R. Gerber

Optical and magneto-optical properties of Co-Ti-substituted barium hexaferrite single crystals and thin films produced by laser ablation deposition

Ellipsometry, reflectance photometry and Kerr polarimetry have been used to determine the optical and magneto-optical properties of bulk single-crystal platelets and thin films of Co-Ti-doped barium hexaferrite. The single crystals were grown by a slow-cooling flux method and the thin films were produced by pulsed laser ablation deposition onto single-crystal Al2O3 substrates at 840°C. The complex refractive index (n) and magneto-optic Voigt parameter (Q) were determined over the spectral range 350–850 nm and the reliability of the data tested by comparison with photometric measurements of reflectance. Although the optical constants of the thin films could not readily be obtained over the entire spectral range by ellipsometry, their magneto-optical performance in terms of the Kerr rotation and ellipticity compared well with predictions based on the fundamental data for the bulk material.

Substrate temperature and oxygen pressure dependence of pulsed laser-deposited Sr ferrite films

The effect of substrate temperature and oxygen pressure on the microstructure, magnetic and magneto-optical properties of Sr ferrite (SrM) films grown on (001) single-crystal sapphire substrates by pulsed laser deposition has been investigated. Polycrystalline SrM films with perpendicular magnetic anisotropy could be prepared under a wide range of oxygen pressures and relatively high temperatures, sufficient to crystallise the material. However, an almost exclusive c-axis orientation normal to the film plane could be attained only at a narrow operational window centered at 0.1 mbar and 840°C. The magneto-optical properties of the films were comparable to those of the bulk barium hexaferrite single-crystal material. In addition, results obtained by atomic force microscopy provide convincing evidence that the growth of Sr ferrite on sapphire takes place by a spiral growth mechanism.

Cation distribution in Co-Ti-substituted barium hexaferrites: a consistent model

Abstract A consistent model of cation distribution in single-crystal and polycrystalline samples of BaCo x Ti y Fe 12− x − y O 19 for x and y in the range 0–0.8 was deduced from the Mossbauer spectra, magnetic moments and magneto-optical measurements.

Magnetism and magneto-optics of hexaferrite layers

Recent contributions to research in magnetism and magneto-optics of hexaferrite layers, resulting from the collaboration between the above-mentioned institutions, are comprehensively reviewed. The pulsed laser deposition (PLD) technique is described and its main features, relying on the plume diagnostics and correct oxygen pressure, both being important for the deposition of hexaferrites of complex stoichiometry, are highlighted. The fabricated layers were investigated structurally and it was found that they are highly textured with the c-axis perpendicular to the film plane. Their magnetization was measured over a wide temperature range, 4.2–300 K, and in fields up to 12 T. Its dependence upon the cobalt content x in BaFe12−x−yCoxTiyO19 was also determined in the interval 0 ⩽ x ⩽ 0.8. The results were interpreted in terms of Neel theory and this, when combined with our results of Mossbauer spectra measurements, led to the formulation of a consistent model for the cation distribution in CoTi-substituted barium hexaferrites. The hysteresis-loop measurements provided data for obtaining values of anisotropy, which are in agreement with those of the bulk materials. The domain structure of thin hexaferrite layers was also studied, particularly the domain period dependence upon the sample thickness and cobalt content. The domain period dependence was found to be in very good agreement with theoretical micromagnetic calculations. Ellipsometry, reflectance photometry and Kerr/Faraday polarimetry were used to determine the optical and magneto-optical properties of hexaferrite platelets and thin layers. The complex refractive index and magneto-optic parameter were determined over the spectral range 350–850 nm and the reliability of the data was tested by comparison with photometric measurements of reflectance. The Faraday rotation and absorption spectra of substituted hexaferrite thin layers were measured in the 500–2000 nm wavelength range at room temperature and 80 K. The results obtained are interpreted in terms of single-ion optical electron transitions belonging to either cobalt or iron cations occupying the tetrahedral and/or octahedral positions in the spinel block of hexagonal ferrite crystal lattice.

Optical and magneto-optical properties of Co/Ti substituted barium ferrite

The optical and magneto-optical properties of single crystal Co/Ti doped barium hexaferrites have been measured over the wavelength range 400–800 nm. The spectral dependencies of the Kerr effect indicate that at short wavelengths Fe3+, and at long wavelengths Co2+ ions, are responsible for the M.O transitions. In the near IR, and after quadrilayer enhancement, heavily doped ferrite performs better than TbFeCo as an M.O recording medium.

Domain structure of thin single crystal platelets of BaFe12-x-yCoxTiyO19

Abstract The domain structure in hexaferrite platelets was studied for various cobalt concentrations, 0≤x≤0.78, using the colloid-SEM method, with a view towards the possible applications of thin films of such materials in high density magnetic recording. Typical surface undulation of Bloch walls was observed in thicker platelets. The domain period decreases with decreasing crystal thickness and also with increasing cobalt concentration. The critical sample thickness for the transition from undulated to simple stripe structure was measured on a wedge-form sample and compared with the theoretical estimates.

OPTICAL ABSORPTION AND FARADAY ROTATION OF BARIUM HEXAFERRITE FILMS PREPARED BY LASER ABLATION DEPOSITION

The Faraday rotation and optical absorption of the Co- and Ti-substituted barium hexaferrite thin films measured in the 500–2000 nm wavelength range at room temperature are presented together with the results of the Faraday rotation measurements carried out on selected samples at 80 K. The spectral dependences of the Faraday coefficient are described and interpreted using a set of optical electron transitions belonging to either cobalt or iron cations occupying the tetrahedral and/or octahedral positions in the spinel blocks of hexagonal ferrite crystal lattice.

Faraday rotation in cobalt- and titanium-substituted barium hexaferrites

Abstract Spectral dependences of the optical absorption and the Faraday rotation of Co- and Ti-substituted barium hexaferrite single crystals were measured in the 800–2400 nm wavelength region. The observed behaviour was explained by single ion contributions of paramagnetic and diamagnetic-like transitions due to cobalt ions present on tetrahedral sites.

Superconducting and conventional OGMS: Comparative study

Abstract General analysis of superconducting and conventional OGMS systems is carried out and the performance superiority of superconducting separators is confirmed.

Magnetic interaction between a strongly magnetized sphere and weakly magnetized particles carried by fluid flow

Abstract The magnetic interaction between a sphere magnetized to saturation and particles which possess a weak magnetic moment induced by an external magnetic field is analysed in the inviscid approximation of background fluid flow. Analytical and computational methods are used to obtain particle trajectories, capture cross-sections, and criteria for particle retention.