Abstract
From a high-resolution, far-infrared laser study of the antiferromagnetic resonance (AFMR) of Fe${\mathrm{F}}_{2}$, it is shown that a magnetic polariton model not only reproduces the broad (\ensuremath{\sim}4 kOe) complex structures observed in transmission, but is essential to the determination of the intrinsic (radiative and nonradiative) contributions to the linewidth. For the purest Fe${\mathrm{F}}_{2}$ crystal an upper bound of 20 Oe for $\ensuremath{\Delta}H$ was found. The polariton model is extended to include the effects of Mn impurities. The theory correctly predicts the observed frequency pulling of host and impurity modes as a function of Mn concentration $c$ and the large enhancement of the impurity-mode signal. From their dependence on $c$ and sample thickness $t$ the super-radiance and impurity-impurity contributions to $\ensuremath{\Delta}{H}_{\mathrm{imp}}$ are separately obtained and compared with recently developed theories. The thermal relaxation of the host AFMR is shown to arise from fourmagnon scattering.
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