Since the discovery of giant magnetoresistance in 1980s, Spintronics became an exciting field which studies numerous phenomena including the spin transport in magnetic heterostructures, magnetization dynamics and the interplay between them. I have investigated different topics during my graduate research. In this dissertation, I summarize all my projects including spin pumping, spin convertance and spin injection into ballistic medium. First, we develop a linear response formalism for spin pumping effect. Spin pumping refers that a precessing emits a spin current into its adjacent nonmagnetic surroundings, which was originally proposed using scattering theory. The newly developed formalism is demonstrated to be identical the early theory in limiting case. While our formalism is convenient to include the effects of disorders and spin-orbit coupling which can resolve the quantitative controversies between early theory and experiments. Second, the spin pumping experiments indicates a much smaller spin Hall angle compared with the results obtained via the spin transfer torque measurements. We found that such issues can be resolved when taking into consideration the effects of non-local conductivity. And we conclude neither of the two methods measures the real spin Hall angle while the spin pumping methods provides much accurate estimations. Third, we developed the spin transport equations in weak scattering medium in the presence of spin-orbit coupling. Before this, all spin dependent electron transport has been modeled by the conventional spin diffusion equation. While recent spin injection experiments have seen the failure of spin diffusion equation. As the experimental fitting using spin diffusion models led to unrealistic conclusions. At last, we study the spin convertance in anti-ferromagnetic multilayers, where the spin information can be mutually transferred between ferromagnetic/anti-ferromagnetic and conduction electrons. Our theory successfully explained the experiment results that the insertion of thin NiO film between YIG/Pt largely enhances the spin Seebeck currents.

Spintronics is a research field that focuses on the manipulation of the quantum mechanical spin of charge carriers in solid state materials for future technological applications. Creating large spin currents with large relaxation times is sought after in the field of spintronics which may be aided by combining spintronics with superconductivity. This thesis provides a phenomological study of the effective change in ferromagnetic resonance linewidth, by dynamical spin injection into a permalloy-copper-niobium tri-layer in the superconducting state. The ferromagetic resonance linewidth was measured from 2-14 K. It was observed that there was a change in the behavior of the resonance as well as a change in the linewidth from the between 6-8 K. An observed change in the resonance field, Hr, shows a clear non monotonic behavior as a function of temperature below 7-8 K. The decrease in linewidth was attributed to the suppression of the spin sinking mechanisms due to the superconducting state of niobium.

Spintronics is the study of mutual dependence of magnetization and electron transport, which forms a complementary picture in ferromagnetic (FM) materials. Recently, spintronics based on antiferromagnetic (AF) materials has been suggested. However, a systematic study is not yet available, and a complementary picture of the AF dynamics with electron transport is highly desired. By developing a microscopic theory, we predict the occurrence of spintronic phenomena both in bulk AF texture and on the interface of AF with normal metals. For the bulk, we find that the electron dynamics becomes adiabatic when the local staggered field is varying slowly over space and time, by which the spin-motive force and the reactive spin-transfer torque (STT) are derived as reciprocal effects. While the former generates a pure spin voltage across the texture, the latter can be used to drive AF domain wall and trigger spin wave excitation with lower current densities compared to FM materials. For the interface, by calculating how electrons scatter off a normal metal -antiferromagnet heterostructure, we derive the pumped spin and staggered spin currents in terms of the staggered order parameter, the magnetization, and their rates of change; the reactions of an incident spin current on the antiferromagnet is derived as STTs. These effects are applicable to both compensated and uncompensated interfaces with a similar order of magnitude. In contrast to FM materials, the direction of spin pumping is controlled by the circular polarization of driving microwave; and conversely, the chirality of AF spin wave is tunable by the direction of spin accumulation. / text

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.

The field of Spintronics, which utilizes the spin polarization of electrons as a means to transport energy and information, is currently undergoing a massive expansion due to the numerous recent discoveries of electron spin-based effects only visible at the micro and nano scale. With all these new material and current based properties being discovered, it has become increasingly difficult to experimentally isolate the effects of each of them individually. In this work one of the main proposals is an experimental method for separating the voltage signals generated by the spin rectification effect and spin pumping in bilayer samples, a topic currently of much interest to the spintronics community. After demonstrating this new method for separating voltage signals, it is utilized to reveal new details about how the layers of ferromagnetic bilayer samples interact with each other and the behaviours of spin currents within these systems.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第22161号 / 工博第4665号 / 新制||工||1728(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 藤田 静雄, 准教授 掛谷 一弘 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

