Spin-stretching modes in anisotropic magnets: spin-wave excitations in the multiferroic Ba2CoGe2O7

TL;DR

This study investigates unconventional spin excitations in the multiferroic Ba2CoGe2O7, revealing spin-stretching modes that are electric dipole active and can be observed in various spectroscopic experiments.

Contribution

The paper introduces a multi-boson spin-wave theory to explain novel spin-stretching modes in a non-centrosymmetric multiferroic material, expanding understanding of spin dynamics.

Findings

Identification of spin-stretching modes beyond conventional magnons

Demonstration that these modes are electric dipole active

Prediction of observability in neutron scattering and light absorption

Abstract

We studied spin excitations of the multiferroic Ba2CoGe2O7 in high magnetic fields up to 33 T. In the electron spin resonance and far infrared absorption spectra we found several spin excitations beyond the two conventional magnon modes expected for such a two-sublattice antiferromagnet. We show that a multi-boson spin-wave theory can capture these unconventional modes, that include spin-stretching modes associated with an oscillating magnetic dipole (or only quadrupole) moment. The lack of the inversion symmetry allows these modes to become electric dipole active. We expect that the spin-stretching modes can be generally observed in inelastic neutron scattering and light absorption experiments in a broad class of ordered S > 1/2 spin systems with strong single-ion anisotropy and/or non-centrosymmetric lattice structure.