Magnetic Interactions in the Polar Ferrimagnet with a Bipartite Structure

TL;DR

This study investigates the spin dynamics of Mn$_2$Mo$_3$O$_8$, a bipartite polar ferrimagnet, revealing distinct magnon dispersions and their temperature dependence, highlighting the role of bipartite structure in magnetic excitations.

Contribution

The paper provides the first inelastic neutron scattering analysis of Mn$_2$Mo$_3$O$_8$, demonstrating how bipartite structure influences magnon behavior and ferrimagnetic susceptibility.

Findings

Two distinct magnon dispersions were observed.

Magnetic spectra are well described by a Heisenberg and anisotropy model.

Temperature dependence of susceptibility matches theoretical predictions.

Abstract

The polar magnets A$_2$Mo$_3$O$_8$ (A=Fe, Mn, Co, and Ni) feature a bipartite structure, where the magnetic A$^{2+}$ ions occupy two different sites with octahedral and tetrahedral oxygen coordinations. This bipartite structure provides a platform for the emergence of nontrivial magnetoelectric (ME) effects and intriguing excitation behaviors, and thus creates significant research interest. In this study, we conduct inelastic neutron scattering measurements on single crystals of Mn$_2$Mo$_3$O$_8$, an L-type ferrimagnet in the A$_2$Mo$_3$O$_8$ family, to investigate its spin dynamics. The obtained magnetic excitation spectra reveal two distinct magnon dispersions corresponding to the octahedral and tetrahedral spins in Mn$_2$Mo$_3$O$_8$. These magnon bands can be well described by a spin Hamiltonian including Heisenberg and single-ion anisotropy terms. Employing our effective spin model, we successfully reproduce the unusual temperature dependence of the L-type ferrimagnetic susceptibility through self-consistent mean-field theory. This research reveals the significance of the bipartite structure in determining the excitation properties of the polar magnets $\rm{A_{2}Mo_{3}O_{8}}$ and provides valuable insights into the spin dynamics of L-type ferrimagnets.