OBSERVATION OF A METALLIC ANTIFERROMAGNETIC PHASE AND METAL TO NONMETAL TRANSITION IN CA3RU2O7

Single crystal Ca3Ru2O7 shows a metallic antiferromagnetic phase inter mediate between a first-order metal to nonmetal transition at T-M = 48 K and the antiferromagnetic ordering (Neel) temperature, T-N = 56 K. The metallic antiferromagnetic phase is predicted within various Mott- Hubbard models. Magnetization and electrical resistivity reveal strong ly anisotropic metamagnetism in the nonmetallic antiferromagnetic phas e. The charge and spin excitations are strongly coupled: The H-T phase diagrams determined by magnetization and magnetoresistivity are indis tinguishable and reveal a multicritical point. The heat capacity of Ca 3Ru2O7 suggests it is a highly correlated electron system.