Regensburg 2002 – wissenschaftliches Programm

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MA: Magnetismus

MA 19: Spinstrukturen und magnetische Phasenübergänge (incl. Sensoren(19.11) und Messmethoden(19.12-13))

MA 19.1: Vortrag

Donnerstag, 14. März 2002, 15:15–15:30, H23

RPA-CPA theory for magnetism in disordered Heisenberg systems with long range exchange integrals. — •Georges Bouzerar and Patrick Bruno — Max-Planck Institut für Mikrostrukturphysik Weinberg 2, D-06120 Halle Germany

We present a theory based on Green’s function formalism, to study magnetism in disordered Heisenberg systems with long range exchange integrals. It consists in (i) decoupling the higher order disordered Green’s function within Tyablicov scheme and (ii) treating simultaneously and self-consistently RPA and CPA loops. The CPA method is an extension of the Blackmann-Esterling-Beck approach to the case of Goldstone systems (presence of environmental disorder). The summation over the non-crossing diagrams is performed using the cumulant method. It is shown that the contribution to the self-energy coming from the s-scattering can always be performed analytically. However the contribution of the p,d,f.. scattering is not an easy task in the case of long-range exchange integrals. To overcome this difficulty we provide a simplified treatment of the higher order scattering terms. In the case of nearest-neighbor Heisenberg model, the comparison between the full CPA treatment and the approximation shows a very good agreement. Furthermore we show that even at low concentration the Virtual Crystal Approximation (VCA) is wrong. Our theory allows to calculate the magnetization of each ion as function of Temperature , the Curie Temperature and Spectral weights. It is expected that our method combined with first-principle calculations which would provide the input parameters (exchange integrals) should be an efficient tool to study magnetism in disordered systems.