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Magnetic tunnel junctions (MTJs), which consist of a thin insulation layer sandwiched by two ferromagnetic (FM) layers, are among the key devices of spintronics that have promising technological applications for computer hard disk drives, magnetic random access memory (MRAM) and other future spintronic devices. The work presented here is related to the development of relevant techniques for the preparation and characterization of magnetic films, exchanged biased systems and MTJs. The fabrication and characterization of PtMn/CoFe exchange biased systems and MTJs with Al-O barriers were undertaken when the new Aviza StratIon fxP ion beam deposition tool was developed by the project consortium funded by DTI MNT. After the Nordiko 9550 spintronic deposition tool was installed at Plymouth, the work focused on the development of MTJ multilayer stacks with layer structures of CoFeB/MgO/CoFe/IrMn and IrMn/CoFeB/MgO/CoFeB to achieve coherent tunneling with a crystalline MgO barrier. The film deposition, microfabrication, magnetic field annealing, microstructural and nano-scale characterization, magnetic and magneto-transport measurement for these devices have been systematically studied to achieve smooth interfaces and desired crystallographic textures and magnetic properties of layer stacks. Magnetoresistance (MR) of up to 200% was obtained from MTJs with a layer structure of Ta/CuN/Ta/CoFeB/MgO/CoFe/IrMn/Ta and a CuN bottom electrode. Enhanced exchange anisotropy from the bottom pinned IrMn/CoFeB stacks has been obtained, which demonstrated the possibility of fabricating MTJs with CoFeB as both the top and bottom FM electrodes with strong exchange bias. The origin of the enhanced exchange bias field was studied by employing high resolution transmission electron microscopy (HRTEM) and x-ray magnetic circular dichroism (XMCD) to examine the mmicrostructure properties and element specific magnetic properties of the stacks. Results demonstrate that the enhanced exchange anisotropy in the IrMn/CoFeB system is closely associated with the increased uncompensated interfacial spins. MTJs with layered structures of IrMn/CoFeB/MgO/CoFeB were prepared based on this exchange bias system. However, further work is required for the optimisation of the (001) crystallographic textures of the CoFeB/MgO/CoFeB stack to achieve coherent tunneling.

The field of molecular magnetism has expanded rapidly since the discovery of single molecule magnets (SMMs) in the 1990’s and has witnessed extraordinary advances in the last several decades. One of the current trends in molecular magnetic research is to incorporate metal ions that have pronounced single-ion anisotropy in an effort to improve magnetic exchange interactions. The 4d and 5d transition metal ions have large spin-orbit coupling parameters which contribute to the orbital angular momentum effects that lead to anisotropic behavior. The work herein describes efforts to synthesize and characterize new cyanide-bridged molecular materials incorporating 4d and 5d transition metal ions, specifically the [Os(CN)_(6)]^(3-), [Mo(CN)_(6)]^(3-) and [W(CN)_(8)]^(3-) ions. The 5d hexacyanometallate [Os(CN)_(6)]^(3-) was incorporated into a trinuclear cyanide bridged molecule and the [Fe(CN)_(6)]^(3-) analog was prepared as a reference compound for assessing the effect of the 5d versus 3d metal ion on the magnetic properties. Both molecules exhibit SMM bistability with a pronounced increase (~90 %) in the blocking temperature (TB) of the OsIII analogue. In addition to typical SMM behavior, both compounds exhibit exchange-biased SMM behavior, a shift in the quantum tunneling of the magnetization (QTM) from zero field. This exchange-bias can be turned “on” or “off” depending on the presence of interstitial methanol molecules. New trigonal bipyramidal (TBP) molecules incorporating the rarely studies hexacyanomolybdate(III) ion are presented in chapter III of this dissertation. The molecules of general formula [M(tmphen)_(2)]_(3)[Mo(CN)_(6)]_(2) (M = V^(II), Mn^(II) and Fe^(II); tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline), represent additions to a large homologous family of TBP molecules reported by the Dunbar group over the years. The [Mo(CN)_(6)]^(3-) ion was prepared in situ by loss of one cyanide ligand from [Mo(CN)_(7)]^(4-). Of particular interest among the compounds reported is the V_(3)Mo_(2) analog which