FERROMAGNETISM IN THE HUBBARD-MODEL - INFLUENCE OF THE LATTICE STRUCTURE

By use of the spectral density approach the influence of the lattice s tructure on the possibility of ferromagnetism in the single-band Hubba rd model is investigated. The d = infinity hypercubic lattice does not show magnetic phase transitions of second order irrespective of the s trength of the Coulomb coupling. However, first order transitions to f inite magnetic moments, not visible as singularities of the paramagnet ic susceptibility, may appear in the very strong coupling regime. In d =3 second order transitions are found but only for very strong couplin gs, where the nonlocality of the electronic self energy acts in favour of the spontaneous magnetic moment. The influence of the non-local pa rt of the self energy is particularly strong for lattices with small c oordination number. The non-bipartite fee lattice exhibits saturated f erromagnetism for all band occupations 1 less than or equal to n less than or equal to 2 while for less than half filled bands (0 less than or equal to n less than or equal to 1) the system remains in any case paramagnetic, and that for d = 3 as well as d = infinity. The Curie te mperature runs through a maximum at about n = 1.4 and vanishes for n - -> 1 and n --> 2. (C) 1997 Elsevier Science Ltd.