O estudo das ligas de Heusler tem despertado grande interesse devido às diversas funcionalidades que as propriedades destas ligas proporcionam. Essas ligas em particular, apresentam comportamentos de semi-metais1 ferromagnéticos, polarização total de spin e estruturas atômicas superordenadas, aspectos que ainda representam novidades no campo de pesquisas [1-3]. Neste trabalho foi investigada a influência dos parâmetros de crescimento nas propriedades magnéticas e estruturais de filmes finos de ligas de Heusler a base de Co-Fe-Al. Os filmes foram obtidos por codeposição via pulverização catódica utilizando alvos dos elementos puros (Co, Fe e Al) e de ligas de FeAl e caracterizados por espectroscopia Mössbauer por elétrons de conversão (CEMS), difração de raios X em ângulo rasante GIXRD e magnetometria de amostra vibrante (VSM). Os nossos resultados mostraram que os filmes de CoFeAl obtidos via codeposição de dois alvos (Co e FeAl), formam mais de uma fase, identificadas por CEMS como CoFeAl, com estrutura tipo B2/ L21 e FeAl (clusters). Por outro lado, os filmes de CoFeAl obtidos por codeposição de três alvos dos elementos puros (Co, Fe e Al) apresentaram somente a fase de CoFeAl com estrutura do tipo L21 com alto grau de ordenamento químico. Não foi observado a formação de clusters (FeAl). Os filmes Co2FeAl como obtidos por codeposição a partir de dois alvos (Co e FeAl) formam mais de uma fase de Co2FeAl com estruturas desordenados do tipo A2 e B2. Por outro lado, os filmes de Co2FeAl (3 alvos) apresentaram uma única fase com alto grau de ordenamento, possivelmente com estrutura do tipo L21. / Studies of Heusler alloys have attracted great interest due to several features provided by these alloys. They exhibit half-metallic ferromagnetism, high spin polarization and ordered atomic structures, properties that still represent new aspects in the field of research [1-3]. In this study, we investigated the growth parameters influence on the magnetic and structural properties of Heusler alloys thin films based on Fe-Co-Al. The films were obtained by co-deposition via sputtering with targets of pure elements (Co, Fe and Al) and FeAl alloys. Conversion Electron Mössbauer Spectroscopy (CEMS), Grazing Incidence X-ray Diffraction (GIXRD) and Vibrating Sample Magnetometer (VSM) were used to characterize such films. CEMS results of CoFeAl films obtained by co-deposition of two targets (FeAl and Co) showed more than one CoFeAl phase with B2 - L21 structures and clusters of FeAl. Samples grown by three pure targets (Co, Fe and Al) had only one phase formation of L21-type structure with a high chemical order, and clusters (FeAl) were not observed. Co2FeAl films obtained by co-deposition from two targets (Co and FeAl) had more than one phase with A2 and B2 disordered structures. In addition, Co2FeAl films grown by three targets showed single phase with high chemical order, indicating L21-type structure.

Made available in DSpace on 2016-12-23T14:20:01Z (GMT). No. of bitstreams: 1 EMC_Heusler- parte 1.pdf: 1976712 bytes, checksum: 54cbe3c62f71ef7121b79c6f4e1efaa0 (MD5) Previous issue date: 2008-03-28 / Conselho Nacional de Desenvolvimento Científico e Tecnológico / In this work, magnetic entropy change of nonstoichiometric Ni54[Mn(1-x)Fex]19Ga27 Heusler alloys was experimentally verified by partial substitution (up to 50%) of the Mn atoms by magnetic Fe atoms. Such study was mainly concentrated in the magneto-structural transformation region, at low temperatures. The analysis of X-ray diffraction patterns indicate that the partial substitution of Mn atoms by Fe atoms causes predominance of the L21 -phase, however accompanied by spurious phases. The magnetization measurements as a function of the temperature in the low magnetic field reveals that, in all concentration range (0 x 0.5), the system presents a magnetic transition (Ferromagnetic Paramagnetic) at a temperature, TC, near the room temperature, when the material lay in the austenitic phase. Moreover, as many others Heusler alloys, the material undergoes a martensitic structural transition at low temperatures, TM. As the Fe concentration increases, the Curie temperature, TC, undergoes a little variation, increasing around 5%, while TM decreases slowly and monotonically. The magnetic entropy change, for a field of 5T, presents a maximum SM = - 9,3 J/kg.K, for x = 0.1, at a temperature of 250K, and than decreases for x 0.3, changing linearly with the maximum applied field. / Neste trabalho, verificou-se experimentalmente a variação da entropia magnética nas ligas Heusler não-estequiométricas Ni54[Mn(1-x)Fex]19Ga27, substituindo-se parcialmente (até 50%) os átomos de Mn por átomos de Fe. Tal estudo focalizou-se principalmente na região de transformação magneto-estrutural a baixas temperaturas. A análise dos difratogramas de raios-X indica que a substituição parcial de Mn por Fe nessa faixa de concentração, ocasiona uma predominância da fase- L21, porém acompanhada de outras fases espúrias. As medidas de magnetização em função da temperatura realizadas a campos baixos revelam que, em toda a faixa de concentração estudada (0 x 0,5), o sistema apresenta uma transição magnética (Ferromagnético Paramagnético) ao atingir a temperatura de Curie, TC, um pouco acima da temperatura ambiente, quando o material se encontra na fase austenítica. E, assim como muitas outras ligas Heusler, o material sofre uma transição estrutural ao ser resfriado a temperaturas inferiores à temperatura de transição martensítica, TM. À medida que se aumenta a concentração de Fe, a temperatura de Curie, TC, sofre uma pequena variação, aumentando em torno de 5%, enquanto que TM diminui lenta e monotonicamente. A variação da entropia, para um campo de 5T, apresenta um máximo SM = - 9,3 J/kg.K para x = 0,1, a uma temperatura T = 250K e, então, diminui para x 0,3, variando linearmente com o campo aplicado.

