Organic semiconductors have recently been found to have a comparably large spin diffusion time and length. This makes them ideal candidates for spintronic devices. However, spin injection, transport and detection properties in organic materials have yet to be fully understood. This work studies spin injection from ferromagnets into organic semiconductors via spin pumping. Furthermore, work towards thermal spin injection, and detection is presented and discussed. The first part of this thesis comprises the spin pumping experiments. Measuring linewidth broadening of the microwave absorption at ferromagnetic resonance due to increase in effective Gilbert damping by spin pumping from a ferromagnetic substrate into an adjacent non-magnetic semiconductor allows us to quantify the spin-mixing conductance. This technique is employed to demonstrate spin injection from a ferromagnetic metal, permalloy (Ni81Fe19), into organic small molecules and conjugated polymers as well as to quantify the spin injection efficiency. The results highlight the importance of structural properties of organic semiconductors at the interface to permalloy. Significant suppression of spin injection due to alkyl side-chains separating the core of the small molecules from the interface is exemplary for this finding. Furthermore, the spin-mixing conductance depends very sensitively on the charge carrier density within a certain range of doping level. This suggests a strong link between spin injection efficiency and spin concentration in the organic semiconductor at the interface to permalloy. The second part of the thesis aims to explore spin caloritronic effects. We study spin injection into organic semiconductors by probing the spin Seebeck effect by making use of the inverse spin Hall effect for spin-to-charge conversion. Moreover, we present experimental work towards observation of a novel effect, the inverse spin Nernst effect, for thermal spin detection.

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Irreversible thermodynamics is a widely-applicable toolset that extends thermodynamics to describe systems undergoing irreversible processes. It is particularly useful for describing macroscopic flow of system components, whether conserved (e.g., particle number) or non-conserved (e.g., spin). We give a general introduction to this toolset and calculate the entropy production due to bulk and interfacial flow. We compare the entropy production and heating rate of bulk and interfacial transport, as well as interfacial charge and spin transport. We then demonstrate the power and applicability of this toolset by applying it to three systems. We first consider metal oxide growth, and discuss inconsistency in previous theory by Mott. We show, however, that Mott's solution is the lowest order of a consistent asymptotic solution, with the ion and electron concentrations and fluxes going as power series in t^-k/2, where k = 1, 2, .... We find that this gives corrections to the "parabolic growth law" that has oxide thickness going as t^1/2; the lowest order correction is logarithmic in t. We then consider the effect on spin of electric currents crossing an interface between a ferromagnet (FM) and non-magnetic material (NM). Previous theories for electrical potential and spin accumulation neglect chemical or magnetic contributions to the energy. We apply irreversible thermodynamics to show that both contributions are pivotal in predicting the spin accumulation, particularly in the NM. We also show that charge screening, not considered in previous theories, causes spin accumulation in the FM, which may be important in ferromagnetic semiconductors. Finally, we apply irreversible thermodynamics to thermal equilibration in a thin-film FM on a substrate. Recent experiments suggest that applying a thermal gradient across the length of the system causes a spin current along the thickness; this spin current is present much farther from the heat sources than expected. We find that, although the interaction between the separate thermal equilibration processes increases the largest equilibration length, thermal equilibration does not predict a length as large as the experimentally measured length; it does predict, however, a thermal gradient along the thickness that has the shape of the measured spin current.

Magnetic oxides have become of interest source for spin transport devices due to their high spin polarization. But the real applications of these oxides remains unsatisfactory up to date, mostly due to the change of properties as a result of nano structuring. Magnetite (Fe3O4) is one such a material. High Curie temperature and the half metallicity of Fe3O4 make it a good potential candidate for spin transport devices. Studies have shown that the nano structuring Fe3O4 changes most of it's important properties. This includes high saturation magnetization and drop of conductivity by a few orders of magnitude in Fe3O4 thin films. In this study, we have successfully grown Fe3O4 by reactive sputtering and studied the effect of transition metal buffer layers on structural, transport, and magnetic properties of Fe3O4. It is shown that the lattice strain created by different buffer layers has major impacts on the properties of Fe3O4 thin films. Also for the first time the magnetic force microscopic measurements were carried out in Fe3O4 thin films through Verwey transition. MFM data with the magnetization data have confirmed that the magnetization of Fe3O4 thin films rotate slightly out of the plane below the Verwey transition. Fe3O4 thin films were also successfully used in fabricating spin valve structures with Chromium and Permalloy. Here, the Fe3O4 was used to generated the spin polarized electrons through reflection instead of direct spin injection. This is a novel method that can be used to inject spins into materials with different conductivities, where the traditional direct spin injection fails. Also the effect of growth field on Fe3O4 and Fe3O4/Cr/Py spin valves were investigated. In Fe3O4 the growth field induced an uni-axial anisotropy while it creates a well defined parallel and anti-parallel states in spin valves. Magneto thermal phenomenon including spin dependent Seebeck effect, Planar Nernst effect, and Anomalous Nernst effect were measured in ferromagnetic thin films and spin valves. Spin dependent Seebeck effect and planar Nernst effect were directly compared with the charge counterpart anisotropic magneto resistance. All the effects exhibited similar behavior indicating the same origin, namely spin dependent scattering.

