Index of content:
Volume 87, Issue 11, 01 June 2000
- MAGNETISM AND SUPERCONDUCTIVITY (PACS 74-76)
87(2000); http://dx.doi.org/10.1063/1.373487View Description Hide Description
Pure ultrafine particles have been obtained from mechanosynthesis of the ZnO and oxides. The average grain diameter was estimated from x-ray diffraction to be Refinement of neutron diffraction data showed that the resulting cubic spinel structure is oxygen deficient, with ∼7% of ions occupying the tetrahedral A sites. Magnetization curves taken at 4.2 K showed the absence of saturation in fields up to associated with a spin-canting produced by the milling process. Field-cooled (FC) and zero-field cooled (ZFC) curves showed irreversible behavior extending well above room temperature, which is associated with spin disorder. Annealing samples at 300 °C yields an average grain size and ∼16% of ions at A sites. Partial oxygen recovery is also deduced from neutron data refinement in annealed samples. Concurrently, decrease of magnetic irreversibility is noticed, and assigned to partial recovery of the collinear spin structure. Complex Mössbauer spectra were observed at room temperature and 80 K, with broad hyperfine field distributions spanning from ∼10 to ∼40 T. At hyperfine field distributions indicate high disorder in Fe local environments. The above data suggest the existence of Fe-rich clusters, yielding strong superexchange interactions between Fe ions at A and B sites of the spinel structure.
87(2000); http://dx.doi.org/10.1063/1.373488View Description Hide Description
A nanocompositematerial has been characterized that contains nanometer size magnets that are free to rotate in response to an applied magnetic field. The composite consists of 5–10 nm crystals of dispersed in a solid methanol polymer matrix. The material was prepared by freezing a methanol-based ferrofluid of and subjecting it to a magnetic field applied in alternate directions to anneal the matrix. Before the field treatment, the solid displays magnetic behavior characteristic of an ordinary nanoscopic magnetic material. It is superparamagnetic above the blocking temperature (160 K) and hysteretic below, showing magnetic remanence and coercivity. After the field treatment to anneal the matrix, the same solid shows only Curie–Weiss behavior above and below the blocking temperature over the temperature range from 4.2 to 200 K and in response to applied magnetic fields as low as 1.59 kA/m. The data are consistent with a solid containing rotationally free, nanoscopic magnets encased in cavities of very small dimensions. The free rotation of the particles precludes the observation of magnetic relaxation phenomena that are characteristic of magnetic solids and ferrofluids. The present solid portends a class of magnetic materials with very little or no electrical and magnetic loss.
87(2000); http://dx.doi.org/10.1063/1.373489View Description Hide Description
For a number of years now, siloxanes have been the materials of choice for coating vessels used in the production and short-term storage of hyperpolarized xenon. The methods used to apply this material, however, often vary from one research group to another and it is commonly reported that it is difficult to obtain cells with consistently long spin-lattice relaxation times and high-polarization levels. In a series of controlled experiments individual production variables were altered and optimized, leading to improved protocols for the reliable production of high-quality siloxane-coated cells. During these studies we discovered that the surface-induced relaxation rates in bare and coated Pyrex cells differ profoundly. This information on Xe relaxation helps to define the limits on the way pumping cells can be improved and suggests the need for further fundamental work on relaxation mechanisms.