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The continuous high demand on permanent magnets in industries opened new research plateau to develop alternative magnetic material. The current used permanent magnet materials in the market still suffer from high cost and insufficient magnetic or thermal properties. The central focus of this dissertation work is the optimization of cobalt carbide based nanomaterial by means of modifying polyol synthesis assisted by nucleation agent and systematic statistics using JMP software tool. In most existing literatures, producing cobalt carbide (Co2C or Co3C) lack reproducibility and consistency resulting in nonsolid magnetic properties results. The practical requirements for cobalt carbide to be used as permanent magnet are high coercivity (Hc), high magnetization (Ms), resulting in a high-energy product (HcxMs). Previous literatures have shown coercivities of 1.5 to 2.5 kOe for cobalt carbides under aggressive temperatures conditions (300oC) or after aligning the particles under magnetic field. A statistical guided method performed a sequence of experiments toward producing high coercivities using surface response design. Primarily, the statistical study to optimize cobalt carbide was made by analyzing experimental condition to fulfill high magnetic properties with tuned conditions as much as possible. Therefore, having the advantage for superior control on process variable when shifting cobalt carbide for scale up production in flow chemistry set up using microreaction system (MMRS). The optimization is based upon selecting the most important conditions in polyol reaction to produce cobalt carbide (Co2C or Co3C) and feed JMP software model e.g. reaction temperature, reaction time, and or precursor concentration…etc. These factors called (effects) used to design experiments and generate tables to run minimum experiments. Points of each effect (levels) are selected based on previous knowledge and experience with the synthesis. The output called (response) can be any of the magnetic properties of our interest e.g. magnetization (Ms), coercivity (Hc), or energy magnetic product (HcxMs). In the first model fit of cobalt carbide magnetic was studied in a polyol reaction to increase its magnetic energy product and optimize the experimental conditions. The results disclosed increase in magnetic energy product (6.2 MGOe) when validating the prediction model conditions suggested by JMP: shorter reaction time, and lower precursor concentration conditions at maximum reaction temperature. Finally, to my knowledge studying the effect of the nucleating agent to alter cobalt carbide growth have not been studied so far. Therefore, statistical study design using central composite design (CCD) to investigate the nucleating agent effect of silver nitrite on cobalt carbide coercivity was made. The importance of nucleating agent on coercivity is vigorous to attain and control the growth direction of cobalt carbide nanoparticles. This is due to the shape anisotropy contribution to enhance coercivity unlike weak shape anisotropy attributed to agglomeration of nanoparticles demonstrated in previous studies. Enhancement of coercivity reached 3 kOe with aspect ratio control as a function of silver nitrite concentration under lower reaction temperature. The continuous high demand on permanent magnets in industries opened new research plateau to develop alternative magnetic material. The current used permanent magnet materials in the market still suffer from high cost and insufficient magnetic or thermal properties. The central focus of this dissertation work is the optimization of cobalt carbide based nanomaterial by means of modifying polyol synthesis assisted by nucleation agent and systematic statistics using JMP software tool. In most existing literatures, producing cobalt carbide (Co2C or Co3C) lack reproducibility and consistency resulting in nonsolid magnetic properties results. The practical requirements for cobalt carbide to be used as permanent magnet are high coercivity (Hc), high magnetization (Ms), resulting in a high-energy product (HcxMs). Previous literatures have shown coercivities of 1.5 to 2.5 kOe for cobalt carbides under aggressive temperatures conditions (300oC) or after aligning the particles under magnetic field. A statistical guided method performed a sequence of experiments toward producing high coercivities using surface response design. Primarily, the statistical study to optimize cobalt carbide was made by analyzing experimental condition to fulfill high magnetic properties with tuned conditions as much as possible. Therefore, having the advantage for superior control on process variable when shifting cobalt carbide for scale up production in flow chemistry set up using microreaction system (MMRS). The optimization is based upon selecting the most important conditions in polyol reaction to produce cobalt carbide (Co2C or Co3C) and feed JMP software model e.g. reaction temperature, reaction time, and or precursor concentration…etc. These factors called (effects) used to design experiments and generate tables to run minimum experiments. Points of each effect (levels) are selected based on previous knowledge and experience with the synthesis. The output called (response) can be any of the magnetic properties of our interest e.g. magnetization (Ms), coercivity (Hc), or energy magnetic product (HcxMs). In the first model fit of cobalt carbide magnetic was studied in a polyol reaction to increase its magnetic energy product and optimize the experimental conditions. The results disclosed increase in magnetic energy product (6.2 MGOe) when validating the prediction model conditions suggested by JMP: shorter reaction time, and lower precursor concentration conditions at maximum reaction temperature. Finally, to my knowledge studying the effect of the nucleating agent to alter cobalt carbide growth have not been studied so far. Therefore, statistical study design using central composite design (CCD) to investigate the nucleating agent effect of silver nitrite on cobalt carbide coercivity was made. The importance of nucleating agent on coercivity is vigorous to attain and control the growth direction of cobalt carbide nanoparticles. This is due to the shape anisotropy contribution to enhance coercivity unlike weak shape anisotropy attributed to agglomeration of nanoparticles demonstrated in previous studies. Enhancement of coercivity reached 3 kOe with aspect ratio control as a function of silver nitrite concentration under lower reaction temperature.

