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Ni2MnGa cylinders and tubes are solidified in water chilled copper molds, a few millimeters in external diameter and 5 cm long, by the suction casting technique. At room temperature, all samples are in cubic austenitic phase. Microstructure and crystallographic texture of the as-cast rods and tubes are characterized by XRD, SEM, EBSD and TEM. Because of the heat extraction geometry samples exhibit a strong texture, with the [100] direction preferentially oriented in the radial direction, together with a random distribution on the long axis. This texture is more marked in the tubes. XRD and TEM results indicate that the major austenitic phase is fcc, with L21 order. A minority volume of the equilibrium B2′ disordered phase is detected by the presence of two close Curie temperatures in cylinders and tubes 2 mm in external diameter, but not when this diameter is near 1 mm. Precipitates of the stable compounds α-Mn(S,Se), with a NaCl-type structure, and monoclinic P4S5 are observed in all the samples. Cylinders and tubes in austenitic phase are magnetically soft. Hysteresis loops in martensitic phase exhibit local steps associated to a magnetization mechanism involving twin boundary displacement, indicating that a field-induced variant reordering takes place. The switching field Hsw, corresponding to the magnetization step observed, is identified as the field at which twin boundaries become mobile. The measured values of 0.37 T–0.49 T are consistent with those corresponding to the onset of Type I twin boundaries displacement in 5 M martensite, with an equivalent threshold stress of 1 MP. Fil: Pozo Lopez, Gabriela del Valle. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentina Fil: Condo, Adriana Maria. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comision Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones no Nucleares. Gerencia de Física (Centro Atómico Bariloche). División Física de Metales; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina Fil: Limandri, Silvina Paola. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentina Fil: Mutal, Ruben Hector. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentina Fil: Winkler, Elin Lilian. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comision Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones no Nucleares. Gerencia de Física (Centro Atómico Bariloche). División Resonancias Magnéticas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina Fil: Urreta, Silvia Elena. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentina Fil: Fabietti, Luis Maria Rodolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentina

The magnetization measurements at 5 K were carried out for Ni2Mn1 − xCuxGa (0 ≤ x ≤ 0.40) and Ni2MnGa1 − yCuy (0 ≤ y ≤ 0.25) alloys. All of the magnetization curves are characteristic of ferromagnetism or ferrimagnetism. By using Arrott plot analysis the spontaneous magnetization of all samples was determined from the magnetization curves. The magnetic moment per formula unit, μs, at 5 K was estimated from the spontaneous magnetization. For Ni2Mn1 − xCuxGa (0 ≤ x ≤ 0.40) alloys μs at 5 K decreases linearly with increasing x. On the other hand, the μs at 5 K for Ni2MnGa1 − yCuy (0 ≤ y ≤ 0.25) alloys decreases more steeply with increasing x compared to the μs for Ni2Mn1 − xCuxGa (0 ≤ x ≤ 0.40) alloys. On the basis of the experimental results, the site-occupation configurations of Ni2Mn1 − xCuxGa (0 ≤ x ≤ 0.40) and Ni2MnGa1 − yCuy (0 ≤ y ≤ 0.25) alloys are proposed.

The detrimental effects of repeated thermal/field cycling across the martensitic transition on the transport, magnetic and thermal properties of Ni(45)Co(5)Mn(38)Sb(12) are studied. With the help of resistivity and magnetization measurements, it is shown that the resistivity increases and the magnetization decreases monotonically with continuous temperature or field cycling across the martensitic transition. It is also found that the martensite to austenite transition temperature increases with repeated structural change. The changes occurring in these physical properties are seen to be irreversible in nature. The possible changes in the lattice and the microstructure by repeated structural change seem to be responsible for the observed variations. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

