Antiferromagnetic interactions in single crystalline Zn1-xCoxO thin films

In a rather contradictory situation regarding magnetic data on Co-doped ZnO, we have succeeded in fabricating high-quality single crystalline Zn1-xCoxO (x=0.003-0.07) thin films. This gives us the possibility, for the first time, to examine the it intrinsic magnetic properties of ZnO:Co at a quantitative level and therefore to address several unsolved problems, the major one being the nature of the Co-Co interaction in the ZnO structure.Comment: 4 pages, 4 figures,accepted for publication in PR

Spin-Exchange Interaction in ZnO-based Quantum Wells

Wurtzitic ZnO/(Zn,Mg)O quantum wells grown along the (0001) direction permit unprecedented tunability of the short-range spin exchange interaction. In the context of large exciton binding energies and electron-hole exchange interaction in ZnO, this tunability results from the competition between quantum confinement and giant quantum confined Stark effect. By using time-resolved photoluminescence we identify, for well widths under 3 nm, the redistribution of oscillator strengths between the A and B excitonic transitions, due to the enhancement of the exchange interaction. Conversely, for wider wells, the redistribution is cancelled by the dominant effect of internal electric fields, which dramatically reduce the exchange energy.Comment: 14 pages, 3 figure

Caractérisation de lubrifiant par approche semi – industrielle pour le forgeage de précision

En forgeage de précision, la réussite d’une gamme de déformation dépend en grande partie des performances du lubrifiant utilisé. Celui - ci doit permettre d’éviter un contact direct entre l’acier forgé et l’outil et favoriser l’écoulement du métal dans les outillages. L’étude proposée porte sur la mise en place d’essais de caractérisation de lubrifiants pour le forgeage de précision en conditions semi industrielles. Cette étude repose d’une part sur une démarche d’identification et de maîtrise des paramètres influant sur l’opération de forgeage et d’autre part sur la mise en place d’outillages instrumentés permettant un suivi de certains paramètres de forgeage. L’effet du lubrifiant est évalué suivant des critères liés aux efforts de forgeage et à la géométrie de la pièce forgée. Le protocole expérimental mis en place a permis de mener une étude comparative sur les performances de plusieurs lubrifiants de type polymère et la phosphatation.The success of a cold forging process strongly depends on the effectiveness of the lubricant system. The two main functions of this one is to avoid direct contact between the forged metal and the tools and to help metal flow in the dies. This study presents results obtained with an experimental procedure in order to characterize the lubricant effectiveness during cold forging under semi-industrial conditions. This procedure is based on the identification of the parameters related to the lubricant and driving the forging process. It is based too on the instrumentation of the forging tool in order to follow some particular forging parameters. The effectiveness of the lubricant is estimated through the measurement of the forging load and some dimensions on the forged part. This experimental procedure allowed comparing the effectiveness of two polymer type lubricants and the classical zinc phosphate precoat

Magnetic Anisotropy of Co2+ as Signature of Intrinsic Ferromagnetism in ZnO:Co

We report on the magnetic properties of thoroughly characterized Zn1-xCoxO epitaxial thin films, with low Co concentration, x=0.003-0.005. Magnetic and EPR measurements, combined with crystal field theory, reveal that isolated Co2+ ions in ZnO possess a strong single ion anisotropy which leads to an "easy plane" ferromagnetic state when the ferromagnetic Co-Co interaction is considered. We suggest that the peculiarities of the magnetization process of this state can be viewed as a signature of intrinsic ferromagnetism in ZnO:Co materials.Comment: 4 pages, 4 figure

Transport of indirect excitons in ZnO quantum wells

We report on spatially- and time-resolved emission measurements and observation of transport of indirect excitons in ZnO/MgZnO wide single quantum wells

Shifting donor-acceptor photoluminescence in N-doped ZnO

We have grown nitrogen-doped ZnO films grown by two kinds of epitaxial methods on lattice-matched ScAlMgO$_4$ substrates. We measured the photoluminescence (PL) of the two kinds of ZnO:N layers in the donor-acceptor-pair transition region. The analysis of excitation-intensity dependence of the PL peak shift with a fluctuation model has proven that our observed growth-technique dependence was explained in terms of the inhomogeneity of charged impurity distribution. It was found that the inhomogeneity in the sample prepared with the process showing better electrical property was significantly smaller in spite of the similar nitrogen concentration. The activation energy of acceptor has been evaluated to be $\approx 170$ meV, which is independent of the nitrogen concentration.Comment: 4 pages, 3 figures, 1 table, RevTeX4, to appear in the July issue of J. Phys. Soc. Jp

