Magnetic phase diagrams of axial third-nearest-neighbour Ising model

The magnetic phase diagrams of S = 1/2 axial third-nearest-neighbour Ising (A3NNI) model under an external field have been studied by means of the mol ecular field approximation. The phase boundaries among paramagnetic, (anti) ferromagnetic and various modulated phases are determined by analysing the frequency-dependent susceptibility, or by solving the coupled equations fo r magnetisations for spins up to 17, iteratively. The resultant phase diagr am is found to be very consistent with the exact results for the ground sta te spin ordering and to be far improved than existing finite temperature ph ase diagrams for the same model. (C) 2000 Elsevier Science B.V. All rights reserved.