O efeito Hall quântico surge em gases de elétrons bidimensionais (2DEG) na presença de altos campos magnéticos B. O campo magnético quantiza o movimento planar dos elétrons em órbitas ciclotrônicas caracterizadas pelos níveis de Landau. Neste regime a resistividade transversal (ou Hall) ρxy em função de B exibe platôs em submúltiplos inteiros de e2/h, i.e., ρxy = ν-1 e2/h, sendo ν o fator de preenchimento dos níveis de Landau. Por sua vez, a resistividade longitudinal ρxx apresenta picos nas transições entre platôs de ρxy. Em primeira instância, ρxx é uma medida indireta da densidade de estados no nível de Fermi g(εF), e os picos dos mesmos indicam cruzamentos do nível de Fermi εF com niveis de Landau. Assim, o diagrama de densidade de elétrons n2D e B dos picos de ρxx ~ g(εF) fornece um mapa topológico da estrutura eletrônica do sistema. Em sistemas de duas subbandas, ρxx(n2D, B) exibe estruturas em forma de anel devido a cruzamentos de níveis de Landau de subbandas distintas [experimentos do grupo do Prof. Jiang (UCLA)]. Estes cruzamentos podem ainda levar a instabilidades ferromagnéticas. Investigamos estas instabilidades usando a teoria do funcional da densidade (DFT) para o cálculo da estrutura eletrônica, e o modelo de Ando (formalismo de Kubo) para o cálculo de ρxx e ρxy. Para temperaturas mais altas (340 mK) obtemos as estruturas em forma de anel em ρxx. Para temperaturas mais baixas (70 mK), observamos uma quebra dos anéis devido a transições de fase ferromagnéticas. Variando-se o ângulo θ de B com relação ao 2DEG observa-se o encolhimento do anel. Nossos resultados mostram que o ângulo de colapso total do anel depende de uma competição entre o termo de troca da interação de Coulomb (princípio de Pauli) e cruzamentos evitados devido ao ângulo θ finito. As transições de fase exibem ainda o fenômeno de histerese. Na região de instabilidade ferromagnética obtemos diferentes soluções variando B de forma crescente ou decrescente. Estas soluções possuem energias total diferentes, de forma que representam estados fundamental e excitado de muitos corpos. Esta observação, juntamente com resultados anteriores do grupo [Freire & Egues (2007)], representam as primeiras realizações teóricas da previsão da possibilidade de estados excitados como mínimos locais do funcional de energia do estado fundamental [Perdew & Levy (1985)]. O modelo aqui proposto fornece excelente acordo com os experimentos considerados. Adicionalmente, a observação sistemática e experimentalmente verificada dos estados excitados valida as previsões de Perdew & Levy. Aplicamos ainda estas mesmas ideias no cálculo da estrutura eletrônica e condutância de fios quânticos na presença de campos magnéticos, mostrando que cruzamentos de modos transversais também exibem instabilidades ferromagnéticas observadas em experimentos recentes [Dissertação de Mestrado de Filipe Sammarco, IFSC/USP], fortalecendo a validade do modelo apresentado nesta tese. / The quantum Hall effect arises in two dimensional electron gases (2DEG) under high magnetic fields B. The magnetic field quantizes the planar motion of the electrons into cyclotron orbits given by the Landau levels. In this regime the transversal (Hall) resistivity ρxy shows plateaus as a function of B at integer sub-multiples of e2/h, i.e., ρxy = ν-1 e2/h, where n is the filling factor of the Landau levels. The longitudinal resistivity ρxx shows peaks at the transition between the plateaus of ρxy. In principle, ρxx is an indirect measure of the density of states at the Fermi level g(εF), so that the peaks indicate when the Fermi level εF crosses a Landau level. Therefore, a density-B-field diagram n2D-B of the ρxx ~ g(εF) peaks shows a topological map of the electronic structure of the system. In two-subband systems, ρxx( n2D, B) shows ringlike structures due to crossings of spin-split Landau levels from distinct subbands [experiments from the group of Prof. Jiang (UCLA)] that could lead to ferromagnetic instabilities. We study these instabilities using the density functional theory (DFT) to calculate the electronic structure, and Ando\'s model (Kubo formalism) for ρxx and ρxy. At higher temperatures (340 mK) we also obtain the ringlike structures in ρxx. At lower temperatures (70 mK) we see broken rings due to quantum Hall ferromagnetic phase transitions. Tilting B by theta with respect to the 2DEG normal we find that the ring