This thesis presents the results from investigations of ultrafast magnetisation dynamics driven by pure spin currents. Spin orbit coupling in heavy metal layers - such as tungsten, tantalum or platinum - allows for the generation of pure spin currents, whereby spin up and spin down electrons move in opposite directions. Hence, a flow of angular momentum can be controlled through the manipulation of charge current through a heavy metal layer. When a spin current is injected into a ferromagnet, a torque is exerted on its magnetisation, with the potential to induce a wide variety of ultrafast dynamics. The experimental investigation of these phenomena employed a variety of high-frequency electrical techniques and time-resolved scanning Kerr microscopy (TRSKM) methods. In addition, various simulative and analytical approaches were used to gain insight into the underlying mechanisms. Spin Hall nano-oscillators (SHNOs) have recently been shown to support a tunable GHz spin wave `bullet’ under injection of direct current (DC), making it an exciting candidate for microwave communication applications. This thesis will show how TRKSM can be used to measure the torques within these devices, revealing that radio frequency (RF) current does not possess the same distribution as the DC. The competition between self-inductance and focusing within the device geometry results in a modified distribution of spin current. Further TRSKM measurements show the modified torque landscape to promote the mobility of the `bullet' within the magnetic layer. Devices that exploit spin currents for magnetisation reversal have received interest from academia and industry for their potential use as memory elements. The perpendicular magnetic anisotropy present in Ta/CoFeB/MgO leads to lower write currents and higher thermal stability. However, ultrafast processes have not been previously observed in such devices. TRSKM measurements of Hall bar devices were compared with a macrospin model to understand the underlying torques, and to investigate the conditions required to promote switching. Square elements built from the same stack structure exhibited contrasting static and dynamic behaviour. Pulsed currents drove differing dynamics at the edge and center of the device, while enabling the realignment of magnetic domains. The domains themselves could be driven directly by the spin current leading to domain wall dynamics. Measurements with a bipolar electrical pulse demonstrated that meta-stable switching can be achieved in micron-scale elements.

Transport of spin angular momentum (spin currents) in magnetic thin films is important for non-volatile spin-based memory devices and other emerging information technology applications. It is especially important to understand how a spin current propagates across interfaces and how a spin current interacts with magnetic moments. The great interest in devices based on ferromagnetic metals generated intensive theoretical and experimental studies on the basic physics of spin currents for the last few decades. Of particular interest recently is the so-called "pure" electronic spin current, which is carried by electrons and yet unaccompanied by net charge flow, in part because of the prospect of transporting spin with minimal Joule heating. However, in contrast to ferromagnetic metals, spin transport in antiferromagnetic metals, which are promising materials for next-generation magnetic information technology, is not well understood yet. This dissertation addresses the mechanisms of transport by pure spin current in thin-film multilayers incorporating metals with antiferromagnetic order. We focus on two specific materials: (1) CoGd alloys with ferrimagnetic sublattices, which resemble antiferromagnets near the compensation composition, and (2) elemental antiferromagnetic Cr, which can be grown as epitaxial films and hence serve as a model system material. For both the CoGd and Cr studies, spin-valve-like structures of NiFe/Cu/CoGd and NiFe/Cu/Cr/CoFe are prepared to conduct ferromagnetic resonance spin pumping experiments. Precessing magnetization in the NiFe "spin source" pumps a transverse spin current to the adjacent layers. We measure the loss of the spin angular momentum in the "spin sink" layer by measuring the broadening of the resonance linewidth, i.e., the non-local damping enhancement, of the spin source. The antiparallel magnetic moments of Co and Gd sublattices partially cancel out the dephasing of a transverse