Field-induced ferromagnetic exchange interaction in metamagnetic transitions of heavy-electron liquids

The metamagnetic transition or crossover in ${\mathrm{CeRu}}_2{\mathrm{Si}}_2$ is investigated using the Hubbard model, with a pseudogap structure of the density of states of quasiparticles phenomenologically taken into account. Local quantum spin fluctuations are considered through mapping to the Anderson model. Intersite effects are considered by the $1/d$ expansion method, with $d$ being the spatial dimensionality. There are two main driving forces in the metamagnetic transition: a field-dependent exchange interaction and the magnetostriction or the Kondo volume-collapse effect. The exchange interaction due to the virtual exchange of pair excitations within the quasiparticle band changes its signs with increasing magnetization. It is antiferromagnetic in the absence of fields and is ferromagnetic in the vicinity of the metamagnetic point. The pseudogap structure plays a critical role in this sign change. This exchange interaction scales with the bandwidth of the quasiparticles, and the single-parameter scaling approximately holds for the magnetization and the magnetostriction processes.