From valence fluctuations to long-range magnetic order in EuPd2(Si1−xGex)2 single crystals

Abstract

${\mathrm{EuPd}}_{2}{\mathrm{Si}}_{2}$ is a valence-fluctuating system undergoing a temperature-induced valence crossover at ${T}_{V}^{\ensuremath{'}}\ensuremath{\approx}160\phantom{\rule{4pt}{0ex}}\mathrm{K}$. We present the successful single-crystal growth using the Czochralski method for the substitution series ${\mathrm{EuPd}}_{2}{({\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x})}_{2}$, with substitution levels $x\ensuremath{\le}0.15$. A careful determination of the germanium content revealed that only half of the nominal concentration is built into the crystal structure. From thermodynamic measurements it is established that ${T}_{V}^{\ensuremath{'}}$ is strongly suppressed for small substitution levels and antiferromagnetic order from stable divalent europium emerges for $x\ensuremath{\gtrsim}0.10$. The valence transition is accompanied by a pronounced change of the lattice parameter $a$ of order 1.8%. In the antiferromagnetically ordered state below ${T}_{N}=47\phantom{\rule{4pt}{0ex}}\mathrm{K}$, we find sizable 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 ${\mathrm{EuPd}}_{2}{({\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x})}_{2}$.