A new magnetic phase transition above Morin temperature in DyFeO<sub>3</sub>
Authors: SU Haobin  , ZHENG Shiyun  , WANG Ning  , ZHU Guofeng  , JU Xuewei  , HUANG Feng  , CAO Yiming  , WANG Xiangfeng  ,   1 Fujian Provincial Key Laboratory of Terahertz Functional Devices and Intelligent Sensing, School of Mechanical Engineering and Automation, Fuzhou University, Fuzhou, 350108, China;,   2 School of Physics and Electronic Engineering, Qujing Normal University, Qujing, 655011, China;,   3 Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen, 361102, China
Published: February 2025

Rare-earth orthoferrites (<em>R</em>FeO<sub>3</sub>​) have garnered significant attention due to their intricate magnetic interactions and potential applications in ultrafast spintronic devices. Among them, DyFeO<sub>3</sub> exhibits rich magnetic phase transitions driven by the interplay of Fe³⁺ and Dy³⁺ sublattices. While previous studies focused on temperature-induced spin reorientation near the Morin temperature (T<sub>M</sub>∼50 K), the magnetic phase behavior under external fields above T<sub>M</sub> remains less explored. This work aims to systematically investigate the temperature- and magnetic-field-dependent magneto-dynamic properties of <em>a</em>-cut DyFeO<sub>3</sub> single crystals, with an emphasis on identifying novel phase transitions and elucidating the underlying mechanisms involving Fe³⁺-Dy³⁺ anisotropic exchange interactions. High-quality <em>a</em>-cut DyFeO<sub>3</sub> single crystals were grown using the optical floating zone method and characterized via XRD and Laue diffraction. Time-domain terahertz spectroscopy (THz-TDS) coupled with a superconducting magnet (0-7 T, 1.6-300 K) was employed to probe the ferromagnetic resonance (FM)and antiferromagnetic resonance (AFMR) modes. By analyzing the frequency trends in the spectra, we infer the response of internal magnetic moments to external stimuli. In the zero magnetic field experiment, we find that the temperature induced spin reorientation ( <strong><em>Γ</em></strong> <sub>4</sub>→ <strong><em>Γ</em></strong> <sub>1</sub>) occurs at Morin temperature(~50 K) with decreasing temperature. A broadband electromagnetic absorption (0.45-0.9 THz) emerged below 4 K, attributed to electromagnons activated by broken inversion symmetry in the Dy³⁺ antiferromagnetic state. Above the Morin temperature, we measured the absorption spectra of the sample at constant temperatures (70 K, 77 K, 90 K, and 100 K) in the 0 to 7 T magnetic field range. The experimental results show that with the increase of magnetic field, there exists a new magnetic phase transition ( <strong><em>Γ</em><sub>4</sub>→<em>Γ</em><sub>24</sub>→<em>Γ</em><sub>2</sub>→<em>Γ</em><sub>24</sub>→<em>Γ</em><sub>2</sub></strong> ), and the critical magnetic field of the phase transition changes with temperature. The phase transitions arise from the competition between external magnetic fields and internal effective fields generated by anisotropic Fe³⁺-Dy³⁺ exchange. These findings advance the understanding of magnetoelectric effects in <em>R</em>FeO<sub>3</sub>​ systems and provide a roadmap for designing spin-based devices leveraging field-tunable phase transitions.