Possible Existence of Magnetic Polaron in Nearly Ferromagnetic Semiconductor β-US2

We studied the magnetic properties of the paramagnet beta-US2 with an orthorhombic crystal structure by measuring its magnetization, magnetic susceptibility, specific heat, and electrical resistivity under magnetic field and pressure. The 5f-crystalline electric field (CEF) scheme was determined from the experimental results: a singlet in the ground state and a pseudo-doublet in the first excited state, separated by 85 and 91 K from the ground state. The insulating resistivity markedly changes to a metallic one under a high pressure of 8 GPa similarly to that of the semimetallic Ising-type ferromagnet UTeS with a Curie temperature T-C = 87 K. A resistivity anomaly was also observed in the temperature dependence of the resistivity under pressure, most likely corresponding to ferromagnetic ordering. From these experimental results, it was concluded that beta-US2 is in the vicinity of ferromagnetic ordering at ambient pressure. Furthermore, we found that the easy-axis magnetization at low fields for H parallel to [001] consists of a linear magnetization based on the 5f-CEF scheme and a ferromagnetic-like magnetization with a saturated moment of 0.042 mu(B)/U. The latter magnetization was analyzed on the basis of the ferromagnetic cluster and discussed from the viewpoint of the magnetic polaron.