spin current, thereby resulting in a long spin dephasing length of ≈ 5-6 nm near the magnetic compensation point. We find evidence that the spin current interacts somewhat more strongly with the itinerant transition-metal Co magnetism than the localized rare-earth-metal Gd magnetism in the CoGd alloy. We also examine spin transport via structurally clean antiferromagnetic Cr, epitaxially grown with BCC crystal order. We observe strong spin reflection at the Cu/Cr interface, which is surprising considering that thin layers of Cu and Cr individually are transparent to spin currents carried by electrons. Further, our results indicate other combinations of electrically conductive elemental metals (e.g., Cu/V) can form effective spin-reflecting interfaces. Overall, this thesis advances the basic understanding of spin transport in metallic thin films with and without magnetic order, which can aid the development of next generations of efficient spintronic devices. This work was supported in part by the National Science Foundation, Grant No. DMR-2003914. / Doctor of Philosophy / Manipulation of electronic flow, i.e., net charge flow, underlies modern electronic devices such as computers, mobile phones, and electric cars. However, the conventional charge transport inevitably results in wasted energy, due to resistive (Joule) heating in the devices. A new research area which uses the electron's spin has recently emerged, namely spintronics. Spintronics uses the spin of electrons rather than just the charge, thereby reducing the dependence on charge flow. The flow of spin angular momentum carried by electrons, i.e., "electronic spin current," underpins numerous phenomena in condensed matter physics. An important example is switching and excitation of magnetic order driven electrically by spin current rather than external magnetic field. Spin currents can interact not only with ferromagnetic order consisting of parallel magnetic moments – but also with antiferromagnetic order consisting of alternating magnetic moments that cancel the net magnetization of the material. Indeed, experiments from the last few years demonstrate the ability to rotate antiferromagnetic order (a.k.a. Néel vector) by spin current, which offers new physics not achievable in ferromagnets, such as ultrafast spin dynamics in the THz regime and superfluid spin transport analogous to superconducting electronic transport. However, interaction of a spin current with antiferromagnetic order is not well understood yet. The aim of this thesis is to build a better understanding of spin currents in antiferromagnetic metals. Specifically, we experimentally study basic spin-current physics in a ferrimagnet (CoGd) and an antiferromagnet (Cr). We choose CoGd because adjusting its chemical composition allows us to easily tune its magnetism from ferromagnet-like (uncompensated magnetization) to antiferromagnet-like (compensated magnetization). In antiferromagnet-like CoGd, we find that the oppositely oriented Co and Gd magnetic moments partially cancel the scrambling (dephasing) of spins, so that the spin current is able to propagate over a longer distance - about 3-4 times more than in ferromagnetic metals. The mechanisms behind the longer spin propagation is somewhat akin to the spin "rephasing" technique for lengthening the lifetime of spin-based qubits for quantum computers, but what is remarkable is that we observe this effect in rather disordered magnetic alloys at room temperature. In the other major project of this thesis, we investigate spin transport through multilayers that contain Cr, a structurally and chemically clean antiferromagnetic material. We find that Cr by itself is a good spin transmitter, i.e., effectively allowing a pure spin current to pass through. Surprisingly, when Cr and Cu (another good spin transmitter) are stacked together, we observe strong reflection of a pure spin current at the interface of Cr and Cu. We find that the antiferromagnetic order in Cr is not responsible for this peculiar spin reflection and that other pairs of spin-transmitting metals (for example, V and Cu) can form spin-reflecting interfaces as well. Our work shows an interesting example of "emergent" phenomena where the interface behaves in a way that is not intuitively expected from the properties of the constituent materials. The basic scientific findings from this thesis may help the development of more efficient information-technology devices that run on spin currents.

