METAMAGNETISM AND THERMODYNAMICAL PROPERTIES IN MODULATED SYSTEMS - MODELIZATION AND APPLICATION TO PRNI2SI2

A periodic field model taking into account explicitly the crystal field anisotropy is presented, in order to describe the thermodynamical properties and the magnetization processes of incommensurate modulated magnetic systems. An application is made to the tetragonal PrNi2Si2 compound, where the crystal field and exchange parameters have been unambiguously determined, from a careful joint analysis of specific heat, magnetization, magnetic susceptibility and inelastic neutron scattering experiments. The ground state is a nonmagnetic singlet weil isolated from another singlet and a magnetic doublet as the first two excited states. The existence of an amplitude modulated magnetic structure persisting from T(N) = 20 K down to 0 K is then well explained. It is shown that the periodic field model does not require any additional parameter to account quantitatively very well for the observed properties in the modulated ordered phase, in particular the variation of the specific heat and the metamagnetic process along the [001] easy magnetization direction.