensated ferrimagnets, revealing critical points where stable motion ceases and explosive spin wave emissions occur, contrasting with ferromagnets and antiferromagnets.

Contribution

It identifies specific critical points in ferrimagnet domain wall dynamics where stability is lost, leading to nonlinear excitations and spin wave emissions, a phenomenon absent in ferromagnets and antiferromagnets.

Findings

Stable domain wall motion is limited to P < P_cr.

At P = P_cr, domain walls exhibit explosive, nonlinear dynamics.

Domain walls emit spin waves during instability cycles.

Abstract

Nanoscale self-localized topological spin textures, such as domain walls and skyrmions, are of interest for the fundamental physics of magnets and spintronics applications. Ferrimagnets (FiMs), in the region close to the angular momentum compensation point, are promising materials because of their ultrafast spin dynamics at nonzero magnetization. In this work, we study specific features of the FiM domain wall (DW) dynamics, which are absent in both ferromagnets (FM) and antiferromagnets (AFM). In low-damping FMs and AFMs ($\alpha \ll 1$), the non-stationary forced motion of DWs is characterized by slow ($t_{diss}\propto 1/\alpha$) changes of the DW's velocity and internal structure for all accepted values of the DW energy $E$ and its linear momentum $P$ -- a consequence of the stability of DWs for any value of $P$. In contrast, the dispersion law of FiM DWs has specific points, $P=P_{cr}$ and $E_{cr}=E(P_{cr})$, such that stable DWs are only present for $P<P_{cr}$, i.e., $P_{cr}$ and $E_{cr}$ act as endpoints in the $E(P)$ dependence. We show that when a field-like torque driven DW reaches this endpoint, it falls into a highly-non-equilibrium state with the excitation of fast ($t \ll t_{diss}$) and highly-nonlinear intra-wall magnetization dynamics, covering a wide frequency range up until the frequencies of propagating spin waves. The domain wall finally throws off an "excessive" energy by a short "burst" of the propagating spin waves and returns to the stationary state; the full picture of the forced motion is a periodic repetition of such "explosive" events.