Academic literature on the topic 'Ni-Mn-Ga Thin Films'

The Ni-Mn-Fe-Ga shape memory alloy thin film was deposited onto silicon substrates by using radio-frequency (R.F.) magnetron sputtering technique. Chemical composition, surface morphology and crystallographic structure were systematically investigated by means of X-ray fluorescence (XRF), atomic force microscope (AFM) and X-ray diffraction (XRD). The experimental results show that the magnetron sputtering process has remarkable influence on the chemical compositions and surface characteristics of Ni-Mn-Fe-Ga alloy thin films. As the sputtering power ranging between 245W and 405W, Ni content of the thin films decreases with the sputtering power increasing, whereas Mn and Fe contents increase with increasing the sputtering power and Ga content almost keep a constant. The surface roughness and the average particle size of thin films increase with the increase of Ar working pressure and sputtering power. The film deposited at room temperature has a cubic L21 structure.

We report magneto-optical spectra of the Heusler bulk alloys Ni-Mn-In, thin films Ni-Mn-Ga, microwires Ni-Mn-In and Ni-Mn-Ga in martensitic and austenitic states. Transversal Kerr effect (TKE) was studied at an angle of light incidence of 68° with respect to the sample plane, in the energy range 0.5 eV < E < 4.0 eV, at 50 350 K temperatures, and in magnetic fields up to 3.5 kOe. The TKE spectra profile does not change too much at martensitic transformation in Ni2MnGa thin films, only magnitudes of characteristic maxima decrease. The magneto-optical response of Ni2MnGa microwires is very similar to that for Ni2MnGa thin films. For most of studied bulk samples, the TKE signal is very weak (about 10-5), about two orders of magnitude smaller than for thin films, and in many cases could not be detected at all. It indicates the strong dependence of the magneto-optical response of Heusler alloys on the quality of optically or electrochemically polished surfaces and their microstructure.

Shape memory alloys (SMAs) thin films have attracted much attention in recent years as intelligent and functional materials because of their unique properties. Ferromagnetic shape memory alloys (FSMAs) show large straining output, high impetus and short response time induced by the magnetic field, compared with traditional shape memory alloys. In this paper, Ni-Mn-Ga ferromagnetic shape memory alloys flims prepared by magnetron sputtering are introduced, and the research direction of Ni-Mn-Ga films is presented.

Magnetische Formgedächtnislegierungen sind Festkörper, die eine Phasenumwandlung erster Ordnung von einer hochsymmetrischen Phase (Austenit) zu einer niedersymmetrischen Phase (Martensit) durchlaufen. Dies kann in der Nähe von Raumtemperatur stattfinden und sowohl durch Temperaturänderung, als auch durch äußere Magnetfelder, mechanische Spannungen oder hydrostatischen Druck induziert werden. Daraus ergeben sich funktionale Eigenschaften, wie der magnetokalorische und der elastokalorische Effekt, eine magnetfeldinduzierte Dehnung und ein großer Magnetowiderstand. Zwillingsgrenzen im Martensit können durch äußere Magnetfelder bewegt werden, was zu großen reversiblen Längenänderungen führt. Der Ablauf der Phasenumwandlung und das Gefüge des Martensits werden dabei durch die elastischen Randbedingungen an der Phasengrenze bestimmt. In dieser Arbeit werden deshalb die Nukleation und das Wachstum des Martensits untersucht. Als Modellsystem werden epitaktische Schichten der Heuslerlegierung Ni-Mn-Ga verwendet. In der martensitischen Phase weist diese Legierung eine modulierte Kristallstruktur auf, die im Konzept des adaptiven Martensits durch eine Verzwillingung auf der atomaren Skala interpretiert werden kann. Im ersten Teil wird mit Röntgenbeugung die modulierte Struktur untersucht. Die Intensität der Überstrukturreflexe wird mit einer kinematischen Beugungssimulation verglichen. Dabei wird nachgewiesen, dass es sich um ein nanoverzwillingtes Gefüge mit einer hohen Dichte an Stapelfehlern handelt. Im zweiten Teil wird das martensitische Gefüge mit Elektronenbeugung im Rasterelektronenmikroskop und Texturmessungen durch Röntgenbeugung untersucht. Das martensitische Gefüge kann im Rahmen der phänomenologischen Martensittheorie quantitativ erklärt werden. Daraus ergibt sich ein geometrisches Modell des martensitischen Nukleus und seiner Wachstumsstadien. Die Phasenumwandlung wird temperaturabhängig im Elektronen- und im Atomkraftmikroskop untersucht und mit dem geometrischen Modell verglichen. Die begrenzte Gültigkeit des geometrischen Modells an makroskopischen Zwillingsgrenzen und an der Grenzfläche zum Schichtsubstrat werden diskutiert. Schließlich kann die Bildung des gesamten hierarchischen Zwillingsgefüges erklärt werden. Im dritten Teil wird die Energiebarriere der Nukleation untersucht. Da die Umwandlung bei konstanter Temperatur abläuft, wird geschlussfolgert, dass Autonukleationsprozesse zu einer starken Verringerung der Nukleationsbarrieren führen. Schließlich wird gezeigt, dass durch Nanoindentation die Nukleation gezielt beeinflusst werden kann
