This work is dedicated to the study of spin dynamics in systems based on the semiconductor gallium arsenide (GaAs) that are suitable for use in spintronic devices. We explored two types of model structures using experimental methods of ultrafast laser spectroscopy and transport measurements. In the ferromagnetic semiconductor (Ga,Mn)As, we investigated laser-induced magnetization precession. We found out that transfer of both energy and angular momentum from the circularly polarized laser light can trigger magnetization precession, the latter one being identified as a new phenomenon, the "optical spin transfer torque". Furthermore, we demonstrate the possibility to control the energy-transfer-induced magnetization dynamics both optically and electrically using piezo-stressing. When dealing with purely non-magnetic structures for spintronics, we studied the Spin-Injection Hall Effect (SIHE) in GaAs/AlGaAs heterostructures with a special type of spin- orbit (SO) coupling that are lithographically patterned to create nanodevices. We managed to observe precession of the electron spin in the SO field directly in the space domain by extending the original detection method. This finding, together with the direct detection of a pure spin current, helped to propose a working spin Hall effect transistor.

La spintronique est principalement basée sur le phénomène d’accumulation de spin, inhérent à la circulation d’un courant électrique aux interfaces entre des matériaux ferromagnétiques et non magnétiques. Ces accumulations sont classiquement obtenues dans des empilements multicouches pour lesquels les épaisseurs des couches sont inférieures aux longueurs caractéristiques du transport dépendant en spin. Il est ainsi possible de générer dans ces multicouches des effets de magnétorésistance ou de transfert de spin.Le développement de procédés de nanofabrication permet aujourd’hui de créer des nanodispositifs dont les dimensions latérales sont inférieures aux longueurs caractéristiques du transport dépendant en spin, et donc de mettre en jeu ces mêmes phénomènes. Au cours de ma thèse j’ai étudié différentes nanostructures latérales F/N, montrant qu’il est possible de tirer avantage de la géométrie tridimensionnelle des structures et des différentes orientations possibles des spins injectés. Des études de transport ont en particulier été réalisées dans les régimes colinéaires et non colinéaires, afin d’étudier les conséquences de la non-colinéarité sur les effets d’accumulation de spin et de magnétorésistance.Après un chapitre d’introduction au transport électronique dépendant en spin, le second démontre l’intérêt de l’utilisation de l’alliage CoFe dans la réalisation de structures latérales. Le troisième chapitre explore les nouvelles opportunités offertes par les structures latérales dans le cas du transport colinéaire. Le cas non-colinéaire du transport de spin au travers d’un matériau ferromagnétique est ensuite examiné à l’aide de mesures d’absorption de spin et de mesures d’effet Hanle. Enfin, l’exploitation des purs courants de spin en vue de réaliser des structures fonctionnelles à effets de magnétorésistance est étudiée au cours des Chapitres V et VI. Des nanostructures dont la géométrie tire parti des trois directions de l’espace, basées sur un transport de spin à la fois vertical et latéral, sont notamment présentée dans le Chapitre VI. / Spintronics is mainly based on the phenomenon of spin accumulation, which is inherent to the circulation of an electric current at the interfaces between ferromagnetic and non-magnetic materials. These accumulations are conventionally obtained in multilayers for which the thicknesses of the layers are smaller than the characteristic lengths of the spin-dependent transport. It is thus possible to generate in these multilayers magnetoresistances or spin transfer effects.The development of nanofabrication processes makes it nowadays possible to create nanodevices whose lateral dimensions are less than the characteristic lengths of the spin-dependent transport, and thus to bring into play these same phenomena. During my thesis I studied different F / N lateral nanostructures, showing that it is possible to take advantage of the three-dimensional geometry of the structures, and of the different possible orientations of the injected spins. In particular, transport studies have been carried out in collinear and non-collinear regimes, in order to study the consequences of the non-collinearity on the spin accumulations and magnetoresistances.After an introductory chapter on spin-dependent electron transport, the second chapter demonstrates the interest of the CoFe alloy in lateral structures. The third chapter explores the new opportunities offered by lateral structures in the case of collinear transport. The non-collinear case of spin transport through a ferromagnetic material is then examined using spin absorption measurements and Hanle measurements. Finally, the exploitation of pure spin currents in order to realize functional devices is studied in Chapters V and VI. In particular, new nanostructures whose geometry takes advantage of the three directions of space (based on both vertical and lateral spin transport) are presented in Chapter VI.

