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

TL;DR

This study explores the Eu(Rh$_{1-x}$Co$_{x}$)$_2$Si$_2$ system, demonstrating how Co substitution tunes magnetic and valence states, revealing a critical endpoint and a valence crossover regime through detailed phase diagram analysis.

Contribution

It provides the first detailed phase diagram showing the critical endpoint and valence crossover in Eu(Rh$_{1-x}$Co$_{x}$)$_2$Si$_2$, highlighting the effects of chemical pressure on valence and magnetic transitions.

Findings

Identification of a critical end point near x=0.119-0.166.

Observation of a temperature-induced first-order phase transition.

Construction of a temperature-substitution phase diagram.

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 $x\leq0.23$ 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 $\leq$ $x$ $\leq$ 0.119. The critical end point of this valence transition is located in the phase diagram in the vicinity of 0.119 $<x_{\rm EDX}<$ 0.166. At higher substitution level, the system reaches a valence-crossover regime. The obtained results are presented in a temperature-substitition phase diagram.