Vladimir I. Grebennikov

Extended dynamic spin-fluctuation theory of metallic magnetism

A dynamic spin-fluctuation theory that directly takes into account nonlocality of thermal spin fluctuations and their mode-mode interactions is developed. The Gaussian approximation in the theory is improved by a self-consistent renormalization of the mean field 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 for the Gaussian approximation, and yields a proper second-order phase transition. The effect of nonlocal spin correlations is enhanced by taking into account uniform fluctuations in the single-site mean Green function. Explicit computational formulae for basic magnetic characteristics are obtained. The extended theory is applied to the calculation of magnetic properties of Fe-Ni Invar. Almost full agreement with experiment is achieved for the magnetization, Curie temperature, and local and effective magnetic moments.

Effect of atomic magnetic moments on the relative intensity of the Lβ and Lα components in x-ray emission spectra of 3d transition metal oxides

The integrated-intensity ratio R of the Lβ and Lα lines in x-ray emission spectra of 3d transition metal oxides was measured. The magnitude of the ratio was found to depend nonmonotonically on the atomic number of the 3d elements, with a sharp rise in the middle of the series. In terms of the theory of one-electron resonant x-ray scattering in a solid, a simple relation was derived connecting this ratio with the spin polarization of occupied and empty d states. The calculated values of R qualitatively reflect the behavior of the measured I(Lβ)/I(Lα) ratio on the atomic number of 3d elements. Thus, the existence of a dependence of the integrated-intensity ratio of Lβ and Lα lines on the atomic magnetic moment has been established. All calculated R curves lie above the I(Lβ)/I(Lα)exp data, because not all factors accompanying the x-ray emission process were included in the calculation.

Dynamical theory of thermal spin fluctuations in metallic ferromagnets

A self-consistent dynamical theory of thermal spin fluctuations is developed which describes their spatial correlation. It is based on the functional integral method and utilizes the quadratic representation for the electron free energy in a fluctuating exchange field with renormalized susceptibilities allowing for the interaction of various spin fluctuation modes. Interpolation between the single-site and homogeneous susceptibilities is used, where these susceptibilities are found self-consistently. The average over fluctuations takes account of both long-wavelength and local excitations. A closed system of equations is formed for both unknown quantities: the magnetization and the mean-square exchange field at a site. The basic characteristics of a specific magnet are the density of electron states and the atomic magnetic moment at T=0. A method is proposed for separating the relatively slow thermal-spin fluctuations from the rapid zero-spin fluctuations forming the ground state of the magnets. At T=0 we have a system of equations of mean field theory. The temperature excites thermal spin fluctuations, which are described by taking account of correlation in time and space. The magnetization, susceptibility, magnitude of the spin fluctuations and their distribution over momenta, and the degree of magnetic short-range order in iron are calculated as functions of the temperature in the ferromagnetic and paramagnetic phases, and also at the transition between them, the Curie temperature.

Spin-fluctuation theory beyond Gaussian approximation

A characteristic feature of the Gaussian approximation in the functional-integral approach to the spin-fluctuation theory is the jump phase transition to the paramagnetic state. We eliminate the jump and obtain a continuous second-order phase transition by taking into account high-order terms in the expansion of the free energy in powers of the fluctuating exchange field. The third-order term of the free energy renormalizes the mean field, and the fourth-order term, responsible for the interaction of the fluctuations, renormalizes the spin susceptibility. The extended theory is applied to the calculation of magnetic properties of Fe-Ni Invar.

Virtual excitation of a core-level hole as the origin of extended fine structure of Auger spectra: Theory and experiment

Abstract It has been shown that the contribution from the intermediate excited state of the system at the moment of the Auger transition causes the extended fine structure of secondary electron spectra, which extends several hundred electron volts above the main Auger line. Two types of oscillations are observed in the spectrum, due to the electron scattering on the atomic environment in the intermediate and final states. The comparison between Cu and Ni theoretical and experimental spectra was carried out.

A Fluctuating Field Theory for Systems of Localized Magnetic Moments

Thermodynamics of Heisenberg ferromagnets is described by means of a fluctuating exchange field operating on the atomic magnetic moments. A self-consistent method for calculating field characteristics is developed at arbitrary temperatures. It is shown that transitions in a paramagnetic state may pass both continuously and stepwise on temperature subject to magnitude of single-site susceptibility.

Theory of EXAFS and Comparison with Experimental Data

An analytical expression for an EXFAS oscillating part was obtained. Model spectra with taking into account of PDS oscillations of intermediate and final states were built up. The comparison between model and experimental oscilating parts were performed.

Beyond Gaussian approximation in the spin-fluctuation theory of metallic ferromagnetism

A characteristic feature of the Gaussian spin-fluctuation theory is the jump transition into the paramagnetic state. We eliminate the jump and obtain a continuous second-order phase transition by taking into account the high-order terms of the free energy of electrons in the fluctuating exchange field. The third-order term of the free energy yields a renormalization of the mean field, and fourth-order term, responsible for the interaction of the fluctuations, gives a renormalization of the spin susceptibility. The extended theory is applied to the calculation of magnetic properties of Fe-Ni Invar.

Spin Fluctuations in Disordered Metallic Ferrimagnetic Alloys

We propose a theory for the quantitative description of the thermodynamic, magnetic and electronic properties of disordered alloys of transition metals and normal, for example, the system Fe-Al, as obtained from melting, and in the form of metastable states produced by mechanical grinding. System of itinerant d-electrons with two constants of the intra-atomic exchange interaction is investigated by the fluctuating field approach. Fundamentally new feature of the system is the presence of several ferrimagnetic solutions, transitions between which give variety to the temperature dependence of the magnetic characteristics of disordered alloys. Results of calculation of the temperature dependences of the magnetic characteristics of iron aluminum alloys are considered as an application of the theory.

Extended Dynamic Spin-Fluctuation Theory with Application to Iron

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.