Made available in DSpace on 2015-08-20T17:10:07Z (GMT). No. of bitstreams: 0 Previous issue date: 2015-03-30. Added 1 bitstream(s) on 2015-08-20T17:27:01Z : No. of bitstreams: 1 000842245.pdf: 4480004 bytes, checksum: cf890d88c92e5e1a14fd124dbb193a58 (MD5) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Os corantes cianinas apresentam bom potencial como camada ativa de células solares orgânicas (CSO), como por exemplo, são fortes absorvedores de luz visível e infravermelha, podem ser fabricados utilizando técnicas muito simples, com baixo custo e boa flexibilidade mecânica. No entanto apresentam certos problemas como baixa condutividade eletrônica e as células solares baseadas nestes materiais apresentam baixas eficiências. Neste contexto, este trabalho buscou compreender os processos de geração e transferência de cargas em blendas de corantes em cianina atuando como aceitadores ou doadores de elétrons. Foram utilizados os corantes trimetina (Cy7) com diferentes contraíons. As principais técnicas utilizadas neste estudo foram a de Ressonância de Spin Eletrônico (ESR) e uma variação com aplicação de luz durante a medida (LESR) em temperatura de 77 k. Foram estudados o efeito dos diferentes corantes, bem como o efeito de seus contraíons na geração e transferência de cargas. Para a análise dos resultados foram simulados os espectros de ESR e LESR para cada espécie paramagnética observada utilizando o software EasySpin. Atuando como aceitador de elétrons em blendas com o polímero MEH-PPV, as blendas com corantes Cy3 mostraram uma melhor geração e transferência de cargas fotoinduzidas em comparação com as blendas com corantes Cy7. Já atuando como doadores de elétrons em blendas com o fulereno C60, observou-se um efeito inverso, em que as blendas com corantes Cy7 apresentaram uma melhor geração e transferência de cargas fotoinduzidas. Além disso, os resultados de ESR e LERS mostraram diferentes sinais para o (C60)1 modificando-se apenas os contraíons dos corantes, que pode ser explicado baseando-se em estudos sobre morfologia. Por fim foram realizados estudos de oxidação das amostras a fim de comprovar a origem de alguns sinais de LESR em menores campos magnéticos / The eyanines dyes have good potential as an active layer of organic solar cells, for example, they are strong absorbers of visible and infrared light, can be manufactured using very simple techniques, with low cost and good mechanical flexibility. However, they present problems such as low eletronic conductivity and solar cells based on these materials exhbit low efficiencies. In this context, this study aimed to understand the photoinduced charge generation and transfer processes is blends of dyes based on cyanine acting as electron acceptors or donors for use in organic solar cells. Trimethine dyes (Cy3) and heptamethine dye (Cy7) with different counterions were used. The main technique used in this study were the Electronic Spin Resonance (ESR) and its variation with the application of light during the measurement (LESR) at temperature of 77 K. We studied the effect of different dyes, as well as the effect of its counterions in the charge generation and transfer. For the analysis of results were simulated the ESR and LESR spectra for each paramagnetic species observed through software EasySpin. Acting as electron acceptor in blends with the MEH-PPV polymer, the dyes blends with Cy3 dyes showed better photo-induced charge generation and transfer compared Cy7 dyes. However, working as electron donors in blends with C60 fullerene the opposite effect was observed, where the blends with Cy7 dyes show a better photo-induced charge generation and transfer than blends with Cy3 dyes. Moreover, the ESR and LESR results showed different signals for (C60)1- changing only the conunterions of the dye, which can be explained based on morphological studies. Finally, the samples oxidation studies were conducted to verify the originals of LESR signals at lower magnetic fields / FAPESP: 2010/20692-6

