  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.