structure shrinks. Our results show that the angle of full collapse depends on a competition between the exchange term from the Coulomb interaction (Pauli principle) and the anticrossing of Landau levels due to the finite angle theta. Additionally, at the instabilities we observe hysteresis. Sweeping the B field up or down near these regions we obtain two different solutions with distinct total energies, corresponding to the ground state and an excited state of the many-body system. This result, together with previous results of our group [Freire & Egues (2007)], are the first realizations of the theoretical prediction of the possibility of excited states as local minima of the ground state energy functional [Perdew & Levy (1985)]. The model proposed here shows an excellent agreement with the experiments. Additionally, the systematic and experimentally verified observation of excited states corroborates the predictions of Perdew & Levy. Similar ideas as presented here when applied to the electronic structure and conductance of quantum wires with an in-plane magnetic field show ferromagnetic instabilities at crossings of the wire transverse modes [Master Thesis of Filipe Sammarco, IFSC/USP], also with excellent experimental agreement. This strengthen the range of validity of the model proposed in this Thesis.

Made available in DSpace on 2014-12-17T15:41:43Z (GMT). No. of bitstreams: 1 JoseEMS.pdf: 2230919 bytes, checksum: f0f4e57c89f3eae4282c6daae0fe0aba (MD5) Previous issue date: 2008-04-18 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / It was synthesized different Ni1-xMgxFe2O4 (0,2 ≤ x ≤ 0,7) compositions by use of citrate precursor method. Initially, the precursory citrates of iron, nickel and magnesium were mixed and homogenized. The stoichiometric compositions were calcined from 350?C to 1200?C at ambient atmosphere or in argon atmosphere. The calcined powders were characterized by XRD, TGA/DTG, FTIR, magnetic measures and reflectivity using the wave guide method. I was observed pure magnetic phase formation between 350?C and 500?C, with formation of ferrite and hematite after 600?C at ambient atmosphere. The calcined powder at argon atmosphere formed pure ferromagnetic phase at 1100?C and 1200?C. The Rietveld analyses calculated the cations level occupation and the crystallite size. The analyses obtained nanometric crystals (11-66 nm), that at 900?C/3h presents micrometric sizes (0,45 - 0,70 Om). The better magnetization results were 54 Am2/Kg for x= 0,2 composition, calcined at 350?C/3h and 30 min, and 55,6 Am2/Kg for x= 0,2 1200?C, calcined in argon. The hysteresis shows characteristics of soft magnetic material. Two magnetization processes were considered, superparamagnetism at low temperature and the magnetic domains formation at high temperatures. The materials presented absorption less or equal the 50 % in ranges specific frequency. As for the 2,0 and 3,0 thickness (in 11,0 - 11,8 GHz), the reflectivity of the x= 0,3, 0,5 and 0,4 compositions, all calcined at 900?C/3h showed agreement with MS and O. Various factors contribute for the final radiation absortion effect, such as, the particle size, the magnetization and the polymer characteristics in the MARE composition. The samples that presented better magnetization does not obtaining high radiation absorption. It is not clear the interrelaction between the magnetization and the radiation absorption in the strip of frequencies studied (8,2 - 12,4 GHz) / Foram sintetizadas diferentes composi??es da ferrita Ni1-xMgxFe2O4 com (0,2 ≤ x ≤ 0,7) pelo uso do m?todo dos citratos precursores. Para se obter a fase estequiom?trica Ni1-xMgxFe2O4, inicialmente foram misturados e homogeneizados os citratos precursores de ferro, n?quel e magn?sio. As composi??es estequiom?tricas foram calcinadas entre as temperaturas de 350?C e 1200?C, em atmosfera ambiente ou de arg?nio. Os p?s calcinados foram caracterizados por DRX, TGA/DTG, FTIR, medidas magn?ticas e refletividade pelo m?todo de guia de ondas. Foi observada a forma??o de fase pura ferrimagn?tica entre 350?C e 500?C, formando ferrita e hematita ap?s 600?C, em atmosfera ambiente. O p? calcinado em atmosfera de arg?nio formou fase ferrimagn?tica pura em 1100?C e 1200?C. A an?lise pelo m?todo de Rietveld calculou o n?vel de ocupa??o dos c?tions e o tamanho de cristalito. A an?lise obteve tamanhos de cristais nanom?tricos, (11 - 