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稀土正铁氧体(RFeO3, R为稀土原子)包含Fe3+和R3+两套磁性离子亚晶格, 存在Fe3+-Fe3+, Fe3+-R3+, R3+-R3+三种相互作用, 它们是稀土正铁氧体丰富磁性的来源. 本文利用时域太赫兹磁光谱, 在1.6—300 K的温度范围内, 在不同磁场下, 测量a-cut DyFeO3单晶样品的吸收光谱, 并分析光谱中铁磁(FM)和反铁磁共振(AFMR)吸收峰的温度和磁场依赖特性. 在零磁场变温实验中, 我们发现随温度降低在Morin温度(~ 50 K)出现的温度诱导的自旋重取向(Γ4→Γ1), 以及在4 K温度以下存在一个由于电磁振子导致的宽带吸收. 在Morin温度以上, 我们在恒定温度(70, 77, 90, 100 K)下测量了样品在0—7 T磁场范围的吸收光谱. 实验结果表明, 随着磁场的增大, 存在一个新的磁相变过程(Γ4→Γ24→Γ2→Γ24→Γ2), 相变的临界磁场随温度而变化. 这一相变过程是由于外磁场和Fe3+-Dy3+的各向异性交换相互作用导致的内部有效场的相互竞争和对磁矩的协同作用. 本项研究为深入理解稀土铁氧化物的丰富相变和磁电耦合特性, 以及开发相关的自旋电子学器件提供参考.Rare-earth orthoferrites (RFeO3) have received significant attention due to their intricate magnetic interactions and potential applications in ultrafast spintronic devices. Among them, DyFeO3 exhibits rich magnetic phase transitions driven by the interplay between Fe3+ and Dy3+ sublattices. Previous studies mainly focused on temperature-induced spin reorientation near the Morin temperature (TM~50 K), but there has been limited exploration of magnetic phase behavior under external fields above TM. This work aims to systematically investigate the temperature- and magnetic-field-dependent magneto-dynamic properties of a-cut DyFeO3 single crystals, with an emphasis on identifying novel phase transitions and elucidating the underlying mechanisms involving Fe3+-Dy3+ anisotropic exchange interactions. High-quality a-cut DyFeO3 single crystals are grown using the optical floating zone method and characterized by X-ray diffraction (XRD) and Laue diffraction. Time-domain terahertz spectroscopy (THz-TDS) coupled with a superconducting magnet (0–7 T, 1.6–300 K) is employed to probe the ferromagnetic resonance (FM) and antiferromagnetic resonance (AFMR) modes. By analyzing the frequency trends in the spectra, the response of internal magnetic moments to external stimuli can be inferred. In the zero magnetic field experiment, it is found that the temperature induced spin reorientation (Γ4→Γ1) occurs at Morin temperature(~50 K) with temperature decreasing. A broadband electromagnetic absorption (0.45–0.9 THz) occurs below 4 K, which is attributed to electromagnons activated by broken inversion symmetry in the Dy3+ antiferromagnetic state. Above the Morin temperature, the absorption spectra of the sample are measured at constant temperatures (70, 77, 90, 100 K) and magnetic fields ranging from 0 to 7 T. The experimental results show that with the increase of magnetic field, a new magnetic phase transition occurs (Γ 4 → Γ 24 → Γ 2 → Γ 24 → Γ 2 ), and the critical magnetic field of the phase transition varies 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 contribute to the further understanding of the magnetoelectric effects in RFeO3 systems and provide a roadmap for using field-tunable phase transitions to design spin-based devices .
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Keywords:
- rare-earth orthoferrite /
- terahertz /
- magnetic resonance /
- magnetic phase transition
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图 1 (a) RFeO3体系中3个允许相(Γ1, Γ2, Γ4)的晶胞中Fe3+的4个自旋磁矩; (b) 简化后的示意图
Fig. 1. (a) The four spin magnetic moments of Fe3+ in the cell of three allowable phases (Γ1, Γ2, Γ4) in the RFeO3 system; (b) simplified schematic diagram.
图 2 零磁场下, a-cut DyFeO3样品在不同温度(1.6—300 K)的太赫兹吸收光谱 (a)HTHz平行于晶体的c轴; (b)HTHz平行于晶体的和b轴, 为清晰起见, 不同温度的光谱垂直偏移
Fig. 2. Terahertz absorption spectra of a-cut DyFeO3 samples at different temperatures (1.6–300 K) in zero magnetic field: (a) HTHz parallel to the c-axis of the crystal; (b) HTHz parallel to the b-axis of the crystal, spectra at different temperatures are shifted vertically for clarity.
图 3 在HTHz平行晶体c轴和外加磁场沿晶体a轴方向的实验构型下, a-cut DyFeO3样品在0—7 T磁场下的太赫兹吸收光谱, 温度分别为 (a) 100 K, (b) 90 K, (c) 77 K, (d) 70 K, 清晰起见, 不同磁场的光谱垂直偏移
Fig. 3. Terahertz absorption spectra of the a-cut DyFeO3 sample under a magnetic field from 0–7 T at various temperatures of (a) 100 K, (b) 90 K, (c) 77 K, and (d) 70 K, when the directions of HTHz and the applied magnetic field are along the crystal c axis and a axis, respectively. Spectra are vertically offset for clarity.
图 4 温度100 K时, 0—7 T磁场范围内, a-cut DyFeO3单晶中Γ4→Γ24→Γ2→Γ24→Γ2的相变过程
Fig. 4. The phase transition process Γ4→Γ24→Γ2→Γ24→Γ2 in the a-cut DyFeO3 single-crystal in a magnetic field range of 0—7 T at 100 K.
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