Generalised mean field theory of spin glasses

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Abstract
A generalised mean field theory of an Ising spin glass is presented which takes into account correlations between the spins and their interactions. The susceptibility chi (T) at zero temperature and the coefficient of the linear term in the specific heat for T to 0 tend to zero if spins and interactions are completely correlated (case I) and are largest (i.e. chi (0)=0.64 chi (Tf)) if they are uncorrelated (case II). With increasing correlations the slope of the cusp in the static susceptibility just below the freezing temperature Tf varies by a factor of 4. The temperature Tf is found to be proportional to the concentration c of magnetic impurities in case I and proportional to c1/2 in case II. The dependence of Tf on the mean free path l of the conduction electrons is investigated for the RKKY interaction as a special case. It turns out that Tf is much more sensitive to l in case I than in case II.