| In the new class of materials known as half-metallic ferromagnets, all of the conduction electrons have the same spin at low temperature. This high spin polarization makes half-metals promising candidates for magnetoelectronic applications.; In this work we study the Heusler alloy family of compounds Co2 Cr1-xFe xAl, predicted to be half-metallic for low Fe concentrations. Using DC magnetron sputtering, we have grown for the first time epitaxial thin films of these compounds. Basic characterization shows that the films crystallize in the B2 crystal structure, and that their magnetic and transport properties vary significantly with Fe concentration.; We incorporate these thin films into epitaxial superlattices with Cr and simple spin valve trilayers with Cu and Co90Fe10. Though we do not observe evidence for antiferromagnetic coupling in the superlattices, we find large giant magnetoresistances in the spin valves of up to 7% for intermediate Fe concentrations, the highest to date for any material predicted to be half-metallic. We then measure the spin polarization of CO2Cr 0.6Fe0.4Al with point contact Andreev reflection spectroscopy, finding a value of 50%, lower than the 100% expected for a half-metal.; We use x-ray magnetic circular dichroism to investigate the deficit in the saturation magnetic moments as compared to theory for films with low Fe concentrations. We develop a modified sum rule analysis for the extraction of the spin and orbital moments of Cr, Fe, and Co from the dichroism spectra. The average Cr spin moment has a value approximately 10% of the theoretical value; features of the Cr dichroism spectra suggest that the films may be composed of ordered B2 regions and disordered regions.; We use tight binding theory to calculate the minority spin energy bands, finding a dominance of Co d states near the energy gap. This result, supported by more accurate calculations, suggests that the half-metallicity of these compounds may be especially sensitive to disorder involving Co. Using TEM microscopy, we find that samples with low Fe concentrations contain a second phase. Meanwhile, anomalous x-ray diffraction of Co2FeAl shows atomic disorder involving Co of approximately 10%. Overall, the results suggest that controlling disorder in these films could lead to a highly spin polarized material suitable for magnetoelectronic applications. |