This doctoral work is focused on the study of nanostructured magnetic materials by advanced transmission electron microscopy (TEM) techniques, with emphasis on Ni2MnGa shape memory alloy thin films and magnetite nanoparticles for biomedical applications. The combination of high-resolution transmission electron microscopy and electron diffraction to characterize morphology and crystalline structure, with Lorentz microscopy and Electron Holography, permits to achieve a deep insight in the structural and magnetic nano-characterization of magnetic nanostructured materials. The work, carried out at the CNR-IMEM institute of Parma and partially at the LMA-INA institute of Zaragoza in the framework of the Erasmus Placement, produced remarkable results concerning the correlation between the microstructure of these systems and their magnetic/functional properties, from nano-to micro-to macro scale. In detail, the main topics treated are: 1) The use of Lorentz microscopy to investigate the role of dipolar interaction on hyperthermia of magnetic nanoparticles. Magnetic nanoparticles (NPs) in the superparamagnetic state are suitable for both diagnostic and therapeutic approaches. In particular, the magnetic hyperthermia, performed applying radiofrequency magnetic fields, can be effectively employed to locally induce cancer cell death. In real systems, clusters of magnetic nanoparticles with different size can form and the dipolar interactions that arise among nanoparticles can strongly influence the heating ability of the colloidal suspension. The role of the dipolar interactions in the hyperthermic behaviour of the system, however, is still not completely understood. About this topic, an investigation about magnetite nanoparticles with different degrees of interaction was carried out by Lorentz microscopy in a TEM. With this technique, it was possible to visualize and map the inter-particle interactions and to develop reliable models on the power losses mechanisms for different nanoparticles aggregates. As a result, a deeper understanding of the interactions effects on the performance of different nanoparticles suspensions as hyperthermic mediators was obtained. All the TEM results were supported and complemented by conventional o macroscopic (?) magnetic characterization. 2) Employment of advanced TEM techniques to study the effect of epitaxial strain and film thickness in the twin variants formation in Ni2MnGa martensitic thin films In martensitic thin films, the martensitic phase transition gives rise to a poly-twinned system characterized by a complex microstructure in which two families of twin variants can be displayed, with different magnetic anisotropies. Both the use of different substrates and film thickness can significantly modify the twin variants formation and consequently alter their functional properties. High-resolution TEM (HRTEM), Selected Area Electron Diffraction (SAED) and Electron Holography was employed to fully characterize Ni2MnGa thin films in plan and cross section geometries, by variyng film thicknesses in the range 50-100 nm and substrate type (MgO, MgO/Cr buffer layer). The structural and magnetic properties at the nano-micro scale were obtained by comparing TEM analysis with the morphological, structural and magnetic properties on a larger scale (by atomic force microscopy(AFM) , X-ray diffraction (XRD) and magnetic force microscopy (MFM)). A model for the twin variants selective formation, based on the stress states induced by the different substrates and film thicknesses, is moreover proposed. The model represents a powerful tool to selectively control the twin variants formation in martensitic films with low thicknesses and to tailor their magnetic domains structure. 3) An in depth TEM characterization to study the role of microstructure on magnetically induced reorientation of twin variants in Ni2MnGa 200 nm thin films In Ni2MnGa alloys, giant strains, one order of magnitude higher than the typical magnetostriction and state-of-the-art piezoelectric values, can be obtained by a magnetomechanical effect based on twin variants reorientation induced by magnetic field (MIR). Therefore, the possibility of exploiting the martensitic distortions to create tiny machines while keeping simple design, and high actuation frequencies makes these materials particularly appealing for the integration in active microsystems. However, very limited MIR effects were found in thin films and a full comprehension and exploitation of the effect is still lacking. An in-depth TEM characterization to correlate the crystal structure to the twin variants configurations and the magnetism inside each domain of the Ni2MnGa alloys is proposed by the combination of different advanced TEM techniques such as High Angle Annular Dark Field (HAADF) imaging, HRTEM with the comparison of the experimental images to the simulated ones, SAED, Electron Holography and magnetic domain analysis by in-situ observation. A model based on TEM results, compared with the findings obtained by XRD, AFM, MFM and magnetization curves, has been finally suggested to explain the anisotropic microstructure formation in a 200 nm thick Ni2MnGa film grown on MgO/Cr, displaying a huge anisotropic MIR. The proposed model is crucial for the engineering of the martensitic microstructure and for achieving substantial values of MIR in constrained films. 4) TEM study of the reduction of dimensionality in martensitic Ni2MnGa systems: from thin films to sub-micrometric disks After gaining a good understanding of the microstructure of two-dimensional NiMnGa martensitic films with relation to their application properties, the last chapter is devoted to test the possibility to obtain new functional properties by scaling down the dimensionality of these systems. To this aim, the in-depth TEM characterization was extended to a novel class of nanostructures, Ni2MnGa nano-disks, to investigate the structural and magnetic properties of the disks and the effect of the lateral confinement on the martensitic phase. By employing HRTEM, Lorentz microscopy and electron diffraction analysis as a function of temperature with heating holder, the actuation mechanisms, which remain active with the dimensionality reduction, can be studied. La presente tesi di dottorato è focalizzata sullo studio di materiali magnetici nanostrutturati tramite tecniche avanzate di microscopia elettronica in trasmissione (TEM), con particolare attenzione su film sottili a base della lega metallica Ni2MnGa, materiale magnetico a memoria di forma, e nanoparticelle di magnetite per applicazioni biomediche. La combinazione delle tecniche di microscopia elettronica in alta risoluzione e di diffrazione elettronica con le tecniche di microscopia di Lorentz e olografia