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Le transfert de spin est un moyen de retourner l’aimantation d’une couche dans une jonction tunnel magnétique. Le courant nécessaire à cette tâche dépend des matériaux et dans le contexte actuel consommer moins est devenu un enjeu important. Une solution consiste à utiliser des matériaux ayant une forte polarisation en spin et un faible amortissement magnétique. Ces matériaux sont appelés demi-métaux ferromagnétiques. Du fait de l’existence d’un gap de spin chez les spins minoritaires au niveau de Fermi, ces composés possèdent une polarisation en spin de 100% et un faible amortissement magnétique. En théorie, certains Heusler, tels que Co1.5Fe1.5Ge et Co2MnSi, possèdent ces propriétés s’ils cristallisent dans la bonne phase cristallographique. En pratique, des mesures indirectes semblent confirmer ce comportement mais pourtant aucune preuve directe de cette demi-métallicité n’a été observée jusqu’à présent. C’est dans ce cadre que cette thèse s’inscrit. Après avoir déterminé les conditions de croissance de Co1.5Fe1.5Ge, à l’aide d’une série de mesure et notamment à l’aide de la diffraction anomale, nous avons déterminé l’ordre chimique complet de cet alliage qui est bien celui recherché. Les mesures des propriétés magnétiques donnent des résultats en accord avec la théorie. Mais l’utilisation de ce composé dans des jonctions tunnel magnétiques montre une faible magnétorésistance tunnel. La spectroscopie de photoémission résolue en spin nous a permis d’expliquer ces résultats. Dans le même esprit, nous nous sommes tournés vers le Co2MnSi, un composé qui semble plus prometteur où le gap de spin et de faibles valeurs d’amortissement magnétiques ont été mesurés / Spin transfer is one way of switching the magnetization of a layer in a magnetic tunnel junction. The current needed at this task depends on the materials and in the current context, consume less became an important issue. Materials with a high spin polarization and a low magnetic damping are one solution of this problem. They are called half metal ferromagnets. Because of the existence of a pseudo-gap in the minority spin channel at the Fermi energy, these compounds show a 100% spin polarization and an extremely low magnetic damping. In theory, some Heusler, such as Co1.5Fe1.5Ge and Co2MnSi, possess theses properties if they crystallize in the good crystallographic phase. In practice, there is strong indication of this behavior by mean of indirect techniques. However, no direct evidence of this pseudo-gap has been observed. It is in this context that this thesis is. After having determined growth conditions of Co1.5Fe1.5Ge, by mean of several techniques and especially by anomalous diffraction, we determined the complete chemical order which is the one we were looking for. Magnetic properties measurements show results in agreement with the theory. But the use of this compound in magnetic tunnel junctions shows low tunnel magnetoresistance. Spin resolved photoemission spectroscopy measurements explain very well these results. In the same spirit, we started to study Co2MnSi which seems more promising as this pseudo-gap and low magnetic damping have been observed

Title: Martensite microstructures in thin films and monocrystals of Heusler alloys Ni-Mn-Ga Author: Kristýna Onderková Department: Department of Surface and Plasma Science Supervisor: Mgr. Ing. Oleg Heczko, Dr., Institute of Physics of the Czech Academy of Sciences Abstract: The submitted thesis examines mainly the first thin films from Ni-Mn-Ga Heusler alloy prepared by magnetron sputtering on the new equipment at Institute of Physics of Charles University. However, the work also analysed the thin films prepared in IFW Dresden and bulk material. The main focus of the work is primarily on the martensitic microstructures, because of the significant effect that their twin boundaries have on the magnetic shape memory phenomena. Microscopic techniques used for the research were mainly Scanning Electron Microscopy (SEM), but also Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM). As the Ni-Mn-Ga properties are stronly dependent on chemical composition, the composition was evaluated by two different methods (Electron Dispersive X-ray Spectroscopy and X-ray Fluorescence) and observed differences discussed. Finally the results were compared with X-ray diffraction (XRD) measurements and the films were further characterised by magnetic measurements using Vibrating Sample Magnetometer (VSM)....

