Spin-reorientation induced hidden electric polarization in the noncentrosymmetric berlinite magnetic oxide α- FeP O4

Spin reorientation transition (SRT)-induced electric polarization (P) is a striking manifestation of magnetoelectric coupling in type-II multiferroic materials. This phenomenon typically arises either from complex incommensurate magnetic structures that break spatial inversion symmetry or from changes in the magnetic point-group symmetry of commensurate spin structures during an SRT. In this study, we report an SRT-induced hidden P in a commensurate magnetic arrangement on the background of a noncentrosymmetric lattice α-FePO4, which belongs to the berlinite-type structure stabilized in a nonpolar space group (P3121). The long-range antiferromagnetic ordering of α-FePO4 at TN (∼24 K), followed by SRT at TSR (∼17 K), is confirmed by magnetic susceptibility χ(H,T) and specific heat Cp(H,T) measurements. Interestingly, two significant dielectric (ϵ′) anomalies were observed at TN and TSR, accompanied by a change in the sign of magnetodielectric (%), indicating a coupling between SRT and lattice ϵ′ anomalies. Neutron powder diffraction analysis of the detailed spin structure correlated these ϵ′ and P with the magnetic lattice, where the change in in-plane anisotropy during the SRT results in a shift of the spin moment direction from the ab plane to the c axis supported by the density functional theory calculations. Furthermore, synchrotron x-ray diffraction and Raman scattering revealed a magnetostrictive effect and anomalous spin-phonon coupling at both TN and TSR. The delicate interplay between magnetic anisotropy, magnetic exchange interactions, and crystal field effects at Fe3+ ions within a narrow T window (ΔT = 5 K) establishes α-FePO4 as a unique multiferroic material.