Ferromagnetism in Co-doped ZnO films grown by molecular beam epitaxy: magnetic, electrical and microstructural studies

We studied structural, optical and magnetic properties of high-quality 5 and 15% Co-doped ZnO films grown by plasma-assisted molecular beam epitaxy (MBE) on (0001)-sapphire substrates. Magnetic force microscopy (MFM) and magnetic measurements with SQUID magnetometer show clear ferromagnetic behavior of the films up to room temperature whereas they are antiferromagnetic below 200 K approximately. Temperature dependence of the carrier mobility was determined using Raman line shape analysis of the longitudinal-optical-phonon-plasmon coupled modes. It shows that the microscopic mechanism for ferromagnetic ordering is coupling mediated by free electrons between spins of Co atoms. These results bring insight into a subtle interplay between charge carriers and magnetism in MBE-grown Zn(1-x)CoxO films.Comment: 10 pages, 9 figures, 2 table

Influence of s,p-d and s-p exchange couplings on exciton splitting in (Zn,Mn)O

This work presents results of near-band gap magnetooptical studies on (Zn,Mn)O epitaxial layers. We observe excitonic transitions in reflectivity and photoluminescence, that shift towards higher energies when the Mn concentration increases and split nonlinearly under the magnetic field. Excitonic shifts are determined by the s,p-d exchange coupling to magnetic ions, by the electron-hole s-p exchange, and the spin-orbit interactions. A quantitative description of the magnetoreflectivity findings indicates that the free excitons A and B are associated with the Gamma_7 and Gamma_9 valence bands, respectively, the order reversed as compared to wurtzite GaN. Furthermore, our results show that the magnitude of the giant exciton splittings, specific to dilute magnetic semiconductors, is unusual: the magnetoreflectivity data is described by an effective exchange energy N_0(beta-alpha)=+0.2+/-0.1 eV, what points to small and positive N_0 beta. It is shown that both the increase of the gap with x and the small positive value of the exchange energy N_0 beta corroborate recent theory describing the exchange splitting of the valence band in a non-perturbative way, suitable for the case of a strong p-d hybridization.Comment: 8 pages, 8 figure

L’élevage dans les exploitations françaises

Dans le secteur de l’élevage des ruminants, en France, la forte diversité des systèmes de production est tour à tour considéré comme synonyme de facteur de richesse pour la typicité des produits et de contraintes pour l’organisation de certaines filières. À l’aide d’éclairages quantitatifs issus du traitement des recensements agricoles et qualitatifs en provenance du dispositif des Réseaux d’élevage (réseaux de fermes de référence), les principales tendances d’évolution de cette diversité sont étudiées pour les exploitations laitières, bovines et ovines. Ces évolutions résultent d’une interaction entre les projets des éleveurs toujours marqués par une aspiration à la parité avec les autres catégories socioprofessionnelles (et de plus en plus en matière de conditions de travail) et les caractéristiques de leur environnement dont certaines évoluent de façon permanente (politiques agricoles, filières et marchés) tandis que d’autres délimitent le champ des possibles (potentialités du milieu). Le poids des politiques agricoles et de leurs évolutions paraît très important. Face aux effets attendus du découplage qui pourrait accentuer les avantages des systèmes de production dominants, une politique de développement rural décentralisée pourrait favoriser une diversité de réponses à des enjeux locaux intégrant plus explicitement des dimensions environnementales et sociales associées à la fonction de production.In livestock sector, in France, the strong diversity of production systems is regarded in turn as factor of richness for the products typicity and as constraints for some market organizations and chains of distribution. Using quantitative approach with treatment of agricultural census and qualitative experience from Réseaux d’Elevage (networks of reference farms), the main tendencies of evolution of this diversity are studied for dairy, bovine and ovine farms. These evolutions result from an interaction between projects of stockbreeders always marked by an aspiration for parity with the other social and economic categories (and more and more as regards working conditions) and characteristics of their environment. Some of these characteristics evolve in a permanent way (agricultural policies, chains and markets) while others delimit the field of possible (potentialities of the land). The weight of agricultural policies and their evolutions appears very important. Decoupling is expected to accentuate the advantages of dominant production systems. A decentralized policy of rural development could support a diversity of answers to local stakes integrating more explicitly environmental and social dimensions associated with the production function