<p><p>This thesis concerns design and construction of an ultra high vacuum chamber to fabricate and characterize spintronic devices.  The long term intention is to fabricate spin valve structures with V[TCNE]₂ (hybrid organic inorganic semiconductor room temperature magnet) sandwiched between two ferromagnetic electrodes, which requires better than 10^-8mbar of vacuum. Due to an uncured leak in the chamber, the current vacuum is limited to 4*10^-7mbar. The V[TCNE]₂ thin film prepared in this vacuum, oxidized completely  by  the presence of oxygen during the film growth. Organic magnetoresistance (OMAR) devices which are simple organic diode structures were fabricated and characterized, as they are compatible with high vacuum conditions. A magnetoresistance measurement set up was arranged and the possible problems in fabrication and characterization are analyzed.</p><p> </p><p>To fabricate OMAR devices-ITO/P3HT/Al, RR-P3HT (regio regular poly (3-hexylthiophene)) an effective hole transport polymer with higher hole mobilities was used as an active layer and Al (aluminum) as a cathode. A thermal evaporation setup was added to the vacuum chamber to evaporate Al electrodes. The devices were kept in argon and vacuum environments, while characterizing in dark to suppress the exitons generated by photo illumination. The Organic magnetoconductance of about 1% is observed for the less concentration P3HT (3mg/1ml), and significantly improved to -23% for the high concentration P3HT (10mg/ml) solution. The results support that the negative magnetoconductance is due to the formation of bipolaron under the influence of an external magnetic field.</p><p> </p><p>Finally, suggestions to improve the performance of the vacuum chamber to fabricate and characterize the spintronic devices and OMAR devices are presented.</p></p>

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This thesis concerns design and construction of an ultra high vacuum chamber to fabricate and characterize spintronic devices.  The long term intention is to fabricate spin valve structures with V[TCNE]2 (hybrid organic inorganic semiconductor room temperature magnet) sandwiched between two ferromagnetic electrodes, which requires better than 10-8mbar of vacuum. Due to an uncured leak in the chamber, the current vacuum is limited to 4*10-7mbar. The V[TCNE]2 thin film prepared in this vacuum, oxidized completely  by  the presence of oxygen during the film growth. Organic magnetoresistance (OMAR) devices which are simple organic diode structures were fabricated and characterized, as they are compatible with high vacuum conditions. A magnetoresistance measurement set up was arranged and the possible problems in fabrication and characterization are analyzed.   To fabricate OMAR devices-ITO/P3HT/Al, RR-P3HT (regio regular poly (3-hexylthiophene)) an effective hole transport polymer with higher hole mobilities was used as an active layer and Al (aluminum) as a cathode. A thermal evaporation setup was added to the vacuum chamber to evaporate Al electrodes. The devices were kept in argon and vacuum environments, while characterizing in dark to suppress the exitons generated by photo illumination. The Organic magnetoconductance of about 1% is observed for the less concentration P3HT (3mg/1ml), and significantly improved to -23% for the high concentration P3HT (10mg/ml) solution. The results support that the negative magnetoconductance is due to the formation of bipolaron under the influence of an external magnetic field.   Finally, suggestions to improve the performance of the vacuum chamber to fabricate and characterize the spintronic devices and OMAR devices are presented.

Ce travail de thèse concerne l'étude de l'influence de la nanostructuration du système Cu-Co sur ses propriétés magnétiques et magnétorésistives. Dans un premier temps, l'alliage granulaire Cu 80 Co 20 a été synthétisé sous différentes formes : poudres, couches minces et nanofils. Les poudres d'alliage ont été obtenues par broyage mécanique et les couches minces et nanofils par électrodépôt. Cela a permis d'étudier, d'une part, l'influence de la forme de l'échantillon et, d'autre part, l'influence de la technique d'élaboration sur la nanostructure et les propriétés magnétiques et magnétorésistives des échantillons. Dans un second temps, des nanofils multicouches Cu/Co ont été réalisés par électrodépôt. Des protocoles expérimentaux pour l'analyse à l'échelle nanométrique par microscopie électronique à transmission et par sonde atomique tomographique ont été mis en place. De telles analyses se sont avérées indispensables à la compréhension et à la corrélation complète des propriétés magnétiques et magnétorésistives. Contrairement aux nombreuses études publiées, qui ont souvent conclu à l'obtention de solutions solides sur la base de caractérisations microstructurales, les analyses à l'échelle nanométrique par sonde atomique tomographique et par microscopie électronique à transmission de nos alliages granulaires ont montré qu'aucune solution solide Cu-Co n'a pu être obtenue. De plus, un effet positif de magnétorésistance sous faible champ magnétique appliqué a été observé, et corrélé à la présence d'oxydes.

