Theory of tunneling magnetoresistance in a junction with a nonmagnetic metallic interlayer

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.