Field Induced Phase Transitions in Systems with a Crystalline Field Singlet Ground State

抄録

Magnetic field induced phase transitions are studied in a simplified model to represent systems with a crystalline field singlet ground state such as some rare earth compounds by the mean field approximation. In the case where the magnetic field yields the level crossing of the ground and excited states in the absence of the exchange interaction, then its presence brings two types of the phase transition. One of them is a second order transition accompanied with a polarization perpendicular to the field. This is a transition found by Tsuneto and Murao, and Tachiki and Yamada. Another is a first order transition, which occurs for the ferromagnetic coupling when the perpendicular susceptibility can not be large enough to yield a spontanous polarization. The system skips over the level crossing from the lower to higher magnetization states without change of symmetry. The condition and the nature of these transitions are investigated.