Academic literature on the topic 'NiMnGa Thin Films'

In this work, NiMnGa thin film composed of non-modulated martensite (NM) and seven-layered modulated martensite (7M) was produced. The crystal structure and lattice constants were determined by X-ray diffractometer (XRD). The preferred crystallographic orientation of martensite was determined using the four-circle XRD. SEM/EBSD was employed to verify the crystal structure of the martensite and to reveal its crystallographic features correlated with the microstructure. According to the XRD patterns, the crystal structure of NM and 7M was determined as tetragonal and monoclinic crystal structure, respectively. Pole figures measured by four-circle diffractometer revealed that the NM martensite possesses (004)NM and (220)NM preferred plane texture close to the substrate surface, whereas the 7M martensite has (2 0 20)7M, (2 0 )7M and (040)7M preferred plane texture close to the substrate surface. SEM/EBSD analysis shows that the surface layer of the film is mainly composed of NM martensite that is organized in variant groups. In each variant group, all the martensite plates consist of paired lamellar (112)NM compound twins and there are eight orientation variants in each variant group.

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). [...]

We report the fabrication of Ni2.7Mn0.9Ga0.4/InGaAs bilayers on GaAs (001)/InGaAs substrates by molecular beam epitaxy. To form freestanding microtubes the bilayers have been released from the substrate by strain engineering. Microtubes with up to three windings have been successfully realized by tailoring the size and strain of the bilayer. The structure and magnetic properties of both, the initial films and the rolled-up microtubes, are investigated by electron microscopy, X-ray techniques and magnetization measurements. A tetragonal lattice with c/a = 2.03 (film) and c/a = 2.01 (tube) is identified for the Ni2.7Mn0.9Ga0.4 alloy. Furthermore, a significant influence of the cylindrical geometry and strain relaxation induced by roll-up on the magnetic properties of the tube is found
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The structural and magnetic properties of nickel manganite (NiMn(_2)O(_4)) have been investigated, for bulk, and thin film samples. NiMn(_2)O(_4) has partially inverted spinel structure, Mn(_v)Ni(_1-v)[Ni(_v)Mn(_2-v]O(_4), where v is the inversion parameter. Bulk samples were produced from co-precipitated metal hydroxides at various firing times and temperatures. Particular attention was given to determining the optimum preparation route. Nickel oxide was the major impurity encountered due to sub-optimal preparation conditions, but was difficult to detect using diffraction, due to considerable Bragg reflection overlap with those NiMn(_2)O(_4). Nickel oxide is believed to have been present in most samples studied by other researchers in the field. Pure material formed in air after firing for 48 hours in the region 780 C to 820 C; a much smaller range than previously reported. By controlling the cooling rate after firing, 0.7483(19) ≤ v ≤ 0.8830(22) was obtained; as determined by neutron diffraction measurements. Ferrimagnetic Curie temperatures (T(_c)) between 100 K and 147 K were obtained for the range of v studied; somewhat lower than previously reported. The magnetization below T(_c) exhibits P-type behaviour, which has hitherto not been observed in this compound. Evidence compatible with a local canted state at temperatures below -10 K was observed using muon spectrometry. The magnetic properties of electron-beam evaporated thin films of NiMn(_2)O(_4) were investigated with a custom built Alternating Gradient Field Magnetometer. The AGFM was initially constructed for a study of Dilute Magnetic Semiconductor (DMS) materials, and was capable of temperatures down to 77 K, and resolution of 14 pJT(^1). This instrument used a mechanically resonant quartz fibre sample holder, and piezoelectric detection. The response of the instrument to temperature drift, applied magnetic field, and differing sample properties is reported. The T(_c) of a typical thin film sample was measured, and v= 0.788(8) inferred from the relationship with T(_c), as determined for bulk material.

In this study reliable film type NTCR thermistors based on NiMn(_2)O(_4)+δ were produced and their electrical properties were studied in detail. Electron-beam evaporation procedures have been applied to produce thin film NTCR thermistors. Phase pure NiMn(_2)O(_4)+δ target material was produced via a traditional ceramic precursor oxide route and thin films were deposited in an optimised procedure. The thickness distribution of evaporated films showed good agreement with a theoretical model, derived from evaporation theory and the sticking coefficient of the vapour on the substrates was approximately 80% ± 1.5%. The composition of electron-beam evaporated films was found to be not controllable in terms of the phase purity and the Ni : Mn ratio. In order to avoid these problems thick film NiMn(_2)O(_4)+δ NTCR thermistors were developed using direct screen-printing techniques. Detailed Rietveld refinement analysis was carried out for the source powder used for screen-printing. The main focus of the work was the measurement of resistance-temperature (R-T) characteristics of thin and thick films and pellets. In the temperature range of concern (77 K -550 K) conduction was found to be by variable-range hopping (VRH) and nearest-neighbour hopping (NNH); R ~ exp (TʆT)(^p), where the index p depends on the mode of hopping. Detailed analysis of R-T data showed that screen-printed films and pellets exhibited a p-value of 0.5, which was identified with VRH with a parabolic density of states (DOS) with an exponential dependence of resistance: R ~ exp (TʆT)(^0.5). For electron-beam evaporated films the mechanisms detected were NNH: R ~ exp (TʆT); and VRH with a constant DOS {p = 0.25) following: R ~ exp (TʆT)(_0.25). For screen-printed films with incorporated glass phase the electrical conduction mechanism was analysed using a.c. impedance spectroscopy and at low frequencies the hopping conduction was in agreement with the d.c. behaviour. The time constant of this mechanism could be described by an equivalent circuit containing a RC element. For higher frequencies a second mechanism was found, best described by a CRL element.

