Magnetic properties and effect of pressure on the electronic state of EuCo₂Ge₂

EuCo₂Ge₂ with the tetragonal structure is a Eu-divalent antiferromagnet with the Néel temperature T_N=23 K. The magnetic easy-axis corresponds to the [100] direction (a-axis), while the [001] direction (c-axis) is a hard-axis. The magnetization for H∥[100] indicates a metamagnetic transition at 25 kOe and saturates above 75 kOe. On the other hand, the hard-axis magnetization increases approximately linearly and saturates above 110 kOe. The magnetic phase diagram was constructed. A characteristic feature in EuCo₂Ge₂ is known as a valence transition under pressure, from Eu ^2+δ to Eu ^3−δ′(δ, δ′<1). We also clarified the valence transition by measuring the electrical resistivity under pressure. The valence transition occurs at 3 GPa, with a hysteresis, and terminates at about 4.5 GPa. Further increasing pressure, the electronic state is changed into a moderate heavy fermion state and approaches the nearly trivalent electronic state.