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

TL;DR

This study investigates how magnetic order and valence states in EuPd$_2$(Si$_{1-x}$Ge$_x$)$_2$ evolve under pressure and Ge substitution, revealing a strong pressure dependence of valence crossover and a transition from magnetic to intermediate-valence states.

Contribution

It provides new insights into the pressure-induced valence and magnetic state transitions in Eu-based intermetallic compounds with Ge substitution.

Findings

Valence-change crossover around 160 K at ambient pressure for x=0.

Strong pressure dependence of the valence crossover temperature, d$T'_V$/d$p$ = (80 ± 10) K/GPa.

Transition from antiferromagnetic order to intermediate-valence state with small applied pressure.

Abstract

We present results of magnetic susceptibility and thermal expansion measurements performed on high-quality single crystals of EuPd$_2$(Si$_{1-x}$Ge$_x$)$_2$ for 0 $\leq$ x $\leq$ 0.2 and temperatures 2 K $\leq T \leq$ 300 K. Data were taken at ambient pressure and finite He-gas pressure $p$ $\leq$ 0.5 GPa. For x = 0 and ambient pressure we observe a pronounced valence-change crossover centred around $T'_V$ $\approx$ 160 K with a non-magnetic ground state. This valence-change crossover is characterized by an extraordinarily strong pressure dependence of d$T'_V$ /d$p$ = (80 $\pm 10)$ K/GPa. We observe a shift of $T'_V$ to lower temperatures with increasing Ge-concentration, reaching $T'_V$ $\approx$ 90 K for x = 0.1, while still showing a non-magnetic ground state. Remarkably, on further increasing x to 0.2 we find a stable Eu$^{(2+\delta)+}$ valence with long-range antiferromagnetic order below $T_N$ = (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 $p$ $<$ 0.1 GPa to an intermediate-valence state for $p$ $\geq$ 0.1 GPa.