Ph. Tailhades

Spin waves in periodic magnetic structures—magnonic crystals

Propagation of spin waves (SWs) through a periodic multilayered magnetic structure is analyzed. It is assumed that the structure consists of ferromagnetic layers having the same thickness but different magnetizations. The wave spectrum obtained contains forbidden zones (stop bands) in which wave propagation is prohibited. Introduction into the structure of the ferromagnetic layer with a different thickness breaks the structural symmetry and leads to a localization of the SW mode with the frequency lying in the stop band. Reflection of the wave by the structure of the finite length and transmission of the wave through the structure are also investigated. Numerical calculations of the wave dispersion and the transmission coefficients for symmetrical periodic structures as well as the structures with a defect are presented. Drawing an analogy from photonic crystals known in optics, such magnetic structures can be called one-dimensional (1-D) magnonic crystals (MCs). The possibilities of existence of the 2-D MCs are also discussed.

Ferromagnetic films with magnon bandgap periodic structures: Magnon crystals

A new type of photonic crystals is proposed. The new crystals have a forbidden gap in the microwave spectrum of magnetostatic spin waves, and, by analogy with photonic crystals, they are called magnon crystals. Specimens of such crystals were fabricated on the basis of yttrium iron garnet films. The surfaces of ferromagnetic films containing two-dimensional etched hole structures were studied by atomic force and magnetic force mag-netometry. The propagation of spin waves through the magnon crystals was investigated.

Measurement of Young's modulus of nanocrystalline ferrites with spinel structures by atomic force acoustic microscopy

Abstract Using Atomic Force Acoustic Microscopy, the Youngs moduli of two thin films of nanocrystalline ferrites with spinel structures have been measured as a function of the oxidation temperature on a nanoscale. There is an overall decrease of the Youngs moduli as a function of the oxidation temperature with an intermediate minimum and maximum. These measurements corroborate the existence of chemical gradients from the surface layer to the interior of the films occurring during the oxidation process in the γ-phase. They lead to stress gradients which influence the Youngs modulus and the coercivity Hc. In our measurement technique, we measure the flexural resonance frequencies of an atomic force microscope cantilever and exploit their dependence on the tip-sample contact forces, here an elastic contact described by Hertzian contact theory. The technique has been extended allowing quantitative measurements using a self-consistent calibration. Comparison to nanoindentation measurements have been made. We discuss possible nonlinear effects occurring in the contact which may lend AFAM itself to measure nonlinear parameters on a nanoscale.

Cation valencies and distribution in the spinels CoxCuyMnzFeuO4+δ (δ≥0) thin films studied by X-ray photoelectron spectroscopy

Abstract X-ray photoelectron spectroscopy (XPS) has been used to elucidate the valencies and cation distribution of the copper and manganese in the spinels Co x Cu y Mn z Fe u O 4+ δ ( δ ≥0) as thin films prepared by r.f. sputtering on glass substrates. The results obtained show that identification of the two Cu (Cu + and Cu 2+ ) and three Mn (Mn 2+ , Mn 3+ , Mn 4+ ) species in the stoichiometric ( δ =0) thin film is possible using XPS in the Cu2 p 3/2 region and in the Mn2 p 3/2 , Mn3 p and Mn3 s regions. By a fitting process yielding the amounts of the Cu and Mn ions in the different oxidation states it is possible to determine the cation distribution among the two sublattices of the spinel structure. Significant changes were noted when the thin film was oxidized in cation deficient spinel. These changes included the total oxidation of Cu + and Mn 2+ ions with a variation in the relative amounts of Mn 3+ and Mn 4+ ions in the oxide film. A good agreement was obtained with the cation distribution established by thermogravimetry of the oxidation of thick films.

Optical modes conversion in magneto-photonic crystal waveguides

Abstract We study the propagation of optical radiation in magnetic waveguides with the periodic domain structures (magneto-photonic crystals). Two types of periodic structures are studied––one- and two-dimensional (1D and 2D). Optical modes conversion is possessed by both the first- and second-order magneto-optic effects and scattering by the domains. In the 2D periodic structures conversion becomes anisotropic and strongly depends on the parameters of the structure. The parameters of the domain structures depend on the external magnetic field, therefore the conversion efficiency can be modulated by the variation of this field. A numerical analysis is provided of the dispersion of the interacting modes as well as of the dependence of the intensity of the converted radiation on the parameters of the domain structure.

