Existence of a critical canting angle of magnetic moments to induce multiferroicity in the Haldane spin-chain system, Tb2BaNiO5

TL;DR

This study reveals a critical canting angle of magnetic moments in Tb2BaNiO5 that induces multiferroicity, supported by neutron diffraction and first-principles calculations, highlighting a new spin-driven polarization mechanism.

Contribution

It demonstrates the existence of a critical canting angle triggering multiferroicity in a Haldane spin-chain system, supported by experimental and theoretical evidence.

Findings

Mutually canted magnetic moments in Tb2BaNiO5 below TN

Sudden increase in canting angle at 25 K correlates with electric polarization

Supports a new spin-driven polarization mechanism involving critical canting angle

Abstract

We report an unusual canted magnetism due to 3d and 4f electrons, occupying two different crystallographic sites, with its consequence to electric dipole order. This is based on neutron powder diffraction measurements on Tb2BaNiO5 (orthorhombic, Immm centrosymmetric space group), exhibiting Neel order below (TN) 63 K, to understand multiferroic behavior below 25 K. The magnetic structure is made up of Ni and Tb magnetic moments, which are found to be mutually canted in the entire temperature range below TN, though collinearity is seen within each sublattice, as known in the past. First-principles density functional theory calculations (GCA plus SO and GCA plus U plus SO approximations) support such a canted ground state. The intriguing finding, being reported here, is that there is a sudden increase in this Tb-Ni relative cantingle angle at the temperature (that is, at 25 K) at which spontaneous electric polarization sets in, with bond distance and bong angle anomalies. This finding emphasizes the need for a new spin-driven polarization mechanism, that is, a critical canting angle coupled with exchangestriction, to induce multiferroicity in magnetic insulators with canted spins.