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

In the past two decades, the interest in materials with strong spin-orbit coupling has attracted substantial attention because of the novel physical mechanisms they display and their potential for applications. The interface displaying large spin-orbit coupling has been recognized as a powerful platform to investigate the spin transport in ferromagnetic, antiferromagnetic, and non-magnetic materials, as well as their interfaces. Besides its rich physics, the related applications are also worth studying. The current-induced spin-orbit-torque arising from angular momentum transfer from the lattice to the spin system has substantial potential in recent state-of-art spin-orbit torque magnetic random access memory. In this dissertation, we have been interested in better understanding and characterizing the spin-orbit torque and spin Hall transport in various heterostructures of interest. We used the second harmonic method to determine the magnitude of the spin currents generation and transmission in Cu-Au alloy and Ir-Mn compound, respectively. We also characterized the device performance in selected heterostructures displaying either perpendicular MgO-based tunnel magnetoresistance or unusual surface states. Finally, we used these properties to approach spin-orbit torque magnetic random access memory through designing, fabricating, and characterizing the devices that focused on current-induced spin-orbit-torque magnetization switching.

At present, electronic devices depend upon electric charge to transfer and record information. However, such devices are approaching a scaling limit due to Joule heating. Spintronics offers a solution by exploiting the spin rather than the charge of the electron, since the propagation of spin current can in principle occur without dissipation. Immediate applications lie in magnetic random access memory and novel media for hard disk recording. Within this thesis, the Magneto-optical Kerr effect (MOKE) has been used to measure the static and dynamic magnetic properties of a number of different thin film samples that are of interest for spintronic applications. A femtosecond laser has been used to perform time-resolved MOKE (TRMOKE) and time resolved reflectivity (TRR) measurements simultaneously, which probe the spin and charge dynamics respectively. Measurements have been performed upon a continuous thin film of CrO2 that is known to be half-metallic in bulk form, and a series of YIG/Cu/Ni81Fe19 based structures that are expected to exhibit the spin Seebeck effect (SSE). Chemical vapour deposition (CVD) was used to fabricate the continuous CrO2 thin film on a (100)-oriented TiO2 substrate. Precessional magnetisation dynamics were studied by means of the TRMOKE technique. The dependence of the precession frequency and the effective damping parameter upon the static applied magnetic field were investigated. The precession frequency exhibited a minimum at the hard axis saturation field as expected. However precession was also observed for fields greater than the hard axis saturation value, perhaps suggesting the presence of a twisted magnetic state within the film. TRMOKE and TRR measurements were performed upon the YIG/Cu/Ni81Fe19 based structures for different values of the pump fluence and applied magnetic field. For fixed pump fluence and varying applied field, the frequency of precession is well described by a numerical solution of the Landau-Lifshitz equation for the Ni81Fe19 (permalloy, Py) layer. The frequency, amplitude, damping, phase and chirp of the precessional oscillations was extracted from measurements made with a field of 3 kOe applied at 2.8° from the normal to the sample plane, in a configuration designed to maximise any spin transfer torque (STT) generated by the SSE. The oscillation parameters extracted for trilayer samples and a Py reference sample were found to be very similar. Features indicative of STT predicted by simulations were not observed. This suggests that either the YIG/Cu interface was unable to efficiently transmit spin current within the samples studied here, or else that the STT generated by means of the SSE is too small to be of practical use.

This thesis consists of three distinct components: (1) a test of Slocnzewski's theory of spin-transfer torque using the Boltzmann equation, (2) a comparison of macrospin models of spin-transfer dynamics in spin valves with experimental data, and (3) a study of spin-transfer torque in continuously variable magnetization. Slonczewski developed a simple circuit theory for spin-transfer torque in spin valves with thin spacer layer. We developed a numerical method to calculate the spin-transfer torque in a spin valve using Boltzmann equation. In almost all realistic cases, the circuit theory predictions agree well with the Boltzmann equation results. To gain a better understanding of experimental results for spin valve systems, current-induced magnetization dynamics for a spin valve are studied using a single-domain approximation and a generalized Landau-Lifshitz-Gilbert equation. Many features of the experiment were reproduced by the simulations. However, there are two significant discrepancies: the current dependence of the magnetization precession frequency, and the presence and/or absence of a microwave quiet magnetic phase with a distinct magnetoresistance signature. Spin-transfer effects in systems with continuously varying magnetization also have attracted much attention. One key question is under what condition is the spin current adiabatic, i.e., aligned to the local magnetization. Both quantum and semi-classical calculations of the spin current and spin-transfer torque are done in a free-electron Stoner model. The calculation shows that, in the adiabatic limit, the spin current aligns to the local magnetization while the spin density does not. The reason is found in an effective field produced by the gradient of the magnetization in the wall. Non-adiabatic effects arise for short domain walls, but their magnitude decreases exponentially as the wall width increases.

