Magnetoelastic effects in multiferroic HoMnO$_3$

TL;DR

This study investigates how magnetic ordering influences the lattice structure of multiferroic HoMnO₃ below its antiferromagnetic transition temperature using neutron diffraction, revealing anomalous lattice parameter behaviors and magnetoelastic coupling.

Contribution

It provides detailed measurements of magnetoelastic effects and magnetic moments in HoMnO₃, highlighting the temperature-dependent lattice anomalies associated with magnetic ordering.

Findings

Lattice parameter a contracts abruptly below T_N

Lattice parameter c increases anomalously below T_N

Unit cell volume shows abrupt contraction below T_N

Abstract

We have investigated magnetoelastic effects in multiferroic HoMnO$_3$ below the antiferromagnetic phase transition $T_N \approx 75$ K by neutron powder diffraction. The lattice parameter $a$ of the hexagonal unit cell of HoMnO$_3$ decrease in the usual way at lower temperatures and then shows abrupt contraction below $T_N$ whereas the lattice parameter $c$ increases continuously with decreasing temperature and shows an anomalous increase below $T_N$. The unit cell volume decreases continuously with decreasing temperature and undergoes abrupt contraction below $T_N$. By fitting the background thermal expansion for a nonmagnetic lattice with the Einstein-Gr\"uneisen equation we determined the lattice strains $\Delta a$, $\Delta c$ and $\Delta V$ due to the magnetoelastic effects as a function of temperature. We have also determined the temperature variation of the ordered magnetic moment of Mn ion by fitting the measured Bragg intensities of the nuclear and magnetic reflections with the known crystal and magnetic structure models.