Tesi sul tema "Alliages Heusler"
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Tuzcuoglu, Hanife. "Corrélation entre les propriétés structurales et magnétiques des couches minces et nanostructures de Co₂FeAl". Thesis, Paris 13, 2014. http://www.theses.fr/2014PA132035/document.
ACorrelation between structural and magnetic properties of Co₂FeAl thin films and nanostructures Co₂FeAl (CFA) is a very attractive Heusler alloy for spintronic applications. Their structural and magnetic properties depend strongly on the crystalline orientations and the interfaces quality. Therefore, the aim of this thesis is the study effects of the film thickness (dCFA), the substrate (MgO, Si and SrTiO₃(STO)) as well as the annealing temperature (Ta) on these properties. The structural analysis revealed a good epitaxial growth for films deposited on MgO and STO, in contrast to the Si substrate. The chemical order varies from the partially ordered B2 phase to the disordered A2 phase as dCFA or Ta decreases, regardless of the substrate. The ferromagnetic resonance (FMR) measurements show the superposition of a uniaxial and fourfold anisotropies for films grown on MgO and STO and only a weak uniaxial anisotropy for the samples grown on Si. The fourfold anisotropy is directly correlated to the crystal structure of the samples. The FMR and Brillouin light scattering measurements reveal the presence of a large negative perpendicular uniaxial anisotropy induced by CFA/MgO interface, which increases with 1/dCFA and with Ta. The relaxation mechanisms have carefully been studied and Gilbert damping coefficients of 0.0011 have been measured making CFA as a potential candidate for spin transfer torque-based devices. Finally, the study of submicron arrays of stripe obtained by patterning of the continuous CFA films reveals a spin waves quantization due to the finite stripes width
Gabor, Mihai Sebastian. "La spintronique avec des matériaux alternatifs : alliages full-Heusler et oxydes magnétiques dilués". Thesis, Nancy 1, 2011. http://www.theses.fr/2011NAN10040/document.
The polarization of the spin current in a spintronic device is at the origin of magneto-resistive effects that define its functional properties. Typically, in order to spin-polarize a current of electrons two alternatives are used: the electron current crosses either a ferromagnetic metal or a magnetic oxide barrier. Within this thesis, we addressed both alternatives. In a first step, layers of TiO2 diluted magnetic oxide type doped with cobalt were prepared by sputtering. Their structural, morphological and magnetic properties were studied to optimize their potential for integration into a complex spin filter type structure.In a second step, we focused our efforts on the elaboration and the study of single-crystal magnetic tunnel junction based on the full-Heusler alloy Co2FeAl and the MgO (001) tunnel barrier. From a theoretical point of view, the Heusler alloys are expected to be half-metallic. This leads to a spin polarization of 100%. Following a parallel crystallographic and magnetic study of thin Co2FeAl alloy films, we demonstrated a direct correlation between structural and magnetic anisotropies. Using these layers as magnetic electrodes, we elaborated and studied UV micro-structured crystalline magnetic tunnel junctions. Our analysis demonstrates that the spin polarized tunneling and the quality of the half-metallicity of the alloy has a clear dependence on the degree of crystallinity of the system. Beyond the purely experimental aspects, this thesis includes studies of analytical, numerical and ab-initio modeling of the magnetic and electronic properties as well as for the spin polarized transport in the studied systems and devices
Guillemard, Charles. "Half-metal magnets Heusler compounds for spintronics". Electronic Thesis or Diss., Université de Lorraine, 2019. http://www.theses.fr/2019LORR0110.