Magnetic shape memory alloys are solids that undergo a first order phase transition from a high symmetry phase (austenite) into a low symmetry phase (martensite). This can happen close to room temperature and can be induced by changes of temperature, external magnetic fields, mechanical stresses or hydrostatic pressure. This leads to functional properties like the magnetocaloric and elastocaloric effect, a magnetic-field-induced strain and giant magnetoresistance. Twin boundaries in the martensite can be moved by external magnetic fields, which leads to giant reversible length changes. The process of the phase transition and the microstructure of martensite are determined by the elastic boundary conditions at the phase interface. In this work, nucleation and growth of the martensite are studied. Epitaxial films of the Heusler alloy Ni-Mn-Ga are used as a model system. This alloy exhibits a modulated crystal structure which is interpreted as twinning on the atomic scale in the framework of adaptive martensite. In the first part, the modulated structure is studied by X-ray diffraction. The intensity of the superstructure is compared to a kinematic diffraction simulation and it is shown that it is a nanotwinned microstructure with a high density of stacking faults. In the seond part, the martensitic microstructure is studied by electron diffraction in the scanning electron microscope and by texture measurements using X-ray diffraction. The martensitic microstructure can be explained quantitatively in the framework of the phenomenological theory of martensite. This leads to a geometrical model of the martensitic nucleus and its growth stages. The phase transformation is studied as a function of temperature in the scanning electron microscope and atomic force microscope and is compared to the geometric model. The limits of the geometrical model at macroscopic twin boundaries and at interfaces to the substrate are discussed. Finally, the formation of the entire twin microstructure can be explained. In the third part, the energy barrier of nucleation is studied. The transformation is isothermal which leads to the conclusion that autonucleation processes decrease the nucleation barrier significantly. Finally, the influence of nanoindentation on the nucleation is shown

Les couches minces épitaxiales de Ni-Mn-Ga ont attiré une attention considérable, car ils sont des candidats prometteurs pour les capteurs et actionneurs magnétiques dans des microsystèmes électromécaniques. Des informations complètes sur les caractéristiques de la microstructure et de la cristallographie des films NiMnGa et leur relation avec les contraintes du substrat sont essentielles à l'optimisation des propriétés. Dans le présent travail, les couches minces épitaxiale de Ni-Mn-Ga ont été produites par pulvérisation cathodique magnétron à courant continu et ensuite caractérisées par la technique de diffraction des rayons X (XRD) et la diffraction d'électrons rétrodiffusés dans un microscope électronique à balayage équipé d’analyse EBSD (MEB-EBSD). Des couches minces épitaxiales avec NiMnGa de composition nominale Ni50Mn30Ga20 et d'épaisseur 1,5 µm ont été fabriquées avec succès sur le substrat monocristallin de MgO par pulvérisation cathodique magnétron DC, après l'optimisation des paramètres tels que la puissance de pulvérisation cathodique, la température du substrat et de la couche d'ensemencement dans le cadre du présent travail. Les mesures de diffraction DRX montrent que les couches minces épitaxiales NiMnGa sont composées de trois phases: austénite, martensite NM et martensite modulée 7M. Avec les géométries de mesure optimisées, le nombre maximum possible de pics de diffraction des phases relatives, en particulier compte tenu de la basse symétrie de la martensite 7M, sont acquis et analysés. Les constantes de réseau de l'ensemble des trois phases dans le cadre des contraintes du