elettronica, permette di ottenere un’approfondita conoscenza delle proprietà magnetico-strutturali di materiali magnetici nanostrutturati. Il presente lavoro, svolto principalmente all’istituto IMEM-CNR di Parma and parzialmente presso il laboratorio LMA-INA di Saragozza, ha prodotto importanti nuovi risultati riguardanti la correlazione tra la microstruttura e le proprietà magnetiche/funzionali di questi sistemi, dalla nanoscala alla micro/macro scala. In dettaglio, i punti più importanti in cui il lavoro è stato articolato sono: 1) Utilizzo della microscopia di Lorentz per l’investigazione riguardante il ruolo delle interazioni dipolari nell’ipertermia magnetica di nanoparticelle di magnetite. Nanoparticelle magnetiche (NPs) nello stato superparamagnetico sono molto promettenti sia nel campo diagnostico, che terapeutico. In particolare, l’ipertermia magnetica, realizzata tramite l’applicazione di un campo magnetico alle radiofrequenze, può essere efficace nell’induzione locale della morte di cellule tumorali. In sistemi di nanoparticelle reali, cluster di nanoparticelle magnetiche con differenti dimensioni possono crearsi grazie alle interazioni dipolari, che possono anche fortemente influenzare la capacità di generazione di calore da parte della sospensione colloidale. Il ruolo delle interazioni dipolari nel comportamento ipertermico del sistema, comunque, non è ancora stato del tutto chiarito. Riguardo questo argomento, uno studio su nanoparticelle di magnetite con differenti gradi di interazioni è stato condotto tramite microscopia elettronica in trasmissione usando la tecnica di microscopia di Lorentz. Con questa tecnica, è possibile visualizzare e mappare le interazioni dipolari tra le particelle e sviluppare modelli attendibili riguardo i meccanismi di perdita di energia per differenti aggregati di nanoparticelle. Come risultato di questo studio, è stata ottenuta una più approfondita comprensione degli effetti indotti dalle interazioni dipolari sulla performance di diversi aggregati di nanoparticelle magnetiche come mediatori ipertermici. Le caratterizzazioni e i risultati conseguiti tramite microscopia TEM sono fortemente supportati dalle caratterizzazioni magnetiche convenzionali. 2) Utilizzo di tecniche avanzate di microscopia elettronica in trasmissione per lo studio degli effetti dovuti a strain epitassiale e spessore del film nella formazione delle varianti di twin in film sottili martensitici di Ni2MnGa. In film sottili martensitici, la transizione di fase martensitica produce una complessa microstruttura rappresentata da un sistema poli-twinnato, in cui due famiglie di varianti di twin con differenti anisotropie magnetiche possono essere comunemente osservate. Sia l’impiego di differenti substrati e/o di buffer-layers come la variazione dello spessore del film, possono pesantemente modificare la formazione delle varianti di twin e conseguentemente alterare le proprietà funzionali dei film stessi. Le tecniche di microscopia elettronica in alta risoluzione (HRTEM), di diffrazione elettronica da area selezionata (SAED) e olografia elettronica sono state impiegate per ottenere una completa caratterizzazione dei film sottili di Ni2MnGa in visione planare ed in sezione trasversale, per film con uno spessore variabile nel range 50-100 nm e con differenti substrati (MgO, MgO/buffer layer di Cr). Le proprietà strutturali e magnetiche alla scala nano-micrometrica sono state ottenute comparano l’analisi TEM con la morfologia e le proprietà strutturali e magnetiche su una scala più ampia (ottenute tramite l’utilizzo di tecniche di: microscopie a scansione di sonda (AFM/MFM), diffrazione di raggi X (XRD). Un modello per la formazione selezione selettiva di varianti di twins, basato sullo stato di stress indotto dai differenti substrati e in funzione dei diversi spessori dei films, è stato inoltre proposto. Questo modello rappresenta un potente strumento per controllare la formazione selettiva delle varianti di twin in film martensitici caratterizzati da bassi spessori e, conseguentemente, permette di controllare la loro struttura a domini magnetici. 3) Caratterizzazione, tramite microscopia elettronica in trasmissione, del ruolo della microstruttura sulla ri-orientazione magnetica delle varianti di twin in film sottili di Ni2MnGa spessi 200 nm. Nelle leghe Ni2MnGa, enormi strain con valori un ordine di grandezza più grandi rispetto a quelli osservati tipicamente nei materiali piezoelettrici, possono essere ottenuti grazie ad un effetto magnetomeccanico basato sulla ri-orientazione delle varianti di twin indotta da un campo magnetico applicato (effetto MIR). Le possibilità di sfruttare la distorsione indotta nella fase martensitica per creare piccoli sistemi di attuazione tenendo un design semplice e di compiere cicli di attuazione ad alte frequenze rendono quindi questi materiali particolarmente attraenti per l’integrazione in microsistemi di attuazione. Tuttavia, effetti MIR molto limitati sono stati osservati in film sottili e una completa comprensione e controllo dell’effetto è tuttora mancante. Una caratterizzazione approfondita per correlare la struttura cristallina alle configurazioni di varianti di twins ed il magnetismo di ogni dominio della lega Ni2MnGa è proposto, tramite l’impiego combinato di differenti tecniche avanzate di microscopia elettronica in trasmissione come imaging con detector anulare ad alto angolo (HAADF), HRTEM con il confronto delle immagini sperimentali a quelle simulate, SAED, olografia elettronica, analisi dei domini magnetici tramite osservazione in-situ. Un modello basato sui risultati ottenuti dalla caratterizzazione TEM, confrontati con i risultati sperimentali ottenuti tramite XRD, AFM, MFM e caratterizzazioni magnetiche, è stato inoltre proposto per spiegare l’orientazione preferenziale della microstruttura in film di Ni2MnGa di 200 nm, cresciuti su MgO/Cr e mostranti un enorme effetto MIR anisotropico. I risultati conseguiti sono di cruciale importanza per l’ingegnerizzazione della microstruttura martensitica e per l’ottenimento di un rilevante effetto MIR in film sottili vincolati al substrato. 4) Studio tramite microscopia elettronica in trasmissione degli effetti della riduzione di dimensionalità in sistemi martensitici Ni2MnGa: dal film sottile al disco sub-micrometrico. Un’approfondita analisi TEM è stata condotta su una nuova classe di materiali nanostrutturati, i nanodischi di Ni2MnGa, per la caratterizzazione delle proprietà strutturali e magnetiche dei dischi e l’effetto del confinamento laterale della fase martensitica. Combinando le tecniche di HRTEM, microscopia di Lorentz e diffrazione elettronica in funzione della temperatura, è possibile scoprire quali sono i meccanismi di attuazione che sopravvivono alla riduzione della dimensionalità.

