Anisotropic, thermal, and magnetic properties of (formula presented) Explanation via a crystalline electric field scheme

We have studied the thermal and magnetic properties of a single crystal of (formula presented) which has a small net magnetic moment of about (formula presented) below the ordering temperature (formula presented) by means of specific heat, thermal expansion, and magnetization measurements. The magnetic part of the specific heat shows a broad peak around 30 K, and the magnetic entropy reaches (formula presented) at about 50 K. This indicates that the first excited doublet is situated at about 50 K from the ground state. In addition, the thermal expansion along the tetragonal [001] direction exhibits a clear negative peak around 25 K, which is also attributed to the magnetic excitations between crystalline electric field (CEF) states. These data, together with the anisotropic magnetic susceptibility, were analyzed on the basis of a CEF model, and splitting energies from the ground state to the first and second excited states were estimated to be 48 and 140 K, respectively. Furthermore, the anisotropy in the high-field magnetization was well explained by the present CEF model with the same parameters, where the saturation moment for (formula presented) [001] is found to be determined by the value of (formula presented) of the ground state.