Ni-Mn-Ga films in the austenite and the martensite structures at room temperature: Uniaxial texturation and epitaxial growth

Ni-Mn-Ga films in the austenite and the martensite structures at room temperature have been obtained using the DC magnetron sputtering technique. Two elaboration processes were studied. A first batch of samples was deposited using a resist sacrificial layer in order to release the film from the substrate before vacuum annealing. This process leads to polycrystalline films with a strong (022) fiber texture. The martensitic phase transformation of such polycrystalline freestanding films has been studied by optical and scanning electron microscopy. A second batch of samples was grown epitaxially on (100)MgO substrates using different deposition temperatures. The texture has been analyzed with four-circle X-ray diffraction. Epitaxial films crystallized both in the austenite and the martensite structures at room temperature have been studied

Martensite structures and twinning in substrate-constrained epitaxial Ni-Mn-Ga films deposited by a magnetron co-sputtering process

In order to obtain Ni-Mn-Ga epitaxial films crystallized in martensite structures showing Magnetic-Induced Rearrangement (MIR) of martensite variants, a fine control of the composition is required. Here we present how the co-sputtering process might be helpful in the development of Ni-Mn-Ga epitaxial films. A batch of epitaxial Ni-Mn-Ga films deposited by co-sputtering of a Ni-Mn-Ga ternary target and a pure manganese target has been studied. The co-sputtering process allows a precise control of the film compositions and enables keeping the epitaxial growth of Ni-Mn-Ga austenite during deposition at high temperature. It gives rise to tune the content of the MIR-active 14-modulated martensite in the film at room temperature, as well as micro and macro-twinned domains sizes

Tuning macro-twinned domain sizes and the b-variants content of the adaptive 14-modulated martensite in epitaxial Ni-Mn-Ga films by co-sputtering

In order to obtain modulated-martensite in our epitaxial Ni-Mn-Ga films, we have tuned the composition by using a co-sputtering process. Here we present how the composition affects the variant distribution of the 14-modulated martensite at room temperature. The nature of such modulated-martensites is still strongly debated for magnetic shape memory alloys. It has been very recently demonstrated that the modulated-martensites in Ni-Mn-Ga are adaptive phases. The results presented here corroborate this theory for the first time, for three different compositions. Moreover, we demonstrate with the help of the adaptive modulations theory that b-variants of the 14-modulated martensite form close to the free-surface of the film to release the stress induced by branching of macro-twinned domains during the martensitic transformation on a rigid substrate. At room temperature, the content of such b-variants is found to strongly decrease when the macro-twinned domain sizes increase

Fabrication and characterization of a Ni-Mn-Ga uniaxially textured freestanding film deposited by DC magnetron sputtering

Homogeneous freestanding films have been obtained by the direct current (DC) magnetron sputtering technique using a sacrificial layer. After annealing, the films are crystallized with a strong out-of-plane texture along the (022) direction. The stoichiometry of the annealed films is close to the target composition and leads to a martensitic transformation around 255K. The annealed films demonstrate ferromagnetic behavior with a Curie temperature of about 362K. The magnetization process has been studied on the both states and during the martensitic transition. The saturation magnetizations have been determined by fitting the experimental data with a saturation approach law in the range 1-5T. Results show the saturation magnetization of the martensite is around 10% higher than that of the austenite. A model based on intrinsic magnetic properties of each state allowing the description of the magnetization M=f (H, T) of such polycrystalline films during the martensitic transformation is presented. The mass fraction of martensite inside the austenite phase can be determined using this model. The shape memory effect is analyzed both by scanning electron microscopy and by optical microscopy with in-situ measurement of the resistance temperature dependence

Films Ni-Mn-Ga et mémoire de forme magnétique : Elaboration et caractérisation des propriétés structurales et magnétiques

