Rhombohedral distortion-driven ferroelectric order and exchange bias effect in geometrically frustrated ZnFe2O4

Abstract

The rhombohedral distortion-driven occurrence of the spontaneous electric polarization at a reasonably high temperature $(T)$ and an exchange bias (EB) effect below the antiferromagnetic (AFM) N\'eel temperature $({T}_{N})$ are revealed in ${\mathrm{ZnFe}}_{2}{\mathrm{O}}_{4}$. We observe the magnetic memory effect, suggesting a cooperative glassy magnetic state below ${T}_{N}$. The phase separation between the long-range AFM and the glassy magnetic component leads to the EB effect below ${T}_{N}$. The synchrotron diffraction studies confirm a structural transition to a polar $R3m$ structure from the cubic spinel structure, involving a strong rhombohedral distortion. This distortion correlates with the occurrence of the spontaneous electric polarization $(P)$ below $\ensuremath{\sim}110$ K $({T}_{\text{FE}1})$, which is accompanied by a short-range order (SRO). The $P$-value enhances further due to an additional rhombohedral distortion below ${T}_{N}$. A considerable magnetoelectric (ME) coupling is detected below ${T}_{\text{FE}1}$. The increase of $P$ is $\ensuremath{\sim}7.2$% for 5 T at the liquid nitrogen temperature. There has been a fundamental interest among the community in the unique result of the occurrence of ferroelectric (FE) order coexisting with SRO and having a significant ME coupling, which is accompanied by a strong rhombohedral structural distortion analogous to that observed in ${\mathrm{BiFeO}}_{3}$.