Nowadays, the miniaturization development is a key parameter in order to fabricate increasinglycomplex microsystems. Research on smart materials aroused a great interest. In this context, westudy the magnetic shape memory alloy Ni2MnGa as a thin layer. This material can be activatedby mechanical, thermal and magnetic stresses. The coupling of these effects would allow thedevelopment of micro-actuators usually made from complex assemblies. This multidisciplinary studyfocuses on the impact of the process on the functional properties of thin films. The originality of thiswork lies in the use of silicon substrates in the context of a technological implementation. A methoddeveloped by PVD was qualified to obtain a film with magnetic shape memory alloy properties.; De nos jours, l’essor de la miniaturisation est un paramètre clé pour la réalisation de microsystèmesde plus en plus complexes. Les recherches sur l’élaboration de matériaux « intelligents », onttoujours suscité un grand intérêt. Dans ce cadre, on se propose d’étudier l’alliage à mémoirede forme magnétique Ni2MnGa sous la forme de couche mince. Ce matériau a la propriétéparticulière de répondre aux sollicitations mécaniques, thermiques et magnétiques. Le couplage deces effets permettrait l’élaboration de micro-actionneurs usuellement réalisés à partir d’assemblagescomplexes. Cette étude pluridisciplinaire traite de l’influence des paramètres d’élaboration sur lespropriétés fonctionnelles des couches minces. L’originalité de ce travail de thèse réside dansl’emploi de substrats en silicium dans la perspective d’une transposition technologique. Un procédéd’élaboration par PVD a été qualifié afin d’obtenir un film aux propriétés AMF magnétique

