This thesis is an experimental study of spin dependent transport in nanoscale ferromagnetic tunnel junction arrays and lateral multi-terminal devices with normal metal and superconducting spin transport channels. Two-, three-, and five-junction arrays have been fabricated in the form of lateral circuits and characterized using variable temperature magneto-transport measurements. The smallest inter-junction separation achieved was 65 nm. No significant enhancement in the sequential magneto-resistance (MR) was observed, which is attributed to the combined effect of short spin diffusion length in the ferromagnetic electrodes and high resistance of the tunnel barriers used. A substantially weaker bias dependence of the MR is observed for double junctions than for single junctions, consistent with the theoretical expectations. Spin diffusion and relaxation in one-dimensional normal metal channels is studied using a novel multi-terminal device. The device has multiple ferromagnetic detector electrodes for an in-situ determination of the spin transport parameters. Such configuration has a great advantage as it eliminates sample-to-sample uncertainties in the physical properties studied. A three terminal device having a pair of detector electrodes placed symmetrically about the injection point is used to directly demonstrate decoupling of spin and charge current in nanostructures. Furthermore, by varying the thickness of the normal metal channel on the scale of the mean free path the surface contribution to spin relaxation is measured and compared to the bulk spin scattering rate. It is found that for Al surface scattering makes a weak contribution to the overall spin relaxation rate, the result that should be important for a number of proposed thin film spin-based devices. The interplay between non-equilibrium magnetism and superconductivity is studied in a ferromagnetic/superconductor single electron transistor. Spin imbalance in the base is controlled by the bias voltage applied to the magnetic emitter/collector as well as the relative orientation of their magnetic moments. A strong magneto-transport effect is observed and attributed to a suppression of the superconducting gap in the center electrode by the spin imbalance in the antiparallel state of the device. The intrinsic spin relaxation parameters for the center electrode, important for interpreting the data are studied in a separate experiment using spin injection into a one-dimensional superconducting channel. It is found that the spin accumulation increases substantially on transition into the superconducting state while the spin diffusion length is reduced. These results represent a new way of combining magnetism and superconductivity on the nano-scale. / QC 20100924

We present a study on perpendicular magnetic tunnel junctions with W as buffer and capping layers. A tunneling magnetoresistance of 138% and an interfacial magnetic anisotropy of 1.67 erg/cm(2) were obtained in optimally annealed samples. However, after extended annealing at 420 degrees C, junctions with W layers showed extremely small resistance due to interdiffusion of W into the MgO barrier. In contrast, in Ta-based junctions, the MgO barrier remained structurally stable despite disappearance of magnetoresistance after extended annealing due to loss of perpendicular magnetic anisotropy. Compared with conventional tunnel junctions with in-plane magnetic anisotropy, the evolution of tunneling conductance suggests that the relatively low magnetoresistance in perpendicular tunnel junctions is related to the lack of highly polarized Delta(1) conducting channel developed in the initial stage of annealing. Published by AIP Publishing.

