A renormalisation group study of the BNNNI model

Abstract

The phase diagram of the two-dimensional biaxially next-nearest-neighbour king (BNNNI) model is obtained using the real-space renormalisation group method with the mean-field approximation. The transitions between disordered and commensurate phases are observed to be single transitions for the ferromagnetic phase at intermediate and high temperatures and for the antiphase structure at high temperatures. However, within the approximation, successive transitions are observed between the antiphase structure and disordered phase at intermediate and low temperatures.