Improvements in thin film elaboration methods and a deeper understanding of condensed matter physics have led to new exciting phenomena in spin electronics (spintronics). In particular, magnetization reversal by spin-orbit and spin-transfer torque as well as the development of spin waves based devices have placed the Gilbert magnetic damping coefficient as a key parameter for future data storage and information processing technologies. The prediction of ultralow magnetic damping in Co2MnZ Heusler half-metal magnets is explored in this study and the damping response is shown to be linked to the underlying electronic structure. By substitution of the Z element in high quality Co2MnZ (Z=Al, Si, Ga, Ge, Sn and Sb) epitaxial thin films grown by molecular beam epitaxy, electronic properties such as the minority-spin band gap, Fermi energy position in the band gap, and spin polarization can be tuned and the consequences for magnetization dynamics analyzed. Experimental results allow us to directly explore the interplay of spin polarization, spin gap and Fermi energy position, with the magnetic damping obtained in these films (together with predictions from ab initio calculations). The ultralow magnetic damping coefficients measured in the range from 4.1 x10-4 to 9 x10-4 for Co2MnSi, Co2MnGe, Co2MnSn and Co2MnSb are the lowest values ever reported in conductive layers and offer a clear experimental demonstration of theoretical predictions on half metal magnetic Heusler compounds. Then, the relation between the Gilbert damping and the ultrafast demagnetization time in quaternary Co2MnSixAl1-x compounds with a tunable spin polarization is analyzed. This way, it is possible to confront theoretical models unifying those two quantities that live in different timescales. Finally, structural and magnetic properties of Mn3Ga/Co2YZ Heusler superlattices are investigated in order to combine ultralow Gilbert damping coefficient, minority spin band gap and perpendicularly magnetized heterostructures, another requirement for low energy consumption devices. Through the present work, we aim to prove that Heusler compounds provide an excellent playground to study fundamental magnetism and offer a pathway for future materials design
Guillemard, Charles. "Half-metal magnets Heusler compounds for spintronics". Thesis, Université de Lorraine, 2019. http://www.theses.fr/2019LORR0110.
Improvements in thin film elaboration methods and a deeper understanding of condensed matter physics have led to new exciting phenomena in spin electronics (spintronics). In particular, magnetization reversal by spin-orbit and spin-transfer torque as well as the development of spin waves based devices have placed the Gilbert magnetic damping coefficient as a key parameter for future data storage and information processing technologies. The prediction of ultralow magnetic damping in Co2MnZ Heusler half-metal magnets is explored in this study and the damping response is shown to be linked to the underlying electronic structure. By substitution of the Z element in high quality Co2MnZ (Z=Al, Si, Ga, Ge, Sn and Sb) epitaxial thin films grown by molecular beam epitaxy, electronic properties such as the minority-spin band gap, Fermi energy position in the band gap, and spin polarization can be tuned and the consequences for magnetization dynamics analyzed. Experimental results allow us to directly explore the interplay of spin polarization, spin gap and Fermi energy position, with the magnetic damping obtained in these films (together with predictions from ab initio calculations). The ultralow magnetic damping coefficients measured in the range from 4.1 x10-4 to 9 x10-4 for Co2MnSi, Co2MnGe, Co2MnSn and Co2MnSb are the lowest values ever reported in conductive layers and offer a clear experimental demonstration of theoretical predictions on half metal magnetic Heusler compounds. Then, the relation between the Gilbert damping and the ultrafast demagnetization time in quaternary Co2MnSixAl1-x compounds with a tunable spin polarization is analyzed. This way, it is possible to confront theoretical models unifying those two quantities that live in different timescales. Finally, structural and magnetic properties of Mn3Ga/Co2YZ Heusler superlattices are investigated in order to combine ultralow Gilbert damping coefficient, minority spin band gap and perpendicularly magnetized heterostructures, another requirement for low energy consumption devices. Through the present work, we aim to prove that Heusler compounds provide an excellent playground to study fundamental magnetism and offer a pathway for future materials design
Ortiz, Hernandez Guillermo. "Élaboration et étude des propriétés physiques de couches minces monocristallines d'alliage de Heusler à faible amortissement magnétique pour composants hyperfréquences". Toulouse 3, 2013. http://thesesups.ups-tlse.fr/2045/.