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

L'interaction entre électrons de conduction itinérants et électrons localisés dans les hétérostructures magnétiques est à l'origine d'effets tels que le transfert de moment de spin, le pompage de spin ou l'effet Hall de spin. Cette thèse est centrée sur le phénomène de pompage de spin : une couche ferromagnétique (FM) en précession injecte un courant de spin pur dans les couches adjacentes. Ce courant de spin peut être partiellement ou totalement absorbé par une couche, dite réservoir de spin, placée directement en contact avec le matériau ferromagnétique ou séparée par une couche d'espacement. L'absorption de la composante transverse du courant de spin induit une augmentation de l'amortissement de la précession ferromagnétique de la couche libre. Cet effet à été mesuré par des expériences de résonance ferromagnétique avec, pour la couche en précession FM, trois matériaux ferromagnétiques différents (NiFe, CoFeB et Co), et pour la couche de réservoir de spin, différents matériaux paramagnétiques (Pt, Pd, Ru), ferromagnétiques et antiferromagnétiques. Dans un premier temps, nous avons vérifié que le facteur d'amortissement non-local généré est de type amortissement de Gilbert, et qu'il est inversement proportionnel à l'épaisseur de la couche en précession FM. L'analyse de l'augmentation de l'amortissement a été réalisée dans le cadre du modèle de pompage de spin adiabatique proposé par Tserkovnyak et al.. Dans un second temps et suivant ce modèle, nous avons extrait les paramètres de conductance avec mélange de spin à l'interface g↑↓ pour différentes interfaces, ces paramètres déterminent le transport du courant de spin à travers des interfaces ferromagnétique/métal non-magnétique. Un troisième résultat important de cette thèse porte sur la longueur d'absorption du courant de spin dans des matériaux ferromagnétiques et paramagnétiques. Celle-ci varie considérablement d'un matériau à l'autre. Pour les matériaux ferromagnétiques, la longueur d'absorption du courant de spin est linéaire par rapport à l'épaisseur de la couche réservoir de spin, avec pour longueur caractéristique ~ 1 nm. Ce résultat est en cohérence avec les théories antérieures et avec les valeurs de longueur de déphasage de spin pour le transfert de moment de spin dans les matériaux ferromagnétiques. Dans les paramagnétiques tels que Pt, Pd, Ru, la longueur d'absorption est soit linéaire soit exponentielle selon que le réservoir paramagnétique est directement en contact avec la couche en précession ou bien séparé par une couche mince d'espacement en Cu. La longueur caractéristique correspondante est inférieure à la longueur de diffusion de spin. Des mesures complémentaires de dichroïsme circulaire magnétique par rayons X ont révélé une induction de moments magnétiques dans les matériaux paramagnétiques comme Pd, Pt, lorsque couplé directement ou indirectement avec une couche FM. Ce résultat fournit une explication de la dépendance en épaisseur linéaire observée dans les hétérostructures en contact direct. Etant donné que le pompage de spin et le couple de transfert de spin (STT) sont des processus réciproques, les résultats de cette thèse sur la conductance avec mélange de spin, la longueur d'absorption de spin et les moments de spin induits sont également d'un grand intérêt pour les études de transfert de moment de spin, ainsi que d'effet Hall de spin, direct et inverse. L'avantage des études présentées ici réside dans le fait qu'elles sont effectuées sur des couches minces continues, sans aucune étape de nanofabrication.

