Structural and magnetic interplay in multiferroic Ho0.5Nd0.5Fe3(BO3)4

Tripathy, A., Gautam, K., Dey, K., (...), Sathe, V.G., Shukla, D.K.// Physical Review B//

Rare-earth iron borates, RFe3(BO3)4 (R = rare earth), are of considerable interest because of their structural and magnetic complexities due to the involvement of both 3d and 4f magnetic interactions in a helical lattice. Among them Ho0.5Nd0.5Fe3(BO3)4 has attracted the most attention, as it exhibits a multiferroic property with a large, spontaneous, and magnetic-field-induced electric polarization (P) below the antiferromagnetic transition temperature (TN32 K). This compound has been reported to be noncentrosymmetric (nonpolar) down to the lowest temperature, and the origin of spontaneous electric polarization below TN is not known. By utilizing temperature-dependent synchrotron x-ray powder diffraction, we report here the observation of structural phase transition, i.e., the lowering of crystal symmetry from R32 to P3121 below TN in Ho0.5Nd0.5Fe3(BO3)4, which is further confirmed by single-crystal x-ray diffraction measurements as well as substantiated by dielectric and Raman spectroscopic results. With the help of x-ray resonant magnetic scattering, we have studied the element-specific magnetic ordering behavior, and by combining all the results, we show that the spontaneous electric polarization below the antiferromagnetic ordering transition emerges through the p-d hybridization mechanism associated with the FeO6 octahedra. The observations of subtle structural changes at low temperature and element-specific magnetic ordering behavior provide a comprehensive understanding of the multiferroicity in this family of compounds.