From magnetic order to valence-change crossover in EuPd$_2$(Si$_{1-x}$Ge$_x$)$_2$ using He-gas pressure

Abstract

We present results of magnetic susceptibility and thermal expansion measurements performed on high-quality single crystals of EuPd_22(Si_{1-x}1−xGe_xx)_22 for 0 \leq x \leq≤x≤ 0.2 and temperatures 2 K \leq T \leq≤T≤ 300 K. Data were taken at ambient pressure and finite He-gas pressure pp\leq≤ 0.5 GPa. For x = 0x=0 and ambient pressure we observe a pronounced valence-change crossover centred around T'_VT′V\approx≈ 160 K with a non-magnetic ground state. This valence-change crossover is characterized by an extraordinarily strong pressure dependence of dT'_VT′V /dpp = (80 \pm 10)±10) K/GPa. We observe a shift of T'_VT′V to lower temperatures with increasing Ge-concentration, reaching T'_VT′V\approx≈ 90 K for x = 0.1x=0.1, while still showing a non-magnetic ground state. Remarkably, on further increasing xx to 0.2 we find a stable Eu^{(2+\delta)+}(2+δ)+ valence with long-range antiferromagnetic order below T_NTN = (47.5 \pm± 0.1) K, reflecting a close competition between two energy scales in this system. In fact, by the application of hydrostatic pressure as small as 0.1 GPa, the ground state of this system can be changed from long-range antiferromagnetic order for pp<< 0.1 GPa to an intermediate-valence state for pp\geq≥ 0.1 GPa.