Ni-Mn-Ga is a ferromagnetic shape memory alloy that can be used for future sensors and actuators. It has been shown that magnetic field can induce phase transformation and consequently large strain in stoichiometric Ni2MnGa. Since then considerable progress has been made in understanding the underlying science of shape memory and ferromagnetic shape memory in bulk materials. Ni-Mn-Ga thin films, however is a relatively under explored area. Ferromagnetic shape memory alloy thin films are conceived as the future MEMS sensor and actuator materials. With a 9.5 percent strain rate reported from magnetic reorientation, Ni-Mn-Ga thin films hold great promise as actuator materials. Thin films come with a number of advantages and challenges as compared to their bulk counterparts. While properties like mechanical strength, uniformity are much better in thin film form, high stress and constraint from the substrate pose a significant challenge for reorientation and shape memory behavior. In either case, it is very important to understand their behavior and examine their properties. This thesis is an effort to contribute to the literature of Ni-Mn-Ga thin films as ferromagnetic shape memory alloys. The focus of this project is to develop a recipe for fabricating NiMnGa thin films with desired composition and microstructure and hence unique properties for future MEMS actuator materials and characterize their properties to aid better understanding of their behavior. In this project NiMnGa thin films have been fabricated using magnetron sputtering on a variety of substrates. Magnetron sputtering technique allows us to tailor the composition of films which is crucial for controlling the phase transformation properties of NiMnGa films. The composition is tailored by varying several deposition parameters. Microstructure of the films has been investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. Mechanical properties of as-deposited films have been probed using nano-indentation technique. The chemistry of sputtered films is determined quantitatively by wavelength dispersive X-ray spectroscopy (WDS). Phase transformation is studied by using a combination of differential scanning calorimetry (DSC), in-situ heating in TEM and in-situ XRD instruments. Magnetic properties of films are examined using superconducting quantum interface device (SQUID).

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.

The last decade has witnessed the discovery of materials combining shape memory behavior with ferromagnetic properties (FSMAs), see, e.g., James & Wuttig1, James et al.2. These materials feature the so-called giant magnetostrain effect, which, in contrast to conventional magnetostriction, is due to the motion of martensite twins. It was first observed in NiMn2Ga single crystals, Ullakko et al.3, but later discovered in polycrystals as well, see Ullakko4. This effect has motivated the development of a new class of active materials transducers, which combine intrinsic sensing capabilities with superior actuation speed and improved efficiency when compared to conventional shape memory alloys. The effect has also been found in thin films, Rumpf et al.5, and this technology is currently being developed intensively in order to pave the way for applications in micro- and nanotechnology. As an example, Kohl et al.6,7, recently proposed a novel actuation mechanism based on NiMnGa thin film technology, which makes use of both the ferromagnetic transition and the martensitic transformation allowing the realization of an almost perfect antagonism in a single component part. The implementation of the mechanism led to the award-winning development of an optical microscanner8. Possible applications in nanotechnology arise, e.g., by combination of smart NiMnGa actuators with scanning probe technologies. The key aspect of Kohl’s device is the fact that it employs electric heating for actuation, which requires a thermo-magneto-mechanical model for analysis. The research presented in this paper aims at the development of a model that simulates this particular material behavior. It is based on ideas originally developed for conventional shape memory alloy behavior, (Mueller & Achenbach9, Achenbach10, Seelecke11, Seelecke & Mueller12) and couples it with a simple expression for the nonlinear temperature-and position-dependent effective magnetic force. This early and strongly simplified version does not account for a full coupling between SMA behavior and ferromagnetism yet, and does not incorporate the hysteretic character of the magnetization phenomena either. It can however be used to explain the basic actuation mechanism and highlight the role of coupled magnetic and martensitic transformation with respect to the actuator performance. In particular will we be able to develop guidelines for desirable alloy compositions, such that the resulting transition temperatures guarantee optimized actuator performance.