Finite-field chiral tetracritical behavior in a distorted triangular antiferromagnet

Abstract

The magnetic-field–temperature phase diagram of the antiferromagnetic planar model on a hexagonal lattice with a small basal-plane distortion is studied within mean-field theory. The convergence of four critical lines is shown to occur at a field ${\mathit{H}}_{\mathit{x}}$>0 as a consequence of in-plane anisotropy of the form ${\mathit{Es}}_{\mathit{x}}^2$. This is in contrast with the previously studied H=0 tetracritical behavior of the regular triangular lattice, as realized in ${\mathrm{CsMnBr}}_3$ [B. D. Gaulin et al., Phys. Rev. Lett. 62, 1380 (1989)]. The present system also exhibits the same chiral degeneracy and should be relevant to the previously misidentified multicritical behavior of (${\mathrm{CH}}_3$ ${)}_4$ ${\mathrm{NMnCl}}_3$.