Distinct magnetic regimes through site-selective atom substitution in the frustrated quantum antiferromagnet Cs$_2$CuCl$_{4-x}$Br$_x$

TL;DR

This study systematically investigates how site-selective atom substitution in Cs$_2$CuCl$_{4-x}$Br$_x$ affects magnetic regimes, revealing three distinct magnetic phases separated by critical concentrations due to structural peculiarities.

Contribution

It introduces a detailed analysis of magnetic behavior across the solid solution, highlighting the impact of site-selective substitution on magnetic frustration and phase transitions.

Findings

Identification of three distinct magnetic regimes separated by critical concentrations

Observation of non-smooth variation in magnetic correlations with Br concentration

Correlation between structural peculiarities and magnetic phase behavior

Abstract

We report on a systematic study of the magnetic properties on single crystals of the solid solution Cs$_2$CuCl$_{4-x}$Br$_x$ (0 $\leq$ x $\leq$ 4), which include the two known end-member compounds Cs$_2$CuCl$_4$ and Cs$_2$CuBr$_4$, classified as quasi-two-dimensional quantum antiferromagnets with different degrees of magnetic frustration. By comparative measurements of the magnetic susceptibility $\chi$($T$) on as many as eighteen different Br concentrations, we found that the inplane and out-of-plane magnetic correlations, probed by the position and height of a maximum in the magnetic susceptibility, respectively, do not show a smooth variation with x. Instead three distinct concentration regimes can be identified, which are separated by critical concentrations x$_{c1}$ = 1 and x$_{c2}$ = 2. This unusual magnetic behavior can be explained by considering the structural peculiarities of the materials, especially the distorted Cu-halide tetrahedra, which support a site-selective replacement of Cl- by Br- ions. Consequently, the critical concentrations x$_{c1}$ (x$_{c2}$) mark particularly interesting systems, where one (two) halidesublattice positions are fully occupied.