Coupled ferroelectric polarization and magnetization in spinel FeCr ₂S ₄

Abstract

One of the core issues for multiferroicity is the strongly coupled ferroelectric polarization and magnetization, while so far most multiferroics have antiferromagnetic order with nearly zero magnetization. Magnetic spinel compounds with ferrimagnetic order may be alternative candidates offering large magnetization when ferroelectricity can be activated simultaneously. In this work, we investigate the ferroelectricity and magnetism of spinel FeCr ₂S ₄ in which the Fe ²⁺ sublattice and Cr ³⁺ sublattice are coupled in antiparallel alignment. Well defined ferroelectric transitions below the Fe ²⁺ orbital ordering termperature T _oo = 8.5 K are demonstrated. The ferroelectric polarization has two components. One component arises mainly from the noncollinear conical spin order associated with the spin-orbit coupling, which is thus magnetic field sensitive. The other is probably attributed to the Jahn-Teller distortion induced lattice symmetry breaking, occuring below the orbital ordering of Fe ²⁺. Furthermore, the coupled ferroelectric polarization and magnetization in response to magnetic field are observed. The present work suggests that spinel FeCr ₂S ₄ is a multiferroic offering both ferroelectricity and ferrimagnetism with large net magnetization.