L'interaction entre électrons de conduction itinérants et électrons localisés dans les hétérostructures magnétiques est à l'origine d'effets tels que le transfert de moment de spin, le pompage de spin ou l'effet Hall de spin. Cette thèse est centrée sur le phénomène de pompage de spin : une couche ferromagnétique (FM) en précession injecte un courant de spin pur dans les couches adjacentes. Ce courant de spin peut être partiellement ou totalement absorbé par une couche, dite réservoir de spin, placée directement en contact avec le matériau ferromagnétique ou séparée par une couche d'espacement. L'absorption de la composante transverse du courant de spin induit une augmentation de l'amortissement de la précession ferromagnétique de la couche libre. Cet effet à été mesuré par des expériences de résonance ferromagnétique avec, pour la couche en précession FM, trois matériaux ferromagnétiques différents (NiFe, CoFeB et Co), et pour la couche de réservoir de spin, différents matériaux paramagnétiques (Pt, Pd, Ru), ferromagnétiques et antiferromagnétiques. Dans un premier temps, nous avons vérifié que le facteur d'amortissement non-local généré est de type amortissement de Gilbert, et qu'il est inversement proportionnel à l'épaisseur de la couche en précession FM. L'analyse de l'augmentation de l'amortissement a été réalisée dans le cadre du modèle de pompage de spin adiabatique proposé par Tserkovnyak et al.. Dans un second temps et suivant ce modèle, nous avons extrait les paramètres de conductance avec mélange de spin à l'interface g↑↓ pour différentes interfaces, ces paramètres déterminent le transport du courant de spin à travers des interfaces ferromagnétique/métal non-magnétique. Un troisième résultat important de cette thèse porte sur la longueur d'absorption du courant de spin dans des matériaux ferromagnétiques et paramagnétiques. Celle-ci varie considérablement d'un matériau à l'autre. Pour les matériaux ferromagnétiques, la longueur d'absorption du courant de spin est linéaire par rapport à l'épaisseur de la couche réservoir de spin, avec pour longueur caractéristique ~ 1 nm. Ce résultat est en cohérence avec les théories antérieures et avec les valeurs de longueur de déphasage de spin pour le transfert de moment de spin dans les matériaux ferromagnétiques. Dans les paramagnétiques tels que Pt, Pd, Ru, la longueur d'absorption est soit linéaire soit exponentielle selon que le réservoir paramagnétique est directement en contact avec la couche en précession ou bien séparé par une couche mince d'espacement en Cu. La longueur caractéristique correspondante est inférieure à la longueur de diffusion de spin. Des mesures complémentaires de dichroïsme circulaire magnétique par rayons X ont révélé une induction de moments magnétiques dans les matériaux paramagnétiques comme Pd, Pt, lorsque couplé directement ou indirectement avec une couche FM. Ce résultat fournit une explication de la dépendance en épaisseur linéaire observée dans les hétérostructures en contact direct. Etant donné que le pompage de spin et le couple de transfert de spin (STT) sont des processus réciproques, les résultats de cette thèse sur la conductance avec mélange de spin, la longueur d'absorption de spin et les moments de spin induits sont également d'un grand intérêt pour les études de transfert de moment de spin, ainsi que d'effet Hall de spin, direct et inverse. L'avantage des études présentées ici réside dans le fait qu‘elles sont effectuées sur des couches minces continues, sans aucune étape de nanofabrication. / In magnetic heterostructures, the interaction between itinerant conduction electrons with localized electrons is at the origin of effects such as the spin momentum transfer, spin pumping or the spin Hall effect. This thesis is centred on the phenomenon of spin pumping, which states that a precessing ferromagnetic (FM) layer injects a pure spin current into its adjacent metallic layers. This spin current can be partially or fully absorbed by a spin sink layer, placed directly in contact with the ferromagnet or separated by a spacer layer. The absorption of the transverse component