exhibits extraordinarily strong antiferromagnetic coupling (estimated J = -134 cm^(-1). The observed exchange coupling parameter is more than twice the current record for the antiferromagnetic coupling parameter for a cyanide-bridged magnetic molecule. Another set of results were obtained using the octacyanometallate anion [WV(CN)_(8)]^(3-) as a building block for the synthesis and magnetic studies of a family of new cyanide-bridged magnetic materials. The compounds exhibit several different structural motifs including three 0-D molecular compounds (two pentanuclear molecules and a linear trinuclear molecule) and a 1-D chain, findings that illustrate the structural versatility of the octacyanotungstate(V) ions. The TBP molecule, [Mn(tmphen)_(2)]_(3)[W(CN)_(8)]_(2), exhibits evidence for an out-of-phase signal when subjected to ac measurements in zero applied field. The 1-D chain also reveals evidence for the beginning of an out-of-phase signal under zero applied field which hints at single chain magnet behavior.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In this work a study of the exchange interaction between ferromagnetic (NiFe) and antiferromagnetic (IrMn) layers was done through structural, magnetic and electric characterization. NiFe/IrMn/Ta films were grown with different number of repetitions of this basic structure aiming to evaluate possible changes on the magnetic anisotropies presented by the samples. It was implanted in the Laboratório de Magnetismo e Materiais Magnéticos a system in order to measure the Anisotropic Magnetorresistance (AMR) as function of the applied field angle. The AMR consists in a change of the eletric resistance of a ferromagnetic material as function of the angle between the electric current and the magnetization of the material and, by this way, sensible to changes in the anisotropy presented by the samples. It is presented a simple model to calculate the AMR as function of the angle field and, by comparing with the experimental curves, to obtain the magnetic parameters who describe the system. In the model the equilibrium direction of the magnetization is obtained from the minimization of the free magnetic energy, which is given by the Zeeman, uniaxial, unidirectional and manetostatic energies. The AMR curves present an assymetry around 180 degrees when measured at fields below the bias one. According to the fittings, it was found that such assymetry can be due by a misalignment between the anisotropy axis (uniaxial and unidirectional) or by a misalignment between the measuring current and the easy magnetic axis of the samples. It was not observed, as expected, an expressive increase of the anisotropy dispersion of the uniaxial anisotropy with the increase of the number of trilayers. The same was observed with the bias and coercive fields. / Neste trabalho, o estudo da interação de troca entre as camadas ferromagnética (NiFe) e antiferromagnética (IrMn) em multicamadas foi realizado através da caracterização estrutural, magnética e de transporte elétrico. Foram crescidos filmes de NiFe/IrMn/Ta com diferentes números de repetições dessa estrutura básica com o objetivo de avaliar as possíveis modificações nas anisotropias apresentadas pelas amostras. Foi implantado no Laboratório de Magnetismo e Materiais Magnéticos (LMMM) um sistema para medidas de Magnetorresistência Anisotrópica (AMR) em função do ângulo de aplicação do campo. A AMR consiste na variação da resistência elétrica e a magnetização do material e, portanto, sensível as modificações de anisotropia nas amostras. É apresentado um modelo simples para calcular as curvas de AMR em função do ângulo do campo e, na comparação com as curvas experimentais obter os parâmetros magnéticos que descrevem o sistema. No modelo, a direção de equilíbrio da magnetização é obtida a partir da minimização da energia livre magnética, que por sua vez é dada pela soma da energia Zeeman, uniaxial, unidirecional e magnetostática. As curvas de AMR apresentam uma assimetria em torno de 180 graus quando medidas em valores de campo menores que o campo de Bias. De acordo com os ajustes, foi verificado que esta assimetria pode ser causada tanto por um desalinhamento entre os eixos de anisotropias (uniaxial ou unidirecional) como por um desalinhamento entre o eixo da corrente de medição e o eixo de fácil magnetização da amostra. Não foi observado, conforme esperado, um aumento expressivo na dispersão da anisotropia uniaxial com o aumento do número de repetições das tricamadas. O mesmo ocorrendo com os valores dos campos de Bias e coercivo.