The triangular planar antiferromagnet (TPA) in an external magnetic field H is widely studied both analytically and numerically. Low-temperature expansion of the free energy shows that the infinite degeneracy of the minimum-energy configurations is lifted by thermal fluctuations and Monte Carlo simulation provides a rich phase diagram in the H-T plane where three different configurations are encircled by the paramagnetic saturated phase. Here, we give explicitly the elementary excitation dispersion curves. The field dependence of the uniform modes is closely related to the minimum-energy configuration selected by thermal fluctuations. Some real compounds are indicated as possible candidates to test experimentally the phenomenology of the TPA model. We have also explained the stability over a finite region in the H-T plane of the phase characterized by two spins parallel and one spin antiparallel to the field in the magnetic cell. The stabilizing mechanism of this phase arises from crucial nonlinear effects. For this phase the presence of long-range order is proven analytically.

• Received 13 November 1991

DOI:https://doi.org/10.1103/PhysRevB.45.7936