This thesis fits in the framework of magnetic materials for microwave applications. The magnetic materials most commonly used in microwave devices are ferrites, especially garnets, which have a very low magnetic damping (about 0. 0001). The needs of miniaturization in current technologies bring some problems related to the integration of ferrites in microwave components. Theses materials, containing iron oxides, generally require very high deposition temperatures that may damage other parts within a same circuit. One solution lies in the use of ferromagnetic materials, which can be deposited at moderate temperatures. However, this type of material usually has high damping factors making them incompatible with the characteristics desired in microwave components. In this context, the Heusler alloys are particularly attractive. Indeed, the latter have high magnetizations and Curie temperatures, and ab initio calculations predict extremely low damping factors. The first step of this work was to develop a Ferromagnetic Resonance setup capable of studying materials in the microwave domain. This step was followed by the preparation of epitaxial thin film of Co2MnSi, by sputtering deposition, on MgO (001) and MgO/Cr (001) substrates. A thorough structural study (RHEED, X-ray diffraction, transmission electron microscopy) allowed verifying the high crystalline quality of samples. Their magnetic properties were studied by static and dynamic techniques in order to provide a complete characterization of their magnetic parameters as a function of thickness: saturation magnetization, exchange constant, damping factor, gyromagnetic ratio, anisotropy constants. The samples studied show low magnetic damping factors ranging between 0. 002 and 0. 007. The magnetic parameters (magnetocrystalline anisotropy, gyromagnetic factor) are strongly modified with the implementation of a Cr buffer layer. This suggests that the magnetic properties of Heusler alloys thin films are closely related to strains at interfaces. This work shows that Co2MnSi could constitute a building bloc for the realization of microwave components. The buffer layer engineering is set to be a key to making these alloys compatible with the needs of current technologies. This work should make possible the study of more complex systems such as magnonic crystals based on Heusler alloys
Creton, Nicolas. "Etude du comportement magnéto-mécanique des alliages à mémoire de forme de type Heusler Ni-Mn-Ga". Besançon, 2004. http://www.theses.fr/2004BESA2045.
Palin, Victor. "Heusler compounds for spin-orbitronics : exploration of topological effects and magnetic anisotropy engineering". Electronic Thesis or Diss., Université de Lorraine, 2023. http://www.theses.fr/2023LORR0031.
Over the last decades, the needs in storage capacity as shot up with computing development. The energy crisis that we are going through in the 21th century requires to develop new fundamental materials for data storage. It was with this purpose that physicist develop new ways to store information in order to reduce device’s scale, energy consumption and manufacturing cost while memories’ size and information’s speed has shot up. The research conducted in this thesis make use of two different ways to improve data storage:- The first one is by using emerging materials in science, called topological insulator, that host peculiar spin texture predicted to generate very high spin-to-charge interconversion. This non-trivial state of matter can be complex to stabilize and image. This is the goal of the first part of this thesis where topological insulators coming from the half-Heusler family are engineered by molecular beam epitaxy. Structural characterization are carried out by X-ray and electronic diffraction along with scanning tunneling microscopy and transmission electron microscopy that confirm an epitaxial growth in the desired structure predicted to host a non-trivial topology. Angle resolved photoemission spectroscopy is performed and reveals the presence of linear states around the Γ point of the Brillouin zone. Nonetheless, the complex Fermi surfaces imaged do not allow to draw clear conclusions on the non-trivial nature of both alloys. Transport measurements were performed to test the potential interconversion efficiency of our compounds and spin Seebeck experiments revealed a spin-to-charge conversion two to three times higher in our TIs compared to a Pt control sample.- The second way chosen to improve conventional magnetic memories is by playing with magnetic anisotropy. Here again, Heusler family offers a vast variety of compounds allowing to fulfill this goal. The Mn3Z family compounds has attracted a lot of attention owing to their tetragonalized unit cell that allows to stabilize perpendicular magnetic anisotropy (PMA) even in a thin film geometry. In this thesis, we investigate Mn(100-x)Ga(x) and Mn(100-x)Ge(x) alloys and manage to stabilize them in their D0(22) structure that offers PMA. A peculiar zoom is then done on Mn3Ge-based stacks composed of a second Heusler alloy with remarkable properties, the Co2MnZ’ family (Z' = Si, Ge). Co2MnZ’ compounds have a half-metallic behavior making them very suitable for spin transfer torque related applications due to their low magnetic damping and full spin polarization at the Fermi level. Here we develop Mn3Ge/Co2MnZ' heterostructures (bilayers and superlattices) and manage to grow both compounds in the desired structures. The overall system is perpendicularly magnetized (thanks to Mn3Ge), terminated with a half-metal magnet (thanks to Co2MnZ') and the thicknesses used for both layers allow to tune the magnetic properties and obtained 100% of remanence
Pradines, Barthélémy. "Structure électronique et propriétés magnétiques statiques et dynamiques d'alliages d'Heusler partiellement désordonnés et d'hétérostructures tout-Heusler". Thesis, Toulouse 3, 2017. http://www.theses.fr/2017TOU30171/document.
