Low-energy magnons in the altermagnet $\alpha$-MnTe

We report high-field electron spin resonance studies of the altermagnetic material $\alpha$-MnTe. In magnetic fields applied parallel to the triangular Mn$^{2+}$ layers we observed a single resonance line, corresponding to an antiferromagnetic resonance (AFMR) mode. The resonance fields of this excitation exhibit an isotropic behavior with $g_\mathrm{eff}=2.01$, which is close to the free-electron $g$-factor value and agrees with the absence of orbital momenta for the Mn$^{2+}$ ions. At low temperatures, the AFMR mode is remarkably sharp ($\sim50$ mT for the full width at the half-maximum). This mode exhibits a noticeable broadening with increasing temperature, indicating the enhanced effect of magnon-magnon interactions. Based on this behavior, we estimate the strength of these interactions.