Observation of surface ferromagnons in the axion-insulating phase of the antiferromagnetic topological insulator EuSn$_2$As$_2$

We report the study of spin dynamics of Eu$^{2+}$ in the antiferromagnetic axion topological insulator EuSn$_2$⁢As$_2$ by means of antiferromagnetic resonance at 9.34 GHz. Below the Néel temperature (𝑇$_N$), two types of resonance modes, the conventional bulk antiferromagnetic resonance and additional surface ferromagnetic resonance, are observed. The latter turns out to be characteristic of the axion-insulating phase. Above 𝑇$_N$, we prove the existence of a Kosterlitz-Thouless scenario that is relevant for the spin relaxation in the Eu$^{2+}$ layers. The absence of Korringa relaxation indicates the strong confinement of the conduction electrons at the Fermi level to the SnAs layers.