of the spin current results in an enhancement of the effective damping of the precessing ferromagnet which we have studied using ferromagnetic resonance experiments for three different ferromagnets (NiFe, CoFeB and Co) as the precessing FM layer and various paramagnets (Pt, Pd, Ru), ferromagnets or an antiferromagnet as the spin sink layer. As a first step we have verified that the additional non-local damping is Gilbert type, and that it depends inversely on the thickness of the FM precessing layer. The analysis of the enhanced damping was done in the frame of an adiabatic spin pumping model proposed by Tserkovnyak et al. Within this model we extracted as a second step the interfacial spin mixing conductance parameters g↑↓ for various interfaces, which determine the spin current transport through FM/NM interfaces. A third important result of the thesis concerns the absorption length of spin currents in ferromagnets and paramagnets which we found can be very different. In ferromagnets the spin current absorption is linear with the spin sink layer thickness, with a characteristic length of ~1nm. This is consistent with theory and the spin dephasing length for spin momentum transfer in ferromagnets. In paramagnets such as Pt, Pd, Ru, the spin current absorption is either linear or exponential depending on whether the paramagnetic is directly in contact with the FM or separated by a thin Cu spacer layer. The corresponding characteristic length is less than the spin diffusion length. Complementary X-ray magnetic circular dichroism measurements revealed induced magnetic moments in paramagnets like Pd, Pt when directly or indirectly coupled with a FM layer. This provides an explanation for the linear thickness dependence for the direct contact heterostructures. Since spin pumping and spin transfer torque (STT) are reciprocal processes the results of this thesis on the spin mixing conductances, spin absorption length scales and induced moments will also be of great interest for studies on spin momentum transfer, Spin Hall effect and Inverse Spin Hall effect. The convenience being that these studies can be done on continuous films and no nanofabrication is required.

In this work, we analyzed samples of thin films Ni81Fe19 and Pd with ZnO spacer. Ni81Fe19 is a ferromagnet with a low saturation magnetic field. Pd is a normal metal with high spinorbit coupling, much used in spin Hall effect and inverse spin Hall effect studies. ZnO is a semiconductor whose role is to reduce the charge current between layers. The sample have 5 nm of Ni81Fe19, 3 nm of Pd, and 2 nm of ZnO, with dimensions of 0.4 mm x 8 mm, deposited by magnetron sputtering. Using spin pumping we analyze the signal of the continuous voltage induced by ferromagnetic resonance. These samples the measured signal is a consequence of anisotropic magnetoresistance, anomalous Hall effect and inverse spin Hall effect. The thicknesses used contributes to a predominant inverse spin Hall effect signal. The ZnO spacer layer 2 nm reduces the effects of spin rectification, while maintaining spin current transfer. / Neste trabalho analisamos amostras de filmes finos de Ni81Fe19 e Pd separados por ZnO. O Ni81Fe19 foi escolhido por ser um ferromagneto com baixo campo magnético de saturação. O Pd é um metal normal com alto acoplamento spin-órbita, muito usado em estudos de efeito Hall de spin e efeito Hall de spin inverso. O ZnO é um semicondutor com o papel de diminuir a transferência de corrente de carga entre as camadas. As amostras tem espessura de 5 nm de Ni81Fe19, 3 nm de Pd e 2 nm de ZnO, com dimensões de 0,4 mm x 8 mm, depositadas por magnetron sputtering. Através da técnica de spin pumping analisamos o sinal de tensão contínua induzida por ressonância ferromagnética. Nestas amostras o sinal medido é consequência de efeitos de magnetorresistência anisotrópica, efeito Hall anômalo e efeito Hall de spin inverso. As espessuras utilizadas permitem um sinal de efeito Hall de spin inverso predominante. A camada espaçadora de 2nm de ZnO reduz os efeitos de retificação de spin, mantendo a transferência de corrente de spin.