Notes: The contribution of shape anisotropy to the total anisotropy of an antiferromagnetic particle is shown to be negligible compared with the contribution from the magnetocrystalline anisotropy of the material. It follows that the superparamagnetic behavior of small antiferromagnetic particles is independent of the particle shape. © 1995 American Institute of Physics.

Notes: We have investigated the effect of ion modification of multilayer growth and ion etching of multilayer interfaces on perpendicular anisotropy and giant magnetoresistance in sputter deposited Co/Pt and Co/Cu multilayers. These two properties are thought to be particularly sensitive to the form of the interface profile in multilayers. We find that the ion-assisted deposition conditions used degrade both perpendicular anisotropy and GMR through interface smoothing and mixing. In contrast, thermal annealing and first experiments in ion etching show that GMR can be increased by smoothing of the Co/Cu interfaces.

Notes: Granular and multilayer Co-Ag and NiFe-Ag thin films have been prepared by magnetron sputtering and by concurrent low-energy (0–500 eV) ion radiation-assisted deposition. The ion beam bombardment slightly increased the giant magnetoresistance of granular films and then progressively decreased it. X-ray data revealed a change in the crystalline structure of these films, dependent on the energy of the beam. Examination using transmission electron microscopy (TEM) also highlighted differences between the as-sputtered and the ion beam-assisted films. For multilayer films, TEM and x-ray analysis shows changes in the interfacial and crystal structure induced by the ion beam. © 1996 American Institute of Physics.

Notes: The design philosophy and objectives for producing phase-optimized trilayer structures for magneto-optic enhancement of the polar Kerr effect is outlined. A number of alternative designs is presented based on glass/Co-Pt/SiO2/Al. The performance of an actual device, as a function of the wavelength of the incident radiation, and of angle of incidence, for both of the principal polarization states, has been evaluated experimentally and theoretically. Results indicate excellent agreement and demonstrate the ease with which devices can be designed and fabricated.

Notes: For bulk applications, high-Tc superconductors generally require high critical current density, Jc. This paper reports two methods of producing high-Tc ceramic wires. The first is an extrusion and sintering route where Jc is routinely 600–1000 A cm−2. The second is by melt texturing or directional solidification where we have achieved in excess of 7000 A cm−2 by direct measurement at 77 K in zero field. However, the ease with which wire may be extruded and sintered is attractive and functioning devices have already been manufactured using this route. Fabrication of devices is facilitated by a unique processing route that has resulted in the strongest bulk ceramic yet reported (bend strength 〉200 MPa). The melt-texturing route and the problems associated with it are examined and compared with the extrusion and sintering route. The processing and the microstructures associated with the directionally solidified YBa2Cu3Ox will be examined in detail as will the resultant superconducting properties, in particular the magnetization.

Notes: Interpretations of Mössbauer measurements of the orientation of magnetic moments in magnetic recording particles have recently been critized. It has been alleged that a serious overestimation of spin-canting angles resulted in part from neglecting thick absorber effects. I show that effects are insignificant. The asymmetric and broadened line shapes which occur in Mössbauer spectra of γ-Fe2O3 at room temperature have the consequence that analyses based on line intensities are erroneous, and that line area measurements are inaccurate unless the analysis is performed upon high-quality data and properly accounts for the asymmetry. Cooling γ-Fe2O3 to 4.2 K reduces the line asymmetry and width.

