Can the Ferroelectric Soft Mode Trigger an Antiferromagnetic Phase Transition?

TL;DR

This study reveals a new multiferroic behavior in a quadruple-perovskite where ferroelectric and antiferromagnetic phases are coupled, with the ferroelectric soft mode potentially triggering magnetic order changes.

Contribution

It demonstrates a novel type of multiferroicity where ferroelectric transition influences antiferromagnetic phase, highlighting the role of soft phonons in magnetoelectric coupling.

Findings

Ferroelectric transition at 125 K involves a displacive to order-disorder crossover.

Ferroelectric polarization increases below 48 K, aligning with Mn spin Néeel temperature.

No change in crystal symmetry below 48 K suggests enhanced magnetoelectric coupling without structural change.

Abstract

Type-II multiferroics, where spin interactions induce a ferroelectric polarization, are interesting for new device functionalities due to large magnetoelectric coupling. We report on a new type of multiferroicity in the quadruple-perovskite $\text{BiMn}_{\text{3}}\text{Cr}_{\text{4}}\text{O}_{\text{12}}$, where an antiferromagnetic phase is induced by the structural change at the ferroelectric phase transition. The displacive nature of the ferroelectric phase transition at 125 K, with a crossover to an order-disorder mechanism, is evidenced by a polar soft phonon in the THz range and a central mode. Dielectric and pyroelectric studies show that the ferroelectric critical temperature corresponds to the previously reported N\'eel temperature of the $\text{Cr}^{\text{3+}}$ spins. An increase in ferroelectric polarization is observed below 48 K, coinciding with the N\'eel temperature of the $\text{Mn}^{\text{3+}}$ spins. This increase in polarization is attributed to an enhanced magnetoelectric coupling, as no change in the crystal symmetry below 48 K is detected from infrared and Raman spectra.