66 nm), que a 900?C/3h apresentam tamanhos microm?tricos (0,45 0,70 Om). Os melhores resultados de magnetiza??o foram 54 Am2/Kg para a composi??o x= 0,2, calcinada a 350?C/3h e 30 min, e 55,6 Am2/Kg para x= 0,2 a 1200?C, calcinada em arg?nio. As histereses mostraram um perfil de materiais magn?ticos moles. Dois processos de magnetiza??o foram considerados, o superparamagnetismo a baixa temperatura e a forma??o de dom?nios magn?ticos em altas temperaturas. Os materiais apresentaram absor??o igual ou inferior a 50 % em faixas de freq??ncias espec?ficas. Para as espessuras 3,0 e 2,0 mm (em 11-11,8 GHz), as refletividades das composi??es x= 0,3, x= 0,5 e x= 0,4, todas a 900?C/3h demonstraram concord?ncia com MS e O. V?rios fatores contribuem para o efeito final de absor??o de radia??o, tais como, o tamanho de part?culas, a magnetiza??o, e as caracter?sticas do pol?mero na composi??o do MARE. As amostras que apresentaram maiores magnetiza??es n?o atingiram alta absor??o de radia??o. N?o ficou esclarecido a interrela??o entre a magnetiza??o e a absor??o de radia??o na faixa de freq??ncia estudada (8,2 12,4 GHz)

O efeito Hall quântico surge em gases de elétrons bidimensionais (2DEG) na presença de altos campos magnéticos B. O campo magnético quantiza o movimento planar dos elétrons em órbitas ciclotrônicas caracterizadas pelos níveis de Landau. Neste regime a resistividade transversal (ou Hall) ρxy em função de B exibe platôs em submúltiplos inteiros de e2/h, i.e., ρxy = ν-1 e2/h, sendo ν o fator de preenchimento dos níveis de Landau. Por sua vez, a resistividade longitudinal ρxx apresenta picos nas transições entre platôs de ρxy. Em primeira instância, ρxx é uma medida indireta da densidade de estados no nível de Fermi g(εF), e os picos dos mesmos indicam cruzamentos do nível de Fermi εF com niveis de Landau. Assim, o diagrama de densidade de elétrons n2D e B dos picos de ρxx ~ g(εF) fornece um mapa topológico da estrutura eletrônica do sistema. Em sistemas de duas subbandas, ρxx(n2D, B) exibe estruturas em forma de anel devido a cruzamentos de níveis de Landau de subbandas distintas [experimentos do grupo do Prof. Jiang (UCLA)]. Estes cruzamentos podem ainda levar a instabilidades ferromagnéticas. Investigamos estas instabilidades usando a teoria do funcional da densidade (DFT) para o cálculo da estrutura eletrônica, e o modelo de Ando (formalismo de Kubo) para o cálculo de ρxx e ρxy. Para temperaturas mais altas (340 mK) obtemos as estruturas em forma de anel em ρxx. Para temperaturas mais baixas (70 mK), observamos uma quebra dos anéis devido a transições de fase ferromagnéticas. Variando-se o ângulo θ de B com relação ao 2DEG observa-se o encolhimento do anel. Nossos resultados mostram que o ângulo de colapso total do anel depende de uma competição entre o termo de troca da interação de Coulomb (princípio de Pauli) e cruzamentos evitados devido ao ângulo θ finito. As transições de fase exibem ainda o fenômeno de histerese. Na região de instabilidade ferromagnética obtemos diferentes soluções variando B de forma crescente ou decrescente. Estas soluções possuem energias total diferentes, de forma que representam estados fundamental e excitado de muitos corpos. Esta observação, juntamente com resultados anteriores do grupo [Freire & Egues (2007)], representam as primeiras realizações teóricas da previsão da possibilidade de estados excitados como mínimos locais do funcional de energia do estado fundamental [Perdew & Levy (1985)]. O modelo aqui proposto fornece excelente acordo com os experimentos considerados. Adicionalmente, a observação sistemática e experimentalmente verificada dos estados excitados valida as previsões de Perdew & Levy. Aplicamos ainda estas mesmas ideias no cálculo da estrutura eletrônica e condutância de fios quânticos na presença de campos magnéticos, mostrando que cruzamentos de modos transversais também exibem instabilidades ferromagnéticas observadas em experimentos recentes [Dissertação de Mestrado de Filipe Sammarco, IFSC/USP], fortalecendo a validade do modelo apresentado nesta tese. / The quantum Hall effect arises in two dimensional electron gases (2DEG) under high magnetic fields B. The magnetic field quantizes the planar motion of the electrons into cyclotron orbits given by the Landau levels. In this regime the transversal (Hall) resistivity ρxy shows plateaus as a function of B at integer sub-multiples