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En électronique de spin, le couple de transfert de spin (STT) et la magnétorésistance tunnel (TMR) dans les jonctions tunnel magnétiques à électrodes ferromagnétiques (F) sont deux phénomènes physiques essentiels. Dans cette thèse, nous présentons une étude théorique du STT dans des jonctions tunnel antiferromagnétiques (AF), où deux électrodes non-plus F mais AF sont séparées par une barrière isolante non-magnétique. Plus concrètement, les comportements du STT et de la TMR sont étudiés dans des jonctions tunnel AF cristallines, et ce, à l´aide de calculs de liaisons fortes dans le cadre du formalisme de Keldysh. Nous avons observé une distribution spatiale de la composante perpendiculaire du STT régulière et de signe alternatif, ce qui est similaire au comportement de la composante parallèle. Ces variations spatiales de la composante perpendiculaire sont cependant spécifiques à l'utilisation d'une barrière tunnel et contrastent avec les effets observés par le passé pour le cas de couches séparatrices métalliques. De plus, contrairement aux jonctions tunnel F conventionnelles, nous avons montré que la TMR peut augmenter avec la tension appliquée et atteindre des valeurs du même ordre de grandeur que pour des vannes de spin usuelles : tout-métallique et à électrodes F.L´analyse effectuée pour des AF est ensuite étendue aux matériaux ferrimagnétiques (FI), pour lesquels les AF constituent, somme toute, des cas limites. La complexité magnétique additionnelle inhérente aux FI se traduit par un comportement spatial du STT beaucoup plus riche dans les jonctions tunnel FI. Nous observons notamment que les paramètres électroniques tels que les largeurs et les décalages de bandes ont une très forte influence sur le STT. Plus particulièrement, la différence entre les couplages d'échange inter-spin locaux des deux sous-réseaux du FI donne lieu à une distribution spatiale du STT ondulatoire qui est modulée par la densité locale de spin. Il est possible d'ajuster cet effet en jouant sur la tension appliquée aux bornes de la jonction tunnel FI. Nous trouvons de plus que la différence entre les couplages d'échange inter-spin locaux constitue un paramètre fondamental pour la quantification des longueurs caractéristiques du STT dans les FIs. Ce paramètre peut être considéré comme un champ d´échange effectif, par similitude avec le cas usuel des Fs qui présentent un champ d´échange homogène.Pour finir, nous avons sondé expérimentalement les longueurs caractéristiques du STT dans des AFs. Pour de l'Ir20Mn80 et du Fe50Mn50, nous avons déterminé les longueurs de pénétration de spin et les mécanismes d'absorption de courants de spin à température ambiante en utilisant la résonance F et le pompage de spin. Plus précisément, nous avons associé les profondeurs de pénétration critiques à deux mécanismes d'absorption distincts: du déphasage pour l´Ir20Mn80 et du retournement de spin pour le Fe50Mn50. / Spin transfer torque (STT) and tunnelling magnetoresistance (TMR) in magnetic tunnel junctions with ferromagnetic (F) leads are two essential underlying phenomena of modern spintronics. We present here a theoretical study of STT in antiferromagnet (AF) based tunnel junctions, where two AF metal electrodes are separated by a thin nonmagnetic insulating barrier. In particular, the behaviour of STT and TMR in epitaxial AF-based tunnel junctions is investigated using tight binding calculations in the framework of the Keldysh formalism. The spatial distribution of the STT out-of-plane component is found to be staggered, similar to the in-plane component. This behaviour is specific to the use of a tunnel barrier and significantly differs from the out-of-plane torques reported in previous works using a metallic spacer. Additionally, we show that unlike conventional ferromagnetic-based tunnel junctions, the TMR can increase with applied bias and reach values comparable to typical magnetoresistances found for usual spin valves.Next, the analysis carried out for AFs is extended to ferrimagnets (FI), for which AFs constitute simpler limiting cases. The additional magnetic complexity inherent to FI materials yields to a richer physics concerning the STT spatial behaviour in FI based tunnel junctions.Electronic structure parameters such as band widths and exchange splittings of the FI are shown to have a strong influence on STT. In particular, the STT spatial distribution within the leads exhibits a striking spin-modulated wave-like behaviour resulting from the interplay between the exchange splittings of the two FI sublattices. This wave-like behaviour can also be tuned via the applied voltage across the junction. Furthermore, the fundamental intrinsic parameter for quantifying STT characteristic lengths in FI metals is identified. This fundamental parameter can be considered as an effective exchange field in FIs, similar to the homogeneous exchange field in the F case.Finally, the STT characteristic lengths in AF materials are investigated experimentally. Here, room temperature critical depths and absorption mechanisms of spin currents in Ir20Mn80 and Fe50Mn50 are determined by F-resonance and spin pumping. In particular, room temperature critical depths are observed to be originated from different absorption mechanisms: dephasing for Ir20Mn80 and spin flipping for Fe50Mn50.

