Coexistence of Ising and XY Spin Systems on a Single Tb Atom in TbCoGa₅

The tetragonal compound TbCoGa₅ exhibits two successive magnetic transitions at T _N1 = 36.2 and T _N2 = 5.4 K. The results of the measurement of the magnetic susceptibility and specific heat indicate that the c- and ab-components of the magnetic moments of Tb³⁺ show independent antiferromagnetic orderings at T _N1 and T _N2, respectively. In this work, we have investigated the physical properties of TbCoGa₅ in a magnetic field. Single crystals of TbCoGa₅ were grown and their specific heat, electrical resistivity, magnetization and elastic constant were measured. The magnetic exchange interactions in TbCoGa₅ are discussed referring to the result of the magnetization process. Additionally, the present and previous results are summarized in a magnetic phase diagram. The magnetic phase diagram indicates that the behavior of T _N1 in a changing magnetic field is similar to that of an Ising spin system, and the behavior of T _N2 is similar to that of an XY spin system. Thus, we conclude that Ising and XY spin systems coexist on a single Tb atom in TbCoGa₅.