Magnetic properties of impurities and impurity pairs in magnetic multilayers

Spin-dependent scattering is considered to be the origin of giant magnetoresistance in magnetic multilayers, although the role of bulk or interface defects is still under discussion. We present calculations of spin-dependent impurity scattering potentials as a function of the impurity position in a Co/Cu-multilayer. The calculations are performed within spin density functional theory using a Green function method (the tight binding Korringa±Kohn±Rostoker method). It is shown that the impurity potential depends strongly on the impurity position, which is re ̄ected in local densities of states and local impurity moments. Furthermore, calculations of magnetic properties of impurity pairs at the interface are presented and compared with results for single impurities. § 1. Method of calculation We present ab initio calculations of self -consistent impurity potentials in magnetic multilayers. Our calculations are based on spin density functional theory within a Green function formalism (Zeller and Dederichs 1979, Podloucky et al. 1980). We start from the solution of the Kohn±Sham equations [E + Ñ V s (r) ]G s (r, r  ; E ) = d (r r  ) (1) with the spin-dependent one particle potential V s (r) to calculate the Green function of the translationally invariant multilayer G Ê s (r, r  ; E ) . This calculations are performed using the advantages of a tight binding (TB) Korringa±Kohn±Rostoker (KKR) scheme (Zeller et al. 1995, Zahn et al. 1997). Since the method scales linearly with the number of atoms N in the unit cell we are able to investigate large supercells. Accordingly, we have chosen a multilayer geometry in the so-called ®rst antiferromagnetic maximum of interlayer exchange coupling (IEC) consisting of 9 monolayers (ML) of Co separated by 7ML of Cu in the (100) direction, denoted as Co9Cu7 . With the introduction of defects in the system the translational invariance is broken. Consequently, the defect Green function G s (r, r  ; E ) has to be calculated by solving a Dyson equation: G s (r, r  ; E ) = G s (r, r  ; E ) + dr  G Ê s (r, r  ; E )DV s (r  )G s (r  , r  ; E ) , (2) 0141±8637/98 $12.00 Ñ 1998 Taylor & Francis Ltd. where DV s (r  ) is the potential di€erence caused by the defect. In our case we concentrate on impurity atoms and impurity pairs. Charge relaxation is allowed up to second nearest neighbours. Starting from the Green functions G Ê s (r, r  ; E ) or G (r, r  ; E ) the local spindependent densities of states (LDOS) nsi (E ) are given by the imaginary part of the diagonal elements of the corresponding Green function, e.g.