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In this work we investigated the magnetization process and the magnetoimpedance in samples exhibiting exchange bias. Ni81Fe19=Fe50Mn50=T a bilayers and multilayers where grown onto glass substrates. The exchange bias was induced during growth by a 2 kOe static magnetic field. Structural, magnetic and impedance characterization was carried out. In the hysteresis curves from all samples a shift was observed. This shifting was also observed in the magnetoimpedance curves, and it had the same magnitude observed in the magnetic hysteresis curve. This displacement caused the impedance versus magnetic field curve to be approximately linear in the region near to H = 0. It is possible to use these systems as a magnetic field sensor with a linear response. The sensitivity, defined as the rate of change of the real part of the impedance with the magnetic field, reached values above 200 m =Oe at frequencies below 1 GHz. / Neste trabalho são investigados os processos de magnetização e a magnetoimpedância em filmes com exchange bias (EB). São estudadas amostras de Ni81Fe19=Fe50Mn50=T a depositadas como bi e multi-camadas, sob substratos de vidro. O Exchange Bias foi induzido durante o crescimento das amostras através da aplicação de um campo magnético estático de 2 kOe. A caracterização estrutural das amostras foi feita através de medidas de difração de raios-x a altos ângulos, utilizando uma configuração θ 2θ. A caracterização magnética estática foi feita através de medidas de magnetização obtidas com um magnetômetro de gradiente de campo alternado. As componentes indutivas da impedância foram medidas em função da freqüência (100 MHz 3 GHz) e do campo estático (H=±200Oe). Em todas as amostras foi observado o deslocamento da curva de histerese magnética devido a ocorrência do fenômenos de Exchange Bias. Este deslocamento também é observado nas curvas de magnetoimpedância e tem mesma magnetude do observado nas curvas de histerese magnética dentro da precisão experimental que temos na medida do campo magnético. Este deslocamento faz com que a variação da impedância com o campo mangnético aplicado seja aproximadamente linear na região próxima a H = 0. Tornando o fenômeno de exchange bias um possivel aliado para a obtenção de assimetrias nas curvas de magnetoimpedância para o possível uso destes sistemas como sensores de campo magnético com resposta linear. Definindo a sensibilidade como a taxa de variação da parte real da impedância com o campo magnético, foram obtidos valores, em módulo, acima de 200m =Oe em frequências abaixo de 1 GHz.

The present rapid increase in the number of space missions demands a decrease in the cost of satellite equipment, but also requires the development of instruments that have low power consumption, low weight, and small size.Anisotropic magnetoresistance (AMR) sensors can answer these needs on account of their small size, weight, and power consumption. AMR sensors also produce lower noise than either giant magnetoresistance (GMR) or tunnel magnetoresistance (TMR) devices and are thus more suitable for space applications.The type of AMR sensor developed in this study was a Planar Hall EffectBridge (PHEB) sensor. The FM layer was also coupled with an AFM layer in order to fix the internal magnetization of the FM layer.One technique that was employed in order to meet the low-noise requirement was to make the FM layer thicker than has previously been attempted.In doing so, the exchange bias field between the AFM layer and the FMlayer is no longer high enough to bias the thicker FM layer, so in order to correct this unwanted effect, the material stack was upgraded to two AFM–FM interfaces. With this configuration, it became possible to increase the exchange field by up to 60%. Stronger exchange bias leads to a thicker FMlayer and so to lower noise in the device performance. Another strategy that was used to lower the resistance of the device was to implement an NiFeX alloy instead of the standard NiFe. NiFeX consists of an alloy of NiFe andCu, Ag, or Au; the last of these is known to have very low resistivity.This solution leads to a significant lowering of the device’s resistance. A recent technological advance used to fabricate devices with lower resistance is to deposit a multilayer of AFM–FM.

