Anomalous magnetic ground state in PrSi evidenced by the magnetocaloric effect

Previous investigations into the physical properties of PrSi have shown this compound to order ferromagnetically at 54 K. However, the magnetic ground state has not yet been determined unambiguously. PrSi crystallizes in the orthorhombic FeB-type structure. Typically, the crystalline electric field (CEF) would uplift the degeneracy of the J = 4 Pr ³⁺ free-ion ground state multiplet, yielding nine (normally non-magnetic) singlets. It is known that magnetic order in such systems may be established via the admixture of two singlets into a doublet state, analogous to TmNi, which crystallizes in the same structure. From symmetry considerations, collinear ferromagnetically ordered moments should lie parallel to the crystallographic b-axis. However, neutron diffraction experiments have shown that the easy magnetic axis lies in the ac-plane. This is the first suggestion that the ground state in PrSi is not determined by the CEF alone. Here, we investigate the ground state properties of PrSi by analyzing its specific heat. A Schottky contribution associated with the thermal population of CEF-split energy levels is absent from the 4 f-electron contribution to the specific heat and the magnetic configurational entropy points to a full nine-fold degenerate J = 4 multiplet magnetic ground state. The strongest evidence for such a ground state is found when the magnetocaloric effect (MCE) in this system is considered. Furthermore, the MCE indicates the presence of a significant higher order exchange term in the magnetic Hamiltonian.