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

TL;DR

This study uses electron spin resonance to identify ferromagnetic nanodefects within an antiferromagnetic crystal EuSn₂As₂, revealing a natural magnetic metamaterial with implications for layered magnetic compounds.

Contribution

It demonstrates the presence of ferromagnetic nanodefects in EuSn₂As₂ and links these to observed magnetic properties, a novel insight into layered antiferromagnetic materials.

Findings

ESR signal splits into two lines below T_N

One line is a conventional AFM resonance

The other is a FM resonance from nanodefects

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, $T \leq T_N\approx 24$\,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 $T< T_N\rightarrow 0$.