Electronic transport and magnetoresistance in polycrystalline and epitaxial CrO2 nanowires

We have studied and compared the electrical and magnetic behavior of sub-micron-sized polycrystalline and epitaxial chromium dioxide (CrO2) wires, grown using a selective-area growth technique. Low-temperature transport measurements have shown that the dc resistivity of polycrystalline CrO2 wires is strongly dependent on the linewidth. Below a critical temperature, a transition from a positive to a negative temperature coefficient of resistivity is observed, which we attribute to a competition between the scattering of the conduction electrons inside the grains and scattering across the grain boundaries. Using a simple model based on grain boundary scattering, we can separate and quantify each type of scattering, and based on this, we estimate a mean transmission probability through the grain boundaries to be on the order of 10(-1). Unlike polycrystalline wires, epitaxial CrO2 wires behave in a highly metallic fashion, and a clear width-dependence is observed in the resistivity data for wires aligned along the b axis, but not for those aligned along the c axis. Furthermore, low-field magnetoresistance (MR) values have been measured as a function of magnetic fields applied both longitudinally and transversely. The results indicate that the MR behavior of polycrystalline CrO2 wires is dominated by the shape anisotropy; however, for epitaxial CrO2 wires, both the shape and magnetocrystalline anisotropy play important roles, and the resulting MR properties are found to be closely related to the orientation of the wire axis. By studying the MR curves, we inferred the internal magnetic domain structures in various single crystal CrO2 wires and found that the spin-dependent transport is much stronger across a grain boundary than a magnetic domain wall.