Heusler compounds are cubic intermetallics with a wide range of interesting properties, which are closely related to the structure of the material. In addition, several exotic physical phenomena have been predicted for different compositions in the family, but have not been experimentally realized. By and large, the lack of success in realization of various properties are due to the issues with intrinsic material attributes, which have been difficult to resolve as the relationship between them is not well understood. The aim of this work has been to unravel the entanglement between the intrinsic material attributes of cobalt-based quaternary Heusler compounds such as the structure, defects (disorder), chemical inhomogeneities etc., and the resulting physical properties.

No description available.

Half-metallic ferromagnets (HMFs), with fully spin-polarized conduction electrons, are prime candidates for optimizing spintronic devices. Many Heusler compounds (a class of ternary and quaternary intermetallics) are predicted to be HMFs, in particular Co$_{2}YZ$ (where $Y$ is usually another transition metal, and $Z$ is an s-p element). Crystal structure is controlled by thermodynamics to a large extent. Ideally, one should be able to control and optimize properties which are of interest by appropriately "tuning" the structure (e.g. annealing), but first one must understand the structure and its relation to observed physical properties. A local structural probe technique such as nuclear magnetic resonance (NMR) is an essential tool for identifying and quantifying the various atomic-scale orderings. Different Heusler structure types and antisite disorders affect the material's physical properties. In this thesis, order-disorder phenomena in both bulk and thin film samples of Co$_2$Mn$_{1-x}$Si$_x$ and Co$_2$Mn$_{1-x}$Fe$_x$Si have been systematically studied using NMR. Though it is the films which are directly implemented in actual devices, studying bulk samples as model systems provides invaluable information regarding the material properties. The evolution of local atomic structure in numerous thin films has been shown to depend greatly on preparation parameters, including post-deposition annealing temperature, and specific stoichiometry. For Co$_2$MnSi films, the ideal post-annealing temperature for promoting the $L2_1$ atomic structure was found; the threshold temperature above which structure continues to become higher-ordered in the bulk, but where too much interdiffusion at the buffer interface occurs, degrading the smooth interfaces necessary for high magnetoresistance ratios. NMR also adds evidence that Co$_2$Mn$_x$Si$_{0.88}$ ($x>$1) electrodes in magnetic tunnel junctions have highest tunneling magneto-resistance because the excess Mn suppresses the formation of detrimental Co$_{Mn}$ antisites. A systematic investigation of several thermal and magnetic properties, including Sommerfeld coefficients, Debye temperatures, saturation magnetic moments, spin-wave stiffness, and magnon specific heat coefficient, were measured for selected Co$_2$-based ternary and quaternary Heusler compounds. Obtained values were compared with theoretical ones calculated using electronic band structure methods. It has been systematically shown that adding a magnon term to the specific heat has a negligible effect on the electronic contribution in all cases.

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

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

In dieser Arbeit werden Resultate elektronischer Transportmessungen von supraleitenden Eisenpniktiden und ferromagnetischen Heusler-Verbindungen diskutiert. Die Eisenpniktide sind eine neuartige Klassen von Hochtemperatursupraleitern, deren Eigenschaften sich aus einem Zusammenspiel von Supraleitung und Magnetismus ergeben. Während die sogenannten 122-Pniktide Antiferromagnetismus aufweisen und unter Dotierung in einen supraleitenden Zustand übergehen, konnte in dotiertem LiFeAs Ferromagnetismus beobachtet werden. Undotiert hingegen zeigt dieses Material interessante supraleitende Eigenschaften. Die Heusler-Verbindungen sind u.a. durch ihren Ferromagnetismus bekannt. Das hier untersuchte Co2FeSi ist einer der stärksten Ferromagnete. Der in diesem Material vorhergesagte vollständig spinpolarisierte elektronische Transport, d.h. alle Leitungselektronen besitzen den gleichen Spin, konnte nachgewiesen werden. Die hier genannten Eigenschaften können exzellent mit der Methode der elektronischen Transportmessungen untersucht werden. Deren Ergebnisse aus Messungen an Einkristallen werden in dieser Arbeit diskutiert. / In this work, results of electronic transport measurements are discussed for superconducting iron pnictides as well as for ferromagnetic Heusler compounds. The iron pnictides are a recently discovered class of high temperature superconductors where magnetism might play a crucial role. While the 122-pnictides show antiferromagnetism and migrate to the superconducting state upon doping, ferromagnetism has been observed in doped LiFeAs. On the other hand, in the undoped state this material shows interesting superconducting properties. Among other propierties, Heusler compounds are well known due to their ferromagnetism. Co2FeSi, which was investigated in this work, is one of the strongest ferromagnets. Beside this, one predicts this compound to be a half-metallic ferromagnet with completely spin polarized electronic transport where all conducting electrons have the same spin. The here addressed properties can well be investigated with the method of electronic transport measurements, whose results on single crystals are discussed in this work.