substrat dans les films sont entièrement déterminées. L’analyse SEM-EBSD en profondeur du film a permis en outre de vérifier la situation de coexistence de trois phases constitutives: austénite, 7M martensite et martensite NM. La martensite NM se trouve près de la surface libre du film, l'austénite au-dessus de la surface du substrat, et la martensite 7M dans les couches intermédiaires entre l'austénite et la martensite NM. La caractérisation de microstructure montre que la martensite 7M et la martensite NM ont une morphologie de plaque et sont organisées en deux zones caractéristiques décrites avec des bas et haut contraste en images d’électrons secondaires. Des plaques de martensite locales similaire en orientation morphologique sont organisées en groupes de plaques ou colonies ou variantes de colonies. Une caractérisation plus poussée en EBSD indique qu'il existe quatre plaques de martensite distinctes dans chaque colonie de variante à la fois pour la martensite NM et 7M. Chaque plaque de martensite NM est composée de variantes lamellaires majeures et mineures en termes d’épaisseurs appariées et ayant une interface interlamellaire cohérente, alors que chaque plaque de martensite 7M contient une variante d'orientation. Ainsi, il existe quatre variantes d'orientation de martensite 7M et huit variantes d’orientation de martensite NM dans une colonie de variantes. Selon l'orientation cristallographique des martensites et des calculs cristallographiques, pour la martensite NM, les interfaces inter-plaques sont constituées de macles de type composées dans des plaques adjacentes de martensite NM. La distribution symétrique des macles composées résulte dans des interfaces de plaques longues et droites dans la zone de contraste relatif faible. La répartition asymétrique conduit à la modification de l’orientation d'interface entre les plaques de la zone de contraste relativement élevé. Pour la martensite 7M, à la fois les interfaces de type I et de type II sont à peu près perpendiculaires à la surface du substrat dans les zones à faible contraste relatif. Les paires de macles de type-I apparaissent avec une fréquence beaucoup plus élevée, par comparaison avec celle des macles de type II. Cependant, il y a deux traces d’interface de macles de type II et une trace d’interface de macles de type I dans les zones de contraste relatifs élevés. [...]
Epitaxial Ni-Mn-Ga thin films have attracted considerable attention, since they are promising candidates for magnetic sensors and actuators in micro-electro-mechanical systems. Comprehensive information on the microstructural and crystallographic features of the NiMnGa films and their relationship with the constraints of the substrate is essential for further property optimization. In the present work, epitaxial Ni-Mn-Ga thin films were produced by DC magnetron sputtering and then characterized by x-ray diffraction technique (XRD) and backscatter electron diffraction equipped in scanning electron microscope (SEM-EBSD). Epitaxial NiMnGa thin films with nominal composition of Ni50Mn30Ga20 and thickness of 1.5 µm were successfully fabricated on MgO monocrystalline substrate by DC magnetron sputtering, after the optimization of sputtering parameters such as sputtering power, substrate temperature and seed layer by the present work. XRD diffraction measurements demonstrate that the epitaxial NiMnGa thin films are composed of three phases: austenite, NM martensite and 7M martensite. With the optimized measurement geometries, maximum number of diffraction peaks of the concerning phases, especially of the low symmetrical 7M martensite, are acquired and analyzed. The lattice constants of all the three phases under the constraints of the substrate in the films are fully determined. These serve as prerequisites for the subsequent EBSD crystallographic orientation characterizations. SEM-EBSD in film depth analyses further verified the co-existence situation of the three constituent phases: austenite, 7M martensite and NM martensite. NM martensite is located near the free surface of the film, austenite above the substrate surface, and 7M martensite in the intermediate layers between austenite and NM martensite. Microstructure characterization shows that both the 7M martensite and NM martensite are of plate morphology and organized into two characteristic zones featured with low and high relative second electron image contrast. Local martensite plates with similar plate morphology orientation are organized into plate groups or groups