Laurence Bouet

RELATIONS BETWEEN MAGNETO-OPTICAL PROPERTIES AND REACTIVITY IN COBALT-MANGANESE FERRITE THIN FILMS AND POWDERS

Abstract Co-Mn spinel ferrites were prepared as submicron powders and thin films. Because of their finely divided state, these spinels could be oxidized at low temperatures to give novel cation-deficient ferrites. For these two material forms, the magneto-optical properties were found to be strongly dependent on the ferrite oxidation state. The highest coercivities and Faraday rotations were obtained when the ferrites were partially oxidized. These phenomena are attributed to the mechanical stress effect developed during the oxidation of the manganese ions. The properties of these ferrites could be of interest for magneto-optical recording applications. The first static recording tests were performed at 780 nm wavelength.

About the interesting properties of mixed-valence defect spinel ferrites for mass storage media

When the spinel ferrites present a large surface/volume ratio (i.e when they are in the form of thin films or fine powders) they can be oxidized into mixed-valence defect ferrites. These original spinels have magnetic, magneto-optical and optical properties making them interesting materials for high density recording media.

Magneto-optical properties of a sputtered Bi-substituted Dy–Fe garnet-ferrite/hematite bilayer

Rapid thermal annealing was performed after a Bismuth-substituted Dy iron garnet-ferrite (Bi2DyFe4GaO12) was deposited on a quartz glass substrate having a hematite underlayer, and the effect of the hematite underlayer on the magneto-optical characteristics of the garnet ferrite film was evaluated. The presence of the underlayer greatly improved the magneto-optical effect of the garnet film, but the amount of improvement depended on the hematite phase. Big improvement was obtained using underlayer phases that are easily crystallized, i.e., Fe3O4 and γ-Fe2O3 which have a spinel structure, but not with α-Fe2O3 which has a corundum structure.

Controlling garnet film composition by magnetic-field-controlled radio frequency magnetron sputtering

To find a way to control the compositional change in Bi-substituted garnet during sputtering, we studied the effects of controlling the cathode magnetic field. The magnetic-field-controlled rf magnetron sputtering method that we developed can create a garnet film whose composition is the same as that of the sputtering target. When we deposited a film of Bi-substituted Dy iron garnet–ferrite (Bi2DyFe4GaO12) by this method, there was no compositional change between the target and the film even after a long sputtering process. Therefore, this sputtering method is effective at suppressing compositional change during the film formation process.

Magneto-optical properties of a sputtered Bi-substituted Dy iron garnet-ferrite/spinel-ferrite bilayer

Abstract Bi-substituted Dy iron garnet-ferrites, Bi 2 DyFe 4 GaO 12 , were prepared by RF magnetron sputtering on a spinel-ferrite underlayer on a quartz glass substrate. This study investigated the effect of the spinel underlayer, a mixed crystal of Fe 3 O 4 and γ-Fe 2 O 3 , on improving the magneto-optical properties. It was found that the presence of the spinel-ferrite underlayer increased the Faraday effect, compared to that of a garnet layer with no underlayer, for which the Faraday effect was almost non-existent. It is concluded that the high-temperature-treated spinel-ferrite underlayer enhanced the magneto-optical characteristics. This bilayer structure is therefore promising for magneto-optical recording media.

Cation-Deficient Spinel Ferrites : Application for High-Density Write-Once Optical Recording

Thin films of defect spinel ferrites can be used as write-once read-many media working with blue wavelengths. In fact, because these non-stoichiometric ferrites are metastable, they can be transformed into corundum phases at moderate temperatures by a laser spot. The transformed regions have different optical indices from the starting ferrite film, making the readout process possible. Recording density of about 5 bits/µm2 was demonstrated.

CoMn cation-deficient ferrite thin films: Interesting ways to tailor their magneto-optical properties

Abstract Cation-deficient CoMn ferrites were prepared as thin films. Their magneto-optical properties were studied and correlated with the oxidation reactions occurring in the films. The coercivity and remanent Faraday rotation variations could be, in part, attributed to the development of mechanical stresses, which could be revealed by use of grazing-angle X-ray diffraction. Moreover, a directional order could be created in thin films by slow cooling from 350°C. The creation of this reversible phenomenon led to an increase in coercivity without detectable modification of the oxidation and stress states of the samples. Control of the oxidation state and the creation of a directional order allowed modification of the magneto-optical properties of the ferrite thin films over a wide range.

The role of cobalt in the oxidation mechanism modification of the Mo3+ ions of molybdenum-substituted magnetites

Abstract A kinetic study of the oxidation of Mo 3+ ions has been performed on two submicrometer Mo and MoCo ferrites of spinel structure with a nearly identical molybdenum content. We show that for the Co-free sample, oxidation proceeds by cationic diffusion in a cation-deficient spinel with a variable chemical diffusion coefficient. For the Co sample, which showed a pronounced stability towards oxygen, the oxidation process was divided into three steps. In step I, oxidation declines with time in an initial period (degree of oxidation α α α > 0.25) oxidation is best described by the diffusion-controlled Jander equation. Comparing the vibrational spectra of these two spinels after total oxidation for various thermal treatments, we conclude that the coordination around Mo 6+ ions changes from tetrahedral to octahedral with Co substitution and that the location of cobalt and molybdenum ions in octahedral sites changes with the annealing process.