Universal field dependence of magnetic resonance near zero frequency

TL;DR

This paper uncovers a universal power law governing magnetic resonance near zero frequency across various magnets, linking symmetry properties to the resonance behavior and suggesting ways to measure critical fields via spin current signals.

Contribution

It reveals a universal power law for zero-frequency magnetic resonance and connects symmetry considerations to the critical behavior in magnetic systems.

Findings

Resonance frequency follows a universal power law near critical field.

Symmetry determines the power law exponent ($p=1$ or $p=1/2$).

Spin current can be enhanced and measured near zero resonance frequency.

Abstract

Magnetic resonance is a widely-established phenomenon that probes magnetic properties such as magnetic damping and anisotropy. Even though the typical resonance frequency of a magnet ranges from gigahertz to terahertz, experiments also report the resonance near zero frequency in a large class of magnets. Here we revisit this phenomenon by analyzing the symmetry of the system and find that the resonance frequency ($\omega$) follows a universal power law $\omega \varpropto |H-H_c|^p$, where $H_c$ is the critical field at which the resonance frequency is zero. When the magnet preserves the rotational symmetry around the external field ($H$), $p = 1$. Otherwise, $p=1/2$. The magnon excitations are gapped above $H_c$, gapless at $H_c$ and gapped again below $H_c$. The zero frequency is often accompanied by a reorientation transition in the magnetization. For the case that $p=1/2$, this transition is described by a Landau theory for second-order phase transitions. We further show that the spin current driven by thermal gradient and spin-orbit effects can be significantly enhanced when the resonance frequency is close to zero, which can be measured electrically by converting the spin current into electric signals. This may provide an experimentally accessible way to characterize the critical field. Our findings provide a unified understanding of the magnetization dynamics near the critical field, and may, furthermore, inspire the study of magnon transport near magnetic transitions.