Notes: Abstract The lepidocrocite (γ-FeOOH) to maghemite (γ-Fe2O3), and the maghemite to hematite (α-Fe2O3) transition temperatures have been monitored by TGA and DSC measurements for four initial γ-FeOOH samples with different particle sizes. The transition temperature of γ-FeOOH to γ-Fe2O3 and the size of the resulting particles were not affected by the particle size of the parent lepidocrocite. In contrast, the γ-Fe2O3 to γ-Fe2O3 transition temperature seems to depend on the amount of excess water molecules present in the parent lepidocrocite. Thirteen products obtained by heating for one hour at selected temperatures, were considered. Powder X-ray diffraction was used to qualify their composition and to determine their mean crystallite diameters. Transmission electron micrographs revealed the particle morphology. The Mössbauer spectra at 80 K and room temperature of the mixed and pure decomposition products generally had to be analyzed with a distribution of hyperfine fields and, where appropriate, with an additional quadrupole-splitting distribution. The Mössbauer spectra at variable temperature between 4.2 and 400 K of two single-phase γ-Fe2O3 samples with extremely small particles show the effect of superparamagnetism over a very broad temperature range. Only at the lowest temperatures (T⩽55 K), two distributed components were resolved from the magnetically split spectra. In the external-field spectra the ΔmI=0 transitions have not vanished. This effect is an intrinsic property of the maghemite particles, indicating a strong spin canting with respect to the applied-field direction. The spectra are successfully reproduced using a bidimensional-distribution approach in which both the canting angle and the magnetic hyperfine field vary within certain intervals. The observed distributions are ascribed to the defect structure of the maghemites (unordered vacancy distribution on B-sites, large surface-to-bulk ratio, presence of OH- groups). An important new finding is the correlation between the magnitude of the hyperfine field and the average canting angle for A-site ferric ions, whereas the B-site spins show a more uniform canting. The Mössbauer parameters of the two hematite samples with MCD104 values of respectively 61.0 and 26.5 nm display a temperature variation which is very similar to that of small-particle hematites obtained from thermal decomposition of goethite. However, for a given MCD the Morin transition temperature for the latter samples is about 30 K lower. This has tentatively been ascribed to the different mechanisms of formation, presumably resulting in slightly larger lattice parameters for the hematite particles formed from goethite, thus shifting the Morin transition to lower temperatures.

Notes: Abstract Magnetism in fine particles of goethite which contain 5, 11, 18 and 30 mole % aluminium has been investigated. Mössbauer spectroscopy was used, with the samples at 4.2 K and in applied magnetic fields of 0–8 T. Analysis was via a mean field model, in which the spectra from each sample were simultaneously least-squares fitted. The presence of aluminium increases the effective anisotropy, introduces a range of anisotropy fields, and induces ferrimagnetism. Quantitative measurements of these effects are presented. It is surmised that the anisotropy is principally determined by the local environment of ferric ions, and that the distribution of anisotropy fields is the result of a variety of atomic neighbour configurations. Ferrimagnetism is understood to arise from a strong preference, with 100% success at intermediate concentrations, for aluminium ions to crystallise on the same sublattice. Simulations of the net magnetic moment in small particle systems are presented, and it is shown that statistical fluctuations and the effects of surfaces are unimportant in the particles studied.

Notes: Abstract Examination has been made on aged and fresh Ag−Sn alloys and on commercial Cu−Ag−Sn dental alloys. Although x-ray diffractograms of aged Ag−Sn showed only λ Ag−Sn and free silver,119Sn Mössbauer spectra exhibited Sn(IV) oxide also. A low Debye temperature showed the oxide to be in intimate dynamical contact with the metallic matrix. Upon adding mercury, the phases λ1 Ag−Hg and η′ Cu−Sn were observed in a commercial specimen. Conversion-electron spectra of a mercury-coated disk showed the presence of λ2 Sn−Hg and a distribution of line positions smaller than that for particulate amalgams. Internal oxidation was found to prevent amalgamation.

Notes: Abstract Ultrafine antiferromagnetic FeF2 particles (〈10 nm) were prepared by a new technique, viz, the SF6-sensitized infrared photodecomposition of Fe(CO)5 induced by a transversely excited atmospheric (TEA) CO2 laser. The magnetic properties have been examined by57Fe Mössbauer spectroscopy. At low temperatures the magnetic hyperfine field decreases faster with increasing temperature than the hyperfine field of the bulk; this behavior appears to be consistent with collective magnetic excitations. The transition between the paramagnetic and antiferromagnetic states takes place at a higher temperature and over a broader range as compared to the bulk. FeF2 ultrafine particles are relatively sensitive to oxidation; cubic-type iron oxide is formed.