Unveil the spin-reorientation induced dielectric anomaly and electric polarization in α-FePO4

Y. H. Tseng¹, T. W. Yen¹, Ajay Tiwari¹, C. W. Wang², H. C. Wu¹, J.-Y Lin³, H. L. Liu⁴, H. D. Yang¹, D Chandrasekhar Kakarla^1*

¹Physics, National Sun Yat-sen University, KAOHSIUNG, Taiwan
²National Synchrotron Radiation Research Center, Hsinchu, Taiwan
³Institute of Physics, National Yang Ming Chiao Tung University, Hsinchu, Taiwan
⁴Department of Physics, National Taiwan Normal University, Taipei, Taiwan

* Presenter:D Chandrasekhar Kakarla, email:chandu@g-mail.nsysu.edu.tw

Spin reorientation transition (SRT)-induced electric polarization is a striking example of magnetoelectric coupling, typically seen in type-II multiferroic materials. This coupling occurs due to complex magnetic structures that break spatial inversion symmetry, generating electric polarization. In this study, we report SRT-induced polarization within a commensurate magnetic structure in a polar lattice. The structural, magnetic, thermodynamic, and electrical properties of polar α-FePO4 were investigated through various techniques, including synchrotron X-ray powder diffraction (SXRD), magnetic susceptibility (𝜒(𝐻,𝑇)), heat capacity (Cp(𝐻,𝑇)), neutron diffraction, dielectric (𝜀′(𝐻,𝑇)) and pyroelectric (P(H, T)) measurements, along with density functional theory (DFT) calculations. α-FePO4 crystallizes in the polar P3121 space group and exhibits long-range antiferromagnetic ordering at TN1 (~25 K), followed by a spin reorientation at TN2 (~17 K). These transitions were confirmed by SXRD, magnetic susceptibility, and heat capacity measurements. Notably, two distinct dielectric anomalies were observed at TN1 and TN2, along with a reversal in magnetodielectric (MD) behavior, pointing to a close link between the SRT and dielectric anomalies. Neutron diffraction analysis revealed how changes in spin orientation correlate with these dielectric properties. During the SRT, the spin moment direction shifts from the ab-plane to the c-axis, a phenomenon supported by DFT calculations. Moreover, SXRD detected magnetostrictive effects at both TN1 and TN2. The intricate balance between magnetic anisotropy, exchange interactions, and crystal field effects at Fe3+ ions within a narrow temperature range (~5 K) highlights α-FePO4 as a distinctive multiferroic material.

Keywords: Spin reorientation transition , Multiferroic, Neutron diffraction, Magnetic anisotropy, DFT calculations