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(Rh₁₋ₓCoₓ)₂Si₂ with x≤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 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(Rh₁₋ₓCoₓ)₂Si₂, a sharp temperature-induced first-order phase transition is observed in magnetization, resistivity and heat capacity for 0.081 ≤ x ≤ 0.119. The critical end point of this valence transition is located in the phase diagram in the vicinity of 0.119 <xEDX< 0.166. At higher substitution level, the system reaches a valence-crossover regime. The obtained results are presented in a temperature-substitition phase diagram.