Magnetic phase diagram of the triangular antiferromagnetic $Cs_2CuCl_{4-x}Br_x$ mixed system

TL;DR

This study maps the magnetic phases of the $Cs_2CuCl_{4-x}Br_x$ system using neutron diffraction, revealing two ordered magnetic states depending on Cl/Br concentration and a non-magnetic range at intermediate compositions.

Contribution

It provides the first detailed magnetic phase diagram of the mixed system, identifying how magnetic order depends on composition and proposing exchange path mechanisms based on DFT calculations.

Findings

Two long-range magnetic ordered phases identified.

A non-magnetic range exists at intermediate compositions.

Exchange interactions are non-frustrated according to DFT.

Abstract

The novel magnetic phase diagram of the $Cs_2CuCl_{4-x}Br_x$ mixed system is established by means of single crystal neutron diffraction in the lowest temperature region and zero magnetic field. Two long-range ordered magnetic phases exist in this mixed system depending on the Cl/Br concentration. In the rich Cl concentration range, the ordered magnetic state occurs below the ordering temperature $T_N = 0.51(1)K$ for $Cs_2CuCl_3Br_1$ and at $Cs_2CuCl_{2.6}Br_{1.4}$ below $T_N = 0.24(2)K$. Magnetic order with a temperature-independent position $(0, 0.573(1), 0)$ below the ordering temperature $T_N = 0.63(1)K$ appears in the rich Br concentration for $Cs_2CuCl_{0.6}Br_{3.4}$. Between the rich Cl and rich Br concentration ranges (two magnetic phases), there is a range of x without magnetic order down to $50mK$. A suggestion about the magnetic exchange paths in the $bc$-layer for different regimes is presented, which can be controlled depending on the preferred Br-occupation in the [CuX4] tetrahedra. The density functional theory (DFT) calculations of the exchange coupling constants J, J^' for some ordered compositions of the mixed system $Cs_2CuCl_{4-x}Br_x$ indicate that these are not frustrated.