The Heusler alloy family contains several compounds considered to be prime candidates to be integrated as magnetic electrode into high-?performance spintronic devices. Some of these alloys indeed exhibit high Curie temperatures, have been predicted theoretically to be half-?metallic, and display a low Gilbert damping parameter. Nevertheless, the experimental results are generally less convincing than those reported numerically. The first part of this thesis is devoted to the ab initio study of hypotheses that are usually used to explain the differences between experimental and theoretical results. Calculations, based on the density functional theory, are then used to understand the impact of structural defects (partial chemical disorders, tetragonal deformation, vacancies) on the static and dynamic electronic and magnetic properties of Co-?based bulk full-Heusler alloys (Co2MnSi, Co2MnSn, Co2MnAl et Co2FeAl). In the second part of this thesis we propose to study, with the same numerical tools, the physical characteristics of "all-Heusler" heterostructures promising for spintronics and rare in the literature. We have focused on the variations of electronic properties in the vicinity of the "half-metal/insulator" (Fe2TiSi/Co2MnSi, CoTiAs/Co2MnSi) or "half-metal/non-magnetic metal" (Fe2VAl/Co2MnSi, RhNiSi/Co2MnSi) interfaces composing the studied multilayers. The obtained results highlight the interest of these structures for spintronic devices such as spin valves or magnetic tunnel junctions
Neggache, Amina. "Propriétés électroniques des alliages d'Heusler Co1.5Fe1.5Ge et Co2MnSi". Thesis, Université de Lorraine, 2014. http://www.theses.fr/2014LORR0229/document.
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
Wederni, Asma. "Synthèse, caractérisation et étude magnétique des alliages à mémoire de forme de type Heusler Ni-Mn-Sn-X (X= Pd, Cu)". Doctoral thesis, Universitat de Girona, 2021. http://hdl.handle.net/10803/673935.
Els aliatges tipus Heusler (tant estequiomètrics com no estequiomètrics) es troben entre els sistemes més estudiats, ja que permeten el canvi de les temperatures de transició a causa de les variacions de la composició. Els primers aliatges de Heusler que s’han estudiat en profunditat són els aliatges de Ni-Mn-Ga. No obstant això, per superar l’elevat cost del gal i la baixa temperatura de transformació martensítica, recentment s’ha intentat buscar aliatges sense Ga, principalment, introduint In, Sn o Sb. L’objectiu d’aquest treball és la producció i caracterització d’aliatges de Heusler no estequiomètrics a partir d’una composició de Ni-Mn-Sn, amb dopatge de cobalt o pal·ladi. L'efecte de l'element dopant es determinarà, sobre l'estructura cristal·lina, les temperatures de transició de fase, els paràmetres termodinàmics i magnetocalòrics. D’altra banda, s’exercirà un tractament tèrmic sobre els aliatges de Ni-Mn-Sn-Cu per després caracteritzar-los de la mateixa manera. Els aliatges de memòria de forma es produiran en el primer pas, mitjançant la tècnica de la fusió de l’arc, per produir l’aliatge massiu i després per la fusió per obtenir cintes de memòria de forma. Les mostres produïdes es caracteritzen per la tècnica de microscòpia electrònica d’escombratge (SEM), per comprovar l’estructura morfològica dels aliatges, la tècnica de microanàlisi (EDX) per tenir la composició final de manera experimental, la tècnica de difracció de raigs X (XRD) per extreure informació de l’estructura cristal·lina, calorimetria d’escaneig diferencial (DSC) per estudiar les variacions tèrmiques i termodinàmiques induïdes per transformacions de fase estructurals i finalment tècniques de magnetometria (PPMS: VSM, resistivitat, cicles d’histèresi) per caracteritzar el comportament de les transformacions de fase i l’efecte magnetocalòric sota camps magnètics externs aplicats. Entre els resultats obtinguts en el present treball es pot concloure que tant els aliatges dopats amb Cu com els aliatges dopats amb Pd tenen una morfologia similar. La transformació reversible d'austenita-martensita es va detectar a totes les cintes. El dopatge Cu o Pd tendeix a augmentar les temperatures de transició estructural. Aquests canvis en les temperatures de transformació magnètica i martensítica es confirmen directament a la relació (e / a) de l'aliatge. A més, s’ha destacat el fet que els aliatges dopats amb Pd mostren un bon efecte magnetocalòric. Aquesta dependència ha de permetre seleccionar la composició adequada per a la producció d'aliatges amb transformació en un rang de temperatura desitjat. D’altra banda, les mesures de DRX, DSC i magnometria realitzades sobre aliatges Ni-Mn-Sn-Cu després de l’aplicació de cicles tèrmics de calefacció-refrigeració de 100 vegades, ens van permetre certificar la gran estabilitat dels aliatges i la efecte memòria de forma
