Ferromagnetismus in schmalen Energiebändern
The method of spectral moments is applied to the Hubbard-model for six various densities of states. We find for each density of states a criterion for ferromagnetism as a function of the electron concentrationn. The most surprising result may be that sometimes there are two distinct ferromagnetic solutions, a fact which was pointed out also by Meyer and Schweitzer using the Roth-method. Looking for the reason of the possibility of magnetic ordering we discuss the behaviour of some important physical quantities (energy, magnetization, spin dependent band shift, band width and Fermi-energy).