Magnetic ordering in pressure-induced phases with giant spin-driven ferroelectricity in multiferroic TbMnO3

TL;DR

This study investigates how pressure and magnetic fields influence magnetic ordering and ferroelectricity in TbMnO3, revealing a transition from cycloid to E-type spin order and the mechanisms behind giant polarization effects.

Contribution

It provides new insights into pressure-induced magnetic phase transitions and their relation to ferroelectric polarization in TbMnO3, highlighting the role of exchange striction and field-induced magnetic structures.

Findings

Incommensurate cycloid magnetic order is stable below 4.5 GPa.

Above 4.5 GPa, E-type Mn spin order and noncollinear Tb spin order appear.

Magnetic field along the a axis enhances ferroelectric polarization via a field-induced antiferromagnetic structure.

Abstract

In order to clarify the mechanism associated with pressure/magnetic-field-induced giant ferroelectric polarization in TbMnO3, this work investigated changes in magnetic ordering brought about by variations in temperature, magnetic field, and pressure. This was accomplished by means of neutron diffraction analyses under high pressures and high magnetic fields, employing a single crystal. The incommensurate magnetic ordering of a cycloid structure was found to be stable below the reported critical pressure of 4.5 GPa. In contrast, a commensurate E-type spin ordering of Mn spins and a noncollinear configuration of Tb spins with k=(0,1/2,0) appeared above 4.5 GPa. The application of a magnetic field along the a axis (H_{||a}) under pressure induces a k=(0,0,0)antiferromagnetic structure in the case of Tb spins above H_{||a}, enhancing the ferroelectric polarization, while the E-type ordering of Mn spins is stable even above the critical field. From the present experimental findings, we conclude that the E-type ordering of Mn spins induces giant ferroelectric polarization through an exchange striction mechanism. The H_{||a}-induced polarization enhancement can be understood by considering that the polarization, reduced by the polar ordering of Tb moments in a zero field, can be recovered through a field-induced change to nonpolar k=(0,0,0) ordering at H_{||a} ~ 2T.