Towards a critical endpoint in the valence fluctuating Eu(Rh1−xCox)2Si2 system

We report on the successful single crystal growth of pure EuRh₂Si₂ and of Eu(Rh${}_{1-x}$Co${}_x$)₂Si₂ with $x\le 0.23$ by the flux method. Through Co substitution, EuRh₂Si₂ 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 EuRh₂Si₂ comprising multiple magnetic phases and showing a sizeable 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${}_x$)₂Si₂, a sharp temperature-induced first-order phase transition is observed in magnetization, resistivity and heat capacity for 0.081 $\le$ $x$ $\le$ 0.126. The critical end point of this valence transition is located in the phase diagram in the vicinity of 0.126 $<x_{\mathrm{EDX}}<$ 0.23. At higher substitution level, the system reaches a valence-crossover regime. The obtained results are presented in a temperature-substitution phase diagram.