Magnetic domain configurations and huge wall resistivity in half-metallic chromium dioxide nanostructures

We have fabricated, studied and compared the electrical and magnetic behavior of several sub-micron-sized polycrystalline and epitaxial chromium dioxide (CrO2) nanostructures, grown using selective-area growth technique. Magnetic domain structures were studied by magnetic force microscopy, and in-plane, lamellar domain structure with fragmented walls aligned along the magnetic easy axis direction have been observed, indicating the existence of a large magnetocrystalline anisotropy in epitaxial CrO2 nanostructures. Low-temperature transport measurements on nanowires 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 have been 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 model based on grain boundary scattering, we estimate a mean transmission probability through the grain boundaries to be on the order of 10−1 . Furthermore, magnetoresistance (MR) measurement indicates 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. We have also studied the magnetotransport properties of CrO2 nanoscale continuous contacts. Manipulating the domain walls using a large dc current in the contact area yields a magnetoresistance of up to 25%, which is the largest ever seen in a single ferromagnetic film. The single domain-wall-resistance (DWR) is determined to be three orders of magnitude larger than that of conventional 3d ferromagnets, as a result of the material's half-metallicity. We have measured DWR and the spin-torque effect along different crystallographic axes and at varying temperatures. Finally, we present the results of a theoretical analysis of this system, based on its half-metallic character and on the intrinsic magnetic behavior of CrO2