CORRELATED-ELECTRON THEORY OF STRONGLY ANISOTROPIC METAMAGNETS

Metamagnetic transitions in strongly anisotropic antiferromagnets are investigated within a quantum mechanical theory of correlated electron s. We employ the Hubbard model with staggered magnetization m(st), alo ng an easy axis e in a magnetic field H//e. On the-basis of the dynami cal mean-field theory(DMFT) this model is studied both analytically an d numerically. At intermediate couplings the self-consistent DMFT equa tions, which become exact in the limit of a large coordination number, are solved by finite temperature quantum Monte Carlo techniques. The temperature and magnetic-field dependence of the homogeneous and stagg ered magnetization are calculated and the magnetic phase diagram is co nstructed. At half filling the metamagnetic transitions are found to c hange from first order at low temperatures to second order near the Ne el temperature, implying the existence of a multicritical point. Dopin g;with holes or elections has a strong effect: the system becomes meta llic, the electronic compressibility increases, and the critical tempe ratures and fields decrease. These results are related to known proper ties of insulating metamagnets such as FeBr2, metallic metamagnets suc h as UPdGe, and the giant and colossal magnetoresistance found in a nu mber of magnetic bulk systems.