Ferromagnetism and the temperature-dependent electronic structure in thin Hubbard films

The magnetic behavior of thin ferromagnetic itinerant-electron films is investigated within the strongly correlated single-band Hubbard model. For its approximate solution we apply a generalization of the modified alloy analogy (MAA) to deal with the modifications due to the reduced translational symmetry. The theory is based on exact results in the limit of strong Coulomb interaction which are important for a reliable description of ferromagnetism. Within the MAA the actual type of the alloy analogy is determined selfconsistently. The MAA allows, in particular, the investigation of quasiparticle lifetime effects in the paramagnetic as well as the ferromagnetic phase. For thin fcc(100) and fcc(111) films the layer magnetizations are discussed as a function of temperature as well as film thickness. The magnetization at the surface-layer is found to be reduced compared to the inner layers. This reduction is stronger in fcc(100) than in fcc(111) films. The magnetic behavior can be microscopically understood by means of the layer-dependent spectral density and the quasiparticle density of states. The quasiparticle lifetime that corresponds to the width of the quasiparticle peaks in the spectral density is found to be strongly spin- and temperature-dependent.