Temperature dependence of the EPR linewidth of CrBr3nearTC

Measurements of the temperature dependence of the EPR linewidth $\Delta H$ of Cr${\mathrm{Br}}_3$ at 9 GHz and between 20 and 550 °K are reported. The data were taken for the applied field ${\mathrm{H}}_0\parallel c$ axis and ${\mathrm{H}}_0\perp c$ axis. The most significant features of the data are (i) a general narrowing of the line as $T$ is lowered from the high-temperature side, (ii) the increasing anisotropy between the two directions as $T$ approaches the Curie point $T_C$ so that near $T_C$, $\frac{{\left(\Delta H\right)}_{\parallel }}{{\left(\Delta H\right)}_{\perp }}\simeq 2$, and (iii) the observation of a crossover from narrowing to broadening for ${\left(\Delta H\right)}_{\parallel }$ near 50 °K. For $34<T<\mathrm{}50\mathrm{}$ °K, ${\left(\Delta H\right)}_{\parallel }$ increases as $T$ decreases, confirming the predictions of recent theories of Kawasaki, Huber, and Tomita and Kawasaki. The effect of the magnetic field on the spin dynamics in a ferromagnet as reflected via EPR is discussed. An extension of Huber's calculations to a uniaxial ferromagnet satisfactorily explains the observed anisotropy of the linewidth. Some other ferromagnetic systems, where the crossover effect might be observed, are suggested.