Made available in DSpace on 2014-12-02T11:16:55Z (GMT). No. of bitstreams: 0 Previous issue date: 2014-02-28Bitstream added on 2014-12-02T11:20:54Z : No. of bitstreams: 1 000796475.pdf: 2212773 bytes, checksum: 83e5eed0cc62a3b455003a862d382477 (MD5) / Foi estudado teóricamente a densidade local de estados (LDOS), por meio de uma ponta de STM, de sistemas bidimensionais hospedando um adatom e uma segunda impureza localizada abaixo da superfície hospedeira, ambos capacitivamente acoplados a pontas de AFM e atravessados por campos magnético antiparalelos. São analisados dois sistemas, uma monocamada de grafeno e um gás de elétrons bidimensional (2DEG). As pontas de AFM ajustam os níveis das impurezas com respeito à energia de Fermi, onde dois comportamentos constrastantes emergem: o fator de Fano para o grafeno diverge, enquanto no 2DEG ele se aproxima de zero. Como resultado, a degenerescência de spin da LDOS é levantada exclusivamente no sistema do grafeno, em particular para o regime assimétrico da interferência Fano. O resultado deste limite é um fenômeno não intuitivo, que consiste de um fator de Fano dominante devido à impureza que se encontra abaixo do plano de grafeno, mesmo com um forte acoplamento entre a ponta do STM e o adatom. Assim, foi obtida uma condutância completamente polarizada, por meio do deslocamento vertical da posição da ponta de STM. Para melhor conhecimento, este trabalho é a primeira proposta que utiliza o efeito Fano como mecanismo para filtrar spins no grafeno. Esta característica surge a partir de elétrons de Dirac que se comportam como se não tivessem massa, por conta da estrutura de bandas do grafeno, e nos permite empregá-lo como um filtro de spins relativístico Fano / We explore theoretically the density of states (LDOS) probed by an STM tip of 2D systems hosting an adatom and a subsurface impurity, both capacitively coupled to AFM tips and traversed by antiparallel magnetic fields. Two kinds of setups are analyzed, a monolayer of graphene and a two-dimensional electron gas (2DEG). The AFM tips set the impurity levels at the Fermi energy, where two contrasting behaviors emerge: the Fano factor for the graphene diverges, while in the 2DEG it approaches zero. As result, the spin-degeneracy of the LDOS is lifted exclusively in the graphene system, in particular for the asymmetric regime of Fano interference. The aftermath of this limit is a counterintuitive phenomenon, which consists of a dominant Fano factor due to the subsurface impurity even with a stronger STMadatom coupling. Thus we find a full polarized conductance, achievable just by displacing vertically the position of the STM tip. To the best knowledge, our work is the first to propose the Fano effect as the mechanism to filter spins in graphene. This feature arises from the massless Dirac electrons within the band structure and allows us to employ the graphene host as a relativistic Fano spin-filter

Orientador: Antonio Carlos Ferreira Seridonio / Co-orientador: Ezequiel Costa Siqueira / Banca: João Carlos Silo Moraes / Banca: Eduardo Miranda / Resumo: Foi estudado teóricamente a densidade local de estados (LDOS), por meio de uma ponta de STM, de sistemas bidimensionais hospedando um adatom e uma segunda impureza localizada abaixo da superfície hospedeira, ambos capacitivamente acoplados a pontas de AFM e atravessados por campos magnético antiparalelos. São analisados dois sistemas, uma monocamada de grafeno e um gás de elétrons bidimensional (2DEG). As pontas de AFM ajustam os níveis das impurezas com respeito à energia de Fermi, onde dois comportamentos constrastantes emergem: o fator de Fano para o grafeno diverge, enquanto no 2DEG ele se aproxima de zero. Como resultado, a degenerescência de spin da LDOS é levantada exclusivamente no sistema do grafeno, em particular para o regime assimétrico da interferência Fano. O resultado deste limite é um fenômeno não intuitivo, que consiste de um fator de Fano dominante devido à impureza que se encontra abaixo do plano de grafeno, mesmo com um forte acoplamento entre a ponta do STM e o adatom. Assim, foi obtida uma condutância completamente polarizada, por meio do deslocamento vertical da posição da ponta de STM. Para melhor conhecimento, este trabalho é a primeira proposta que utiliza o efeito Fano como mecanismo para filtrar spins no grafeno. Esta característica surge a partir de elétrons de Dirac que se comportam como se não tivessem massa, por conta da estrutura de bandas do grafeno, e nos permite empregá-lo como um filtro de spins relativístico Fano / Abstract: We explore theoretically the density of states (LDOS) probed by an STM tip of 2D systems hosting an adatom and a subsurface impurity, both capacitively coupled to AFM tips and traversed by antiparallel magnetic fields. Two kinds of setups are analyzed, a monolayer of graphene and a two-dimensional electron gas (2DEG). The AFM tips set the impurity levels at the Fermi energy, where two contrasting behaviors emerge: the Fano factor for the graphene diverges, while in the 2DEG it approaches zero. As result, the spin-degeneracy of the LDOS is lifted exclusively in the graphene system, in particular for the asymmetric regime of Fano interference. The aftermath of this limit is a counterintuitive phenomenon, which consists of a dominant Fano factor due to the subsurface impurity even with a stronger STMadatom coupling. Thus we find a full polarized conductance, achievable just by displacing vertically the position of the STM tip. To the best knowledge, our work is the first to propose the Fano effect as the mechanism to filter spins in graphene. This feature arises from the massless Dirac electrons within the band structure and allows us to employ the graphene host as a relativistic Fano spin-filter / Mestre