Cette thèse s'inscrit dans la thématique des TA-MRAM, nouvelles mémoires non volatiles qui utilisent des impulsions de courant pour chauffer et ainsi permettre le renversement de l'aimantation d'une couche ferromagnétique. Un premier but de ce travail a été de comprendre les mécanismes d'écriture (chauffage) des cellules TA-MRAM. Mesures électriques et simulations thermiques sont alors comparées. De manière générale les TA-MRAM associent deux couches ferromagnétiques dont les directions relatives des aimantations sont stabilisées par des couches antiferromagnétiques. Une action développement matériaux a été menée dans la conception de jonctions tunnel magnétiques utilisant l'alliage FeMn dans la couche de stockage. Nous avons alors cherché à optimiser les couches ferromagnétiques et antiferromagnétique de la couche de stockage afin de minimiser le champ magnétique nécessaire au renversement de l'aimantation de ces couches et donc de diminuer la consommation d'énergie. Enfin ce manuscrit présente une partie gravure par faisceau d'ion (IBE) de points mémoires magnétiques qui donnent les clefs de fabrication des mémoires MRAM. / This thesis addresses a current topic of TA-MRAM, new non volatile memories using pulses to heat and thus allows the reversal of the magnetization of a ferromagnetic layer. A first goal was to learn the writing mechanism (heating) of TA-MRAM cells. Therefore, we compared electrical measurements and thermal simulations. Generally speaking, TA-MRAM combines two ferromagnetic layers where the relative direction of the magnetization is pinned by antiferromagnetic layers. Moreover, we developed new magnetic tunnel junctions using FeMn in the storage layer. We thus optimized the ferromagnetic and antiferromagnetic layers of the storage layer in order to minimize the magnetic field needed to reverse the magnetization of these layers and thus to decrease the power consumption. To finish, the etching of magnetic memory dots is explored, giving the pattern key of magnetic devices.

Magnetic, magnetocaloric, magnetotransport and magnetoelastic properties of Ni-Mn-X (X = In, and Ga) Heusler alloys and La-Fe-Si based rare earth compounds have been synthesized and investigated by x-ray diffraction, magnetization, strain, and electrical resistivity measurements. The phase transitions, magnetic, magnetocaloric, magnetotransport and magnetoelastic properties strongly depend on the composition of these systems. In Ni50Mn50-xInx with x = 13.5, magnetocaloric and magnetotransport properties associated with the paramagnetic martensitic to paramagnetic austenitic transformation were studied. It was shown that magnetic entropy changes (SM) and magnetoresistance (MR) associated with this transformation are larger and the hysteresis effect is significantly lower when compared to that associated with paramagnetic-ferromagnetic transitions or ferromagnetic-antiferromagnetic/paramagnetic transitions in other systems. The Hall resistivity and the Hall angle shows unusual behavior in the vicinity of the martensitic phase transition for Ni50Mn50-xInx with x = 15.2. The observed Hall resistivity and Hall angle are 50 μ*cm and , respectively. It was observed that the presence of Ge, Al and Si atoms on the In sites strongly affects the crystal structure, and the electric and magnetic behaviors of Ni50Mn35In15. It was found that the partial substitution of In atoms by Si in Ni50Mn35In15 results in an increase in the magnetocaloric effect, exchange bias and shape memory effect. In Ni50Mn35In15-xSix, the peak values of positive SM for magnetic field changes H = 5 T were found to depend on composition and vary from 82 Jkg-1K-1 for x = 1 (at T = 275 K) to 124 Jkg-1K-1 for x = 3 (at T = 239 K). The partial substitution of Ni by Co in Ni50Mn35In15 significantly improves the magnetocaloric effect and MR in the vicinity of martensitic transition. In addition, significantly large inverse SM and MR were observed at the inverse martensitic phase transitions of the Ga-based magnetic shape memory Heusler alloys Ni50-xCoxMn32-yFeyGa18. The phase transition temperatures and magnetic properties were found to be correlated with the degree of tetragonal distortion in these samples. In LaFe11.57Si1.43Bx the crystal cell parameters and Curie temperatures were found to increase linearly with increasing B concentration up to ~ 0.1 % and 9 %, respectively. It was found that the characteristics of the magnetocaloric effect of LaFe11.57Si1.43 can be adjusted by a change in B concentration in the LaFe11.57Si1.43Bx system. A study of the influence of a small substitution of Ni, Cu, Cr, and V for Fe in LaFe11.4Si1.6 revealed that the magnetic, magnetocaloric, and magnetovolume coupling constant is related to an increase in the average Fe-Fe interatomic distances, leading to a change in the d-d exchange interaction.

