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Little is currently known about the effects of gamma-ray irradiation on oxide magnet materials. In particular, the effect of particle size on radiationsusceptibility was investigated. Two commercial powders of BaFe12O19 were thoroughly characterized, then exposed to 1 MGy of gamma radiation from a 60Co source. ACsusceptibility and DC magnetometry and Mossbauer spectroscopy were performed after irradiation and compared to pre-irradiated measurements. DC magnetization and ACsusceptibility decreased for both samples with the relative change of DC magnetization being larger for the micrometer-sized particles and the relative change of the ACsusceptibility being larger for the nanometer-sized particles. Mossbauer spectroscopy indicated a decrease in both the hyperfine fields and in their distribution for each Fe site, particularly in the larger particle sample. Decreases in susceptibility are believed to be due to radiation-induced amorphization at the particle surfaces as well as amorphization and nucleation of new crystallites at internal crystallite boundaries, resulting in overall reduction in the particle magnetic moment. This radiation damage mechanism is different than that seen in previous studies of neutron and heavy ion irradiation of BaFe12O19.

We examine the crystal structure and electrical and optical properties of ZnO epitaxial films grown by pulsed laser deposition in a H2 or D2 ambient. n-type electrical conductivity is enhanced by three orders of magnitude as a result of growing in H2 (D2) compared to ZnO films grown in O2. Hall effect measurements reveal very small carrier activation energies and carrier concentrations in the mid-1018 cm−3 range. Optical absorption measurements show that the enhanced conductivity is not a result of ZnO reduction and interstitial Zn formation. Photoluminescence spectra suggest excitonic emission associated with exciton-hydrogen donor complex formation and show no evidence for midgap emission resulting from defects. We have modeled the transport properties of H (D) doped ZnO films using variable range hopping and surface layer conductivity models, but our data do not fit well with these models. Rather, it appears that growth in H2 (D2) promotes the formation of an exceedingly shallow donor state not seen in...

Co-doped rutile TiO2 was synthesized by injecting Co ions into single crystal rutile TiO2 using high energy ion implantation. Microstructures of the implanted specimens were studied in detail using high-resolution transmission electron microscopy (HRTEM), energy dispersive x-ray spectroscopy, electron diffraction, and HRTEM image simulations. The spatial distribution and conglomeration behavior of the implanted Co ions, as well as the point defect distributions induced by ion implantation, show strong dependences on implantation conditions. Uniform distribution of Co ions in the rutile TiO2 lattice was obtained by implanting at 1075K with a Co ion fluence of 1.25×1016Co∕cm2. Implanting at 875K leads to the formation of Co metal clusters. The precipitated Co metal clusters and surrounding TiO2 matrix exhibit the orientation relationships Co⟨110⟩‖TiO2[001] and Co{111}‖TiO2(110). A structural model representing the interface between Co metal clusters and TiO2 is developed based on HRTEM imaging and image sim...

Cobalt-doped titanium dioxide, or CTO, has emerged in the past 2 years as a semiconducting, transparent, room-temperature ferromagnet. Very recently it has been shown that the magnetism in CTO often originates in surface nanoparticles or Co-rich regions that have a much-enhanced substitutional Co content up to 40% of Ti sites, so that magnetic CTO is not a true dilute magnetic semiconductor (DMS), but rather a fairly high-density spin system. In this work we describe a computational study of Co-rich CTO using the generalized gradient approximation to the density functional theory within the supercell model. Our total energy calculations show a strong tendency for Co-atom clustering or segregation on Ti sites. There is also a strong tendency for the oxygen vacancies to form complexes with the Co atoms. In addition, we find that the oxygen stoichiometry plays an essential role in determining the system’s magnetic order. The largest ordered moments require at least enough oxygen vacancies to put all of the C...

Pure anatase TiO2 and CoxTi1−xO2(0.01

Epitaxial ferromagnetic films of Cr-doped TiO2 anatase (CrxTi1−xO2−x∕2) were grown on LaAlO3(001) using oxygen-plasma-assisted molecular-beam epitaxy. CrK-shell x-ray absorption near-edge spectroscopy shows that the formal oxidation state of Cr is +3 throughout the films, with no evidence for either elemental Cr or half-metallic CrO2. Cr is found to substitute for Ti in the lattice, with uniform distribution throughout the doped region of the film. The Cr-doped anatase films exhibit room-temperature ferromagnetism aligned in-plane, with a saturation magnetization of ∼0.6μB/Cr atom.