Directional dichroism in the paramagnetic state of multiferroics: a case study of infrared light absorption in Sr2CoSi2O7 at high temperatures

TL;DR

This study demonstrates the occurrence of non-reciprocal directional dichroism in the paramagnetic phase of Sr2CoSi2O7 at high temperatures and magnetic fields, explained by a single-ion hybridization mechanism without requiring magnetic ion correlations.

Contribution

It provides the first observation of NDD in the paramagnetic state of a multiferroic material at high temperatures and offers a theoretical framework using analytical and numerical methods.

Findings

NDD observed up to 70 K and 30 T in Sr2CoSi2O7

Single-ion hybridization explains NDD without magnetic correlations

Theoretical analysis maps spin excitations and selection rules

Abstract

The coexisting magnetic and ferroelectric orders in multiferroic materials give rise to a handful of novel magnetoelectric phenomena, such as the absorption difference for the opposite propagation directions of light called the non-reciprocal directional dichroism (NDD). Usually these effects are restricted to low temperature, where the multiferroic phase develops. In this paper we report the observation of NDD in the paramagnetic phase of Sr2CoSi2O7 up to temperatures more than ten times higher than its N\'eel temperature (7 K) and in fields up to 30 T. The magnetically induced polarization and NDD in the disordered paramagnetic phase is readily explained by the single-ion spin-dependent hybridization mechanism, which does not necessitate correlation effects between magnetic ions. The Sr2CoSi2O7 provides an ideal system for a theoretical case study, demonstrating the concept of magnetoelectric spin excitations in a paramagnet via analytical as well as numerical approaches. We applied exact diagonalization of a spin cluster to map out the temperature and field dependence of the spin excitations, as well as symmetry arguments of the single ion and lattice problem to get the spectrum and selection rules.