Diese Arbeit befasst sich mit dem Wachstum von Dünnschichtstrukturen, die zur Herstellung eines Spin-selektiven Schottky-Barrier-Tunneltransistors (SS-SBTT) erforderlich sind. Das Bauelement basiert auf dem Transport von Ladungsträgern durch eine dünne halbleitende (SC) Schicht, die zwei ferromagnetische (FM) Kontakte trennt. Daher müssen hochqualitative und gitterangepasste vertikale FM/SC/FM-Trilayer gezüchtet werden, was aufgrund der inkompatiblen Kristallisationsenergien zwischen SC und Metallen eine experimentelle Herausforderung darstellt. Das Problem wurde mit einem Festphasenepitaxie-Ansatz gelöst, bei dem eine dünne amorphe Ge-Schicht (4-8 nm) durch Ausglühen über Fe3Si auf GaAs(001)-Substraten kristallisiert wird. Langsame Glühgeschwindigkeiten bis zu einer Temperatur von 260°C konnten ein neues gitterangepasstes Polymorph von FeGe2 erzeugen, über das ein zweites Fe3Si mittels Molekularstrahlepitaxie gezüchtet werden könnte. SQUID-Magnetometermessungen zeigen, dass die dreischichtigen Proben in antiparallele Magnetisierungszustände versetzt werden können. Vertikale Spin-Ventil-Bauelemente, die mit verschiedenen Trilayern hergestellt wurden, wurden verwendet, um zu demonstrieren, dass der Ladungstransport über die Heteroübergänge spinselektiv ist und bei Raumtemperatur einen Magnetowiderstand von höchstens 0,3% aufweist. Der Effekt nimmt bei niedrigen Temperaturen ab, was mit einem ferromagnetischen Übergang in der FeGe2-Schicht korreliert. Durch TEM- und XRD-Experimente konnte festgestellt werden, dass das neue FeGe2-Polymorph die Raumgruppe P4mm aufweist und bis zu 17% Si-Atome als Ersatz für Ge-Stellen enthält. Die Isolierung von FeGe2 war möglich, indem das Verhältnis von Fe-, Si- und Ge-Atomen so eingestellt wurde, dass die richtige Stöchiometrie bei vollständiger Durchmischung erreicht wurde. Anhand von FeGe2-Dünnschichten wurde ein zunehmender spezifischer Widerstand bei niedriger Temperatur und ein semi-metallischer Charakter beobachtet. / This thesis discusses the growth of thin film structures required to fabricate a Spin-Selective Schottky Barrier Tunnel transistor (SS-SBTT). The device relies on charge carriers being transported through a thin semiconducting (SC) layer separating two ferromagnetic (FM) contacts. Thus, high quality and lattice-matched FM/SC/FM vertical trilayers must be grown, which is experimentally challenging due to incompatible crystallization energies between SC and metals. The problem was solved using a solid-phase epitaxy approach, whereby a thin amorphous layer of Ge (4-8 nm) is crystallized by annealing over Fe3Si on GaAs(001) substrates. Slow annealing rates up to a temperature of 260°C could produce a lattice-matched Ge-rich compound, over which a second Fe3Si could be grown my molecular-beam epitaxy. The compound obtained during annealing is a new layered polymorph of FeGe2. SQUID magnetometry measurements indicate that the trilayer samples can be placed in states of antiparallel magnetization. Vertical spin valve devices created using various trilayers were used to demonstrate that charge transport is spin-selective across the heterojunctions, showing a magnetoresistance of at most 0.3% at room temperature. The effect decreases at low temperature, correlating with a ferromagnetic transition in the FeGe2 layer. TEM and XRD experiments could determine that the new FeGe2 polymorph has a space group P4mm, containing up to 17% Si atoms substituting Ge sites. Isolating FeGe2 was possible by tuning the proportion Fe, Si and Ge atoms required to obtain the right stoichiometry upon full intermixing. Hall bars fabricated on FeGe2 thin films were used to observe an increasing resistivity at low temperature and semimetallic character.

