Temperature-dependent quasiparticle bandstructure of ferromagnetic gadolinium

By a combination of a many-body evaluation of the s-f model (Kondo-lattice model) with an 'ab initio' bandstructure calculation the temperature-dependent electronic quasiparticle structure of the 'local moment' ferromagnet Gd is derived. The s-f model incorporates the exchange interaction between itinerant (5d, 6s) conduction electrons and localized 4f electrons which form the permanent magnetic moments. The s-f model possesses an instructive special case, concerning a ferromagnetically saturated semiconductor, that can be treated rigorously revealing thereby the basic excitation processes and correlation effects induced by the s-f exchange interaction, The model evaluation is finally combined with an ASW bandstructure calculation to get the temperature-dependent quasiparticle bandstructure and the spectral density of Gd. Depending on the effective exchange coupling J/W (J: exchange constant; W bandwidth) strikingly different behaviour of the induced exchange splitting of the (5d, 6s) band states is observed. In the weak coupling case a 'Stoner-like' spin splitting of the band states proportional to the f-magnetization occurs, while for stronger exchange an additional splitting of the quasiparticle dispersions for each spin direction happens, that even persists in the paramagnetic phase (T > T-c). The depolarization for T --> T-c is then no longer due to a 'Stoner shift' but rather to a redistribution of spectral weight between two quasiparticle peaks. Extremely complicated behaviour is found for intermediate couplings J/W that requires a careful interpretation of respective photoemission data. (C) 1999 Elsevier Science B.V. All rights reserved.