Correlated-electron theory of strongly anisotropic metamagnets

Metamagnetic transitions in strongly anisotropic antiferromagnets are investigated within a quantum mechanical theory of correlated electrons. We employ the Hubbard model with staggered magnetization ${\mathbf{m}}_{\mathrm{st}}$ along an easy axis $\mathbf{e}$ in a magnetic field $\mathbf{H}\Vert \mathbf{e}$. On the basis of the dynamical mean-field theory (DMFT) this model is studied both analytically and numerically. At intermediate couplings the self-consistent DMFT equations, 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 staggered magnetization are calculated and the magnetic phase diagram is constructed. At half filling the metamagnetic transitions are found to change from first order at low temperatures to second order near the Néel temperature, implying the existence of a multicritical point. Doping with holes or electrons has a strong effect: the system becomes metallic, the electronic compressibility increases, and the critical temperatures and fields decrease. These results are related to known properties of insulating metamagnets such as FeBr${}_2$, metallic metamagnets such as UPdGe, and the giant and colossal magnetoresistance found in a number of magnetic bulk systems.