This model is not the same as the Stoner Model of itinerant ferromagnetism. In the Stoner model, a high density of states at the Fermi level makes the nonmagnetic state unstable. With SP calculations on covalent ferromagnets, the exchange-correlation integral in the LSDA-DFT takes the place of the Stoner parameter. The density of states at the Fermi level does not play a special role. A significant advantage of the half-metallic model is that it does not rely on the presence of mixed-valency as does the double exchange mechanism and it can therefore explain the observation of CMR in stoichiometric phases like the pyrochlore Tl2Mn2O7. Microstructural effects have also been investigated for polycrystalline samples and it has been found that the magnetoresistance is often dominated by the tunneling of spin polarized electrons between grains, giving rise to an intrinsic grain-size dependence to the magnetoresistance.