Can the Ferroelectric Soft Mode Trigger an Antiferromagnetic Phase Transition?

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 Néel temperature of the ${\text{Cr}}^{\text{3+}}$ spins. An increase in ferroelectric polarization is observed below 48 K, coinciding with the Néel 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.