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

It is demonstrated by numerical evaluation of the real-space Kubo formula that the tunneling magnetoresistance (TMR) due to tunneling between two cobalt electrodes separated by a vacuum gap remains nonzero when one of the electrodes is covered with a copper layer. This contradicts the classical theory of tunneling that predicts zero TMR. It is shown that a nonzero TMR is due to quantum well states in the Cu layer that do not participate in transport. Since these only occur in the down-spin channel, their loss from transport creates a spin asymmetry of electrons tunneling from a Cu overlayer, i.e., nonzero TMR. This mechanism could provide an explanation of the observed nonzero TMR for junctions with Cu or Ag interlayers. A simple method for modifying the classical theory of tunneling so that it can describe correctly tunneling in the presence of quantum well states is proposed and implemented for the Co junction with a Cu interlayer.