Can the ferroelectric soft mode trigger an antiferromagnetic phase transition?

Andre Maia ¹ Christelle Kadlec ¹ Maxim Savinov ¹ Rui Vilarinho ² Joaquim Agostinho Moreira ² Viktor Bovtun ¹ Martin Kempa ¹ Martin Míšek ¹ Jiří Kaštil ¹ Andriy Prokhorov ¹ Jan Maňák ¹ Alexei Belik ³ Stanislav Kamba ¹

¹Department of Dielectrics, FZU - Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic
²IFIMUP, University of Porto, Porto, Portugal
³International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Tsukuba, Japan

Multiferroics with a quadruple perovskite structure and chemical formula (AA`₃)B₄O₁₂ have attracted great interest due to the intriguing, and in some cases large, spin-order-driven ferroelectric polarization. Among them, the cubic quadruple perovskite BiMn₃Cr₄O₁₂ is an intriguing system that exhibits a ferroelectric and magnetic phase transition at the same temperature, with the ferroelectric phase transition of the displacive type rather than induced by the magnetic order. We report on the magnetic and dielectric properties, together with infrared, terahertz, microwave, EPR and Raman spectra of BiMn₃Cr₄O₁₂ ceramics. Infrared reflectivity and time-domain terahertz transmission spectra reveal a polar soft phonon near 26 cm^-1 at 300 K, whose softening down to 12.5 cm^-1 is responsible for the displacive ferroelectric phase transition at 125 K. The system becomes multiferroic at that temperature, since it coincides with the appearance of long‐range G‐type AFM order of the B‐site Cr sublattice, as revealed by neutron powder diffraction measurements [1]. On further cooling to 48 K, another similar collinear G‐type AFM transition occurs for the A′‐site Mn sublattice [1]. Below 48 K, the magnetic order induces a magnetoelectric coupling that enhances the ferroelectric polarization. No pronounced phonon anomalies are seen near 48 K. The soft mode contributes about 94% to the permittivity, as predicted from first-principles calculations [2, 3]. The ferroelectric phase transition is shown to be predominantly of the displacive type, with a small order-disorder character near the critical temperature. A significant spin-phonon coupling below 125 K is revealed by certain Raman-active phonons. The observed phonon activities are compared with the predictions from factor-group analysis and first-principles calculations [2, 3].

References:

[1] L. Zhou et al., Adv. Mater. 29, 1703435 (2017)

[2] J.Q. Dai and C.C. Zhang, J. Am. Ceram. Soc. 102, 6048 (2019)

[3] J.Q. Dai, X.L. Liang, and Y.S. Lu, Chem. Phys. 538, 110924 (2020)