From valence fluctuations to long-range magnetic order in EuPd$_2$(Si$_{1-x}$Ge$_x$)$_2$ single crystals

TL;DR

This study investigates how germanium substitution in EuPd$_2$(Si$_{1-x}$Ge$_x$)$_2$ single crystals suppresses valence fluctuations and induces long-range magnetic order, revealing a sharp transition between these states.

Contribution

It provides the first successful growth of EuPd$_2$(Si$_{1-x}$Ge$_x$)$_2$ single crystals and maps the phase diagram showing the transition from valence fluctuations to magnetic order.

Findings

Valence crossover temperature $T'_V$ decreases with Ge substitution.

Antiferromagnetic order appears for $x \,\gtrsim\, 0.10$.

Lattice parameter $a$ changes by about 1.8% across the transition.

Abstract

EuPd$_2$Si$_2$ is a valence-fluctuating system undergoing a temperature-induced valence crossover at $T'_V\approx160\,$K. We present the successful single crystal growth using the Czochralski method for the substitution series EuPd$_2$(Si$_{1-x}$Ge$_x$)$_2$, with substitution levels $x\leq 0.15$. A careful determination of the germanium content revealed that only half of the nominal concentration is build into the crystal structure. From thermodynamic measurements it is established that $T'_V$ is strongly suppressed for small substitution levels and antiferromagnetic order from stable divalent europium emerges for $x\gtrsim 0.10$. The valence transition is accompanied by a pronounced change of the lattice parameter $a$ of order 1.8%. In the antiferromagnetically ordered state below $T_N = 47$ K, we find sizeable magnetic anisotropy with an easy plane perpendicular to the crystallographic c direction. An entropy analysis revealed that no valence fluctuations are present for the magnetically ordered materials. Combining the obtained thermodynamic and structural data, we construct a concentration-temperature phase diagram demonstrating a rather abrupt change from a valence-fluctuating to a magnetically-ordered state in EuPd$_2$(Si$_{1-x}$Ge$_x$)$_2$.