Fabrication of molecular spin devices with ferromagnetic electrodes coupled with a high spin molecule is an important challenge. This doctoral study concentrated on realizing a novel molecular spin device by the bridging of magnetic molecules between two ferromagnetic metal layers of a ferromagnetic-insulator-ferromagnetic tunnel junction on its exposed pattern edges. At the exposed sides, distance between the two metal electrodes is equal to the insulator film thickness; insulator film thickness can be precisely controlled to match the length of a target molecule. Photolithography and thin-film deposition were utilized to produce a series of tunnel junctions based on molecular electrodes of multilayer edge molecular electrodes (MEME) for the first time. In order to make a microscopic tunnel junction with low leakage current to observe the effect of ~10,000 molecules bridged on the exposed edge of a MEME tunnel barrier, growth conditions were optimized; stability of a ~2nm alumina insulator depended on its ability to withstand process-induced mechanical stresses. The conduction mechanism was primarily 1) tunneling from metal electrode to oranometalic core by tunneling through alkane tether that acts as a tunnel barrier 2) rapid electron transfer within the oranometalic Ni-CN-Fe cube and 3) tunneling through alkane tether to the other electrode. Well defined spin-states in the oranometalic Ni-CN-Fe cube would determine electron spin-conduction and possibly provide a mechanism for coupling. MEME with Co/NiFe/AlOx/NiFe configurations exhibited dramatic changes in the transport and magnetic properties after the bridging of oranometalic molecular clusters with S=6 spin state. The molecular cluster produced a strong antiferromagnetic coupling between two ferromagnetic electrodes to the extent, with a lower bound of 20 erg/cm,2 that properties of individual magnetic layers changed significantly at RT. Magnetization, ferromagnetic resonance and magnetic force microscopy studies were performed. Transport studies of this configuration of MEME exhibited molecule-induced current suppression by ~6 orders by blocking both molecular channels and tunneling between metal leads in the planar 25μm2 tunnel junction area. A variety of control experiments were performed to validate the current suppression observation, especially critical due to observed corrosion in electrochemical functionalization step. The spin devices were found to be sensitive to light radiation, temperature and magnetic fields. Along with the study of molecular spin devices, several interesting ideas such as ~9% energy efficient ultrathin TaOx based photocell, simplified version of MEME fabrication, and chemical switching were realized. This doctoral study heralds a novel molecular spin device fabrication scheme; these molecular electrodes allow the reliable study of molecular components in molecular transport.

In the present thesis, the properties at ferromagnet/semiconductor interfaces, relevant for semiconductor spintronics applications, are addressed. Semiconductor spintronics refers to the possibility of storing information using the electron spin, additional to the electron charge, for enhanced flexibility in nanoscale semiconductor devices. The system under focus is the Fe/ZnSe(001) heterostructure, where ZnSe is a wide gap semiconductor ideally compatible with GaAs. The heterostructures are grown on GaAs(001) substrates by molecular beam epitaxy. From various electron-beam based diffraction, spectroscopy and microscopy techniques, it is shown that Fe grows epitaxially and predominantly in a layer-by-layer mode on ZnSe(001) with no presence of chemically reacted phases or interdiffusion. An in-plane uniaxial magnetic anisotropy (UMA) is detected for thin Fe films on ZnSe(001) by magnetometry, thus opposing the cubic symmetry of bcc Fe. From first principles calculations, the unidirectional sp3-bonds from ZnSe are shown to induce this uniaxiality. Moreover, an in-plane anisotropic lattice relaxation of Fe is found experimentally, seemingly as a consequence of the sp3-bonds, giving an additional UMA contribution via magneto-elastic coupling. It is proposed that these two effects are responsible for the much-debated UMA observed in Fe/semiconductor structures in general. The interface magnetism is probed by x-ray magnetic circular dichroism and Mössbauer spectroscopy. It is found that the magnetic moment at the interface is comparable or even enhanced with respect to the bulk Fe. These two experiments are believed to provide the first unambiguous proof of a persistent bulk magnetic moment at a transition metal/semiconductor interface. Spin-polarised transport measurements are performed on Fe/ZnSe/FeCo magnetic tunnel junctions. A magnetoresistance of 16% is found at low temperature, which evidences both the existence of interface spin polarisation, as inferred from the bulk magnetic moment above, and that the spin polarisation can be transmitted across the semiconductor barrier layer.

