Magnetism, Transport and Superconductivity in Systems of Reduced Dimension

The magnetic, transport and superconducting properties of a number of reduced dimensional systems are presented. Metallic granular solids, with nanometer-sized magnetic metal granules embedded in a host metal matrix, have been prepared using magnetron sputter deposition. The phase-separation of metastable alloys and the resultant granular structure have been examined for many systems. By choosing appropriate processing conditions, the granule size and separation can be controlled. The magnetic properties of these granular solids below percolation show behavior characteristic of single domain particles. The magneto-transport properties of metallic granular solids have been extensively studied. Giant Magnetoresistance (GMR) effect has been observed and its dependence on magnetism, particle size, temperature characterized for many systems. It has been found that GMR originates from the spin-disorder scattering of the conduction electrons by the single-domain magnetic particles. A model is developed to qualitatively account for the experimental results. Further insights into the physical origin of the GMR effect in metallic granular solids have been provided by Hall effect and thermal transport studies. The results suggest that interfacial spin disorder scattering are responsible for GMR effect, the dominant electron scattering events are mostly elastic and the inelastic scattering processes are incoherent, and the scattering is of intermediate range. Using electro-deposition, arrays of ferromagnetic Co and Ni nanowires have been fabricated in porous polycarbonate membrane prepared from nuclear track etching. Detailed structural characterization has been carried out. Magnetic measurements of these materials show very large perpendicular magnetic anisotropy. The easy magnetization direction of a film containing the nanowires is perpendicular to the film as the result of the unique geometry. The magnetic properties of nanowire arrays of different diameter have also been examined. The enhanced coercivity and high remanence suggest these materials might be a new medium for high -density recording applications. In the area of layered structures, the effects of finite size and interfaces on the magnetism and magnetization of magnetic multilayers have been studied. It has been found that magnetization of rare-earth Gd due to polarized conduction electrons is affected when layered with non -magnetic W. The reduction of the ferromagnetic ordering temperature for ultra-thin Gd layers agrees with the predictions of the finite-size scaling theory. Finally, the effect of ferromagnetism on superconductivity has been investigated. The measurements of the superconducting transitions in multilayers and trilayers show an oscillatory dependence of the transition temperature on the ferromagnetic layer thickness. This is in support of early theoretical predictions of "pi-phase" coupling, where the superconducting order parameter tunnels through the ferromagnetic layer and obtains a phase shift of pi in going from one superconductor to the other.