| Fabrication includes deposition of Co, Ni, Fe, and Ni0.8Fe 0.2 by UHV magnetron sputtering and thermal/electron-beam evaporation to form AMR (Anisotropic Magneto-Resistance) thin films and multilayer structures displaying GMR (Giant Magneto-Resistance). Along with ferromagnetic transitional metals and their alloys, Ag, Cu, and Ta were used as either spacer or buffer layers. Structures were patterned by photolithography and advanced electron-beam lithography using lift-off or etching methods and resulting minimal feature sizes from 20μm down to 50nm. The patterned structures were physically characterized by AFM (Atomic Force Microscopy), SEM (Scanning Electron Microscope), and TEM (Transmission Electron Microscope). Other characterizations include SQUID (Super-conducting Quantum Interference Device) magnetometry and AC/DC (Alternating-Current/Direct-Current) transport measurements. Magnetization and MR measurements show a geometrical dependence on width of the wires. Specifically, Fe(50Å)/Co(50Å)/Cu(50Å)/Co(50Å)/Cu(20Å) coercive field increases from 110Oe for 2.2μm structure to 170Oe for 0.8μm width wire. The field of maximum MR increases from 158Oe for 2μm structure to 318Oe for 0.5μm structure. Magnetization loops show widening as coercivity increases with decrease in wire widths. The MR responses show a drastic increase for the saturation values to be reached also with a decrease in wire widths. To produce structures smaller than possible using photolithography, electron-beam lithography was utilized with a bi-level resist of PMMA and co-polymer. An over-hang structure was developed in the bilayer resist that minimizes interaction with the deposited material. Using a physical collimation mask, nanowire structures with integrated leads were produced by sputtering. Resistance vs. temperature data suggest highly disordered wires. Evaporated Ni wires show better metallic transport properties than direct sputtered wires. Although AFM images of sputter deposited nanowires show smooth physical structures, evaporated wires display a series of island-like structures. Resistivity and MR characteristics are further improved by using lift-off to pattern Nb as an etch mask on top of sputter deposited Ni thin films. Using this process, the ratio of the residual resistivity of nanowire structures to room temperature resistivity is 0.298 as compared to 0.748 for electron-beam deposited nanowire produced by lift-off. |
