Coherent tunneling in ferromagnetic planar junctions: Role of thin layers at the barriers

We study theoretically coherent electron tunneling in single-barrier FM/NM/I/NM/FM and NM/FM/I/FM/NM systems, where FM (NM) denotes a ferromagnetic (nonmagnetic) metal and I stands for an insulating nonmagnetic barrier. The metallic films on both sides of the barrier are thin enough, so that partial confinement of electron states plays a significant role. Two different cases are analyzed in detail: (i) the case where the thickness of one of the thin films is constant while that of the second film is varied; and (ii) the case where both thin films are equally thick. The junction resistance depends on the relative orientation of magnetic moments of the ferromagnetic electrodes in FM/NM/I/NM/FM systems and on the relative orientation of magnetic moments of the thin ferromagnetic films in NM/FM/I/FM/NM systems. Tunneling current and magnetoresistance, calculated as a function of the thickness of thin films, show pronounced peaks related to resonant-type tunneling. The variation of the magnetoresistance with bias voltage is also studied and this variation is generally nonmonotonous. (C) 2000 American Institute of Physics. [S0021-8979(00)01722-9].