Hexagonal spin lattice with ferromagnetic intrachain and antiferromagnetic interchain couplings in an external magnetic field

The hexagonal Heisenberg model, in which the spins along the c axis interact via a ferromagnetic exchange coupling and those in the c plane via an antiferromagnetic one, undergoes nontrivial changes when an external magnetic field is applied in the c plane. In the classical approximation this model has the same phenomenology as the two-dimensional planar triangular antiferromagnet: it is characterized by infinitely many minimum-energy configurations, thermal fluctuations select one configuration out of the manifold, and an intermediate collinear phase intervenes between the low-field 120^∘{} three-sublattice configuration and the high-field asymmetric fan phase. We find that the quantum nature of the model leads to first-order phase transitions between the helix and fan phase and between the fan and saturated phase. This model is suitable to describe CsCuCl₃ and agreement between theoretical expectations and experiment is found.