Conselho Nacional de Desenvolvimento Científico e Tecnológico / Magnetic tunnel junctions (MTJ) ofCoFeB=MgO=CoFeB and multilayers of (CoFeB=MgO)x3 were produced using the technique of magnetron sputtering, where the insulating film was grown in an atmosphere reactive Ar +O. Multilayers were produced on measures of X-ray difraction and magnetization. Junctions for transport measurements. All curves IxV, nonlinear, were measured at room temperature, and adjustments made using the Simmons model for symmetric barrier. Adjustments were made firt for the positive voltages and then to negative voltages, where the height and thickness of the barrier and the effective area of tunneling was always considered free parameters. Since the effective area of tunneling, much smaller than the area produced during deposition,thus indicating the existence of points where the current tunneling through the barrier,due to fluctuations in the thickness of the insulation. The post was seen exponential growth of the resistance multiplied by the effective area of tunneling as a function of thickness, using only the values calculated from the simulation curves IxV. We also observed the curve of conductance versus voltage, for the investigation of oxidation or not the interface between electrode and barrier, showing that almost 100% of samples of the tunnel junctions was low oxidation of the electrode (positive). / Junções túnel magnéticas (MTJ) deCoFeB=MgO=CoFeB e multicamadas deCoFeB=MgO)x3 foram produzidas utilizando a técnica de magnetron sputtering, onde o filme isolante foi crescido em atmosfera reativas, Ar+O. As multicamadas foram produzidas visando medidas de difração de raio-X (XRD) e magnetização. As junções, para medidas de transporte. Todas as curvas IxV, não lineares, foram medidas a temperatura ambiente, e os ajustes realizados utilizando o modelo de Simmons para barreira simétrica. Os ajustes foram realizados primeiro para as tensões positivas e depois para tensões negativas, onde a altura e a espessura da barreira, e a área efetiva de tunelamento foram considerados parâmetros livres sempre. Sendo a área efetiva de tunelamento, muito menor, do que a área produzida durante a deposição, indicando assim a existência de pontos onde a corrente de tunelamento atravessa a barreira, devido a flutuações na espessura do isolante. A posteriori foi verificado o crescimento exponencial da resistência multiplicada pela área efetiva de tunelamento em função da espessura, utilizando somente valores calculados através das simulações das curvas IxV. Também foi verificada a curva de condutância versus a tensão, para a investigação da oxidação ou não da interfase entre eletrodo e barreira, mostrando que quase 100% das amostras das junções túnel ocorreu oxidação do eletrodo de baixo (positivo).

Nanomagnetic devices are highly energy efficient and non-volatile. Because of these two attributes, they are potential replacements to many currently used information processing technologies, and they have already been implemented in many different applications. This dissertation covers a study of nanomagnetic devices and their applications in various technologies for information processing – from simulating and analyzing the mechanisms behind the operation of the devices, to experimental investigations encompassing magnetic film growth for device components to nanomagnetic device fabrication and measurement of their performance. Theoretical sections of this dissertation include simulation-based modeling of perpendicular magnetic anisotropy magnetic tunnel junctions (p-MTJ) and low energy barrier nanomagnets (LBM) – both important devices for magnetic device-based information processing. First, we propose and analyze a precessionally switched p-MTJ based memory cell where data is written without any on-chip magnetic field that dissipates energy as low as 7.1 fJ. Next, probabilistic (p-) bits implemented with low energy barrier nanomagnets (LBMs) are also analyzed through simulations, and plots show that the probability curves are not affected much by reasonable variations in either thickness or lateral dimensions of the magnetic layers. Experimental sections of this dissertation comprise device fabrication aspects from the basics of material deposition to the application-based demonstration of an extreme sub-wavelength electromagnetic antenna. Magnetic tunnel junctions for memory cells and low barrier nanomagnets for probabilistic computing, in particular, require ultrathin ferromagnetic layers of uniform thickness, and non-uniform growth or variations in layer thickness can cause failures or other problems. Considerable attention was focused on developing methodologies for uniform thin film growth. Lastly, micro- and nano-fabrication methods are used to build an extreme sub-wavelength electromagnetic antenna implemented with an array of magnetostrictive nanomagnets elastically coupled to a piezoelectric substrate. The 50 pW signal measured from the approximately 250,000-nanomagnet antenna sample was 10 dB above the noise floor.