of e2/h, i.e., ρxy = ν-1 e2/h, where n is the filling factor of the Landau levels. The longitudinal resistivity ρxx shows peaks at the transition between the plateaus of ρxy. In principle, ρxx is an indirect measure of the density of states at the Fermi level g(εF), so that the peaks indicate when the Fermi level εF crosses a Landau level. Therefore, a density-B-field diagram n2D-B of the ρxx ~ g(εF) peaks shows a topological map of the electronic structure of the system. In two-subband systems, ρxx( n2D, B) shows ringlike structures due to crossings of spin-split Landau levels from distinct subbands [experiments from the group of Prof. Jiang (UCLA)] that could lead to ferromagnetic instabilities. We study these instabilities using the density functional theory (DFT) to calculate the electronic structure, and Ando\'s model (Kubo formalism) for ρxx and ρxy. At higher temperatures (340 mK) we also obtain the ringlike structures in ρxx. At lower temperatures (70 mK) we see broken rings due to quantum Hall ferromagnetic phase transitions. Tilting B by theta with respect to the 2DEG normal we find that the ring structure shrinks. Our results show that the angle of full collapse depends on a competition between the exchange term from the Coulomb interaction (Pauli principle) and the anticrossing of Landau levels due to the finite angle theta. Additionally, at the instabilities we observe hysteresis. Sweeping the B field up or down near these regions we obtain two different solutions with distinct total energies, corresponding to the ground state and an excited state of the many-body system. This result, together with previous results of our group [Freire & Egues (2007)], are the first realizations of the theoretical prediction of the possibility of excited states as local minima of the ground state energy functional [Perdew & Levy (1985)]. The model proposed here shows an excellent agreement with the experiments. Additionally, the systematic and experimentally verified observation of excited states corroborates the predictions of Perdew & Levy. Similar ideas as presented here when applied to the electronic structure and conductance of quantum wires with an in-plane magnetic field show ferromagnetic instabilities at crossings of the wire transverse modes [Master Thesis of Filipe Sammarco, IFSC/USP], also with excellent experimental agreement. This strengthen the range of validity of the model proposed in this Thesis.

A study is presented detailing experimental investigations of magnetization dynamics in nanostructured systems which are coupled magnetically. This work seeks to characterize the anisotropy of such systems through experimental techniques which probe microwave resonant absorption in the materials. A custom-built experimental setup, designed and assembled in our labs, is explained in detail. This setup allows for angular-dependent ferromagnetic resonance (FMR) measurements in the sample plane through vector network analyzer spectroscopy and is adaptable to two different types of coplanar waveguides. This technique has proven effective for characterization of multiple types of magnetic systems, including multilayered structures as detailed here, with different types of anisotropies while allowing us to draw analogies with more common characterization techniques. The angular FMR setup has been used to study coupled systems, such as those coupled through the Ruderman–Kittel–Kasuya–Yosida interaction as well as exchange-biased structures. These types of coupled systems have technological impacts and are highly applied in the components of magnetoresistive random access memory. Using this new characterization technique, properties of synthetic antiferromagnets have been revealed which had not been observed before. In addition to these experiments, magnetic susceptibility and FMR in exchange biased systems have been investigated at temperatures as low as 2 K. This investigation used a new FMR spectrometer and was one of the first studies to use this instrument. For the first time a new method of identifying several types of coupling which can be present in layered nanostructures is presented and supported through comparison with known techniques, thus connecting a new characterization technique for layered structures with decades-old procedures. Many results within this work are also supported theoretically with computer simulations.