or variant colonies. Further EBSD characterization indicates that there are four distinct martensite plates in each variant groups for both NM and 7M martensite. Each NM martensite plate is composed of paired major and minor lamellar variants in terms of their thicknesses having a coherent interlamellar interface, whereas, each 7M martensite plate contains one orientation variant. Thus, there are four orientation 7M martensite variants and eight orientation NM martensite variants in one variant group. According to the crystallographic orientation of martensites and the crystallographic calculation, for NM martensite, the inter-plate interfaces are composed of compound twins in adjacent NM plates. The symmetrically distribution of compound twins results in the long and straight plate interfaces in the low relative contrast zone. The asymmetrically distribution leads to the change of inter-plate interface orientation in the high relative contrast zone. For 7M martensite, both Type-I and Type-II twin interfaces are nearly perpendicular to the substrate surface in the low relative contrast zones. The Type-I twin pairs appear with much higher frequency, as compared with that of the Type-II twin pairs. However, there are two Type-II twin interface trace orientations and one Type-I twin interface trace orientation in the high relative contrast zones. The Type-II twin pairs are more frequent than the Type-I twin pairs. The inconsistent occurrences of the different types of twins in different zones are originated from the substrate constrain. The crystallographic calculation also indicates that the martensitic transformation sequence is from Austenite to 7M martensite and then transform into NM martensite (A→7M→NM). [...]

Um den ständig steigenden Energiebedarf durch Kälteanlagen wie Kühlschränke oder Klimaanlagen zu verringern, sind in den vergangenen Jahren Kühlprozesse in den Mittelpunkt aktueller Forschungen gerückt, die auf Phasenumwandlungen in Festkörpern beruhen. Ein Beispiel dafür sind magnetokalorische Materialien, zu denen auch das in der vorliegenden Arbeit untersuchte Ni-Mn-Ga-Co gehört. In dieser Heusler-Legierung tritt eine Phasenumwandlung erster Ordnung von einer ferromagnetischen, kubischen Hochtemperaturphase (Austenit) in eine tetragonal verzerrte Tieftemperaturphase (Martensit) mit geringerer Magnetisierung auf. Der Unterschied in den Magnetisierungen beider Phasen erlaubt es auch, diese Phasenumwandlung durch ein Magnetfeld zu induzieren. Hierbei kühlt sich das Material durch eine Verringerung der Gitterentropie in dem System ab. Ein Nachteil von Phasenumwandlungen erster Ordnung ist die damit verbundene Hysterese. Außerdem lässt sich der magnetokalorische Effekt durch die scharfe Umwandlung nur in einem kleinen Temperaturbereich effektiv nutzen. Das Ziel dieser Arbeit besteht darin, anhand epitaktisch gewachsener Ni-Mn-Ga-Co-Schichten auf PMN-PT-Substraten zu untersuchen, ob und wie die Umwandlungstemperatur und damit auch die Hysterese der Heusler-Legierung durch mechanische Spannung beeinflusst werden kann. Dafür soll durch Anlegen eines elektrischen Feldes an das piezoelektrische Substrat die Ni-Mn-Ga-Co-Schicht reversibel mechanisch verspannt und die daraus resultierenden Veränderungen der strukturellen und magnetischen Eigenschaften untersucht werden. Im ersten Ergebnisteil wird zunächst gezeigt, dass epitaktische Ni-Mn-Ga-Co-Schichten auf PMN-PT wachsen können und diese einen strukturellen und magnetischen Phasenübergang zeigen. Eine Beeinflussung der bei Raumtemperatur vorliegenden Phase ist dabei über eine Variation der chemischen Zusammensetzung der Probe möglich. Im Anschluss werden die Auswirkungen eines angelegten elektrischen Feldes auf die strukturellen und magnetischen Eigenschaften analysiert. Röntgenuntersuchungen zeigen, dass die piezoelektrische Dehnung des Substrats vollständig auf das Ni-Mn-Ga-Co übertragen werden kann. Allerdings treten bei hohen Temperaturen aufgrund einer Phasenumwandlung im PMN-PT nichtlineare Dehnungseffekte auf. Eine Veränderung der Umwandlungstemperaturen durch die Dehnung des Ni-Mn-Ga-Co ist jedoch nicht möglich. Als wahrscheinliche Ursache dafür wird eine Besonderheit des martensitischen Gefüges der Ni-Mn-Ga-Co-Schichten diskutiert. Im Austenit wurde jedoch eine vollständig reversible Änderung der Magnetisierung um bis zu 7 % gemessen. Diese Magnetisierungsänderung bietet einen interessanten Anknüpfungspunkt für weitergehende Untersuchungen dieses Systems für multikalorische Anwendungen.

