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Magnetic resonance is a widely-established phenomenon that probes magneticproperties such as magnetic damping and anisotropy. Even though the typicalresonance frequency of a magnet ranges from gigahertz to terahertz, experimentsalso report the resonance near zero frequency in a large class of magnets. Herewe revisit this phenomenon by analyzing the symmetry of the system and findthat 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 resonancefrequency is zero. When the magnet preserves the rotational symmetry around theexternal field ($H$), $p = 1$. Otherwise, $p=1/2$. The magnon excitations aregapped above $H_c$, gapless at $H_c$ and gapped again below $H_c$. The zerofrequency is often accompanied by a reorientation transition in themagnetization. For the case that $p=1/2$, this transition is described by aLandau theory for second-order phase transitions. We further show that the spincurrent driven by thermal gradient and spin-orbit effects can be significantlyenhanced when the resonance frequency is close to zero, which can be measuredelectrically by converting the spin current into electric signals. This mayprovide an experimentally accessible way to characterize the critical field.Our findings provide a unified understanding of the magnetization dynamics nearthe critical field, and may, furthermore, inspire the study of magnon transportnear magnetic transitions.

Universal field dependence of magnetic resonance near zero frequency