Le domaine de recherche de la spintronique vise a concevoir des dispositifs électroniques misant sur le degré de libre de spin pour transporter de l'information. An d'intégrer ces courants de spin dans des dispositifs électroniques, il est particulièrement intéressant d'étudier l'inter-conversion d'un pur courant de spin en un courant de charge par l'effet Hall de spin, ainsi que le transfert de moment angulaire entre les électrons de conduction d'un métal normal (NM) et l'aimantation d'un ferromagnétique (FM) (couple de transfert de spin / pompage de spin). An de mieux comprendre ces différentes interaction, cette thèse se concentre sur l'étude du système hybride constitué de la juxtaposition d'un ferrimagnétique isolant, le grenat d'yttrium fer (YIG), et d'un métal normal _a fort couplage spin-orbite (Pt ou Ta), nécessaire pour bénéficier de la polarisation en spin de l'interface par un courant électrique dans le plan. Nous avons étudié le pompage de spin et la magnétorésistance produite par l'effet Hall de spin a l'interface entre des bicouches de YIG j Pt et YIG j Ta, et ceci sur des lms étendus de YIG de 200 nm d'épaisseur, produits par épitaxie en phase liquide. Nous observons que la tension électrique, produite par l'effet Hall de spin inverse, change de signe entre du Pt et du Ta confirmant ainsi l'inversion des signes de l'angle de Hall entre ces deux matériaux. En outre, en mesurant la variation de la tension de Hall inverse en fonction de l'épaisseur de la couche de Ta, nous avons réussi à borner la longueur de diffusion de spin dans le Ta. Tant le YIG j Pt et le YIG j Ta affiche une variation semblable de la magnétorésistance a effet Hall de spin en fonction de l'orientation du champ magnétique. Pour étudier l'inuence interfaciale du pompage de spin… / Spintronics aims at designing electronic devices which capitalize on the spin degree of freedom to transport information using spin currents. In order to incorporate spin currents intoelectronic devices, it is particularly interesting to study the interconversion from a spin current, the motion of spin angular momentum, to a charge current (Spin Hall Effect) as well as the transfer of spin angular momentum between the conduction electrons of a normal metal (NM) and the magnetization of a ferromagnet (FM) (Spin Transfer Torque/Spin Pumping). To investigate the interplay of those effects this thesis studies hybrid systems of the ferromagnetic insulator Yttrium Iron Garnet and normal metals with large spin-orbit coupling, a prerequisite for spin Hall e_ect. We study spin pumping and spin hall magnetoresistance in YIGjPt and YIGjTa bi-layers using extended _lms of 200 nm thick YIG, grown by liquid phase epitaxy. The inverse spin Hall voltages in Pt and Ta confirm the opposite signs of spin Hall angles in these two materials. Moreover, from the dependence of the inverse spin Hall voltage on the Ta thickness, we constrain the spin di_usion length in Ta. Both the YIGjPt and YIGjTa systems display a similar variation of resistance upon magnetic eld orientation, the spin Hall magnetoresistance. To study the inuence of interfacial spin pumping and a possible reverse e_ect, it is desirable to work with thin _lm thicknesses. A high quality 20 nm thick YIG _lm was grown by pulsed laser deposition, showing a damping similar to that of bulk YIG. We use nano-lithography to pattern series of YIG(20nm) and YIG(20nm)jPt(13nm) discs with diameters between 300 and 700 nm. The ferromagnetic resonance (FMR) spectra of the individual sub-micron sized samples are recorded through magnetic resonance force microscopy. . Passing dc-current through micron sized YIGjPt disks reveal a variation of the FMR linewidth consistent with the geometry and amplitude of the expected SHE transfer torque. In the absence of exciting microwave _elds, a variation in the magnetization is detected when the dc-current reaches the expected threshold for auto oscillations.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18056号 / 理博第3934号 / 新制||理||1567(附属図書館) / 30914 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)准教授 戸塚 圭介, 教授 石田 憲二, 教授 川上 則雄 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM