Anthony C. Nunes

Polarized neutron study of the magnetization density distribution within a CoFe2O4 colloidal particle II

Abstract We describe an X-ray and polarized neutron powder diffraction study of finely divided (diameter ≈ 100 A) CoFe 2 O 4 particles coated with oleic acid and naked. (4,0,0) line shapes, and neutron flipping ratio indicates that a disordered and magnetically anomalous shell approximately 25 A thick may be induced in the ferrite by the presence of the surfactant.

Experimental study of magnetic colloidal fluids behavior in power transformers

A newly developed colloidal magnetic fluid (MF) has been investigated for use as high-voltage insulation. Being a liquid magnet, MF interacts with the magnetic field of transformer windings and improves their cooling, while at the same time MF increases the lightning impulse withstand voltage and decreases the discharge propagation velocity, probably due to an enhanced electric field dissipation due to the presence of magnetite nanoparticles.

Fractionation of a water-based ferrofluid

Abstract Column chromatography of a water-based magnetic colloid produces fractions differing in their mean particle size. Particle size distributions of the fractions determined from electron micrographs are similar in shape to that of the original. Preliminary results of neutron scattering and magnetization measurements are presented.

Flux lattice behavior in high‐Tc materials studied by neutron depolarization

The depolarization of a neutron beam passing through a sample of the high‐Tc superconductor YBa2Cu3O7−δ has been measured as a function of temperature and applied field. The difference in behavior between field‐cooled and zero‐field‐cooled states, the observation of hysteresis correlated with Hc1, and the disappearance of the effect near 55 K (below Tc) suggest an explanation in terms of vortex line lattice formation with possible connection to recently proposed flux line entanglement and melting.

Calculated ferrite nanocrystal relaxation and its magnetic implications

Electrostatically driven relaxation and resulting strain-induced magnetic anisotropy may contribute significantly to the magnetic properties of colloidal preparations of ferrite nanocrystals. Preliminary molecular dynamic calculations are described which predict an expansion of a few per cent in a nanocrystal relative to the bulk. A linear extrapolation of strain-induced magnetic anisotropy suggests that such an expansion may give rise to a stress-effective field comparable to the magnetic anomalies observed in these systems.

Size and quality dispersion of a single preparation of colloidal magnetic particles

Abstract Using a variety of techniques (including electron microscopy, neutron and X-ray scattering and magnetization measurements) we have characterized fractions of a single preparation of water-based magnetic colloid. The fractions, produced by column chromatography, not only vary in size, as expected, but also in quality as indicated by a reduced saturation moment density and increased polycrystallinity in the larger particles. This result emphasizes the necessity of careful characterization of such samples and suggests an alternate explanation of the magnetic ‘dead layer’ or dual regions reported in some earlier papers.

Neutron depolarization effects in a high‐Tc superconductor (abstract)

Using the polarized beam small‐angle neutron scattering spectrometer at the Rhode Island Nuclear Science Center Reactor, we have observed significant depolarization of a neutron beam by passage through polycrystalline high‐Tc superconductors, specifically 123 Y‐Ba‐Cu‐O prepared and characterized at the IBM Watson Research Center. We believe that this technique will prove useful in studying aspects of these materials, such as the penetration depth of shielding currents, the presence and structure of trapped flux vortices, and grain size effects on the supercurrent distribution in polycrystalline samples. The two samples showed sharp transitions at 87 and 89 K, and have been studied at temperatures of 77 K; the second sample has also been studied at 4 K. The transition to the superconducting state was monitored by the shift in resonant frequency of a coil surrounding the sample. No measurable depolarization was observed in either sample at 77 K in both the field‐cooled and zero‐field‐cooled states, using ap...

High‐resolution transmission electron microscopy and x‐ray study of colloidal cobalt ferrite particles

High‐resolution transmission electron micrographs, including high‐resolution structure images, and x‐ray powder diffraction patterns have been observed of oleic acid‐coated colloidal particles of cobalt ferrite. Particles were prepared by grinding. Particle defect content, size distribution and aspect ratio, and crystallite size and lattice constant were determined. Particles are irregularly shaped of log normally distributed dimension peaking near 5 nm and with an average aspect ratio of 1.3. They appear to be single crystals with few volume defects. Their lattice constant (0.8393±0.0003 nm) is indistinguishable from that of the bulk material. This supports earlier assertions that the anomalous magnetic behavior of these particles originates at their surface or the ferrite‐surfactant interface.

Neutron depolarization study of a high-Tc epitaxial thin film

Abstract Neutron depolarization measurements on an epitaxial thin film of YBaCuO superconducting material are reported and interpreted on the basis of formation of an Abrikosov flux lattice by field cooling. The temperature and field dependences of this effect have been measured and suggest that the technique may be useful in obtaining information on flux lattice stability and pinning forces.

Neutron measurements on an epitaxial thin film of YBaCuO

We report evidence for an ordered flux line lattice in a 2‐μm epitaxial film of YBaCuO from both neutron depolarization and diffraction measurements. Depolarization occurred when field cooling was done normal to the film plane but not within it; this is consistent with frozen‐in vortices. Evidence for ordering of the flux lines comes from intensity maxima observed in difference patterns between field cooled and zero field cooled conditions.