Spin-filter effect of the europium chalcogenides: An exactly solved   many-body model
  A model Hamiltonian is introduced which considers the main features of the experimental spin filter situation as s-f interaction, planar geometry and the strong external electric field. The proposed many-body model can be solved analytically and exactly using Green functions.   The spin polarization of the field-emitted electrons is expressed in terms of spin-flip probabilities, which on their part are put down to the exactly known dynamic quantities of the system.   The calculated electron spin polarization shows remarkable dependencies on the electron velocity perpendicular to the emitting plane and the strength of s-f coupling. Experimentally observed polarization values of about 90% are well understood within the framework of the proposed model. 
Robert Metzke and Wolfgang Nolting