Dans la première partie de cette thèse, nous étudions le renversement de l'aimantation de nanofils d'alliage FePt à forte anisotropie magnétocristalline. Lorsque la largeur du fil devient inférieure à la taille des dendrites, nous avons montré qu'il existe une transition du processus de renversement de l'aimantation, de la croissance de dendrites vers la propagation d'une paroi magnétique unique qui renverse tout le fil. Au-delà, la diminution de la largeur du fil jusqu'à la taille caractéristique du désordre et/ou de la rugosité moyenne conduit au renforcement de la coercivité. Ceci conduit finalement dans les fils ultra-fins à un renversement consistant en un mélange de nucléation de domaines et de propagation de parois magnétiques. Dans la deuxième partie, nous rapportons l'utilisation de la magnétorésistance de Magnon (MMR), qui provient de la contribution des magnons à la résistivité, pour mesurer le renversement d'aimantation, dans des nanostructures avec aimantation perpendiculaire (FePt) ou planaire (NiFe). Nous avons montré que la MMR peut être utilisée pour détecter le retournement de l'aimantation dans les nanofils et nano-aimants, et en particulier pour détecter la position d'une paroi magnétique le long d'un nanofil fabriqués à partir d'une couche unique. Enfin, nous étudions dans une dernière partie la dynamique de dépiégeage de paroi magnétique sous champ et sous courant, dans les deux systèmes FePt et NiFe. Nous observons trois types de dépiégeage de paroi, qui dépendent de la nature des défauts ou de la géométrie de la constriction. L'analyse statistique du temps de piégeage montre que le processus de dépiégeage peut être décrit comme procédant d'un chemin simple, de chemins en série, ou de chemins alternatifs. En outre, l'effet du courant sur tous ces mécanismes de dépiégeage s'est révélé équivalent à l'effet du champ appliqué, ce qui permet de mesurer l'efficacité du transfer de spin dans ces systèmes. Keywords: Magnetization reversal, magnon magnetoresistance, domain wall, spins transfer torque.

This thesis describes a research of suppression of superconducting gap in a superconducting island of a Ferromagnetic-Superconducting-Ferromagnetic Single-Electron-Transistor due to the fringing magnetic fields produced by the ferromagnetic leads. The devices are working below the critical temperature of the superconducting gap. A model is proposed to explain how the fringing magnetic field produced by the leads is strong enough to suppress the superconducting gap. The peak of the fringing magnetic field produced by one lead reaches 5000 oe. It is observed an inverse tunneling magneto resistance during the suppression of the superconducting gap, obtaining a maximum absolute value 500 times greater than the TMR in the normal state where the efficiency of the spin injection is low. It is concluded that the suppression of the superconducting gap is due to fringing magnetic field and not to the spin accumulation because the low efficiency of the spin injection. It is suggested a new geometry to reduce the effect of the fringing magnetic field so it can be obtained a suppression of the superconductivity due to the spin accumulation. It is described the qualitatively behavior of the IV characteristic when the suppression of the superconductivity is due to spin accumulation.

