Theory of a Two-Dimensional Ising Model with Random Impurities. III. Boundary Effects

Abstract

We study the effects which immobile random impurities may have on magnetic properties and spin-spin correlation functions of a ferromagnetic system properties the Curie temperature. This is done within the context of the model introduced in the first paper of this series by computing the spin correlation functions on the boundary of a half-plane of random Ising spins, where the boundary row is allowed to intereact with a magnetic field $\mathfrak{H}$. We find that as $T\to T_c-$ the average boundary spontaneous magnetization vanishes as $T_c-T$. However, the average boundary magnetic susceptibility is shown not to exist for a finite range of temperature about $T_c$. Furthermore, at $T_c$ the average boundary magnetization behaves as $-N^{-1\mathrm{}}$ sign $\left(\mathfrak{H}\right){\left[\ln N{\beta }_c\right|\mathfrak{H}\mathrm{}\left|\right]\mathrm{}}^{-1\mathrm{}}$, where $N^{-1\mathrm{}}$ is a measure of the width of the distribution of random bonds. Whenever $T-T_c=O\left(N^{-2\mathrm{}}\right)$, the average spin-spin correlation function for two spins on the boundary is shown to approach its limit at infinite separation as some inverse power of the separation instead of as an exponential. At $T_c$ this average correlation function behaves asympotically as $N^{-2\mathrm{}} {\left(\ln m{N}^{-2\mathrm{}}\right)}^{-1\mathrm{}}$ when $m$ the separation between boundary spins, is large. Finally, we make the probabilistic nature of the boundary spontaneous magnetization more precise by computing its probability distribution function.