Advanced Ceramics and Nanocomposites of Half-metallic Ferromagnetic CrO2 for Magnetic, GMR and Optical Sensors

The physical properties of metal oxides are diverse, including semiconductors or insulators (ZnO, ZrO2, TiO2), good metals (RuO2), and metal-insulator systems (VO2) in terms of the electron band structure and variation of electrical resistivity (σ) as a function of temperature (T), i.e., thermal coefficient of σ, expressed as In terms of distributing the magnetic spins (of total value S) in the metal cations via O2− anions, a metal oxide behaves to be paramagnetic (C2O3), S ≠0, or diamagnetic (YBa2Cu3O7 and other ceramic superconductors1–5) with S=06–8 Depending on the relative strength of the spin-ordering over thermal effects, a paramagnetic oxide often behaves as a ferrimagnet (γ-Fe2O3 or Fe3O4) or antiferromagnet (FeO or MnO2). Metallic CrO2, with a Curie temperature T C =390 K, is the only ferromagnet in this class. Schwarz9 used local-spin-density-approximation (LSDA) band theory to predict that the S-moment would be the full 2µB required by Hund’s rules for Cr4+ (3d2) state in CrO2. The Fermi levelE F lies in a partly filled (metallic) band for the majority (up-spin)electrons, but for minority (down) spins lies in a semiconductor like energy bandgap aqueous precursor solution of CrO3 or in general a Cr6+ -compound. In the proposed polymer precursor method, the Cr6+ cations ultimately are reduced to Cr4+ cations as soon as admixing to a polymer, whichreacts with Cr6+ to form a metal ion-polymer complex capping in part of the polymer molecules.