Mestrado em Ciência e Engenharia de Materiais
Ligas de forma ferromagnética em sistemas Ni-Mn-Ga são uma classe recente de materiais activos que podem gerar deformações de até 10% induzidas por um campo magnético por um rearranjo de maclas. Esta e outras propriedades físicas destas ligas têm importância tecnológica. Este trabalho investiga as propriedades de filmes finos de ligas de Ni-Mn-Ga sobre diferentes substratos, incluindo substratos activos (piezeléctricos). Para estudar as propriedades de filmes finos da liga, heteroestruturas sob a forma de Ni-Mn-Ga/substrato foram produzidas por RF sputtering com magnetrão utilizando temperaturas de deposição de 3200C, 3700C, 4000C sobre substratos de Al2O3, MgO, SrTiO3 e PMN-PT. A influência da temperatura do substrato durante a deposição nas propriedades estruturais e magnéticas de filmes finos foi estudada. Além disso, o acoplamento magnetoeléctrico entre Ni-Mn-Ga como filme fino material ferromagnético e PMN-PT como material piezoeléctrico foi investigada. O efeito magnetoeléctrico foi investigado apenas em filmes depositados a temperatura do substrato de 3700C e 4000C. As propriedades estruturais foram estudadas por difração de raios-X, as propriedades magnéticas foram investigadas por VSM, SQUID, e MFM, e o efeito magnetoeléctrico foi estudado por técnica lock-in. A medida estrutural mostrou que os filmes depositados são parcialmente cristalinos e o grau de cristalinidade aumenta como o aumento da temperatura do substrato. Fases austenita e martensita foram observadas nesses filmes. Os resultados da medição magnética mostram que todos os filmes depositados exibem comportamento ferromagnético e o comportamento ferromagnético é favorecido com o aumento da temperatura do substrato. Todos os filmes depositados na temperatura do substrato de 400ºC apresentam temperaturas dev Curie acima da temperatura ambiente: 337K para Ni-Mn- Ga/PMN-PT, 345K para Ni-Mn-Ga/STO e 348K para Ni-Mn-Ga/Al2O3. Nenhuma evidência separada de temperatura de transição estrutural foi observada para nos filmes. Os resultados das medições magnetoeléctricas mostram que as heteroestruturas multiferróicas Ni-Mn-Ga/PMN-PT apresentam efeito magnetoelétrico. O valor máximo medido para a tensão magnetoeléctrica induzida para filmes depositados à temperatura do substrato de 3700C e 4000C são 3.16mV/cmOe e 3.02mV/cmOe, respectivamente.
