Multiple magnetic ordering phenomena in multiferroic o-HoMnO3

TL;DR

This study investigates the complex magnetic ordering phenomena in multiferroic o-HoMnO3, revealing how Ho 4f magnetic order influences Mn magnetic order and ferroelectric polarization without altering lattice distortions.

Contribution

It provides the first detailed analysis of Ho 4f magnetic ordering in o-HoMnO3 and its impact on magnetic and ferroelectric properties using resonant X-ray diffraction.

Findings

Ho 4f order resembles an ac-cycloid similar to TbMnO3

Ho order affects Mn magnetic order but not lattice distortions

Ferroelectric polarization is influenced by Ho order independently of lattice changes

Abstract

Orthorhombic HoMnO3 is a multiferroic in which Mn antiferromagnetic order induces ferroelectricity. A second transition occurs within the multiferroic phase, in which a strong enhancement of the ferroelectric polarization occurs concomitantly to antiferromagnetic ordering of Ho 4f magnetic moments. Using the element selectivity of resonant X-ray diffraction, we study the magnetic order of the Mn 3d and Ho 4f moments. We explicitly show that the Mn magnetic order is affected by the Ho 4f magnetic ordering transition. Based on the azimuthal dependence of the (0 q 0) and (0 1-q 0) magnetic reflections, we suggest that the Ho 4f order is similar to that previously observed for Tb 4f in TbMnO3, which resembles an ac-cycloid. This is unlike the Mn order, which has already been shown to be different for the two materials. Using non-resonant diffraction, we show that the magnetically-induced ferroelectric lattice distortion is unaffected by the Ho ordering, suggesting a mechanism through which the Ho order affects polarization without affecting the lattice in the same manner as the Mn order.