  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.