Ferromagnetic shape memory alloys (FSMAs) in Ni-Mn-Ga systems are a recent class of active materials that can generate large magnetic field induced strains up to 10% by twin rearrangement. This and other physical properties these alloys have many technological importance. This work investigates the properties of Ni-Mn-Ga alloy thin films on different substrates including active substrate (piezoelectric). To study the properties of thin films of the alloy, the heterostructures in the form of Ni-Mn-Ga/substrate were produced by RF magnetron deposition system using substrate deposition temperatures of 3200C, 3700C, and 4000C, where the substrates used were Al2O3, MgO, SrTiO3 and PMN-PT. The influences of deposition substrate temperature on structural and magnetic properties of sputtered thin films on the aforementioned substrates were studied. Moreover, magnetoelectric coupling between Ni-Mn- Ga thin film as ferromagnetic material and PMN-PT as piezoelectric material was investigated. The magnetoelectric effect was investigated only on films deposited at substrate temperature of 3700C and 4000C. The structural properties were studied by x-ray diffraction, magnetic properties were investigated by VSM, SQUID, and MFM, and the magnetoelectric effect was studied by lock-in technique. The structural measurement has shown that asdeposited films are partially crystalline and degree of crystallinity increases as substrate temperature increase. Austenite and martensite phases have been observed in these films. The magnetic measurement results show that all films as-deposited display ferromagnetic behaviour and ferromagnetic behaviour improvements are observed as substrate temperature increases. All films deposited at substrate temperature of 4000C exhibit Curie temperatures above room temperature which are 337K for Ni-Mn-Ga/PMN-PT, 345K for Ni-Mn- Ga/STO, 348K for Ni-Mn-Ga/Al2O3. No separate signature of structural transition temperature was observed for all these films. The magnetoelectric measurement results show that a heterostructure of Ni-Mn-Ga/PMN-PT multiferroic exhibit magnetoelectric effect. The measured maximum induced magnetoelectric voltage for films deposited at substrate temperature of 3700C and 4000C are 3.16mV/cmOe and 3.02mV/cmOe, respectively.

碩士
東海大學
物理學系
101
Ni-Mn-Ga alloy is a multifunctional smart material, which has various features, such as: shape memory effect, magnetostriction, magnetocaloric effect, exchange bias and so on. Among them, the most striking of which include the magnetocaloric and magnetostrictive effects. In this study, structure, magnetic, magnetocaloric, and magnetostrictive properties of Ni50MnxGa50-x (x = 26-32) alloy ingots and films have been investigated. For Ni50MnxGa50-x bulks, Ni50Mn26Ga24 and Ni50Mn28Ga22 alloys show a ferromagnetic martensite phase at low temperature. When temperature is rised, the ferromagnetic martensite phase transforms into the ferromagnetic austenite phase, and then into paramagnetic austenite phase at higher temperature. However, Mn content is increased to 30-32 at%, only transformation of ferromagnetic into paramagnetic phase is observed. Besides, magnetization of the alloys is decreased and thus the maximum value of the magnetic entropy is decreased with increasing Mn content. Their values are ΔSM = 1.1 J/kgK for x = 26, 1.4 J/kgK for x = 28, 0.74 J/kgK for x = 30 and 0.4 J/kgK for x = 32, respectively. On the other hand, structure of Ni50MnxGa50-x films prepared by PLD at room temperature then followed by various post-annealing temperature can be divided into three regions: 1. A (austenite) phase: lower annealing temperature and low Mn content is A; 2. M (martensite) phase: low annealing temperature and the high Mn content; 3. A+M phases: high annealing temperature and high Mn content. Phase constitution area is not distinct for the thickness region between 300-500 nm. The maximum magnetic entropy change is increased with increasing Mn content, which might be due to increased volume fraction of M phase with higer magnetization. Large strain of 5054 ppm is obtained for Ni50Mn30Ga20 film at applied magnetic field of 2 kOe. Two possible reasons are proposed for such a large strain value: the magnetostrictive effect of the films induced and also nonuniformly applied magnetic field may contribute.

