| The magnetic and magneto-transport properties of one-dimensional nanowires, two-dimensional porous magnetic networks, and three-dimensional perovskite-type oxides are presented.; Arrays of multilayered nanowires, with wire diameter and layer thickness on the nanometer scale, have been successfully fabricated using a novel pulsed electrodeposition method. They not only have the characteristics of nanowires, but also those of multilayer thin films and granular solids, manifested in the magnetic shape anisotropy and the interlayer exchange coupling strength. Giant magnetoresistance (GMR) effect has been realized in a rare current-perpendicular-to-plane geometry. Spin disorder from both interlayer exchange coupling and uncorrelated magnetic moments has contributed to the GMR effect. The characteristic length scale in perpendicular spin transport, the spin-flip diffusion length, has also been determined.; Semimetallic bismuth (Bi) nanowires made by electrodeposition exhibit very large positive magnetoresistance (MR) with a quasi-linear field dependence. In contrast to the small MR effects in most Bi nanostructures previously reported, the large MR in Bi nanowires is due to the large grain sizes realized by the electrodeposition process. As the small nanowire dimensions effectively restrict the carrier mean free path, pronounced classical finite size effects have been observed. Quantum size effects due to the comparable nanowire diameter and the Fermi wavelength of Bi have also been evidenced.; A novel type of nanoporous magnetic networks has been realized by sputtering onto porous alumina membranes. As the dimensions of the networks, the thickness and the width, are both on the nanometer scale, domain wall formation and motion are impeded. Consequently, large enhancement of the coercivity and its dependence on the network thickness and width have been observed. Anisotropic magnetoresistance (AMR) effects have been suppressed in these magnetic networks, as the nanostructures of the networks have randomized the relative orientation of the sensing current and the magnetization direction.; Bulk perovskite-type oxides exhibiting colossal magnetoresistance (CMR) effects have been investigated. Strong lattice effects have been demonstrated in a series of rare earth (RE)-doped (RE)0.67Ca0.33MnO 3 samples. As the structure distortion increases with doping by the smaller size RE ions, the reduction in double exchange strength as a consequence leads to a higher resistivity, lower transition temperature, suppressed magnetism and larger CMR effect. The lattice effects have also been utilized to weaken the double exchange in a series of Nd1-xCaxMnO 3 samples, which displays a variety of phenomena due to the competing mechanisms against the double exchange. For example, charge ordering over a large composition range has been observed, accompanied by an abrupt increase in resistivity and a sudden change of the lattice parameters. In certain composition range, the magnetic field induces a metastable first-order transition from an AF insulator to a FM metal and reduces the resistance by several orders of magnitude. Finally, the lattice effects have been intentionally bypassed in the La1-xCaxMn1-yFeyO 3 samples to probe the effect of Fe doping on the Mn site. The replacement of the Mn3+ by the same size Fe3+ has effectively depopulated the hopping electrons participating in the double exchange. As a result, ferromagnetism and metallic conduction have both been suppressed. |
