Cs2CoBr4

A "lost" quantum magnet from Cs2BX4 generic family, Cs2CoBr4, was re-discovered in our group. The primary antiferromagnetic exchange interactions are along the "legs" and "rungs" of a "zig-zag spin ladder", which makes the spin subsystem highly frustrated [1]. There is an additional  twist: cobalt ions have S=3/2 that are subject to extreme easy plane ionic anisotropy, such that only the pseudospin-1/2 degrees of freedom are active at low temperature. The orientation of the easy plane is alternating between the sites and thus the effective S=1/2 Hamiltonian is full of strongly anisotropic direction-dependent interactions.

In applied magnetic fields thios material has a very complicated phase diagram [2]. First, there is a longitudinal spin-density wave phase [3]. Its incommensurate propagation vector scales with the magnetization of the sample. At higher field, the spin-density wave ordering vector locks at 1/3 value in an up-up-down 1/3 magnetization plateau phase. Then the system goes through a spin flop transition. Finally, at higher fields there is an incommensurate fan phase, with a propagation vector given not by magnetization, but by the exchange frustration ratio.

The most exciting physics concerns the magnetic excitations. In zero field these are an hierarchy of spinon bound states known as a "Zeeman ladder". At higher magnetic fields more conventional spin wave like excitations are observed instead [4]