Récemment, les études des propriétés de vortex Abrikosov dans des systèmes fortement anisotropes et magnétiques ont été stimulées par la découverte des supraconducteurs à base de fer et des supraconducteurs ferromagnétiques.Dans cette thèse nous étudions la statique et la dynamique de vortex dans ces systèmes. D’abord, le problème de l'interaction de vortex avec un petit défaut a été examiné dans le cadre de la théorie de Ginzburg-Landau. Le potentiel de pinning pour une cavité cylindrique elliptique a été calculé. D'autre part, la conductivité d'un supraconducteur anisotrope à l'état mixte a été analysée en détail dans le cadre de la théorie de Ginzburg-Landau dépendant du temps.Une partie significative de la thèse est consacrée à l'étude de l'interaction entre lesondes de spin (magnons) et vortex dans les supraconducteurs ferromagnétiques.Nous avons démontré que le spectre de magnon acquiert une structure de bande en présence d'un réseau de vortex idéal. En utilisant les équations phénoménologiques de London et de Landau-Lifshitz-Gilbert, nous avons étudié les réponses ac et dc de vortex dans les supraconducteurs ferromagnétiques. Enfin, nous avons examiné l'état de vortex dans des structures hybrides supraconducteur(S)-ferromagnétique(F)(par exemple, super-réseaux FS) avec une forte dispersion spatiale de la susceptibilité magnétique. Dans ces systèmes l'électrodynamique supraconductrice peut être fortement non locale, qui mène à l'attraction des vortex et à une transition de phase du premier ordre dans la phase de vortex. / Recently, the studies of the properties of Abrikosov vortices in strongly anisotropicand magnetic media have been stimulated by the discovery of the iron-based andferromagnetic superconductors. In this thesis an analysis of vortex statics anddynamics in such systems has been carried out. Firstly, the problem of vortex pinningon a small defect has been considered. Within the Ginzburg-Landau theory thepinning potential for a cavity in the form of an elliptical cylinder has been derived.Secondly, the flux-flow conductivity of an anisotropic superconductor has beenanalyzed in detail within the time-dependent Ginzburg-Landau theory.A significant part of the thesis is devoted to the study of interplay between spinwaves (magnons) and vortices in ferromagnetic superconductors. We havedemonstrated that the magnon spectrum acquires a Bloch-like band structure in thepresence of an ideal vortex lattice. Using the phenomenological London and Landau-Lifshitz-Gilbert equations, we studied the ac and dc responses of vortices inferromagnetic superconductors. Finally, we investigated the vortex state insuperconductor-ferromagnet (FS) hybrid structures (e. g., FS superlattices) withstrong spatial dispersion of the magnetic susceptibility. In such systems thesuperconducting electrodynamics may be strongly nonlocal, which leads to theattraction of vortices and to a first order phase transition at the lower critical field.

The purpose of this work is to reveal the governing mechanisms responsible for the magnetic field-induced i) martensite reorientation in Ni2MnGa single crystals, ii) stress-assisted phase transformation in Ni2MnGa single crystals and iii) phase transformation in NiMnCoIn alloys. The ultimate goal of utilizing these mechanisms is to increase the actuation stress levels in magnetic shape memory alloys (MSMAs). Extensive experimental work on magneto-thermo-mechanical (MTM) characterization of these materials enabled us to i) better understand the ways to increase the actuation stress and strain and decrease the required magnetic field for actuation in MSMAs, ii) determine the effects of main MTM parameters on reversible magnetic field induced phase transformation, such as magnetocrystalline anisotropy energy (MAE), Zeeman energy (ZE), stress hysteresis, thermal hysteresis, critical stress for the stress induced phase transformation and crystal orientation, iii) find out the feasibility of employing polycrystal MSMAs, and iv) formulate a thermodynamical framework to capture the energetics of magnetic field-induced phase transformations in MSMAs. Magnetic shape memory properties of Ni2MnGa single crystals were characterized by monitoring magnetic field-induced strain (MFIS) as a function of compressive stress and stress-induced strain as a function of magnetic field. It is revealed that the selection of the operating temperature with respect to martensite start and Curie temperatures is critical in optimizing actuator performance. The actuation stress of 5 MPa and work output of 157 kJm−3 are obtained by the field-induced variant reorientation in NiMnGa alloys. Reversible and one-way stress-assisted field-induced phase transformations are observed in Ni2MnGa single crystals under low field magnitudes (<0.7T) and resulted in at least an order of magnitude higher actuation stress levels. It is very promising to provide higher work output levels and operating temperatures than variant reorientation mechanisms in NiMnGa alloys. Reversible field-induced phase transformation and shape memory characteristics of NiMnCoIn single crystals are also studied. Reversible field-induced phase transformation is observed only under high magnetic fields (>4T). Necessary magnetic and mechanical conditions, and materials design and selection guidelines are proposed to search for field-induced phase transformation in other ferromagnetic materials that undergo thermoelastic martensitic phase transformation.

