Abstract
A dynamic spin-fluctuation theory that directly takes into account nonlocality of thermalspin fluctuations and their mode–mode interactions is developed. The Gaussianapproximation in the theory is improved by a self-consistent renormalization of the meanfield and spin susceptibility due to the third- and fourth-order terms of the free energy,respectively. This eliminates the fictitious first-order phase transition, which is typical forthe Gaussian approximation, and yields a proper second-order phase transition. The effectof nonlocal spin correlations is enhanced by taking into account uniform fluctuations in thesingle-site mean Green function. Explicit computational formulae for basic magneticcharacteristics are obtained. The extended theory is applied to the calculation of magneticproperties of Fe–Ni Invar. Almost full agreement with experiment is achieved for themagnetization, Curie temperature, and local and effective magnetic moments.
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