Submitted by Automa??o e Estat?stica (sst@bczm.ufrn.br) on 2016-05-10T22:17:08Z No. of bitstreams: 1 AngelaMartaDaSilva_DISSERT.pdf: 7504975 bytes, checksum: a6fd1e70c9103a34433fbcc2f7ad91e6 (MD5) / Approved for entry into archive by Arlan Eloi Leite Silva (eloihistoriador@yahoo.com.br) on 2016-05-16T22:49:04Z (GMT) No. of bitstreams: 1 AngelaMartaDaSilva_DISSERT.pdf: 7504975 bytes, checksum: a6fd1e70c9103a34433fbcc2f7ad91e6 (MD5) / Made available in DSpace on 2016-05-16T22:49:04Z (GMT). No. of bitstreams: 1 AngelaMartaDaSilva_DISSERT.pdf: 7504975 bytes, checksum: a6fd1e70c9103a34433fbcc2f7ad91e6 (MD5) Previous issue date: 2015-09-08 / Neste trabalho foram produzidas e estudadas ligas de CFA, do tipo full-Heusler, onde obtivemos resultados de filmes finos simples de Co2FeAl (CFA) e multicamadas de CFA/Ag/CFA, produzidas por magnetron Sputtering em substratos amorfos (vidro), enfatizando o estudo de suas propriedades estruturais, magn?ticas quasi-est?ticas e din?micas. Atrav?s da an?lise de raios-X verificou-se a forma??o da estrutura cristalina do CFA, obtendo a fase A2 (desordenada estruturalmente, onde os ?tomos encontram-se distribu?dos aleatoriamente) e a fase B2 (estrutura semi-ordenada). As medidas revelaram um comportamento magn?tico quasi-est?tico que para camadas simples de MGO/CFA apresentam clara anisotropia do tipo uniaxial, enquanto para a camada simples e multicamada de CFA apresentam anisotropia uniaxial induzida no plano. Medidas de magnetoimped?ncia (MI) s?o realizadas numa vasta gama de frequ?ncias variando de 1.0 GHz a 3.0 GHz e variando o ?ngulo entre o campo magn?tico externo e a corrente de sonda com rela??o aos eixos de anisotropias. As curvas de MI apresentam comportamento t?pico verificada para amostra anisotr?pica, com estrutura simples e de duplo pico de acordo com as frequ?ncias e ?ngulos como esperado. / In this work, full-Heusler Co2FeAl (CFA) single layer and multilayer CFA/Ag/CFA samples were deposited by magnetron sputtering on amorphous glass substrates. We investigated their structural, quasi-static and dynamic magnetic properties. Through the x-ray analysis, the formation of a CFA crystalline structure was verified, A2 phase (structurally disordered, where atoms are randomly distributed) and B2 phase (semi-ordered structure). The quasi-static magnetic behavior measurements revealed that the single layered MgO/CFA sample presentes anisotropy in the plane, while the single layered CFA and the multilayer present uniaxial has uniaxial magnetic anisotropy. Magnetompedance (MI) measurements are performed in a wide range of frequencies, from 1.0 GHz to 3.0 GHz, and varying the angle between the external magnetic field and probe electrical current with respect to the anisotropy axis. The MI curves for each sample present typical behavior verified for anistropic samples, with single and double peak structure according to the frequency and angle, as expected.