The purpose of this work is to reveal the governing mechanisms responsible for the magnetic field-induced i) martensite reorientation in Ni2MnGa single crystals, ii) stress-assisted phase transformation in Ni2MnGa single crystals and iii) phase transformation in NiMnCoIn alloys. The ultimate goal of utilizing these mechanisms is to increase the actuation stress levels in magnetic shape memory alloys (MSMAs). Extensive experimental work on magneto-thermo-mechanical (MTM) characterization of these materials enabled us to i) better understand the ways to increase the actuation stress and strain and decrease the required magnetic field for actuation in MSMAs, ii) determine the effects of main MTM parameters on reversible magnetic field induced phase transformation, such as magnetocrystalline anisotropy energy (MAE), Zeeman energy (ZE), stress hysteresis, thermal hysteresis, critical stress for the stress induced phase transformation and crystal orientation, iii) find out the feasibility of employing polycrystal MSMAs, and iv) formulate a thermodynamical framework to capture the energetics of magnetic field-induced phase transformations in MSMAs. Magnetic shape memory properties of Ni2MnGa single crystals were characterized by monitoring magnetic field-induced strain (MFIS) as a function of compressive stress and stress-induced strain as a function of magnetic field. It is revealed that the selection of the operating temperature with respect to martensite start and Curie temperatures is critical in optimizing actuator performance. The actuation stress of 5 MPa and work output of 157 kJm−3 are obtained by the field-induced variant reorientation in NiMnGa alloys. Reversible and one-way stress-assisted field-induced phase transformations are observed in Ni2MnGa single crystals under low field magnitudes (<0.7T) and resulted in at least an order of magnitude higher actuation stress levels. It is very promising to provide higher work output levels and operating temperatures than variant reorientation mechanisms in NiMnGa alloys. Reversible field-induced phase transformation and shape memory characteristics of NiMnCoIn single crystals are also studied. Reversible field-induced phase transformation is observed only under high magnetic fields (>4T). Necessary magnetic and mechanical conditions, and materials design and selection guidelines are proposed to search for field-induced phase transformation in other ferromagnetic materials that undergo thermoelastic martensitic phase transformation.

This work presents the development of a GaN-based dilute magnetic semiconductor (DMS) by metal organic chemical vapor deposition (MOCVD) that is ferromagnetic at room temperature (RT), electrically conductive, and possesses magnetic properties that can be tuned by n- and p-doping. The transition metal series (TM: Cr, Mn, and Fe) along with the rare earth (RE) element, Gd, was investigated in this work as the magnetic ion source for the DMS. Single- phase and strain-free GaTMN films were obtained. Optical measurements revealed that Mn is a deep acceptor in GaN, while Hall measurements showed that these GaTMN films were semi-insulating, making carrier mediated exchange unlikely. Hysteresis curves were obtained for all the GaTMN films, and by analyzing the effect of n- and p-dopants on the magnetic properties of these films it was determined that the magnetization is due to magnetic clusters. These findings are supported by the investigation of the effect of TM dopants in GaN nanostructures which reveal that TMs enhance nucleation resulting in superparamagnetic nanostructures. Additionally, this work presents the first report on the development of GaGdN by MOCVD providing an alternate route to developing a RT DMS. Room temperature magnetization results revealed that the magnetization strength increases with Gd concentration and can be enhanced by n- and p-doping, with holes being more efficient at stabilizing the ferromagnetic signal. The GaGdN films obtained in this work are single-phase, unstrained, and conductive making them suitable for the development of multifunctional devices that integrate electrical, optical, and magnetic properties.