Towards a Microscopic Model of Magnetoelectric Interactions in Ni3V2O8

TL;DR

This paper develops a microscopic model of magnetoelectric interactions in Ni3V2O8, combining neutron scattering and first-principles calculations to identify phonons inducing polarization and elucidate spin-phonon coupling mechanisms.

Contribution

It introduces a detailed microscopic magnetoelectric coupling model for Ni3V2O8, linking phonon modes to ferroelectric polarization and spin interactions, advancing understanding of multiferroic behavior.

Findings

Identified eleven phonons capable of inducing polarization.

Small atomic distortions (~0.001 Å) can produce observed polarization.

Highlighted the importance of Dzyaloshinskii-Moriya interaction in multiferroicity.

Abstract

We develop a microscopic magnetoelectric coupling in Ni$_3$V$_2$O$_8$ (NVO) which gives rise to the trilinear phenomenological coupling used previously to explain the phase transition in which magnetic and ferroelectric order parameters appear simultaneously. Using combined neutron scattering measurements and first-principles calculations of the phonons in NVO, we determine eleven phonons which can induce the observed spontaneous polarization. Among these eleven phonons, we find that a few of them can actually induce a significant dipole moment. Using the calculated atomic charges, we find that the required distortion to induce the observed dipole moment is very small (~0.001 \AA) and therefore it would be very difficult to observe the distortion by neutron-powder diffraction. Finally, we identify the derivatives of the exchange tensor with respect to atomic displacements which are needed for a microscopic model of a spin-phonon coupling in NVO and which we hope will be obtained from a fundamental quantum calculation such as LDA+U. We also analyze two toy models to illustrate that the Dzyaloskinskii-Moriya interaction is very important for coexisting of magnetic and ferroelectric order but it is not the only mechanism when the local site symmetry of the system is low enough.