In this thesis, we have performed systematical study of the complex impedance spectra(CIS) with the manganeses oxide thin films by the equivalent circuit model(ECM) composed of resistance and capacitance. The ECM has been utilized in analog of the electrical and dielectric properties of the granular films. The purpose of this research is to understand how the electrical- and magneto-transport properties in La0.67Ca0.33MnO3(LCMO),La0.8Ba0.2MnO3(LBMO),La0.67Sr0.33MnO3(LSMO(113)) and La0.67Sr1.33MnO4 (LSMO(214)) thin films, at various magnetic fields and temperatures. First of all, we demonstrate that the LSMO(214) and LSMO(113) can be sensitively affected by magnetic states on the manganite films. Our result provides further understanding of the dielectrics variation during the phase transition from an AFM insulating phase and/or a ferromagnetic metallic phase to a paramagnetic PM metallic phase. It is known that the strong correlation between the itinerant carriers and the local magnetic moments is the mechanism for FM/PM phase transition for LSMO(113), while the direct magnetic exchange coupling governed the AFM/PM phase transition and an indirect coupling to the status of intrinsic carriers for LSMO(214) films. These transitions can not be concludes directly by using a dc resistance measurement but can be clearly distinguished by the CIS measurement. On the other hand, the dc resistance (Rdc) and the relaxation time(£n) have the same tendency that this indicates the changes of £n matches to the electric transport properties for LCMO_90min and LSMO(214) thin films. We focus on the the dielectric properties of both samples are insensitive to temperature, revealing that the dielectric behavior is independent of magnetic phase transition but strongly associated with the transport properties. Therefore, the magnetic transitions can be most thoroughly investigated by combining CIS measurements and RC ECM, as well as by making dc resistance measurements. Moreover, the relative change of M£q(ac) is nearly larger than the dc resistive variation. This phenomenon, called giant magneto-impedance effect (GMI), implies that thehigh-frequency magnetotransport effect may enhance the performance of these manganese oxides for sensing the magnetic field. The CMI, have been analyzed by ECM, including two sets of parallel R and capacitance (C) components in series. The analyzing results the specific feature of grain boundaries(GBs) can be attributed to the interplay of magnetic moment spin disorder to ordering. The grain boundary (GB) effect can enhance low field magnetoresitance (LFMR) for artificial GBs, but shows very limited enhancement for those GBs in epitaxial films. This study finds that artificial GBs, which exhibit large LFMR, can be modeled as a non-conductive layer which disconnects the lattice periodicity of adjacent grains and contains no magnetic ions. The GBs in the present fully strained epitaxial film, which shows a relatively smaller LFMR, are more similar to a semi-continuous grain with continuous distribution of magnetic ions that align loosely parallel to the grain magnetic moment. In addition, we report in this study the high frequency magneto-transport properties, based on the classical model, of La0.8Ba0.2MnO3 and La0.67Ca0.33MnO3 thin films around their ferromagnetic transitions and under an external magnetic field. It is found that the specific features of magneto-impedance can be correlated with the complex magnetization response and the dielectric relaxation in corresponding phase states. The fast dielectric relaxation time, £nE, and the slow magnetic response, £nH, reflect the interplay of itinerant carriers and the magnetic coupling to the ac electromagnetic wave, indicating that the double exchange, or hopping, of carriers between O 2P and Mn 3d-eg states occur prior to the indirect magnetic coupling of adjacent Mn ions via strong Hunt¡¦s rules. Applied magnetic field enhances both electric and magnetic dipoles are now responding faster to the electromagnetic wave. The results of our work may provide a fundamental understanding of high frequency magnetic and electrical properties of the manganite films, and imply tips for device application of the films.

Hole-doped maganite with middle to narrow bandwidth La1-xCaxMnO3 was extensively studied because of its colossal magnetoresistance (CMR) characteristic under a magnetic field. These kind of materials show un- common magnetic and electric properties. The charge order phase only happens to the region x> 0.5, and along with decreasing temperature, its phase goes from para-insulator to charge-ordered then to antiferromagne- tism. In our studies, we apply correlation function of Green¡¦s function to LCMO and get susceptibility of charge and spin. Then we can get the cri- tical value of Coulomb repulsion inside the material by substituting the experimental values of phase transition temperature. This critical values is the key point of charge-ordered. Then we can also get the size of char- ge gap which decides the stability of charge-ordered phase. After know- ing the Coulomb repulsion and charge gap, we can picture the relation of inside and on-site Coulomb repulsion qualitatively while the transition happens. Here the on-site Coulomb repulsion means to the Hund¡¦s coupl- ing between d electrons. And by this we¡¦ll understand the physics inside CMR materials.

The work presented in this thesis shows how tunneling spectroscopy techniques can be applied to metallic nanoparticles to obtain useful information about fundamental physical processes in nanoscopic length scales. At low temperatures, the discrete character of the energy spectrum of these particles, allows the study of spin-polarized current via resolved "electron-in-a-box" energy levels. In samples consisting of two ferromagnetic electrodes tunnel coupled to single aluminum nanoparticles, spin accumulation mechanisms are responsible for the observed spin-polarized current. The observed effect of an applied perpendicular magnetic field, relative to the magnetization orientation of the electrodes, indicates the suppression of spin precession in such small particles. More generally, in the presence of an external non-collinear magnetic field, it is the local field "felt" by the particle that determines the character of the tunnel current. This effect is also observed in the case where only one of the electrodes is ferromagnetic. In contrast to the non-magnetic case, ferromagnetic nanoparticles exhibit a much more complex energy spectrum, which cannot be accounted for, using the simple free-electron picture. It will be shown that interactions between quasi-particle excitations due to sequential electron tunneling and spin excitations in the particle are likely to play an important role in the observed temperature/voltage dependence of magnetic hysteresis loops.

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