Effective field theory for the Ising model with a fluctuating exchange integral in an asymmetric bimodal random magnetic field: A differential operator technique

The spin-1/2 Ising model on a square lattice, with fluctuating bond interactions between nearest neighbors and in the presence of a random magnetic field, is investigated within the framework of the effective field theory based on the use of the differential operator relation. The random field is drawn from the asymmetric and anisotropic bimodal probability distribution P(h(i)) = p delta(h(i) - h(1)) + q delta(h(i) - ch(1)), where the site probabilities p, q take on values within the interval [0, 1] with the constraint p + q = 1; h(1) is the random field variable with strength h(1) and c the competition parameter, which is the ratio of the strength of the random magnetic field in the two principal directions +z and -z; c is considered to be positive resulting in competing random fields. The fluctuating bond is drawn from the symmetric but anisotropic bimodal probability distribution P(J(ij)) = 1/2{delta(J(ij)-(J+Delta))+ delta(J(ij)-(J-Delta))}, where J and Delta represent the average value and standard deviation of J(ij), respectively. We estimate the transition temperatures, phase diagrams (for various values of the system's parameters c, p, h(1), Delta), susceptibility, and equilibrium equation for magnetization, which is solved in order to determine the magnetization profile with respect to T and h(1). (c) 2012 Elsevier B.V. All rights reserved.