Spin-reorientation induced hidden electric polarization in α-FePO₄polar magnetic oxide

The structural, magnetic, thermodynamic and electrical properties of polar α-FePO₄ were comprehensively investigated. The α-FePO₄, which belongs to the Berlinite-type structure stabilized in a polar space group (P3₁21), exhibits long-range antiferromagnetic ordering at T_N1 (~25 K), followed by spin reorientation at T_N2 (~17 K). This has been confirmed by SXRD, 𝜒(𝐻,𝑇), and Cp(𝐻,𝑇) measurements. Notably, two significant dielectric anomalies were observed at T_N1 and T_N2, accompanied by a change in the sign of MD (%), indicating a coupling between Spin reorientation transition (SRT) and lattice dielectric anomalies. Neutron analysis of the detailed spin structure correlated these dielectric properties 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 DFT calculations. Further, SXRD revealed a magnetostrictive effect at both T_N1 and T_N2. The delicate interplay between magnetic anisotropy, magnetic exchange interactions, and crystal field effects at Fe³⁺ ions within a narrow temperature window (ΔT = 5 K) establishes α-FePO₄ as a unique multiferroic material.