Dans cette thèse, nous étudions le transport diffusif de la charge, du spin et de la chaleur dans les structures métalliques incluant des métaux ferromagnétiques. En particulier, nous nous sommes intéressés à la partie de ce transport dont la polarisation n'est pas colinéaire à l'aimantation ambiante.Par exemple un courant polarisé en spin arrivant sur une couche magnétique dont l'aimantation pointe dans une autre direction verra sa partie transverse (i.e. non colinéaire à l'aimantation) précesser et être absorbée par l'aimantation sur une distance de quelques nanomètres au plus. Nous présentons un état de l'art sur ces deux distances caractéristiques, de précession et d'absorption transverse. Nous montrons aussi que ce comportement a un impact majeur sur la dynamique, notamment des murs de domaines magnétiques "longs" (plus de dix nanomètres).Nous étudions aussi le torque de transfert de spin dans ces structures magnétiques. Cette étude a porté sur deux aspects majeurs. Tout d'abord l'amplitude du torque, pour savoir s'il est capable de démarrer une des dynamiques magnétiques connues : l'inversion magnétique, ou la précession en état stationnaire. Ensuite, la dépendance du torque avec l'angle relatif des aimantations : dans certains cas, une configuration non-colinéaire peut-être stabilisée. Deux sources d'énergies ont été considérées, une différence de potentiel, ou une différence de température (en incluant les effets thermoélectriques dépendant du spin).Toute cette étude est réalisée dans le cadre de notre théorie, la Continuous Random Matrix Theory, que nous présentons dans son intégralité, de son origine avec la théorie de scattering, jusqu'aux équations différentielles de diffusion, résultat majeur de cette thèse.Nous présentons aussi l'outil numérique basé sur cette théorie, grâce auquel nous avons effectué toutes nos simulations. Cet outil permet d'évaluer le transport diffusif dans les structures métalliques tridimensionnelles, en utilisant des paramètres matériaux facilement disponibles dans la littérature pour la plupart. / In this thesis we study the diffusive transport of the charge, spin and heat in metallic structures involving ferromagnets. In particular, we focused on the part of the transport which polarization is not collinear to the surrounding magnetization.For example, a spin-polarized current arriving on a magnetic layer with a magnetization pointing in another direction will have its transverse part (i.e. non-collinear to the magnetization) precess and be absorbed by the magnetization, over a distance of up to a few nanometers. We present a state-of-the-art collection of values for those two characteristic lengths, of precession and transverse absorption. We also show that this behavior as a tremendous impact over the dynamics, notably that of "long" magnetic domain walls (over ten nanometers).We also study the spin-transfer torque in those magnetic structures, and focus on two major aspects. First the amplitude of the torque, to know if it is strong enough to start one of the known dynamics: magnetic switching or steady-state precession. Second, the dependence of the torque with the relative angle between the magnetizations: in some cases, a non-collinear configuration may be stabilized. Two driving forces have been considered, a voltage bias or a temperature difference (by including spin-dependent thermoelectric effects).This whole study is performed within the framework of our theory, the Continuous Random Matrix Theory, that we present in its entirety, from its origin with the scattering theory, to the diffusion differential equations, one of the main results of this thesis.We also present the numerical tool we developed, based on this theory, which we used to perform all of our simulations. This tool allows for the evaluation of the diffusive transport in three-dimensional metallic structures, using (mostly) readily available material parameters.

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京都大学 / 新制・課程博士 / 博士(工学) / 甲第23512号 / 工博第4924号 / 新制||工||1769(附属図書館) / 京都大学大学院工学研究科電子工学専攻 / (主査)教授 白石 誠司, 教授 山田 啓文, 教授 引原 隆士 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

Title: Investigation of spin structure and dynamics in magnetically ordered thin films Author: Tomáš Janda Abstract: This thesis is devoted to the development of methods for imaging and ultrafast manipulation of magnetic textures, such as magnetic domains and domain walls (DWs), and to the investigation of the corresponding magnetiza- tion dynamics. We focused on techniques that are, besides ferromagnets (FMs), applicable also to antiferromagnets (AFs), in particular, AF thin films. The employed excitation techniques were mostly based on direct or indirect effects of ultrashort laser pulses. We studied the DW motion induced by a transfer of angular momentum from circularly polarized femtosecond laser pulses in a FM GaMnAsP and we found that the observed macroscopic DW displacement is only possible due to its inertia. In a distinct experiment, picosecond current pulses were generated by an absorption of the ultrashort laser pulses in a verti- cal PIN diode-based photoconductive switch and used to excite a magnetic film deposited directly on top of the diode. For imaging of AF domain patterns, we developed a simple table-top laser-scanning technique, which is based on a magneto-thermoelectric response of the AF textures in the presence of a local laser-induced heating. We first used the method to image the...

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