The two-dimensional Ising model with two kinds of bonds –ferro and antiferromagnetic– is investigated in both quenched and annealed approximations. High temperature series expansion technique is used to get the respective free energies and susceptibilities. Form the analysis of the ratio method results, the phase diagrams are drawn and critical concentrations are estimated. It is shown that in both approximations the system behaves similarly, although in the annealed case the magnetically ordered phases are more stable against increasing impurity concentration. It is also proved that for Bethe lattices initial susceptibilities for quenced and annealed systems coincide.
We analyze new high-temperature series expansions for the susceptibility $\ensuremath{\chi}$ of the two-dimensional random-bond Ising ferromagnet with a symmetric bimodal distribution of two positive couplings
J1 and
J2. By studying a wide range of coupling ratios
J2/J1 we obtain compelling evidence for a critical behavior of the form $\ensuremath{\chi}\ensuremath{\sim}{t}^{\ensuremath{-}7/4}|\mathrm{ln}t{|}^{7/8}$, as predicted theoretically by Shalaev, Shankar, and Ludwig.
In order to study the influence of quenched disorder on second-order phase transitions, high-temperature series expansions of the susceptibility and the free energy are obtained for the quenched bond-diluted Ising model in
d=3−−5 dimensions. They are analyzed using different extrapolation methods tailored to the expected singularity behaviors. In
d=4 and 5 dimensions we confirm that the critical behavior is governed by the pure fixed point up to dilutions near the geometric bond percolation threshold. The existence and form of logarithmic corrections for the pure Ising model in
d=4 are confirmed, and our results for the critical behavior of the diluted system are in agreement with the type of singularity predicted by renormalization group considerations. In three dimensions we find large crossover effects between the pure Ising, percolation, and random fixed points. We estimate the critical exponent of the susceptibility to be $\ensuremath{\gamma}=1.305(5)$ at the random fixed point.