This thesis is a theoretical study of configurational and magnetic interactions in multicomponent solids. These interactions are the projections onto the configurational and magnetic degrees of freedom of the underlying electronic quantum mechanical system, and can be used to model, explain and predict the properties of materials. For example, the interactions govern temperature induced configurational and magnetic order-disorder transitions in Heusler alloys and ternary nitrides. In particular three perspectives are studied. The first is how the interactions can be derived from first-principles calculations at relevant physical conditions. The second is their consequences, like the critical temperatures for disordering, obtained with e.g. Monte Carlo simulations. The third is their origin in terms of the underlying electronic structure of the materials. Intrinsic defects in the half-Heusler system NiMnSb are studied and it is found that low-energy defects do not destroy the important half-metallic property at low concentrations. Deliberate doping of NiMnSb with 3d-metals is considered and it is found that replacing some Ni with extra Mn or Cr creates new strong magnetic interactions which could be beneficial for applications at elevated temperature. A self-consistent scheme to include the effects of thermal expansion and one-electron excitations in the calculation of the magnetic critical temperature is introduced and applied to a study of Ni1−xCuxMnSb. A supercell implementation of the disordered local moments approach is suggested and benchmarked for the treatment of paramagnetic CrN as a disordered magnetic phase. It is found that the orthorhombic-to-cubic phase transition in this nitride can be understood as a first-order magnetic order-disorder transition. The ferromagnetism in Ti1−xCrxN solid solutions, an unusual property in nitrides, is explained in terms of a charge transfer induced change in the Cr-Cr magnetic interactions. Cubic Ti1−xAlxN solid solutions displays a complex and concentration dependent phase separation tendency. A unified cluster expansion method is presented that can be used to simulate the configurational thermodynamics of this system. It is shown that short range clustering do influence the free energy of mixing but only slightly change the isostructural phase diagram as compared to mean-field estimates.

La réfrigération magnétique attire beaucoup d’attention ces dernières années parce qu’elle est considérée comme une technologie respectueuse de l’environnement et énergétiquement économique. Aujourd’hui, cette technologie avancée est encore en phase de recherche que des dispositifs de réfrigérations magnétiques soient déjà opérationnels. Ce travail de thèse consiste à étudier la potentialité de résistance à la corrosion de différents types de matériaux magnétocaloriques (Gd6Co1.67Si3, Ni2Mn0.75Cu0.25Ga et Pr0.66Sr0.34MnO3) en contact avec un fluide caloporteur. Afin de comprendre les propriétés magnétocaloriques des matériaux, nos recherches se sont aussi focalisées sur les relations entre la transition magnéto-structurales d’alliages Heusler Ni2Mn0.75Cu0.25Ga et (i) la distribution cationique au sein de la structure cristalline et/ou (ii) la microstructure. Finalement, le diagramme de phase magnétique et nucléaire en lien avec les effets magnétocalorique obtenu grâce à la diffraction de neutrons et de pérovskite Pr1-xSrxMnO3 (0.25≤x≤0.45) est également présenté. / Magnetic refrigeration has gained lot of importance and attention as they are highlighted to be environmental friendly, energy efficient. Presently, though at research stage, the magnetic refrigerators are pushed towards realization in domestic application with extensive work on materials and with few working models. One critical issue, the potential resistance to corrosion in case of different class of magnetocaloric materials (Gd6Co1.67Si3, Ni2Mn0.75Cu0.25Ga and Pr0.66Sr0.34MnO3) against the heat transport fluid is addressed. To better understand and improve the observed magnetocaloric properties in Heulser alloys Ni2Mn0.75Cu0.25Ga and to elaborate the same with the magneto-structural relation, studies on (i) cation distribution with in crystal structure and/or (ii) microstructural dependence are presented. Nuclear and magnetic phase diagram based on detailed neutron diffraction and magnetism studies for magnetocaloric perovskite oxide Pr1-xSrxMnO3 (0.25≤x≤0.45) is also presented

