General Green's-function formalism for transport calculations with spd Hamiltonians and giant magnetoresistance in Co- and Ni-based magnetic multilayers
S. Sanvito et al., General Green's-function formalism for transport calculations with spd Hamiltonians and giant magnetoresistance in Co- and Ni-based magnetic multilayers, PHYS REV B, 59(18), 1999, pp. 11936-11948
A general Green's-function technique for elastic spin-dependent transport c
alculations is presented, which (i) scales linearly with system size and (i
i) allows straightforward application to general tight-binding Hamiltonians
(spd in the present work). The method is applied to studies of conductance
and giant magnetoresistance (GMR) of magnetic multilayers in current perpe
ndicular to planes geometry in the limit of large coherence length. The mag
netic materials considered are Co and Ni, with various nonmagnetic material
s from the 3d, 4d, and 5d transition metal series. Realistic tight-binding
models for them have been constructed with the use of density functional ca
lculations. We have identified three qualitatively different cases which de
pend on whether or not the bands (densities of states) of a nonmagnetic met
al (i) form an almost perfect match with one of spin subbands of the magnet
ic metal las in Cu/Co spin valves), (ii) have almost pure sp character at t
he Fermi level (e.g.; Ag), and (iii) have almost pure d character at the Fe
rmi energy (e.g., Pd, Pt). The key parameters which give rise to a large GM
R ratio turn out to be (i) a strong spin polarization of the magnetic metal
, (ii) a large energy offset between the conduction band of the nonmagnetic
metal and one of spin subbands of the magnetic metal, and (iii) strong int
erband scattering in one of spin subbands of a magnetic metal. The present
results show that GMR oscillates with variation of the thickness of either
nonmagnetic or magnetic layers, as observed experimentally. [S0163-1829(99)
03118-5].