Les oscillateurs à transfert de spin (STO) sont des oscillateurs nanométriques (~100nm) prometteurs pour les applications radiofréquence. Ils reposent sur la précession de l'aimantation d'une couche magnétique mince induite par transfert de spin (STT). Un dispositif STO basé sur jonction tunnel magnétique (MTJ) fournira typiquement un signal électrique de l'ordre d'une dizaine de GHz et d'une puissance de plusieurs nW. Comparés aux oscillateurs contrôlés en tension (VCO) utilisés actuellement pour la génération de microondes, les STO ont l'avantage d'être hautement accordables en fréquence. Malgré cela, les critères requis en termes de qualité de signal ne sont pas encore remplis par les STO pour être compétitifs.Deux approches existent pour améliorer la qualité du signal de sortie : (i) optimisation de l'empilement magnétique d'un dispositif STO unique et (ii) synchronisation de plusieurs STOs. C'est la deuxième approche qui a été retenue dans le cadre de cette thèse : ici nous nous intéressons à la synchronisation électrique d'un STO à une source de courant RF stabilisée, dit « injection-locking ». Le cas d'un STO à aimantation homogène, de type précession dans le plan (IPP) est étudié.En particulier, la synchronisation d'un STO à 2f, c'est-à-dire lorsque la fréquence du courant injecté est proche du double de la fréquence de génération du STO, est favorisée par rapport à la synchronisation à f. Les résultats expérimentaux obtenus par plusieurs groupes montrent à la fois une gamme de synchronisation et une réduction de largeur de raie plus prononcées à 2f qu'à f.Ce comportement singulier est examiné dans un premier temps par une étude analytique de la dynamique de l'aimantation couplée aux simulations numériques macrospin dans le but d'identifier les mécanismes de synchronisation qui prennent effet au sein du système.En effet, les modèles actuels (formalisme auto-oscillateur KTS) décrivent la synchronisation d'un STO à un courant RF sans faire de distinction entre la synchronisation à f et 2f, et les prédictions qui en découlent s'avèrent être insuffisantes pour la synchronisation à 2f. Pour combler à cela, nous mettons en évidence par extension du formalisme existant les clés du processus de synchronisation à 2f : l'ajustement de fréquence par ajustement de l'amplitude d'oscillation via la non-linéarité, ainsi que la modification du terme d'anti-damping se faisant par l'intermédiaire de la différence de phase.La caractérisation expérimentale du régime synchronisé pour un STO basé sur jonction tunnel magnétique est également détaillée dans le manuscrit. Grâce aux techniques de mesures en domaine temporel et fréquentiel développées spécialement, les grandeurs caractéristiques (gamme de synchronisation et différence de phase) du système sont extraites et comparées aux prédictions théoriques. Enfin, les effets de l'injection du courant RF sur la cohérence du signal de sortie sont discutés. / Spin transfer oscillators (STOs) are promising nanometer scaled oscillators (~100nm) for radiofrequency applications. They rely on the steady precession of the magnetization of a thin magnetic layer induced by spin-transfer torque (STT). A STO device based on a magnetic tunnel junction (MTJ) will typically generate an electrical signal with a frequency of the order of ten GHz and an output power of several nW. Compared to voltage controlled oscillators (VCO) used today for microwave generation, STOs have the advantage of having an important frequency tunability with current. However, criteria in terms of the quality of the output signal are not yet fulfilled for STO to be competitive.To enhance the STO signal properties, two suggestions are proposed: (i) optimization of the magnetic stack within a single STO device and (ii) synchronization of several STOs. The second approach was examined during this thesis: here we look at the electrical synchronization of a STO to a stabilized RF current source, or “injection-locking”. The case of a STO with homogenous magnetization of in-plane precession (IPP) type is investigated.Interestingly, synchronization of a STO at 2f, i.e. when the frequency of the injected current is close to twice the generation frequency of the STO, is favored compared to synchronization at f. The experimental results from several groups have shown both enhanced synchronization range and a more pronounced linewidth reduction at 2f.This singular behavior is examined first through an analytical study of magnetization dynamics along with numerical macrospin simulations, in order to identify synchronization mechanisms taking effect in the system.Indeed, current models (in particular the KTS auto-oscillator formalism) describe synchronization of a STO with making a clear distinction between synchronization at f and 2f, and the resulting predictions turn out to be insufficient at 2f. Here, by extension of the KTS formalism, the keys to the synchronization process at 2f are presented: frequency adjustment by adjustment of the oscillation amplitude via the STO non-linearity and modification of the anti-damping term through the phase-difference.The experimental characterization of the synchronized regime in a MTJ-based STO is also detailed in the manuscript. Utilizing experimental signal processing techniques in both frequency and temporal domain, we extract characteristic quantities for synchronization such as the locking-range and the phase-difference, and we compare these quantities with the analytical predictions. Finally, the effects of current injection on the coherence of the output signal are also discussed.

