Ferromagnetic resonance in an antiferromagnetic crystal EuSn${}_2$As${}_2$

Abstract

We report results of electron spin resonance (ESR) measurements in single crystals of EuSn${}_2$As${}_2$. In the temperature range of antiferromagnetic (AFM) ordering of Eu atoms, $T\le T_N\approx 24$\,K, the ESR signal splits into two resonance lines, one of which, at high-field (or low-frequency), is the conventional acoustic AFM resonance mode that occurs at temperatures below $T_N$. The lower-field (high-frequency) line, as we have proven here, is the ferromagnetic resonance associated with the presence in the layered AFM crystal of a small amount ($\sim 3\%$) of planar nanodefects with a non-zero ferromagnetic (FM) moment. The existence of ferromagnetic nano-inclusions in the bulk of the antiferromagnetic compound makes EuSn${}_2$As${}_2$ a peculiar example of a natural magnetic metamaterial. We believe that the planar FM nanodefects are also inherent in other layered AFM compounds, which explains often observed increase in their magnetic susceptibility upon cooling at $T<T_N\to 0$.

10 pages, 6 figures