Spin-lattice and magnetoelectric couplings enhanced by orbital degrees of freedom in polar magnets

TL;DR

This study investigates how orbital degrees of freedom enhance magnetoelectric coupling in polar magnets, revealing strong spin-lattice interactions and the role of orbital fluctuations in multiferroic Swedenborgites through spectroscopy experiments.

Contribution

It demonstrates the impact of orbital degrees of freedom on magnetoelectric effects in Swedenborgites, highlighting the role of Jahn-Teller active ions and spin-phonon coupling mechanisms.

Findings

Strong magnetic order induces phonon shifts and dielectric changes.

Spin fluctuations significantly reduce phonon lifetime at magnetic transitions.

Orbital singlet ions show no variation at the Néel temperature.

Abstract

Orbital degrees of freedom mediating an interaction between spin and lattice were predicted to raise strong magnetoelectric effect, i.e. realize an efficient coupling between magnetic and ferroelectric orders. However, the effect of orbital fluctuations have been considered only in a few magnetoelectric materials, as orbital degeneracy driven Jahn-Teller effect rarely couples to polarization. Here, we explore the spin-lattice coupling in multiferroic Swedenborgites with mixed valence and Jahn-Teller active transition metal ions on a stacked triangular/Kagome lattice using infrared and dielectric spectroscopy. On one hand, in CaBa$M_4$O$_7$ ($M$ = Co, Fe), we observe strong magnetic order induced shift in the phonon frequencies and a corresponding large change in the dielectric response. Remarkably, as an unusual manifestation of the spin-phonon coupling, the spin-fluctuations reduce the phonon life-time by an order of magnitude at the magnetic phase transitions. On the other hand, lattice vibrations, dielectric response, and electric polarization show no variation at the N\'eel temperature of CaBaFe$_2$Co$_2$O$_7$, which is built up by orbital singlet ions. Our results provide a showcase for orbital degrees of freedom enhanced magnetoelectric coupling via the example of Swedenborgites.