This thesis investigates the growth of intermetallic compounds by co-sputtering from single elemental targets. The preliminary work involved constructing a sputtering set-up to grow abinary alloy (Sm-Co) and investigating how to control the composition spread that was obtained. Having achieved this, a larger sputtering flange was built up to grow the ternary Heusler alloy, Co₂MnSi. Co₂MnSi has been predicted to be a half-metallic ferromagnet, which means that there is an energy gap in the minority spin band at the Fermi energy. This leads to 100% spin polarised conduction electrons, which would enable ideal spin-device performance to be obtained. Co₂MnSi is particularly promising because it is predicted to have a large energy gap in the minority band of ~0.4 eV and has the highest Curie temperature among the known Heuslers of 985 K. Initially, Co₂MnSi was grown on a-plane sapphire and stoichiometric films were single phase and highly (110) textured, without the use of a seed layer. They exhibited the bulk value of the saturation magnetisation, Ms and films grown at the highest deposition temperature (715 K) showed the lowest resistivity (47 μΩcm at 4.2 K) and the lowest room temperature coercivity (18 Oe). The spin polarisation of the transport current, Pt of a 400 nm film grown at this deposition temperature was 54%, consistent with measurements on bulk single crystals. Ms decreased with decreasing film thickness indicating a graded disorder. By growing on GaAs (001), which has a similar lattice parameter to Co₂MnSi it was expected that this disordered region would be confined to the first few atomic layers. However, this was not the case because interfacial reactions resulted in the formation of an epitaxial Mn-As region, and a thin interfacial layer that was Co-Ga rich. This prevented the lattice matching of the Co₂MnSi to the GaAs(001) hence hindering epitaxial growth of the Heusler. The reaction zone also meant that films exhibited a Ms slightly below the bulk value. The expected fourfold anisotropy was not obtained for this cubic material, which is most likely due to the anisotropy of the reconstructed GaAs surface. Inspite of this anomalous behaviour, Pt was 55%, similar to the result obtained on sapphire, indicating that either Pt is independent of orientation or that the Heusler surface reconstructs in the same way. Films showed some improved properties to films grown on a-plane sapphire, indicating the potential of growing on this technologically important substrate. With this in mind, pseudo spin valves involving Co₂MnSi as one of the ferromagnetic electrodes were fabricated in both the CIP and CPP configurations. Clear low-field spin-valve contributions were observed at 15 K but the MR values are much lower than that expected from a PSV with a predicted 100% spin polarised electrode.

Spintronics, also known as spin electronics, or magnetoelectronics, refers to the study of the role that electron and (less frequently) nuclear spins play in solid state physics, and a group of devices that specifically exploit both the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge. As a principal type of spintronic device, a spin-valve is a device that uses ferromagnetic electrodes to polarize and analyze the electronic spins. The electrical resistance of the device depends sensitively on the relative magnetization of its two ferromagnetic electrodes, a phenomenon referred to as Giant Magnetoresistance (GMR). Having been successfully applied in the field of data storage, GMR also shows potential for future logic devices. Organic semiconductors possess many advantages in electronic device applications. Therefore, using organic semiconductors in spintronics is very interesting and promising, in part, because of their exceptionally long spin-decoherence times. This thesis concerns itself with the scientific study of magnetic field and spin effects in organic spin valves (OSV) and organic light emitting diodes (OLED). Three projects were finished, achieving a better understanding of the transportation of charge and spin carriers inside organic films, and paving the way to enhancing the spin diffusion length and the organic magnetoresistance (OMAR) effect. Firstly, C60 films were used as the spin-transport layer of OSV devices, because of its low hyperfine coupling and high mobility, which prior work suggested to be beneficial. Subsequently we studied the spin injection and transport properties by measuring the devices' magnetoresistance (MR) response at various biasing voltages, V, temperatures, T and different C60 film thickness. But we do not observe a significantly increased spin-diffusion length compared to OSV devices based on other organic semiconductors. We propose conductivity mismatch as a likely cause of the loss of spin-valve signal with increasing C60 layer thickness. There exists some disagreement in the scientific literature regarding whether OSV operate in the so-called tunneling regime or the so-called injection regime. To shed light on this question, we fabricated spin-valve devices made of organic semiconductor thin films of rubrene sandwiched between ferromagnetic cobalt and iron electrodes. Current-voltage (I-V) characteristics in Co/AlOx/rubrene/Fe junctions with a rubrene layer thickness, d, ranging from 5-50 nm, were measured, and we found two different modes of conductivity. The first mode, tunneling, occurs in relatively thin junctions, d < 15 nm, and decays exponentially with increasing rubrene thickness. We determined the tunneling decay length to be 1 nm. The tunneling mode is also characterized by a weak temperature dependence and a nearly parabolic differential conductance. The second mode, injection followed by hopping, occurs in relatively thick devices, d ≥ 15 nm, and can be identified by strongly temperature dependent, highly non-linear I-V traces that are similar to those commonly measured in organic injection devices such as OLEDs. We observed MR in devices with a rubrene thickness of 5 nm and 10 nm. Those devices are clearly in the tunneling regime. For the 15 nm device, for which the tunneling current is just barely measurable we could not observe MR. In the third project, we show that the performance of both OMAR and OSV devices very sensitively depends on whether the metallic layers are deposited by thermal evaporation or electron-beam evaporation. A strongly reduced spin diffusion length and an enhanced OMAR response can be achieved in devices fabricated by electron-beam evaporation. Then we showed that the difference must be attributed to the generation of traps resulting from the exposure of the organic layer to X-ray bremsstrahlung that is generated during the e-beam evaporation process. We also used the thermally stimulated current technique (TSC) to characterize these traps.

