Magnetization and specific heat of TbFe₃(BO₃)₄: Experiment and crystal-field calculations

We have studied the thermodynamic properties of single-crystalline TbFe₃(BO₃)₄. Magnetization measurements have been carried out as a function of magnetic field (up to 50 T) and temperature up to 350 K with the magnetic field both parallel and perpendicular to the trigonal c axis of the crystal. The specific heat has been measured in the temperature range 2–300 K with a magnetic field up to 9 T applied parallel to the c axis. The data indicate a structural phase transition at 192 K and antiferromagnetic spin ordering at T_N≈40 K. A Schottky anomaly is present in the specific-heat data around 20 K, arising due to two low-lying energy levels of the Tb³⁺ ions being split by f-d coupling. Below T_N, magnetic fields parallel to the c axis drive a spin-flop phase transition, which is associated with a large magnetization jump. The highly anisotropic character of the magnetic susceptibility is ascribed mainly to the Ising-like behavior of the Tb³⁺ ions in the trigonal crystal field. We describe our results in the framework of a unified approach which is based on mean-field approximation and crystal-field calculations.