We present results of the magnetic susceptibility on high-quality single crystals of EuPd$_2$(Si$_{ 1-x}$Ge$_x$)$_2$ for Ge concentrations 0 $\leq x \leq$ 0.105 performed under varying hydrostatic (He-gas) pressure 0 $\leq p \leq$ 0.5 GPa. The work extends on recent studies at ambient pressure demonstrating the drastic change in the magnetic response from valence-change-crossover behavior for $x$ = 0 and 0.058, to long-range antiferromagnetic (afm) order below $T_{ \text{ N}}$ = 47 K for $x$ = 0.105. The valence-change-crossover temperature $T'_{ \text{ V}}$ shows an extraordinarily strong pressure dependence of d$T'_{ \text{ V}}$/d$p$ = +(80 $\pm$ 10) K/GPa. In contrast, a very small pressure dependence of d$T_{ \text{ N}}$/d$p \leq$ +(1 $\pm$ 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-change-crossover behavior is observed. Estimates of the electronic entropy, derived from analyzing susceptibility data at varying pressures, indicate that the boundary between afm order and valence-change crossover represents a first-order phase transition. Our results suggest a particular type of second-order critical endpoint of the first-order transition for $x$ = 0.105 at $p_{ \text{ cr}} \approx$ 0.06 GPa and $T_{ \text{ cr}} \approx$ 45 K where intriguing strong-coupling effects between fluctuating charge-, spin- and lattice degrees of freedom can be expected.
Categories: cond-mat.str-el