J. Mathon et A. Umerski, Theory of tunneling magnetoresistance in a junction with a nonmagnetic metallic interlayer, PHYS REV B, 60(2), 1999, pp. 1117-1121
It is demonstrated by numerical evaluation of the real-space Kubo formula t
hat the tunneling magnetoresistance (TMR) due to tunneling between two coba
lt electrodes separated by a vacuum gap remains nonzero when one of the ele
ctrodes is covered with a copper layer. This contradicts the classical theo
ry of tunneling that predicts zero TMR. It is shown that a nonzero TMR is d
ue to quantum well states in the Cu layer that do not participate in transp
ort. Since these only occur in the down-spin channel, their loss from trans
port creates a spin asymmetry of electrons tunneling from a Cu overlayer, i
.e., nonzero TMR. This mechanism could provide an explanation of the observ
ed nonzero TMR for junctions with Cu or Ag interlayers. A simple method for
modifying the classical theory of tunneling so that it can describe correc
tly tunneling in the presence of quantum well states is proposed and implem
ented for the Co junction with a Cu interlayer.