L’objectif de la thèse est de développer un système de type jonction tunnel tout oxyde à base de La2/3Sr1/3MnO3 (LSMO) où il serait possible de contrôler l’aimantation des électrodes magnétiques par des processus à faible consommation d’énergie. Des jonctions tunnel épitaxiées de LSMO/SrTiO3/LSMO ont été obtenues montrant un double renversement de l’aimantation à température ambiante et un taux de magnétorésistance de 71 % à 10 K. En exerçant une contrainte sur le LSMO par le substrat il a été possible de moduler l’anisotropie des couches magnétiques. Des anisotropies perpendiculaire ou dans le plan ont pu être obtenues. Afin de contrôler le renversement des moments magnétiques dans une des électrodes ferromagnétiques trois options ont été envisagées : l’utilisation de l’injection de spin à partir d’un métal à fort couplage spin-orbite (Pt, Ir) ou d’un oxyde contenant de tels ions (ici Ru dans SrRuO3), et l’utilisation du Bi2FeCrO6, un oxyde multiféroïque pouvant présenter un couplage magnétoélectrique. Malgré des résultats prometteurs, aucune solution n’a permis des tests sur des jonctions afin d’estimer leur efficacité. L’objectif final n’est pas encore atteint mais des avancées intéressantes ont été faites afin d’envisager des dispositifs permettant le stockage et la manipulation de l’information. / The objective of this work is to develop La2/3Sr1/3MnO3 (LSMO) based all-oxide magnetic tunnel junction systems where it would be possible to control the magnetization of magnetic electrodes by low energy consumption processes. Epitaxial tunnel junctions of LSMO/SrTiO3/LSMO were obtained showing a double magnetization switching at room temperature and a magnetoresistance ratio of 71 % at 10 K. Using strain engineering, it was possible to modulate the anisotropy of the LSMO magnetic layers. Perpendicular or in plane anisotropies could be thus obtained. In order to control the reversal of the magnetic moments in one of the ferromagnetic electrodes three options were considered : the use of spin injection from a metal with a strong spin-orbit coupling (Pt, Ir) or an oxide containing this type of ions (here Ru in SrRuO3), and the use of Bi2FeCrO6 multiferroic oxide that may exhibit a magnetoelectric coupling. Despite promising results, no solution has allowed tests on junctions to be carried out to estimate their effectiveness. Although the final objective is not yet achieved, interesting progress has been made on the way to information storage and manipulation devices.

Aujourd'hui se pose une question fondamentale sur le futur de l'électronique actuelle. De plus en plus, des circuits hybrides intégrant de nouvelles fonctionnalités sont fabriqués. On envisage même, à plus long terme, des circuits basés sur une technologie différente de l'approche CMOS utilisée actuellement. Une de ces technologies est la spintronique qui tire profit du spin, degré de liberté supplémentaire de l'électron. Elle a rapidement fait ses preuves par le passé dans le stockage non volatile binaire (disques durs) et s'oriente aujourd'hui vers de nouvelles mémoires magnétiques ultra-performantes et basse consommation les MRAMs (Magnetic Random Access Memories). En parallèle, une nouvelle catégorie de matériaux à fort potentiel a émergé : les matériaux bidimensionnels (2D). Ces matériaux, dont le fer de lance est le graphène (une couche d'un atome d'épaisseur de graphite), offrent de nouvelles propriétés inégalées. Leur combinaison via la fabrication d'hétérostructures et la capacité d'avoir un contrôle de leur épaisseur à l'échelle atomique pourrait devenir un atout majeur en électronique et plus particulièrement en spintronique. L'objectif de cette thèse a été l'étude de l'intégration et la démonstration du potentiel en termes de fonctionnalités et de performances de ces nouveaux matériaux 2D au sein de jonctions tunnel magnétiques (MTJs), le dispositif prototype de la spintronique. Au cours de cette thèse, nous avons poursuivi les travaux initiés au laboratoire sur l'intégration dans des MTJs du graphène obtenu via une méthode de dépôt CVD (dépôt chimique en phase vapeur) directe sur l’électrode ferromagnétique inférieure. Nous avons démontré que les propriétés de filtrage en spin