We have systemically studied the effects of annealing temperature and alloy composition on the structural and magnetic properties of bulk Ni$_{2}$MnGe and Ni$_{2.1}$Mn$_{0.9}$Ge Heusler alloys. We have observed that both annealing temperature and the alloy composition drastically alter the phases found in the samples due to the presence of competing ternary phases. Annealing at 900 and 950 $^{\circ}$C for both alloy compositions facilitate the formation of L2$_{1}$ Heusler phase. Nevertheless, formation of Ni$_{5}$Mn$_{4}$Ge$_{3}$ and Ni$_{16}$Mn$_{6}$Ge$_{7}$ phases cannot be prevented for Ni$_{2}$MnGe and Ni$_{2.1}$Mn$_{0.9}$Ge alloys, respectively. In order to estimate the magnetic contribution of the Ni$_{5}$Mn$_{4}$Ge$_{3}$ impurity phase to that of the parent Ni$_{2}$MnGe, we have also synthesized pure Ni$_{5}$Mn$_{4}$Ge$_{3}$ alloy. Antiferromagnetic nature of Ni$_{5}$Mn$_{4}$Ge$_{3}$ with low magnetization response allows us to reveal the magnetic response of the stoichiometric bulk Ni$_{2}$MnGe. Bulk Ni$_{2}$MnGe shows simple ferromagnetic behavior with a Curie temperature of 300 K, in agreement with the previous results on thin films. Despite the divergence of magnetization curves between field cooled (FC) and field heated (FH) modes, stoichiometric Ni$_{2}$MnGe alloy does not undergo a martensitic phase transition based on our variable temperature x-ray diffraction experiments., Comment: 6 pages, 9 figures; to appear in Journal of Alloys and compounds