Programa de Doctorat en Tecnologia
Bandaru, Subrahmanyam. "Design of new Heusler-type thermoelectric materials : application to Fe₂VAl". Thesis, Montpellier, 2017. http://www.theses.fr/2017MONTT170/document.
The requirement of a sustainable and green energy is increasing with the rapid rise in human population and industrialization. The traditional way of utilizing fossil fuels can be replaced by thermoelectric devices which can convert thewasted heat from various sources into electrical energy. However, the present day thermoelectric devices are limited due to their low efficiency, toxic nature and high costs. The current challenge in this field is to design highly efficient thermoelectric materials which are environment friendly and available at a reasonable price. Among promising thermoelectric materials forpower generation, the Heusler-type Fe2VAl attained a great attention due to its semiconducting nature over a wide temperature range up to 1173 K. Nonetheless, the thermoelectric use of this compound is jeopardized by its high thermalconductivity. The aim of this thesis was to find new strategies in enhancing the thermoelectric efficiency of Fe2VAl with the aid of ab initio calculations and experimental studies. First principles calculations have been performed using the computer code VASP (Vienna ab-initio Simulation Package) based on the Density Functional Theory (DFT) to study the electronic structure of the full Heusler compound Fe2VAl. The formation energy of the intrinsic point defects such as vacancies, antisites and interstitials is analyzed and antisite defects are found to be the most probable defects. With the aid of the BoltzTraP code based on the Boltzmann transport theory within the constant relaxation time approach, the electronic transport properties of Fe2VAl taking into account the effect of the most favorable defects have been calculated. The presenceof defects does not lead to a significant improvement of the Seebeck coefficient. The lattice thermal conductivities of Fe2VAl, both in pristine form and in presence of its most stable antisite defect (Al V) have been analyzed by ShengBTE and the recently developed code almaBTE. A significant enhancement of the figure of merit (also known as ZT) is found with the presence of antisite defects. Nanostructured Fe2VAl compounds have been synthesized in parallel by the ball milling technique. The constituent elements have been milled together with different contents of NaCl in order to obtain porous samples. The powders have been later washed thoroughly to remove the traces of NaCl. All the powders have been consolidated using Spark Plasma Sintering (SPS). This novel idea is quite successful in achieving a porosity of around 15–20% with NaCl whereas a porosity of ~5 % is found in the case of the samples without NaCl. The thermoelectric efficiency is enhanced remarkably in the porous samples. Nevertheless, the samples milled with 15 % porosity exhibit higher ZT valuesthan the samples with 20 % porosity. Thus, it is crucial to confine and control the porosity to obtain high thermoelectric efficiencies. Our study thus clearly shows that the thermoelectric performance can be enhanced by off-stoichiometry and the modification of the morphology of the samples.Key words: Fe2VAl, materials, Heusler compounds, thermoelectricity, ab initio calculations, formation enthalpy, defects, ball milling,porosity
Abdallah, Iman. "Spin dynamics and structural modifications of Co2MnSi Heusler alloys by helium ions irradiation". Thesis, Toulouse 3, 2016. http://www.theses.fr/2016TOU30079/document.
