Metamagnetism in Heavy Fermion Compounds: An augmented Landau approach

Many heavy fermion materials exhibit metamagnetic behavior where there is a rapid but gradual rise in the magnetization with field. There is no apparent phase transition at any temperature T > 0, but the possibility exists of a first order transition at T = 0. Here we examine how within a fully classical framework, starting from a Landau-type free energy with an added Bragg-Williams entropy term we can arrive at a number of key experimental signatures of metamagnetism as seen for example in UPt₃. In particular, we show that correctly arriving at the meassured (low-field) higher order susceptibilities necessarily invokes an isobestic (crossing) point at a high field in the magnetization isotherms. We also present a full analysis of the angular dependence of the (low-field) linear and nonlinear susceptibilities which when extended also capture the anisotropic high field response of the magnetization. Integral to our analysis is the proper conversion of the evaluated magnetization from constant volume to a constant pressure situation since heavy fermion materials have large magnetostriction the effects of which become pronounced at large fields.