The present thesis is devoted to the investigation of modulated structures as well as the direct measurement of magnetocaloric effect (MCE) in Ni-Mn based magnetic shape memory (MSM) Heusler compounds in pulsed magnetic fields after analyzing isothermal entropy data taken in static magnetic fields. The emphasis is on the modulated structure of MSM Heusler compounds because of lower twinning stress which facilitates the easy transformation from austenite to martensite structure. Synchrotron x-ray powder diffraction (SXRPD) was carried out to study the modulated structure and NPD for antisite disorder as Ni and Mn have easily the same atomic scattering factor. Direct measurement of the adiabatic temperature change ΔTad was done in pulsed magnetic fields, because of fast response of ~10 to 100 ms to the sample temperature on magnetic field, providing adiabatic conditions. It also gives an opportunity of very high magnetic fields up to 70 T because of short pulse duration during the measurement. The modulated structure has been studied for the off-stoichiometric Ni2Mn1.4In0.6 and Ni1.9Pt0.1MnGa MSM Heusler compounds from SXRPD and NPD. Ni2Mn1.4In0.6 exhibits martensitic transition at TM ~ 295 K and Curie temperature TC ~ 315 K. Rietveld refinement reveals uniform atomic displacement in the modulated structure of martensite phase and the absence of premartensite phase and phason broadening of the satellite peaks which was further confirmed by HRTEM study. Therefore, the structural modulation in Ni2Mn1.4In0.6 can be successfully explained in term of the adaptive phase model. Whereas, Ni1.9Pt0.1MnGa shows the premartensite phase in addition to the martensite and austenite phases and follows the soft phonon model. The temperature dependent ac-susceptibility shows the change in slope at different temperatures 365, 265, 230 and 220 K corresponding to the Curie temperature TC, first premartensite T1, second premartensite T2 and martensite temperature TM, respectively. Temperature-dependent high resolution SXRPD data analysis shows first, a nearly 3M modulated premartensite phase with an average cubic-like feature i.e. negligible Bain distortion of the elementary L21 unit cell results from the austenite phase. This phase then undergoes an isostructural phase transition 3M like premartensite phase with robust Bain distortion in the temperature range from 220 to 195 K. Below 195 K, the martensite phase appears which results from the larger Bain-distorted premartensite phase. In this work, the magnetocaloric properties of Ni2.2Mn0.8Ga and Ni1.8Mn1.8In0.4 magnetic shape memory (MSM) Heusler compounds were studied. Ni2.2Mn0.8Ga exhibits the reversible conventional MCE, measured from isothermal entropy change ΔSM and adiabatic temperature change ΔTad because of the geometric compatibility condition (GCC) for cubic austenite phase to tetragonal martensite phase as a consequence of low thermal hysteresis of the martensite phase transition. The reversible MCE has been confirmed by applying more than one pulse in the hysteresis region at 317 K. Ni1.8Mn1.8In0.4 possess improved reversible behavior of inverse MCE due to the closely satisfying of GCC from cubic austenite to modulated monoclinic martensite structure. The maximum value of ΔSM has been found to the same for both heating and cooling curves measured from isothermal magnetization M(T) curves until a magnetic field of 5 T. The adiabatic temperature change ΔTad results in a value of -10 K by applying a magnetic field of 20 T in a pulsed magnetic field. Furthermore, reversible magnetostriction of 0.3% was observed near the first-order martensite phase transition temperatures 265, 270 and 280 K. A reduction of thermal hysteresis has been found in MSM Heusler compounds Ni2Mn1.4In0.6 and Ni1.8Co0.2Mn1.4In0.6 with the application of hydrostatic pressure followed by GCC from pressure dependent x-ray diffraction in both austenite and martensite phase. By increasing pressure, the lattice parameters of both phases change in such a way that they increasingly satisfy the GCC. The approach of GCC for different kind of martensite structures (tetragonal, orthorhombic and monoclinic) will help to design new MSM Heusler compounds taking advantage of first-order martensite phase transition.