Currently applied active materials, like piezoelectrics and magnetostrictives, can change their shape by applying an electric field and a magnetic field, respectively. The range of relative length changes of these materials is typically around 0.1%. In 1996, a new fundamental actuation mechanism has been discovered in the martensite phase of special magnetic shape memory materials. In Ni-Mn-Ga single crystals, optimization of this new shape memory effect has led, in 2002, to the observation of relative length changes reaching 10%. Such deformations are related to the magnetic-induced rearrangement of the twinned martensite structure and were observed under relatively weak magnetic fields, less than one tesla. Through the unique combination of a very large deformation, a high energy density, and a relatively high frequency of magnetic actuation, magnetic shape memory alloys open the door to new applications, that cannot be achieved with conventional active materials. In collaboration with Schneider Electric, and in order to use the high potential's miniaturization of the magnetic shape memory effect, the research focuses on deposition of Ni-Mn-Ga films in this thesis. The research will revolve around two main axes. The study was initially focused on finding a method that allows the release of Ni-Mn-Ga films, before annealing at high temperature. A second part of the investigations focused on obtaining and studying epitaxially grown Ni-Mn-Ga films. An innovative solution, directly inspired by the techniques of micro-electronics, has been developed to release Ni-Mn-Ga films. Synthesis work was therefore carried out and resulted in the production of polycrystalline free-standing films with a fiber texture, which have magnetic properties comparable to the bulks and exhibit reversible martensitic transformations. A second line of research has focused on obtaining and studying epitaxially grown Ni-Mn-Ga films. Again, an important work of synthesis has been conducted. To precisely control the composition of epitaxially grown films on MgO (001), a co-sputtering method has been developed. The magnetic shape memory effect of Ni-Mn-Ga alloys occurs only in certain modulated martensites phases, whose nature is still strongly debated today. We will see that the detailed analyses of structures and textures of our epitaxially grown films, helped to better understand the nature of modulated martensites of Ni-Mn-Ga alloys. Furthermore, in our films, activation's fields of the twinned martensite's rearrangement are only a few hundredths of a tesla, far below those reported in bulk materials. This result strongly boosts the interest of using films in the production of actuators, which use the magnetic shape memory effect.Les matériaux actifs appliqués actuellement, comme les piézoélectriques et les magnétostrictifs, peuvent respectivement changer leur forme par l'application d'un champ électrique, et d'un champ magnétique. La portée des allongements relatifs de ces matériaux, est typiquement de l'ordre de 0,1%. En 1996, un nouveau mécanisme fondamental d'actionnement a été découvert dans la phase martensite d'alliages magnétiques à mémoire de forme. Dans les monocristaux d'alliages Ni-Mn-Ga, l'optimisation de ce nouvel effet mémoire de forme a conduit, en 2002, à l'observation d'allongements relatifs qui atteignent 10%. De telles déformations sont liées au réarrangement, induit magnétiquement, de la structure martensitique maclée. De plus, elles ont été observées sous des champs magnétiques relativement faibles, inférieurs au tesla. Grâce à la combinaison unique d'une très grande déformation, d'une haute densité d'énergie, et d'une fréquence d'actionnement magnétique qui est relativement élevée, les alliages magnétiques à mémoire de forme ouvrent la porte à de nouvelles applications, qui ne peuvent pas être réalisées avec les matériaux actifs classiques. En collaboration avec Schneider Electric, et dans le but d'utiliser le potentiel élevé de miniaturisation de l'effet mémoire de forme magnétique, les recherches se sont concentrées sur la réalisation de films Ni-Mn-Ga dans ce travail de thèse. Les recherches menées se sont articulées autour de deux grands axes. L'étude s'est dans un premier temps focalisée sur la recherche d'un procédé, qui permet la libération de films Ni-Mn-Ga, avant le recuit à haute température. Une deuxième part des investigations s'est concentrée sur l'obtention et l'étude de films Ni-Mn-Ga déposés épitaxiallement. Une solution innovante, directement inspirée des techniques de la micro-électronique, a été développée pour libérer des films Ni-Mn-Ga. Un travail d'élaboration conséquent a été réalisé et a abouti à l'obtention de films libres polycristallins à texture de fibre, qui possèdent des propriétés magnétiques comparables aux massifs, et qui présentent des transformations martensitiques réversibles. Un deuxième axe de recherche s'est concentré sur l'élaboration et l'étude de films Ni-Mn-Ga déposés épitaxiallement. Là encore, un travail important d'élaboration a dû être mené. Afin de contrôler précisément la composition des films déposés épitaxiallement sur MgO (001), un procédé de copulvérisation a été développé. L'effet mémoire de forme magnétique des alliages Ni-Mn-Ga, se produit uniquement dans certaines phases martensites modulées, dont la nature est encore énormément débattue aujourd'hui. Nous verrons que l'analyse détaillée des structures et textures de nos films déposés épitaxiallement, a permis de mieux comprendre la nature des martensites modulées des alliages Ni-Mn-Ga. De plus, dans nos films, les champs d'activation du réarrangement de la martensite maclée sont de seulement quelques centièmes de tesla, nettement inférieurs à ceux rapportés dans les matériaux massifs. Ce résultat relance fortement l'intérêt d'utiliser des films dans la réalisation d'actionneurs utilisant l'effet mémoire de forme magnétique

