Pressure study on the interplay between magnetic order and valence crossover in EuPd2(Si1−xGex) 2

We present results of the magnetic susceptibility on high-quality single crystals of EuPd₂(Si_{1−xGex) 2} for Ge concentrations 0 ≤x ≤0.105 performed under varying hydrostatic (He-gas) pressure 0 ≤p ≤0.5 GPa. The work extends recent studies at ambient pressure demonstrating the drastic change in the magnetic response from valence-crossover behavior for x =0 and 0.058, to long-range antiferromagnetic (AFM) order below T_N=47 K for x =0.105 . The valence-crossover temperature T_V^' shows an extraordinarily strong pressure dependence of d T_V^'/d p =+(80 ±10 ) K/GPa. In contrast, a very small pressure dependence of d T_N/d p ≤+(1 ±0.5 ) K/GPa is found for the AFM order upon pressurizing the x =0.105 crystal from p =0 to 0.05 GPa. Remarkably, by further increasing the pressure to 0.1 GPa, a drastic change in the ground state from AFM order to valence-crossover behavior is observed. Estimates of the electronic entropy related to the Eu 4 f electrons, derived from analyzing susceptibility data at varying pressures, indicate that the boundary between AFM order and valence crossover represents a first-order phase transition. Our results suggest a particular type of second-order critical end point of the first-order transition for x =0.105 at p_cr≈0.06 GPa and T_cr≈39 K where intriguing strong-coupling effects between fluctuating charge, spin, and lattice degrees of freedom can be expected.