In dieser Arbeit werden Resultate elektronischer Transportmessungen von supraleitenden Eisenpniktiden und ferromagnetischen Heusler-Verbindungen diskutiert. Die Eisenpniktide sind eine neuartige Klassen von Hochtemperatursupraleitern, deren Eigenschaften sich aus einem Zusammenspiel von Supraleitung und Magnetismus ergeben. Während die sogenannten 122-Pniktide Antiferromagnetismus aufweisen und unter Dotierung in einen supraleitenden Zustand übergehen, konnte in dotiertem LiFeAs Ferromagnetismus beobachtet werden. Undotiert hingegen zeigt dieses Material interessante supraleitende Eigenschaften. Die Heusler-Verbindungen sind u.a. durch ihren Ferromagnetismus bekannt. Das hier untersuchte Co2FeSi ist einer der stärksten Ferromagnete. Der in diesem Material vorhergesagte vollständig spinpolarisierte elektronische Transport, d.h. alle Leitungselektronen besitzen den gleichen Spin, konnte nachgewiesen werden. Die hier genannten Eigenschaften können exzellent mit der Methode der elektronischen Transportmessungen untersucht werden. Deren Ergebnisse aus Messungen an Einkristallen werden in dieser Arbeit diskutiert. / In this work, results of electronic transport measurements are discussed for superconducting iron pnictides as well as for ferromagnetic Heusler compounds. The iron pnictides are a recently discovered class of high temperature superconductors where magnetism might play a crucial role. While the 122-pnictides show antiferromagnetism and migrate to the superconducting state upon doping, ferromagnetism has been observed in doped LiFeAs. On the other hand, in the undoped state this material shows interesting superconducting properties. Among other propierties, Heusler compounds are well known due to their ferromagnetism. Co2FeSi, which was investigated in this work, is one of the strongest ferromagnets. Beside this, one predicts this compound to be a half-metallic ferromagnet with completely spin polarized electronic transport where all conducting electrons have the same spin. The here addressed properties can well be investigated with the method of electronic transport measurements, whose results on single crystals are discussed in this work.

Half-metallic ferromagnets (HMFs), with fully spin-polarized conduction electrons, are prime candidates for optimizing spintronic devices. Many Heusler compounds (a class of ternary and quaternary intermetallics) are predicted to be HMFs, in particular Co$_{2}YZ$ (where $Y$ is usually another transition metal, and $Z$ is an s-p element). Crystal structure is controlled by thermodynamics to a large extent. Ideally, one should be able to control and optimize properties which are of interest by appropriately "tuning" the structure (e.g. annealing), but first one must understand the structure and its relation to observed physical properties. A local structural probe technique such as nuclear magnetic resonance (NMR) is an essential tool for identifying and quantifying the various atomic-scale orderings. Different Heusler structure types and antisite disorders affect the material's physical properties. In this thesis, order-disorder phenomena in both bulk and thin film samples of Co$_2$Mn$_{1-x}$Si$_x$ and Co$_2$Mn$_{1-x}$Fe$_x$Si have been systematically studied using NMR. Though it is the films which are directly implemented in actual devices, studying bulk samples as model systems provides invaluable information regarding the material properties. The evolution of local atomic structure in numerous thin films has been shown to depend greatly on preparation parameters, including post-deposition annealing temperature, and specific stoichiometry. For Co$_2$MnSi films, the ideal post-annealing temperature for promoting the $L2_1$ atomic structure was found; the threshold temperature above which structure continues to become higher-ordered in the bulk, but where too much interdiffusion at the buffer interface occurs, degrading the smooth interfaces necessary for high magnetoresistance ratios. NMR also adds evidence that Co$_2$Mn$_x$Si$_{0.88}$ ($x>$1) electrodes in magnetic tunnel junctions have highest tunneling magneto-resistance because the excess Mn suppresses the formation of detrimental Co$_{Mn}$ antisites. A systematic investigation of several thermal and magnetic properties, including Sommerfeld coefficients, Debye temperatures, saturation magnetic moments, spin-wave stiffness, and magnon specific heat coefficient, were measured for selected Co$_2$-based ternary and quaternary Heusler compounds. Obtained values were compared with theoretical ones calculated using electronic band structure methods. It has been systematically shown that adding a magnon term to the specific heat has a negligible effect on the electronic contribution in all cases.

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.