Observation of a Metallic Antiferromagnetic Phase and Metal to Nonmetal Transition in Ca3Ru2O7

Single crystal Ca₃Ru₂O₇ shows a metallic antiferromagnetic phase intermediate between a first-order metal to nonmetal transition at T_M = 48 K and the antiferromagnetic ordering (Néel) temperature, T_N = 56 K. The metallic antiferromagnetic phase is predicted within various Mott-Hubbard models. Magnetization and electrical resistivity reveal strongly anisotropic metamagnetism in the nonmetallic antiferromagnetic phase. The charge and spin excitations are strongly coupled: The H-T phase diagrams determined by magnetization and magnetoresistivity are indistinguishable and reveal a multicritical point. The heat capacity of Ca₃Ru₂O₇ suggests it is a highly correlated electron system.