et de membrane protectrice contre l'oxydation de l'électrode ferromagnétique (FM) sous-jacente s'étendaient à une unique couche de graphène. Par ailleurs, nous avons aussi pu étudier et améliorer significativement l'amplitude du filtrage en spin et du signal de magnétorésistance observé via l'optimisation des procédés de croissance et d'intégration et le choix de différentes configurations de matériaux ferromagnétiques (Ni(111), Co...). De forts effets de filtrage de spin ont ainsi pu être observés avec des magnétorésistances allant de -15% à plus de +80%, soit presque trois fois l'état de l'art. En parallèle, nous nous sommes aussi intéressés à un autre matériau 2D, le nitrure de bore hexagonal (h-BN), isolant isomorphe du graphène qui s'apparenterait à une barrière tunnel d'un seul atome d'épaisseur. Afin d’étudier le h-BN dans une MTJ, nous avons décidé d’exploiter à nouveau le principe d’une croissance directe par CVD du matériau 2D sur le matériau FM. Des mesures CT-AFM (Conductive Tip Atomic Force Microscopy) nous ont permis de démontrer les propriétés de barrière tunnel homogène du h-BN ainsi que le contrôle possible de la hauteur de barrière avec le nombre de couches de h-BN. De plus, des mesures électriques et de magnétotransport nous ont permis de confirmer l’intégration réussie de la barrière tunnel h-BN dans notre MTJ. Nous avons pu obtenir les premiers résultats de forte magnétorésistance pour du h-BN avec une amplitude de la magnétorésistance de +50%, plus d'un ordre de grandeur au-dessus de l'état de l'art, révélant le potentiel du h-BN. Nous avons enfin aussi pu démontrer l'importance du couplage entre le h-BN et l'électrode FM offrant un potentiel de contrôle inédit sur les effets de filtrage en spin et allant jusqu'à rendre le h-BN métallique. Lors de cette thèse, nous avons pu montrer que l’intégration du graphène et du h-BN dans des MTJs via la croissance directe par CVD est un procédé privilégié pour tirer pleinement profit de leurs propriétés. Les résultats obtenus de forte magnétorésistance et de filtrage en spin laissent entrevoir le fort potentiel du graphène, du h-BN mais aussi des autres nouveaux matériaux 2D à venir pour les MTJs. Ces études ouvrent une nouvelle voie d’exploration pour les MTJs : les 2D-MTJs. / Nowadays a critical issue is raised concerning the future of current electronics. Increasingly, hybrid circuits with new functionalities are manufactured. A longer term approach is even contemplated with circuits based on a technology different from the one currently used (CMOS technology). One of these envisioned technologies is spintronics, which benefits from the spin properties, the electron additional degree of freedom. Spintronics has quickly proven its worth in the past in the field of non volatile data storing (hard drives) and is today moving towards new fast and ultra-low-power magnetic random access memories the MRAMs. Meanwhile, these last few years, a new category of materials with high potential has emerged : the bidimensional materials (2D). These materials, with graphene (one atomically thick layer of graphite) as the forerunner, provide new unrivaled properties. Their combination in the form of heterostructures and the ability to obtain a control of their thickness at the atomic scale could be a major asset for electronics and more specifically spintronics. The purpose of this thesis has been the study of the integration and the demonstration of the potential in terms of functionalities and performances of these new 2D materials inside the prototypical spintronic device: the magnetic tunnel junction (MTJ). During this thesis, we have pursued the work initiated by the laboratory on the integration of graphene in MTJs with direct CVD deposition method (chemical vapor deposition) on the underlying ferromagnetic electrode. We demonstrated that the spin filtering and protective membrane properties (preventing the oxidation of the underlying ferromagnetic electrode (FM)) observed earlier expand to a graphene monolayer. Furthermore, we have also studied and improved significantly the amplitude of the spin filtering and the magnetoresistance signal observed. This was done thanks to the optimization of the