Towards a critical endpoint in the valence fluctuating Eu(Rh${}_{1-x}$Co${}_x$)${}_2$Si${}_2$ system

Abstract

We report on the successful single crystal growth of pure EuRh${}_2$Si${}_2$ and of Eu(Rh${}_{1-x}$Co${}_x$)${}_2$Si${}_2$ with $x\le 0.23$ by the flux method. Through Co substitution, EuRh${}_2$Si${}_2$ can be tuned from stable antiferromagnetism via a valence-transition state towards the valence-crossover regime. From magnetization measurements, we constructed a $B-T$ phase diagram for EuRh${}_2$Si${}_2$ comprising multiple magnetic phases and showing a sizable magnetic anisotropy within the basal plane of the tetragonal unit cell. This indicates a complex antiferromagnetic ground state for $x=0$. By applying positive chemical pressure through the substitution series Eu(Rh${}_{1-x}$Co${}_x$)${}_2$Si${}_2$, a sharp temperature-induced first-order phase transition is observed in magnetization, resistivity and heat capacity for 0.081 $\le$ $x$ $\le$ 0.119. The critical end point of this valence transition is located in the phase diagram in the vicinity of 0.119 $<x_{\mathrm{EDX}}<$ 0.166. At higher substitution level, the system reaches a valence-crossover regime. The obtained results are presented in a temperature-substitition phase diagram.