Shape memory alloys are the promising candidate materials for sensing and actuator-based applications. Amongst a variety of shape memory alloys, NiMnGa based magnetic shape memory alloys are technologically most important owing to their superior properties such as large recoverable strain, low field actuation and fast response. In the present study, a systematic study has been performed to understand the effect of growth conditions and substrate selection on the texture, microstructure and magnetic properties of sputter deposited NiMnGa thin films. The film deposited in Zone T condition (500 °C) on Silicon (100) are polycrystalline with preferred out of plane orientation. A gradual transition of crystallographic texture from (220) to (400) fiber with increase in sputtering power has been observed and is correlated with the minimization of surface and strain energies in the films. The higher grain size and better packing density in the film deposited at higher sputtering power have led to superior magnetic properties in terms of lower value saturation field and coercivity. The films deposited under Zone 2 condition (650 °C) on silicon (100) substrate showed the development of a bi-axial texture with both out-of-plane and in-plane preferred orientation. In-depth structural investigations reveal excellent crystal quality in these films with rocking curve measurements. The cube on cube orientation relationship films and the substrate further confirms the bi-axial alignment of unit cell due to imperfect epitaxial growth of the film. Three levels of microstructural hierarchy and formation of epitaxial Ni-Si at the interface are some of the key results from high resolution transmission electron microscopy studies. For the first time, a four-step phase transformation sequence ‘austenite → pre-martensite → martensite → intermediate martensite’ has been observed by temperature dependent magnetization and in-situ high/low temperature X-ray diffraction studies. NiMnGa films deposited on Al2O3 (112 ̅0) substrate under Zone T condition shows bi-axially oriented film with multiple variants. The TEM investigations reveals the presence of secondary nano-twins formed by adaptive modulation within the primary martensitic twins. The presence of six satellite spots within the main diffraction spots confirms the presence of seven modulated orthorhombic martensite phase in the as-deposited film. Overall development of bi-axial texture and epitaxial like growth in the films has been attributed to optimum growth conditions and suitable selection of substrate with low lattice mismatch with the film (0.7%). The last part of the work is dedicated to the epitaxially grown films on Al2O3 (112 ̅0) and MgO (100) substrates under Zone 2 condition. The minimization of lattice strain due to lower lattice mismatch between film and substrate and higher mobility of ad-atoms led to the development of such high-quality films. The coexistence of non-modulated and seven modulated martensite phases observed with TEM imaging provides a clear evidence of adaptive modulation of martensite in the as-deposited film. The magnetic field induced reorientation of martensitic variants (MIR) has been observed as an abrupt change in the slope of magnetic hysteresis.

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)....

Um den ständig steigenden Energiebedarf durch Kälteanlagen wie Kühlschränke oder Klimaanlagen zu verringern, sind in den vergangenen Jahren Kühlprozesse in den Mittelpunkt aktueller Forschungen gerückt, die auf Phasenumwandlungen in Festkörpern beruhen. Ein Beispiel dafür sind magnetokalorische Materialien, zu denen auch das in der vorliegenden Arbeit untersuchte Ni-Mn-Ga-Co gehört. In dieser Heusler-Legierung tritt eine Phasenumwandlung erster Ordnung von einer ferromagnetischen, kubischen Hochtemperaturphase (Austenit) in eine tetragonal verzerrte Tieftemperaturphase (Martensit) mit geringerer Magnetisierung auf. Der Unterschied in den Magnetisierungen beider Phasen erlaubt es auch, diese Phasenumwandlung durch ein Magnetfeld zu induzieren. Hierbei kühlt sich das Material durch eine Verringerung der Gitterentropie in dem System ab. Ein Nachteil von Phasenumwandlungen erster Ordnung ist die damit