Nonlinear effect of perpendicular magnetic field on the antiferromagnetic phase transition in weakly coupled layered systems: Equal access decoupling scheme

A system of equations of motion for the Green functions in layered antiferromagnets such as La2CuO4 and YBa2Cu3O6 has been treated by an ``equal access{''} decoupling scheme. The correlation functions are fully equivalent in this scheme in contrast with the ordinary random-phase approximation (RPA). The method provides a new insight into the nature of the RPA treatment of the localized spin dynamics in magnets. Explicit self-consistent expression for the sublattice magnetization in a perpendicular field is given. The dependence of the sublattice magnetization on a perpendicular magnetic field is studied. High temperature tails of the in-plane sublattice magnetization have been found to result from a nonlinear coupling of the perpendicular magnetic field with the antiferromagnetic order parameter.