Measurement of mechanical properties of nanoscaled ferrites using atomic force microscopy at ultrasonic frequencies

To measure local elasticity by Atomic Force Acoustic Microscopy (AFAM), the sample placed in a commercial AFM is insonified by ultrasonic waves or the cantilever is oscillated at ultrasonic frequencies. In contact with the specimen, the cantilever-tip system vibrates out-of-plane and its resonances are measured. A quantitative model based on linear tip-sample force shows that the stiffness of the specimen can be derived from the shift of the contact resonance frequencies relative to resonance frequencies of the free cantilever. We report AFAM results with a well-known material, silicon single-crystal oriented in 〈100〉 direction, to prove the consistence of the local measurement of elasticiy. Then, in thin films of magnetite Fe3O4 and maghemite γFe2O3 with spinel structure, the influence of the deviation from the stoichiometry on the elasticity for a given grain size is determined.

Study by electrical conductivity, derivative thermogravimetry, infrared spectrometry and X-ray photoelectron spectroscopy of oxidation process of Fe2MoO4 in relation to the cationic distribution

Abstract The thermal behaviour in oxygen of Fe2MoO4 spinel prepared by the ceramic method has been investigated over the temperature range of 150–700°C on ground samples. Below 550°C, Fe2MoO4 is partially oxidized in the cation deficient spinel with a very large content of vacancies. The high electrical conductivity of the initial sample and its temperature dependence in oxygen can be related to a mixture of Fe2+ and Fe3+ valencies on both octahedral (B) and tetrahedral (A) sites, despite the observed positive value of the Seebeck coefficient. Derivative thermogravimetric studies showed that B site Fe2+ and Mo3+ ions will be oxidized more rapidly than A site Fe2+ ions when the oxidation temperature is lower for Fe2+ ions on B sites than that for Mo3+ ions on the same sites, and this discrepancy in reactivity has been used to determine the distribution of cations between the sublattices. In addition, a significant content of Mo4+ ions and a transfer of Mo6+ ions from the B sites to the A sites during oxidation can be derived from XPS and IR spectrometry.

Cationic distribution and oxidation kinetics of trivalent molybdenum ions in submicron molybdenum substituted magnetites

Abstract Valence states of molybdenum and iron ions and their cationic distribution on both octahedral (B) and tetrahedral (A) sites on the spinel structure of submicron molybdenum-substituted magnetites which are oxidized in cation deficient spinels have been performed by derivative thermogravimetry, IR spectroscopy and XPS. It was demonstrated that Fe 2+ B , Mo 3+ B , Mo 4+ B , Fe 2+ A and Mo 4+ A are successively oxidized into Fe 3+ and Mo 6+ ions below 450°C. A quantitative analysis based on this difference of reactivity of iron and molybdenum in relation to occupied sites (A or B) permits us to propose a cationic distribution for initial and oxidized phases. Kinetic studies of the oxidation process of Mo 3+ ions suggest that oxidation proceeds by way of a diffusion-controlled reaction in a cation deficient phase of variable composition with a chemical diffusion coefficient dependent on the vacancy content.

Reactivity toward oxygen and cation distribution in copper-manganese ferrite spinels fine powders

The thermal behaviour of CUyMnxFe3−y−xO4 spinels (0≤x≤1, x+y ≈ 1) obtained at low temperature has been investigated over the temperature range of 50–700 °C mainly by thermogravimetry and infrared spectroscopy. Below 600 °C, Cu-Mn ferrites partly oxidized in cation deficient spinels, CuyMnxFe3−y−xO4+δ, showing an incomplete oxidation of the tetrahedral divalent manganese ions. A quantitative analysis by DTG based on an oxido-reduction mechanism and on a difference of reactivity of copper, iron and manganese ions in relation to the occupied site (octahedral or tetrahedral site) permits a cation distribution to be proposed. For x> 0.70, the vacancies were found to occupy octahedral sites with a vacancy cation ratio closely of 1/5 similar to γ-Fe2O3. Subsequent oxidation of Mn2+ ions required higher temperature and was accompanied by a phase change.

Valence states of copper in copper ferrite spinels CuxFe3−xO4 (0 < x ≤ 1) fine powders: Evidence of copper insertion

Abstract The oxidation in cation deficient spinels of copper iron spinels Cu x Fe 3− x O 4 (0 x ≤ 1) synthesized by soft chemistry with a grain size x per mole of ferrite, three oxidation phenomena corresponding to Cu + at B-sites (130°C), Fe 2+ at B-sites (180°C) and Cu + at A-sites (240°C) have been found in close relation with the cation-oxygen distance of each oxidizable cation. For nonstoichiometric synthesized spinels Cu x Fe 3− x O 4+ δ with δ x , the presence of additional interstitial Cu + ions has been envisaged. The oxidation temperature of these interstitial ions (> 300°C) is higher than that for Cu + ions at Asites suggesting that interstitial copper ions also essentially reside in A-sites.