verbundene Hysterese. Außerdem lässt sich der magnetokalorische Effekt durch die scharfe Umwandlung nur in einem kleinen Temperaturbereich effektiv nutzen. Das Ziel dieser Arbeit besteht darin, anhand epitaktisch gewachsener Ni-Mn-Ga-Co-Schichten auf PMN-PT-Substraten zu untersuchen, ob und wie die Umwandlungstemperatur und damit auch die Hysterese der Heusler-Legierung durch mechanische Spannung beeinflusst werden kann. Dafür soll durch Anlegen eines elektrischen Feldes an das piezoelektrische Substrat die Ni-Mn-Ga-Co-Schicht reversibel mechanisch verspannt und die daraus resultierenden Veränderungen der strukturellen und magnetischen Eigenschaften untersucht werden. Im ersten Ergebnisteil wird zunächst gezeigt, dass epitaktische Ni-Mn-Ga-Co-Schichten auf PMN-PT wachsen können und diese einen strukturellen und magnetischen Phasenübergang zeigen. Eine Beeinflussung der bei Raumtemperatur vorliegenden Phase ist dabei über eine Variation der chemischen Zusammensetzung der Probe möglich. Im Anschluss werden die Auswirkungen eines angelegten elektrischen Feldes auf die strukturellen und magnetischen Eigenschaften analysiert. Röntgenuntersuchungen zeigen, dass die piezoelektrische Dehnung des Substrats vollständig auf das Ni-Mn-Ga-Co übertragen werden kann. Allerdings treten bei hohen Temperaturen aufgrund einer Phasenumwandlung im PMN-PT nichtlineare Dehnungseffekte auf. Eine Veränderung der Umwandlungstemperaturen durch die Dehnung des Ni-Mn-Ga-Co ist jedoch nicht möglich. Als wahrscheinliche Ursache dafür wird eine Besonderheit des martensitischen Gefüges der Ni-Mn-Ga-Co-Schichten diskutiert. Im Austenit wurde jedoch eine vollständig reversible Änderung der Magnetisierung um bis zu 7 % gemessen. Diese Magnetisierungsänderung bietet einen interessanten Anknüpfungspunkt für weitergehende Untersuchungen dieses Systems für multikalorische Anwendungen.

In this chapter, results of our recent investigations on the structural, microstructural and magnetic properties of Cu-based Heusler alloys and MFe2O4 (M = Mn, Fe, Co, Ni, Cu, Zn) nanostructures will be discussed. The chapter is divided into two parts, the first part describes growth and different characterizations of Heusler alloys while in the second part magnetic properties of nano-ferrites are discussed. The Cu50Mn25Al25-xGax (x = 0, 2, 4, 8 and 10 at %) alloys have been synthesized in the form of ribbons. The alloys with x ≤ 8 show the formation of Heusler single phase of the Cu2MnAl structure. Further increase of Ga content gives rise to the formation of γ-Cu9Al4 type phase together with Cu2MnAl Heusler phase. The alloys are ferromagnetically ordered and the saturation magnetization (Ms) decreases slightly with increasing Ga concentration. Annealing of the ribbons significantly changes the magnetic properties of Cu50Mn25Al25-xGax alloys. The splitting in the zero field cooled (ZFC) and field cooled (FC) magnetization curves at low temperature has been observed for alloys. Another important class of material is Nanoferrites. The structural and magnetization behaviour of spinel MFe2O4 nanoferrites are quite different from that of bulk ferrites. X-ray diffraction study revealed spinel structure of MFe2O4 nanoparticles. The observed ferromagnetic behaviour of MFe2O4 depends on the nanostructural shape as well as ferrite inversion degree. The magnetic interactions in Ce doped CoFe2O4 are antiferromagnetic that was confirmed by zero field/field cooling measurements at 100 Oe. Log R (Ω) response measurement of MgFe2O4 thin film was taken for 10–90% relative humidity (% RH) change at 300 K.

This paper presents an investigation of the intrinsic magnetoresistance of a ferromagnetic shape memory alloy (FSMA) microactuator for position sensing. The microactuator is designed as a double-beam cantilever of a polycrystalline Ni-Mn-Ga thin film, which exhibits both, a martensitic transformation in the temperature range 333–359 K and a ferromagnetic transition at about 370 K. The microactuator is placed in the inhomogeneous magnetic field of a miniature Nd-Fe-B magnet causing a mixed thermo-magneto-resistance effect upon actuation. The maximum in-plane magnetic field is about 0.38 Tesla. In this case, the maximum magnetoresistance (MR) is 0.19%.