Spintronic, which involve electron's in addition to its charge, has emerged from the discovery of Giant Magnetoresistance (GMR) by A. Fert and P. Grunberg in 1988, rewarded by a Nobel Prize in 2007. It has revolutionized the field of sensor devices. The basic mechanism of GMR and also of the Tunneling Magneto Resistance (TMR) relies on the spin polarization. Therefore there is today an intense research to find materials with both high spin polarization and low damping coefficient for the development of new generation of spintronic devices. In this field, one promising route concerns the Co2MnSi (CMS) Heusler alloy which is predicted to be half metals (i.e.100% spin polarization), with a weak Gilbert damping coefficient below 10-3, about one order of magnitude below the usual ferromagnetic material used in microelectronics. Its high Curie temperature up to 800° K also provides stability for devices working at room temperature. In this work, we study the correlations between the structural and magnetic properties of the Co2MnSi. To achieve our goal, we measure the evolution of the static and dynamic magnetic parameters of the Co2MnSi alloy in which atomic disorder is induced by He+ ion irradiation at 150 KeV. The samples are grown by magnetron sputtering on MgO substrates and then irradiated with light He+ ions. In order to correlate the structural and magnetic modifications of the alloy we combined several experimental techniques. CMS structure was investigated by X-ray diffraction and Transmission Electron Microscopy (TEM), in particular HAADF-STEM imaging mode. The evolution of the static and dynamic magnetic properties of the samples has been measured by means of Magneto Optic Kerr Effect (MOKE), Physical Properties Measurements System (PPMS) at the LPCNO laboratory in Toulouse and Ferromagnetic Resonance (FMR). The FMR set-up has been developed at the CEMES during this PhD. The main results of this work consists of correlation between the tetragonal deformations of the crystalline structure followed by the appearance of uniaxial anisotropy in the material upon irradiation. Furthermore, we demonstrate that the magnetic parameters of the B2 order are slightly affected by irradiation. But for the L21 phase, static and dynamic magnetic properties are drastically affected by irradiation, by the decrease in magnetization saturation, and exchange constant due to the Mn/Co disorder type and an increase of the cubic anisotropy and dynamic relaxation
Dufour, Hugo. "Etude des effets multicaloriques induits lors de la transformation de phase structurale dans les composés de type Heusler". Thesis, Université Grenoble Alpes, 2020. http://www.theses.fr/2020GRALY024.
This manuscript is devoted to the study of the multicaloric properties, and in particular magnetocaloric and elastocaloric properties possibly coupled between them, of Ni-Mn-X type Heusler alloys (X= In, Co-In,...). This preliminary research can quickly lead to the development of new cooling devices or new functionalities, hence the interest shown by certain players in the socio-economic world. To achieve this, we studied the structural and magnetic transformation that occurs in temperature between the high-temperature cubic phase known as « austenite » and the low-temperature phase known as « martensite ». The application of a magnetic field or a uniaxial strain shifts the transformation temperatures respectively towards low temperatures or high temperatures and also makes it possible to induce the transformation from one phase to the other. The multicaloric properties result from the near-transformation-temperature-entropy-variation due to the application of those external perturbations.A particular effort has been made to determine the non-consensual martensitic structure. However, martensite is responsible for shape memory properties and a knowledge of the structure led to the understanding of the martensitic transformation at the basis of elastocaloric properties.The originality of the study wad both on the study of elastocaloric properties and on a combination of theoretical and experimental approaches. Indeed, neutron diffraction studies have led to a better understanding of the crystallographic structures. They were coupled with experimental measurements to determine the entropy variations. Those measurements were based on the implementation of versatile measurement systems generally combining the application of uniaxial strains, temperature scanning (77K - 400K), fine temperature or transport measurements and the possible application of a magnetic field. This experimental versatility has made it possible to fully understand the elastocaloric effect of shape memory ferromagnetic alloys
KOUACOU, ABAKA MICHEL. "Apparition du ferromagnetisme itinerant dans des composes de type heusler. Relation avec des transitions isolant-metal". Université Joseph Fourier (Grenoble), 1996. http://www.theses.fr/1996GRE10087.
