Interplay of 4f−3d interactions and spin-induced ferroelectricity in the green phase Gd2BaCuO5

Abstract

In most of the spin-induced multiferroics, the ferroelectricity is caused by inversion symmetry breaking by complex spin structures of the transition-metal ions. Here, we report the importance of interplay of 4 f − 3 d magnetic interactions in inducing ferroelectricity in the centrosymmetric (Pnma) green phase compound G d 2 BaCu O 5 . With decreasing temperature, a long-range incommensurate ordering of both G d 3 + and C u 2 + spins at T N = 11.8 K occurs with the modulation vector k = ( 0 , 0 , g ) and a lock-in transition to a strongly noncollinear structure with k c = ( 0 , 0 , ½ ) at T loc ∼ 6 K . Both spin structures induce electric polarization consistent with the polar magnetic space groups P m 1 ′ ( α , 0 , g ) s s and P a c a 2 1 , respectively. Based on the symmetry analysis of magnetoelectric interactions, we suggest that the ferroelectricity in both commensurate and incommensurate phases is driven by a complex interplay of two-spins and single-spin contributions from magnetic ions located in noncentrosymmetric environments. Our study demonstrates that the green phase family of compounds may serve as a playground for studying the multiferroic phenomena, where the interplay of 4 f − 3 d interactions demonstrates an alternative route to find magnetoelectric materials.