Spin-Coupled Local Distortions in Multiferroic Hexagonal HoMnO3

TL;DR

This study investigates local structural changes in hexagonal HoMnO3 across magnetic transitions, revealing strong spin-lattice coupling and asymmetric charge polarization that influence multiferroic behavior.

Contribution

It provides detailed measurements of bond distance changes during magnetic transitions and models the spin-lattice interactions with density functional calculations.

Findings

Bond distances change with magnetic transitions.

Strong spin-lattice coupling observed.

Asymmetric charge polarization along z-axis.

Abstract

Local structural measurements have been performed on hexagonal HoMnO3 in order to ascertain the specific changes in bond distances which accompany magnetic ordering transitions. The transition from paramagnetic to the antiferromagetic (noncollinear) phase near ~70 K is dominated by changes in the a-b plane Mn-Mn bond distances. The spin rotation transition near ~40 K involves both Mn-Mn and nearest neighbor Ho-Mn interactions while the low temperature transition below 10 K involves all interactions, Mn-Mn, Ho-Mn (nearest and next nearest) and Ho-Ho correlations. These changes in bond distances reveal strong spin-lattice coupling. The similarity in magnitude of the change in J(Mn-Mn) and J(Ho-Mn) enhances the system frustration. The structural changes are interpreted in terms of a model of competing spin order and local structural distortions. Density functional calculations are used to estimate the energies associated with ionic displacements. The calculations also reveal asymmetric polarization of the charge density of Ho, O3 and O4 sites along the z-axis in the ferroelectric phase. This polarization facilitates coupling between Ho atoms on neighboring planes normal to the z-axis.