Ristoiu, Delia. "Epitaxie de l'alliage d'Heusler demi-métallique NiMnSb : croissance, propriétés de surface et jonctions tunnel homoépitaxiées". Université Joseph Fourier (Grenoble), 2000. http://www.theses.fr/2000GRE10119.
Zou, Naifu. "Deformation mechanisms of polycrystalline Ni-Mn-Ga alloy induced by mechanical and thermo-mechanical training". Thesis, Université de Lorraine, 2017. http://www.theses.fr/2017LORR0358.
External field training is proven to be an effective way to improve the magnetic-field-induced strain (MFIS) in Ni-Mn-Ga Heusler type alloys by eliminating the unfavorable variants. To guide the training procedure, the training mechanisms of alloys with 5M or NM martensite have been investigated, whereas those for alloys with 7M martensite are not fully clarified. In this work, the mechanisms of mechanical and thermo-mechanical training were studied by analyzing the microstructure and crystallographic orientation evolution during these processes.Firstly, microstructure and crystallographic characterizations were performed on the as-annealed Ni50Mn30Ga20 alloy. 5 colonies transformed from one parent austenite grain were observed with each colony consisting of four variants with Type-I, Type-II and compound Transformation (TrF)-twin relations. By assuming an applied compressive load along the solidification direction (SD), 5 colonies could be divided into two groups with respect of the Schmid factor (SF) of detwinning systems of Type-I/Type-II TrF-twin of the in-colony variants: three of them have high SF and referred to as high SF colonies and the other two low SF colonies.Then unidirectional compression was performed on the alloy with the load applied along the SD. By characterizing the microstructure evolution and crystallographic orientation change, the deformation mechanisms were analyzed. The deformation in the early stage was mainly located in some band regions initiated from the high SF colonies and going through the low SF colonies. The detwinning of Type-II/Type-I TrF-twin occurred primarily in high SF colonies, resulting in the thickening of the favorable 7M variants at the expense of the adjacent variants. The twinning of Type-I/Type-II Deformation (DeF)-twin and shuffling systems of the variants in low SF colonies were activated, leading to the formation of new 7M variants and NM. The corresponding strains in the low SF colonies were highly coordinated with those in the high SF colonies allowing the formation of the deformation bands and the accommodation of the macroscopic strain. During the late stage, twinning of Type-I/Type-II DeF-twin and shuffling further progressed to coordinate the macroscopic strain. Reverse shuffling process was activated to accommodate the local deformation. The numbers of colony and variant were greatly reduced.The path and the product of martensitic transformation were also strongly affected by the imposed macroscopic deformation. Under a small load, austenite transformed to 5M martensite following both the Pitsch and a new OR rather than the self-accommodated 7M martensite under the Pitsch OR. With the increase of the applied load, austenite transformed almost simultaneously to 7M martensite under a new OR and 5M martensite. After the martensitic transformation, 5M further transformed to 7M martensite with the decrease of the temperature under the applied load. The martensitic transformation was modified by the external stress in terms of the transformation product and the transformation strain path to accommodate the imposed macroscopic deformation.This work offers new insights into the deformation mechanisms of the Ni-Mn-Ga alloys under unidirectional compression that are useful for the design of effective training procedures and provides new perspectives on further investigations of external field training on Ni-Mn-Ga alloys