Calorimetry Study of the Phase Diagrams of EuNi^sub 2^Ge^sub 2^ and Eu^sub 2^Ni^sub 3^Ge^sub 5^ under Pressure

We report here the phase diagrams of EuNi^sub 2^Ge^sub 2^ and Eu^sub 2^Ni^sub 3^Ge^sub 5^ studied by ac calorimetry under pressure using a diamond anvil cell. We follow the antiferromagnetic transition for EuNi^sub 2^Ge^sub 2^ up to 1.5 GPa. The sudden disappearance of magnetic order at around 2 GPa is confirmed, consistent with the probable occurrence of a first-order valence transition near that pressure. The ac calorimetry results on Eu^sub 2^Ni^sub 3^Ge^sub 5^ clearly show two antiferromagnetic transitions, and suggest that magnetic order persists up to higher pressure than previously expected. At high pressure, where heavy-fermion behavior has been reported, the Neel temperature is decreasing, and magnetic order is expected to disappear at an extrapolated pressure of 12-14 GPa. A semi quantitative analysis of the pressure dependence of the specific heat does not show any large changes, but is compatible with a moderate enhancement of γ. The phase diagrams of Yb and Ce heavy fermion systems are compared and discussed with our system.