Made available in DSpace on 2014-06-12T18:05:52Z (GMT). No. of bitstreams: 2 arquivo8040_1.pdf: 1356225 bytes, checksum: 102f2a0a220b971a269f68419cf93313 (MD5) license.txt: 1748 bytes, checksum: 8a4605be74aa9ea9d79846c1fba20a33 (MD5) Previous issue date: 2002 / No presente trabalho apresentaremos um estudo das propriedades magnéticas de três sistemas: manganitas, fitas amorfas e bicamadas com anisotropia unidirecional. Foi dado enfoque ao efeito da desordem magnética nesses sistemas. Todas as medidas de magnetização foram feitas em um magnetômetro SQUID desenvolvido durante o período da tese. Estudamos várias amostras de manganitas, do tipo filme e espessas. Essas amostras apresentam o efeito de magneto-resistência colossal. As propriedades de transporte foram ajustadas usando o modelo dos canais de condução. As medidas na amostra dopada com Ho apresentaram um comportamento peculiar com um mínimo na resistência em baixas temperaturas. Esse mínimo foi associado ao espalhamento eletrônico em duas dimensões. Além desse mínimo, encontramos efeitos de termoremanência, histerese e irreversibilidade na resistência. A partir das medidas magnéticas foram levantados diagramas de fase que demonstraram comportamento do tipo vidro de spin. As propriedades em baixas temperaturas foram associadas a espalhamento eletrônico por paredes de domínios e a mistura de fases magnéticas. Também foi utilizado um modelo de transporte bidimensional para explicar a existência do mínimo na resistência. O segundo sistema estudado foi a fita amorfa Co70,4Fe4,6Si10B15. Nesse tema, o estudo da magnetoimpedância-gigante foi estendido até freqüências na faixa de microondas, e mostrada sua equivalência com a ressonância ferromagnética. Por fim, foi feito um estudo das propriedades magnéticas em filmes compostos de uma camada ferromagnética e outra antiferromagnética. Estes filmes apresentam anisotropia unidirecional com o ciclo de histerese deslocado. Os resultados demonstraram efeitos de desordem magnética com propriedades tanto do tipo vidro de spin quanto de campo aleatório. Pela primeira vez esses dois tipos de comportamentos foram encontrados em um único sistema. Os resultados foram comparados com simulações utilizando formalismo de campo médio, aplicado a um modelo de bicamadas com rugosidade. Obtendo-se portanto a evidência definitiva que a rugosidade na interface é fonte de desordem magnética nesses sistemas

Magnetic materials are one of the most interesting and promising class of materials for technological applications [1]. Among them, patterned ferromagnetic systems have an important role especially in the prospect of high density data storage [2], domain wall logic devices [3] and magnetic memory [4, 5]. Coupled systems of ferromagnetic and antiferromagnetic materials have been implemented to design sensors such as giant magnetoresistance (GMR) [6-8] and tunnel magnetoresistance (TMR) [9, 10]. Ferromagnetic nanoparticles have been used for the drug targeting, cancer therapy, MRI and many more applications [11, 12]. In addition, more recently, significant attention has been paid to explore the dynamic properties of magnetic materials in the GHz range and use them for technological applications such as microwave filters, signal processing, phase shifter, nonreciprocal microwave devices, spin wave guide, high frequency memory, logic elements [13-19] Boundary conditions, interactions between individual entities, and lateral confinement of magnetic charges generate diverse magnetic properties especially at nanoscale length [20, 21]. The variation of magnetic properties are even quite different when the size of the magnetic structure is smaller or comparable with the magnetic characteristic length such as mean free path of electron, width of domain wall and even the spin diffusion length [22-24]. In this study, we have considered different magnetic systems. Firstly, the multilayer of coupled ferromagnetic and antiferromagnetic system has been considered to evaluate the exchange bias anisotropy. [FeNi/IrMn]n multilayer systems with different thicknesses of ferromagnetic layer were studied. Static and dynamic properties were revealed through magnetometry measurements (VSM) and VNA-FMR techniques respectively. Angular variation of first order reversal curve (AFORC) and ferromagnetic resonance (AFMR) were performed to learn the intrinsic exchange bias distribution. Secondly, patterned magnetic structures were synthesized to understand the magnetization dynamics in confined geometry. Surface modulated thin films with different periodicity, dumbbell-shaped structures with variable size and three dimensional magnonic crystals have been studied using both static and dynamic measurement techniques. Micromagnetic simulations were performed to understand and explain the experimental results.