Large inverse magnetocaloric effect in Ni45Co5Mn37.5In12.5 single crystal above 300 K

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Large Magneto-Caloric Effect in Ni–Co–Mn–In systems at room temperature

International audienc

Search for new manganese-cobalt oxides as positive electrode materials for lithium batteries

International audienceTwo new mixed manganese-cobalt oxides for lithium battery positive electrode materials were obtained using original synthesis routes. Compound I, "LMCO" is a new form of LiMnCoO4 obtained by ion-exchange from NaMnCoO4. Compound II, "MCO" is a nanometric material with formula Mn1-x CoxO≈2 obtained by quenching in specific conditions. We showed recently that the electro-chemical properties of this materials were dramatically enhanced by cobalt doping [Strobel et al., J. Mater. Chem. 15 (2005) 4799]. The effect of cobalt content on its electrochemical behaviour is reported in detail here. Compound I gives rise to two reversible single-phase intercalation reactions centered at 2.7 and 4.4 V on discharge, corresponding to Co3+/Co4+ and Mn3+/Mn4+ redox couples, respectively. The initial capacity is 160 mAh/g at low regime, but stabilizes at ca.100 mAh/g on extended cycling. Compound II gives a single plateau with a much higher capacity. An optimum ratio of Co:Mn = 0.20 was found and gives a capacity of 175 mAh/g after 60 cycles in the potential window 2.0-4.2 V

Structural differences between Sb- and Nb-doped tin oxides and consequences for electrical conductivity

Sb- and Nb-doped tin oxides have been prepared by a co-precipitation method. Whilst X-ray powder diffraction and EDX mapping indicate similar dopant distributions, more detailed characterisation by variable high-temperature XRD, 119Sn MAS NMR and Nb K-edge X-ray absorption near edge structure spectroscopy reveal clear differences in the oxide structures. This detailed structural information is used to validate the measured differences in electrical conductivity.ISSN:1144-0546ISSN:0398-9836ISSN:1369-926

Electrochemical flow-cell setup for in situ X-ray Investigations: II. Cell for SAXS on a Multi-Purpose Laboratory Diffractometer

A unique electrochemical three-electrode flow-cell design is presented for in situ small-angle X-ray scattering experiments on a multi-purpose laboratory X-ray diffractometer. An electrolyte layer thickness of 2 mm enables sufficient photon transmission to acquire in situ scattering curves at high signal-to-noise ratio within less than one hour despite the restricted photon flux from a standard Cu X-ray tube. Complete tightness of the cell allows electrolyte flow with controlled gas saturation in order to guarantee constant experimental conditions even for long experimental protocols. Good electrochemical performance is achieved by a special arrangement of working and counter electrodes that are deposited on the opposing X-ray transmission windows of the cell. The functionality and reliability of the cell are demonstrated in an in situ small-angle X-ray scattering study of the degradation properties of carbon-supported Pt nanoparticles during electrochemical high-potential cycling. Careful subtraction of background scattering and absolute normalization of the scattering curves yield absolute quantitative structural information about the Pt nanoparticle phase at different stages of the degradation protocol, bringing insights into the real-time evolution during electrochemical characterization.ISSN:0013-4651ISSN:1945-711