Superconducting, magnetic and transport properties of several transition metal systems

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Stampe, Patricia A.
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Detailed measurements of the field and temperature dependent ac susceptibility and resistivity have been made for several bond-disordered systems. Analysis of the susceptibility data shows the influence of disorder and spin-orbit induced anisotropy. Model calculations of the temperature dependence of the resistive anisotropy measured in low magnetic fields predict that the anisotropy should decrease linearly with increasing temperature, vanishing at T$\sb{c}.$ The slope of this decrease is predicted to vary inversely with the degree of disorder present in the system. These calculations are compared to measurements of the anisotropy in these systems. Similar measurements have been done on a series of 16 CuNi alloys, with concentrations between 45 and 55 at.% Ni, before and after aging. As the nickel concentration increases this system develops a ferromagnetic ground state at a critical Ni concentration, $x\sb0.$ AC susceptibilty and resistivity data show a decrease in T$\sb{c}$ with decreasing Ni concentration. Detailed studies of the magnetoresistance as a function of concentration allow an accurate determination of $x\sb0$ from experimental data, and indicate a power-law dependence of spontaneous resistive anisotropy (SRA) on reduced concentration, as noticed previously near $x\sb0$ in Pd-based alloys. The SRA is essentially the difference in resistivity with a magnetic field applied parallel and perpendicular to the current, extrapolated to zero field. Magnetic and tranpsort measurements were done on YNi$\sb2$B$\sb2$C superconductors with partial Fe substitution at the Ni site. The strong dependence of superconducting properties on material preparation technique suggests the presence of a defect structure which enhances the critical current density in the superconducting state. The depression of T$\sb{c}$ caused by Fe-doping is fitted to models of magnetic and non-magnetic scattering to ascertain the magnetic state of Fe in this material. Finally, detailed studies of magnetisation and magnetoresistance were done in naturally layered Ce(Fe$\sb{1-x}$Ru$\sb{x})\sb2$ and Gd$\sb2$In in the vicinity of their antiferromagnetic to ferromagnetic phase transition. The suppression of the antiferromagentic state by applied fields causes a giant magnetoresistance (GMR) in these alloys. Attempts are made to correlate the behaviour of the magnetisation and magnetoresistance using a model successful in the interpretation of GMR in granular systems.