Growth and magnetotransport study of thin ferromagnetic CrO2 films

Highly a-axis-textured CrO2 films have been deposited on Al2O3(0001) and on isostructural TiO2(100) substrates by a chemical vapour deposition technique. For Al2O3 substrates a columnar growth of CrO2(010) on an initial Cr2O3(0001) layer has been found in transmission electron microscopy as well as in x-ray diffraction investigations. The sixfold in-plane symmetry of a (0001)-oriented Cr2O3 initial layer leads to three equivalent in-plane orientations of the CrO2 unit cell as confirmed by electron diffraction and scanning electron microscopy. The growth can be understood by a simple model of the in-plane symmetries of the Al2O3(0001), Cr2O3(0001), and CrO2(010) lattices. The growth on TiO2(100) substrates leads to (100)-oriented CrO2 films of higher crystalline quality than the ones grown on Al2O3(0001). Transmission electron microscope images show growth of CrO2(100) directly on the TiO2(100) substrates and no significant Cr2O3 inclusions within the CrO2(100) layer. All contributions to the magnetoresistance (MR) due to anisotropic MR, Lorentz MR, spin disorder, and intergrain tunnelling MR have been determined and partly correlated with the crystalline properties of the samples investigated. For films of both types the intrinsic linear contribution to the high-field MR does not depend on the crystalline quality of the films and supports the suggested intrinsic double-exchange mechanism for CrO2.