Theory of the metamagnetic crossover in CeRu_2Si_2

Based on the periodic Anderson model, it is shown that the competition between the quenching of magnetic moments by local quantum spin fluctuations and a magnetic exchange interaction caused by the virtual exchange of pair excitations of quasiparticles in spin channels is responsible for the metamagnetic crossover in CeRu_2Si_2, cooperated with the electron-lattice interaction. The strength of the exchange interaction is proportional to the bandwidth of quasiparticles and its sign changes with increasing magnetizations; it is antiferromagnetic in the absence of magnetizations, whereas it is ferromagnetic in the metamagnetic crossover region. Experimental results of static quantities are well reproduced.