PRRU2SI2 - A GIANT ANISOTROPIC INDUCED MAGNET WITH A SINGLET CRYSTAL-FIELD GROUND-STATE

The magnetic properties of PrRu2Si2 have been investigated experimenta lly by specific heat, single-crystal magnetization, Pr-141 Mossbauer a nd muon spectroscopies, neutron powder diffraction, and inelastic neut ron scattering, leading to the determination of its zero-field phase d iagram and its crystal electric-field energy levels below 40 meV. PrRu 2Si2 undergoes a magnetic phase transition at T-N similar or equal to 16 K to an axial incommensurate sine-wave magnetic structure character ized by a wave vector tau = (0.133, 0.133, 0), followed by a first-ord er phase transition at T-C similar or equal to 14.0 K to an axial ferr omagnetic structure. The lowest crystal electric-field states are the two singlets \Gamma(t1)((1))] and \Gamma(t2)] separated by 2.25 meV. T he low-temperature properties are described by a Hamiltonian identical to that of an Ising system with a transverse magnetic field. Since th e ratio of the exchange energy to the energy splitting between the sin glets is sufficiently large, it exhibits spontaneous magnetization. Th e nature of the two singlet states explains the giant magnetic anisotr opy. The random-phase approximation predicts the value of the high-fie ld magnetization but yields a low-field magnetization too small by sim ilar to 15%. Possible application of our results to uranium intermetal lic compounds is painted out.