Origin of spin-driven ferroelectricity and effect of external pressure on the complex magnetism of 6H-perovskite Ba3HoRu2O9

TL;DR

This study uncovers the origin of spin-driven ferroelectricity in Ba3HoRu2O9, demonstrating how non-collinear magnetic structures involving Ru and Ho ions break inversion symmetry via inverse Dzyaloshinskii-Moriya interaction, and explores pressure effects on magnetism.

Contribution

It reveals a rare case of ferroelectricity driven by inverse Dzyaloshinskii-Moriya interaction between two different magnetic ions, supported by neutron diffraction and theoretical calculations.

Findings

Spin-driven ferroelectricity arises from non-collinear magnetic structure involving Ru and Ho ions.

External pressure enhances magnetic ordering temperature by 1.6 K/GPa.

Finite-size magnetoelectric domains explain low ferroelectric polarization.

Abstract

The compound Ba3HoRu2O9 magnetically orders at 50 K (TN1) followed by another complex magnetic ordering at 10.2 K (TN2). The 2nd magnetic phase transition was characterized by the co-existence of two competing magnetic ground states associated with two different magnetic wave vectors (K1=1/2 0 0 and K2=1/4 1/4 0). Here, we have discussed the origin of spin-driven ferroelectricity, which is not known yet. We demonstrate through time-of-flight Neutron diffraction and theoretical calculation that the non-collinear structure involving two different magnetic ions, Ru(4d) and Ho(4f), break the spatial inversion symmetry via inverse Dzyaloshinskii-Moriya (D-M) interaction through strong 4d-4f magnetic correlation, which shifts the oxygen atoms and results in non-zero polarization. Such an observation of inverse D-M interaction from two different magnetic ions which caused ferroelectricity is rarely observed. We have systematically studied the spin and dipolar dynamics, which exhibit intriguing behavior with shorter coherence lengths of 2nd magnetic phase associated with the k2-wave vector. The results manifest the development of finite-size magnetoelectric domains instead of true long-range ordering which justifies the experimentally obtained low value of ferroelectric polarization. The synchrotron XRD analysis predicts a non-centrosymmetric space group P-62c. Furthermore, we have investigated the effect of external pressure on this complex magnetism. The result reveals an enhancement of ordering temperature by the application of external pressure (1.6 K/GPa). The external pressure might favor stabilizing the magnetic ground state associated with 2nd magnetic phase. Our study shows an unconventional mechanism of spin-driven ferroelectricity.