Antiferromagnetic resonance in GdB$_6$

The electron spin resonance has been measured for the first time both in the paramagnetic phase of the metallic GdB$_6$ antiferromagnet ($T_N=15.5\,$K) and in the antiferromagnetic state ($T<T_N$). In the paramagnetic phase below $T^*\sim70\,$K, the material is found to exhibit a pronounced increase in the resonance linewidth and a shift in the $g$-factor, which is proportional to the linewidth $\Delta g(T)\sim\Delta H(T)$. Such behavior is not characteristic of antiferromagnetic metals and seems to be due to the effects related to displacements of Gd$^{3+}$ ions from the centrosymmetric positions in the boron cage. The transition to the antiferromagnetic phase is accompanied by an abrupt change in the position of resonance (from $\mu_0H_0\approx1.9\,$T to $\mu_0H_0\approx3.9\,$ T at $\nu=60\,$GHz), after which a smooth evolution of the spectrum occurs, resulting eventually in the formation of the spectrum consisting of four resonance lines. The magnetic field dependence of the frequency of the resonant modes $\omega_0(H_0)$ obtained in the range of $28$–$69$ GHz is well interpreted within the model of ESR in an antiferromagnet with the easy anisotropy axis $\omega/\gamma=(H_0^2+2H_AH_E)^{1/2}$, where $H_E$ is the exchange field and $H_A$ is the anisotropy field. This provides an estimate for the anisotropy field, $H_A\approx800\,$Oe. This value can result from the dipole-dipole interaction related to the mutual displacement of Gd$^{3+}$ ions, which occurs at the antiferromagnetic transition.