growth process, integration, and choice of the different configurations of ferromagnetic materials in our structures (Ni(111), Co...). High spin filtering effects have been observed as a function of the configurations with magnetoristances ranging from -15% to beyond +80%, which is almost three times the state of the art. Meanwhile, we looked at another 2D material, the hexagonal boron nitride (h-BN), an insulating isomorph of graphene which could be considered as an atomically thin tunnel barrier. In order to study h-BN into a MTJ, we took again advantage of direct CVD growth of the 2D material on a ferromagnet. CT-AFM (Conductive Tip Atomic Force Microscopy) measurements allowed us to demonstrate the homogeneous tunnel barrier properties of h-BN and the possible control of the barrier height with the number of h-BN layers. Simultaneously, electrical and magnetotransport measurement in the complete junction allowed us to confirm the achieved integration of the h-BN tunnel barrier into our MTJ. We have been able to obtain the first results of high magnetoresistance for h-BN with values one order of magnitude beyond the state of the art. A magnetoresistance of +50% has been reached, thanks to the optimization of the growth process revealing the potential of h-BN. We have also been able to show the important role of the coupling between h-BN and the FM electrode offering an unprecedented potential of control on the spin filtering effects, ranging up to making the h-BN metallic. During this thesis, we have been able to demonstrate that the integration of graphene and h-BN in MTJs through direct CVD growth is a promising process in order to fully exploit their properties. The results obtained of high magnetoresistance and spin filtering point to the high potential for MTJs of graphene and h-BN but also to all the new 2D materials to come. These studies pave the way for exploring a new path for MTJs : the 2D-MTJs.

Die vorliegende Arbeit befasst sich mit magnetischen Tunnelkontakten (magnetic tunnel junctions, MTJs) auf Basis des Oxids Zinkferrit (ZnxFe3-xO4). Dabei soll das Potential dieses Materials durch die Demonstration des Tunnelmagnetowiderstandes (tunnel magnetoresistance, TMR) in zinkferritbasierten Tunnelkontakten gezeigt werden. Dazu wurde ein Probendesign für MTJs auf Basis der „pseudo spin valve“-Geometrie entwickelt. Die Basis für dieseStrukturen ist ein Dünnfilmstapel aus MgO (Substrat) / TiN / ZnxFe3-xO4 / MgO / Co. Dieser ist mittels gepulster Laserabscheidung (pulsed laser deposition, PLD) hergestellt. Im Rahmen dieser Arbeit wurden die strukturellen, elektrischen und magnetischen Eigenschaften der Dünnfilme untersucht. Des weiteren wurden die fertig prozessierten MTJ-Bauelemente an einem im Rahmen dieser Arbeit entwickeltem und aufgebautem TMR-Messplatz vermessen. Dabei ist es gelungen einen TMR-Effekt von 0.5% in ZnxFe3-xO4-basierten MTJs nachzuweisen. Das erste Kapitel der Arbeit gibt eine Einführung in die spintronischen Effekte Riesenmagnetowiderstand (giant magnetoresistance, GMR) und Tunnelmagnetowiderstand (TMR). Deren technologische Anwendungen sowie die grundlegenden physikalischen Effekte und Modelle werden diskutiert. Das zweite Kapitel gibt eine Übersicht über die Materialklasse der spinellartigen Ferrite. Der Fokus liegt auf den Materialien Magnetit (Fe3O4) sowie Zinkferrit (ZnxFe3-xO4). Die physikalischen Modelle zur Beschreibung der strukturellen, magnetischen und elektrischen Eigenschaften dieser Materialien werden dargelegt sowie ein Literaturüberblick über experimentelle und theoretische Arbeiten gegeben. Im dritten Kapitel werden die im Rahmen dieser Arbeit verwendeten Probenpräparations- und Charakterisierungsmethoden vorgestellt und technische Details sowie physikalische Grundlagen erläutert. Die Entwicklung eines neuen Probendesigns zum Nachweis des TMR-Effekts in ZnxFe3-xO4-basierten MTJs ist Gegenstand des vierten Kapitels. Die Entwicklung des Probenaufbaus sowie die daraus resultierende Probenprozessierung werden beschrieben. Die beiden letzten Kapitel befassen sich mit der strukturellen, elektrischen und magnetischen Charakterisierung der mittels PLD abgeschiedenen Dünnfilme sowie der Tunnelkontaktstrukturen.