Papers by Author: B.I. Reser

Abstract: A simple low-temperature dynamic spin-fluctuation theory of ferromagnetic metals is developed. The theory is based on the functional integral formalism for the multiband Hubbard Hamiltonian and takes into account both single-site and nonlocal spin fluctuations. We show that our approach correctly reproduces the T^3/2 law at low temperatures. The calculated results of magnetic properties for Fe and Fe_0.65Ni_0.35 Invar demonstrate that the approach works on a much wider temperature interval than the spin-wave approximation.

Abstract: The problem of discontinuous phase transition in the dynamic spin-fluctuation theory is resolved by taking into account large anharmonic spin fluctuations and nonlocality of the mean Green function. The extended theory is applied to the calculation of magnetic properties of iron.

Abstract: In the dynamic spin-fluctuation theory extended by taking into account higher-order terms of the free energy and uniform fluctuations along with the local ones, simple computational formulae for local magnetic moment are obtained. The temperature dependence of local magnetic moment in the Fe_0.65Ni_0.35 Invar is calculated. Good agreement with the experimentally estimated value of the magnetic volume change is achieved.

Abstract: Temperature dependence of magnetic characteristics of Fe and Fe-Ni invar is considered in the dynamic nonlocal approximation of the spin-fluctuation theory. Numerical calculations show that the magnetic characteristics can have a discontinuous jump at high temperatures, well below the Curie temperature. Using methods of the catastrophe theory, we investigate the effect of small changes in the initial data on the results of the calculation. It is demonstrated that the discontinuous jump can only be smoothed but cannot be eliminated entirely without a significant change in the system of equations of the spin-fluctuation theory. Possible variants of such changes are discussed.