Magnetization and neutron diffraction studies have been carried out in Ni50-xCoxMN38Sb12 (x=0 and 5) to investigate the effect or Co on the structural and magnetic properties. A comparison of the neutron diffraction data of Ni45Co5MN38Sb12 and Ni50Mn38Sb12 alloys shows that Co substitution leads to an increase in the Mn moments. Considerably large magnetization difference associate with martensitic transition has also been observed On Co substitution. The analysis of the neutron diffraction data has revealed that the martensitic transition is accompanied by about 70% decrease in the moment and about 0.3% decrease in the cell volume. Enhanced magneto-structural coupling resulting from the Co doping has been evidenced by the magnetization as well as the neutron data. The variations in the individual site moments on Co substitution and their temperature dependencies account for the observed bulk magnetization data. (C) 2014 Elsevier B.V. All rights reserved

The temperature dependence of the magnetic polarization near the Curie temperature in Ni MnGa stoichiometric alloys, directly processed from the melt in a twin-roller melt-spinning device, is investigated. The effect of the solidification rate on the Hopkinson peak detected is evaluated in samples quenched at three different tangential wheel speeds of 10, 15, and 20 m/s. The resulting microstructures were previously characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and by transmission electronmicroscopy (TEM). EDS results indicated that all the alloys have the composition Ni MnGa; at room temperature and above this temperature, a cubic L ferromagnetic ordered austenitic phase is observed. The Curie temperatures and the magnitude of the Hopkinson effect are estimated from the magnetic polarization versus temperature curves measured in a Faraday balance, in the range 300 K-400 K. As expected for samples with identical composition, the Curie temperatures remain insensitive to the processing route. At low fields (10 mT), the magnitude of the Hopkinson effect is larger in samples quenched at lower rates and it practically vanishes in all the alloys for applied fields near 100 mT. Fil: Pozo Lopez, Gabriela del Valle. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina Fil: Fabietti, Luis Maria Rodolfo. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina Fil: Condo, Adriana Maria. Comisión Nacional de Energía Atómica. Gerencia del Area de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche); Argentina Fil: Winkler, Elin Lilian. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes. Gerencia de Investigación y Aplicaciones; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Giordano, Rafael Nicolás. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales; Argentina Fil: Haberkorn, Nestor Fabian. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes. Gerencia de Investigación y Aplicaciones; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Urreta, Silvia Elena. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales; Argentina

Martensitic transitions in shape memory Ni-Mn-Ga Heusler alloys take place between a high temperature austenite and a low temperature martensite phase. However, intermartensitic transformations have also been encountered that occur from one martensite phase to another. To examine intermartensitic transitions in magnetic shape memory alloys in detail, we carried out temperature dependent magnetization, resistivity, and x-ray diffraction measurements to investigate the intermartensitic transition in Ni50Mn50-xGax in the composition range 12 x 25 at. %. Rietveld refined x-ray diffraction results are found to be consistent with magnetization and resistivity data. Depending on composition, we observe that intermartensitic transitions occur in the sequences 7M ! L10; 5M ! 7M, and 5M ! 7M ! L10 with decreasing temperature. The L10 non-modulated structure is most stable at low temperature.

The temperature dependences of the lattice parameters and residual stress have been measured for a fine-grained Ni52.2Mn26.8Ga 21.0 (at. %) thin film fabricated by sputter deposition onto a heated silicon wafer with SiNx buffer layer. The transformation volume strain in the film was found to be a lattice expansion during the forward martensitic transformation which is opposite to a volume contraction exhibited by bulk Ni-Mn-Ga alloys. This unusual effect can be explained by the substrate-induced residual stresses in the film and the difference in the elastic modulus of austenite and martensite.

The structural, magnetic, transport, thermal and magnetothermal properties of quaternary Heusler alloys Ni50Mn38-xFexSb12 have been studied. Powder x-ray diffraction and temperature dependence of magnetization studies reveal that with addition of Fe in Mn site, the martensitic transition shifts to lower temperatures. It is also found that the martensitic transition becomes broader for the higher Fe concentrations. The metamagnetic transition in M(H) isotherms becomes very prominent in x=2 and vanishes for x=3 and 4. A maximum positive magnetic entropy change of 14.2 J/kg K is observed for x=2 at 288 K for 50 kOe. Resistivity shows an abrupt decrease across the martensitic transition in all the alloys, except x=6, which does not have the martensitic transition. Maximum negative magnetoresistance of 21% has been obtained for x=2 at 50 kOe. The same alloy also shows an exchange bias field of 288 Oe., Comment: 28 pages, 10 figures, This paper is accepted to be published in Journal of Applied Physics

