Atomic-scale structure of copper/cobalt multilayers and cobalt/alumina/cobalt magnetic tunnel junctions

Magnetic multilayers consist of alternating ferromagnetic and nonmagnetic metallic layers, each layer approximately 10 to 50 Å thick. Magnetic tunnel junctions are made of two ferromagnetic electrodes separated by an insulating layer approximately 10 Å thick. When an applied field switches the relative magnetization of adjacent magnetic layers, both structures can exhibit large changes in electrical resistance (up to several tens of percentage points).; The microscopic origins of large magnetoresistance phenomena in such devices are not well understood. In magnetic multilayers, it is believed that spin-dependent scattering occurs mainly at interfaces, and that knowledge of interface structure can lead to understanding of magnetoresistance behavior. For magnetic tunnel junctions, magnetoresistance is highly sensitive to barrier structure. This indicates that spin-polarization of the tunneling current is determined by atomic-scale structure of the barrier, which is not well known.; A scanning transmission electron microscope (STEM) was used to probe both physical and electronic structure on an atomic scale. The STEM focussed high-energy electrons to a 2 Å diameter beam that was transmitted through thin cross-sectioned samples. The electron energy loss spectrum of the transmitted beam provided information on the local concentration of atomic species as well as the local density of states partitioned by site, element and angular momentum.; STEM analysis of copper/cobalt multilayers showed that the interfaces are chemically sharp and have a large amplitude, short-wavelength ($\ll$250 Å) roughness. The short-wavelength roughness indicates a high density of spin-dependent scattering potentials, which are predicted to generate or at least contribute to the large magnetoresistance effect.; Cobalt/alumina/cobalt magnetic tunnel junctions have an amorphous alumina barrier with an extensive conduction band tail due to disorder. The conduction band tail lowers the average barrier height and may reduce spin-polarization by allowing coupling of states through the barrier. Both barrier edges are aluminum terminated and the interfacial aluminum is metallic. Strong sp-d hybridization observed between interfacial cobalt and aluminum atoms may explain why the spin-polarization of electrons tunneling from cobalt is positive despite a negative polarization in the density of states of cobalt at the Fermi energy. Further studies promise a more complete understanding of how barrier structure determines spin-polarization of the tunneling current.