In this work, two new spintronic memory cell structures are proposed. The first cell uses the diffusion of polarized spins into ferromagnets with perpendicular anisotropy to tilt their magnetization followed by their dipolar coupling to a fixed magnet (Bhowmik et al., 2014). The possibility of setting the magnetization to both stable magnetization states in a controlled manner using a similar concept remains unknown, but the proposed structure poses to be a solution to this difficulty. The second cell proposed takes advantage of the multiple stable magnetic states that exist in ferromagnets with configurational anisotropy and also uses spin torques to manipulate its magnetization. It utilizes a square-shaped ferromagnet whose stable magnetization has preferred directions along the diagonals of the square, giving four stable magnetic states allowing to use the structure as a multi-bit memory cell. Both devices use spin currents generated in heavy metals by the Spin Hall effect present in these materials. Among the advantages of the structures proposed are their inherent non-volatility and the fact that there is no need for applying external magnetic fields during their operation, which drastically improves the energy efficiency of the devices. Computational simulations using the Object Oriented Micromagnetic Framework (OOMMF) software package were performed to study the dynamics of the magnetization process in both structures and predict their behavior. Besides, we fabricated a 4-terminal memory cell with configurational anisotropy similar to the device proposed, and found four stable resistive states on the structure, proving the feasibility of this technology for implementation of high-density, non-volatile memory cells.

Magnetic random­access memory (MRAM) devices have been widely studied since the 1960s. During this time, the size of spintronic devices has continued to decrease. Conse quently, there is now an urgent need for new low­dimensional magnetic materials to mimic the traditional structures of spintronics at the nanoscale. We also require new effective mechanisms to conduct the main functions of memory devices, which are: reading, writ ing, and storing data. To date, most research efforts have focused on MRAM devices based on magnetic tun nel junction (MTJ), such as a conventional field­driven MRAM and spin­transfer torque (STT)­MRAM devices. Consequently, many efforts are currently focusing on new alterna tives using different techniques, such as spin­orbit torque (SOT) and magnetic skyrmions (a skyrmion is the smallest potential disruption to a uniform magnet required to obtain more effective memory devices). The most promising memory devices are SOT­MRAMs and skyrmion­based memories. This study investigates the magnetic properties of 1T­phase vanadium dichalcogenide (VXY) Janus monolayers, where X, Y= S, Se, or Te (i.e., monolayers that exhibit inversion symme try breaking due to the presence of different chalcogen elements). This study is developed along four directions: (I) the nature of the magnetism and the SOT effect of Janus mono layers; (II) the Dzyaloshinskii Moriya interaction (DMI); (III) investigation of stability en hancement by adopting practical procedures for industry; and (IV) study of the effect of a hexagonal boron nitride (h­BN) monolayer as an insulator on the magnetism of the VXY monolayer. This study provides a clear perspective for the next generation of memory de vices, such as SOT­MRAMs based on transition metal dichalcogenide monolayers.

京都大学 / 新制・課程博士 / 博士(理学) / 甲第22991号 / 理博第4668号 / 新制||理||1670(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 柳瀬 陽一, 教授 川上 則雄, 教授 石田 憲二 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM