Magnetic‐Field‐Controlled Quantum Critical Points in the Triangular Antiferromagnetic Cs 2 CuCl 4‐ x Br x Mixed System

The triangular antiferromagnetic Cs 2 CuCl 4‐ x Br x mixed system is studied by neutron single‐crystal diffraction in magnetic field. It shows a rich magnetic phase diagram consisting of four regimes depending on the Br concentration and is characterized by different exchange coupling mechanisms. For the investigated compositions from regime I (0 < x ≤ 1.5), a critical magnetic field B c is found for a Br concentration x = 0.8 at B c = 8.10(1) T and for x = 1.1 at B c = 7.73(1) T and from regime IV (3.2