The magnetic properties of the cubic Heusler alloys with nominal compositions Ni(2-x)Mn(1-x)In have been investigated experimentally as well as theoretically using first principles electronic structure calculations. Magnetization measurements indicate that all the alloys considered in this work have ferromagnetic ground state. The Curie temperature and the saturation moment of the samples are found to decrease systematically with increasing Ni concentration. No indication of structural transition was evident from our experimental data recorded down to 5 K. Electronic structure calculation indicates that localized magnetic moments are predominantly contributed by the Mn atoms and the calculated magnetic moment as a function of excess Ni concentration agrees well with the experimental data. (C) 2009

Ribbons of ferromagnetic shape memory Ni(2+x)Fe(1-x)Ga (x=0 and 0.12) alloys were prepared at wheel speeds of 12 and 35 m/s by melt spinning technique. The structural (site) disorder and the associated changes in the magnetic properties of the ribbons were investigated by (57)Fe Mossbauer spectroscopy and magnetization measurements respectively. Mossbauer spectra of Ni(2)FeGa ribbon revealed the presence of both L2(1) order and B2-like disorder with some amount of Fe antisite atoms on Ni site. The increase in Ni content was found to decrease the T(C) while its effect on T(M) was the opposite way.

Based on experimental observations of modulated magnetic patterns in a Co(0.5)Ni(0.205)Ga(0.295) alloy, we propose a model to describe a ( purely) magnetic tweed and a magnetoelastic tweed. The former arises above the Curie (or Neel) temperature due to magnetic disorder. The latter results from compositional fluctuations coupling to strain and then to magnetism through the magnetoelastic interaction above the structural transition temperature. We discuss the origin of purely magnetic and magnetoelastic precursor modulations and their experimental thermodynamic signatures.

We study here, within the density-functional theory, the magnetic anisotropy energy (MAE) in Ni2MnGa which is a prototype of a magnetic shape-memory alloy. We calculate the MAE, which is a key property for the magnetic shape-memory effect, for tetragonal structure with different ratios of the c and a lattice constants, reproducing the experimental easy axes both in compression and elongation of the structure. Good agreement between the theory and the experiments in the actual values of the MAE is also found when the nonstoichiometry of the experimental samples is modeled with a simple rigid band approximation. In addition, we estimate the magnetostriction coefficient, confirming the difference between the ordinary magnetostriction and the magnetic shape-memory effect. Equally important, we study the microscopic origin of the MAE in Ni2MnGa with the spin density and the orbital moment anisotropy and extend the analysis of the orbital moment anisotropy to the ternary compounds. These results show that the largest contribution to the MAE comes from Ni, in spite of the larger magnetic moment in the Mn sites.

16th International Conference on Positron Annihilation (ICPA) Univ Bristol, H H Wills Phys Lab, Bristol, ENGLAND AUG 19-24, 2012 Edited by:Alam, A; Coleman, P; Dugdale, S; Roussenova, M We have studied the behaviour of defects in off-stoichiometric Ni-Mn-Ga ferromagnetic shape memory alloys by means of positron lifetime spectroscopy. The measurements presented in this work have been performed in six ternary alloys. The studied samples cover a large composition range. Positron experiments have been performed at room temperature after subsequent isochronal annealing at different temperatures up to a maximum temperature of 700 degrees C. Results show a large variation of the average positron lifetime value with the isochronal annealing temperature in three of these samples, with significant differences between them. In the other three, the response is quite different. The results are discussed in terms of different types of positron trapping defects and their evolution with the annealing temperature. The present work shows a high dependence of recovery behaviour with composition in NiMnGa ferromagnetic shape memory alloys.

The new concept of nanotweezers based on bimetallic composite Ti50Ni25Cu25/Pt with shape memory effect have recently demonstrated the ability to manipulate real nanoobjects, such as CNTs, graphene layers, etc., when heated to 40-60 degrees C by laser radiation. We demonstrate possibility of manipulation of microbiological objects - fibers from the body of Culex pipiens in vacuum chamber of FIB device. Composite microactuator controlled by magnetic field at constant temperature was produced using rapidly quenched ferromagnetic shape memory Heusler alloy Ni53Mn24Ga23. The experiments on control of this microactuator by applying magnetic field of 8 T prove the possibility of manipulation of living objects. (C) 2016 Published by Elsevier B.V.