Investigating the antiferromagnetic and the structural transitions of TbFe₃(BO₃)₄ and GdFe₃(BO₃)₄ by soft X-ray absorption spectroscopies

Bing-Yi Wang¹, Ruei-Tze Hung¹, Hong-Ji Lin², Chien-Te Chen², H. Nakajima³, T. Kimura³, Ashish Chainani², Yi-Ying Chin^1*

¹Department of Physics, National Chung Cheng University, Chiayi, Taiwan
²Condensed Matter Physics Group, National Synchrotron Radiation Research Center, Hsinchu, Taiwan
³Division of Materials Physics, Graduate School of Engineering Science, Osaka University, Osaka, Japan

* Presenter:Yi-Ying Chin, email:yiyingchin@ccu.edu.tw

There are two phase transitions in ferroborates RFe₃(BO₃)₄ (R: rare earth), a structural transition and an antiferromagnetic (AFM) one. At room temperature, their space group is R32, while it turns into P3₁21 below the structural transition temperature (TS) for R=Y, Ho, Dy, Tb, Gd, and Eu. Moreover, the TS as a function of the ionic radius of R ions demonstrates that the size of the R ions has a strong influence on TS. On the other hand, neutron diffraction experiments as well as the heat capacity measurements indicate the presence of an AFM ordering at low temperatures. Different from TS, Neel temperatures (TN) of RFe₃(BO₃)₄ are very close, between 30 K and 40 K. This implies that the AFM behavior is dominated by Fe, as expected for the short distance between the Fe ions. However, for TbFe₃(BO₃)₄, there is AFM coupling between the planes formed by Fe ions, while EuFe₃(BO₃)₄ orders antiferromagnetically into an easy-plane magnetic structure. Furthermore, the AFM structure of GdFe₃(BO₃)₄ is similar to EuFe₃(BO₃)₄ below 36 K, while there is spin rotation phase transition in GdFe₃(BO₃)₄ at 9 K. The variation in the AFM structures indicates the importance of the R ions.
To investigate the structural transition and the AFM structures of TbFe₃(BO₃)₄ and GdFe₃(BO₃)₄, we performed X-ray linear dichroism (XLD) experiments at the Fe-L₂,₃ edges as well as the R-M₄,₅ edges. Because Fe3+ has a half-filled 3d shell, no XLD is expected. However, owing to the low local symmetry of RFe₃(BO₃)₄, a finite XLD signal was observed for both the Fe and R edges at room temperature. Across the TS, the presence of strong modifications at the Fe-L₂,₃ edges as well as the R-M₄,₅ edges is consistent with the structural data. Below TN, the XLD of TbFe₃(BO₃)₄ is distinct from that of GdFe₃(BO₃)₄, especially for the Fe-L₂ edge, indicating the different AFM structures. To determine the AFM structures, we also performed the multiplet calculations. To probe the phase transition of TbFe₃(BO₃)₄ induced by the applied magnetic field, we also conducted high-field X-ray magnetic circular dichroism experiments on TbFe₃(BO₃)₄ to extract the magnetic behaviors of the R and Fe ions under different magnetic fields.

Keywords: Ferroborates, Antiferromagnetic transition, Structural transition, X-ray linear dichroism, X-ray magnetic circular dichroism