Energy enables basic and innovative services to reach a seemingly ever-growing population and when its generation costs are reduced or when its usage is optimized it has the greatest impact on the reduction of poverty. Furthermore, there is a pressing need to decouple energy generation from non-renewable and carbon-heavy sources which has led mayor economies to increase research efforts in these areas. This thesis discusses research on water oxidation using nanostructured iron oxide electrodes and current-induced magnetic domain wall motion in nickel/cobalt bi-segmented nanowires. These two fields may seem disparate at first glance, but are linked by such common theme: materials for energy, and more precisely, materials for energy conversion and economy. The work presented in this document aims also to reflect this theme by using widely available materials like iron and aluminum, and optimizing the methods to produce the final samples using the least resources possible. All samples were prepared by electroplating metals (iron, cobalt and nickel) into anodized alumina templates fabricated inhouse. For water oxidation, iron nanorods were integrated into an electrode and annealed in air, while nickel/cobalt nanowires were isolated and contacted individually to test for spintronics-related effects. Spintronic-based devices aim to reduce energy usage in nowadays microelectronic devices. The nanostructured iron oxide electrode showed its usefulness for water oxidation in a laboratory environment, making it an appropriate complement to other electrodes specially designed for water reduction in a photoelectrochemical cell. This two-electrode design, allows for hydrogen and oxygen to be produced at each electrode and therefore eases their separate collection for, e.g., fuel or fertilizers. On the other hand, this work presents one of the first experimental demonstration of current-induced domain wall motion in soft/hard cylindrical magnetic nanowires at zero applied external magnetic field. These kinds of experiments are expected to be the first of many which will allow researchers in the field to test for spintronic-relevant properties and interactions in cylindrical magnetic nanowires.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第22270号 / 理博第4584号 / 新制||理||1658(附属図書館) / 京都大学大学院理学研究科化学専攻 / (主査)教授 小野 輝男, 教授 吉村 一良, 教授 島川 祐一 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

The polarization response such as ferroelectric and ferroelastic switching in ferroelectrics is the important feature for ferroelectric and electromechanical applications. In polycrystalline form ferroelectrics, effects of the microstructural parameters such as texture, grain size, and residual stress are there and have not fully been understood. Among these effects, (1) the origin of grain size effects on ferroelastic switching, (2) mechanical stress effects on polarization switching, and (3) ferroelectric switching kinetics and the relationship to grain boundaries are investigated.<br>Firstly, the microscopic origin of ferroelastic switching suppression in smaller grains is discovered using a microscopic probing technique (piezoresponse force microscopy). It is demonstrated that there is no independent grain size effect on ferroelastic switching; the grain size affects the domain structure in a grain, and the domain structure plays an important role in the ferroelastic switching suppression. This result suggests that the grain size is not an independent critical parameter for the electromechanical property degradation in a grain < 1 m as the ferroelastic switching is a dominant component for the electromechanical property.<br>The study about the mechanical stress effects on the electric field induced polarization switching rationalizes the emergence of the electric field induced low-symmetry phases observed in tetragonal Pb(Zr,Ti)O3 and BaTiO3 ceramics after poling. It is demonstrated that a shear stress plays an important role in stabilizing the monoclinic phase in Pb(Zr,Ti)O3 whereas a normal stress along the polarization axis is a key for the monoclinic phase in BaTiO3 with a thermodynamic approach. It is suggested that the fraction of the low-symmetry phase, which is important for the large electromechanical property, can be engineered by applying an appropriate stress.<br>For the work about ferroelectric switching kinetics, the first direct Barkhausen noise associated with ferroelectric switching is measured. The domain switching time is quantified by the frequency of the Barkhausen noise. It is discovered that the dominant domain wall pinning site is grain boundaries based on the domain wall jump distance between pinning sites calculated from the switching time. This result suggests that the technique is a good tool for understanding the relationship between microstructure – domain wall kinetics.<br>In sum, the mechanisms of the polarization switching suppression due to domain structure and grain boundaries, and the emergence of the low symmetry phases due to stresses are revealed. These discoveries facilitate further improvements of the device performances with engineering the domain structure, grain boundaries and residual stress.<br>

The past few decades have witnessed a race towards developing smaller, faster, cheaper and ultra high capacity data storage technologies. In particular, this race has been accelerated due to the emergence of the internet, consumer electronics, big data, cloud based storage and computing technologies. The enormous increase in data is paving the path to a data capacity gap wherein more data than can be stored is generated and existing storage technologies would be unable to bridge this data gap. A novel approach could be to shift away from current two dimensional architectures and onto three dimensional architectures wherein data can be stored vertically aligned on a substrate, thereby decreasing the device footprint. This thesis explores a data storage concept based on vertically aligned cylindrical magnetic nanowires which are promising candidates due to their low fabrication cost, lack of moving parts as well as predicted high operational speed. In the proposed concept, data is stored in magnetic nanowires in the form of magnetic domains or bits which can be moved along the nanowire to write/read heads situated at the bottom/top of the nanowire using spin polarized current. Cylindrical nanowires generally exhibit a single magnetic domain state i.e. a single bit, thus for these cylindrical nanowire to exhibit high density data storage, it is crucial to pack multiple domains within a nanowire. This dissertation demonstrates that by introducing compositional variation i.e. multiple segments along the nanowire, using materials with differing values of magnetization such as cobalt and nickel, it is possible to incorporate multiple domains in a nanowire. Since the fabrication of cylindrical nanowires is a batch process, examining the properties of a single nanowire is a challenging task. This dissertation deals with the fabrication, characterization and manipulation of magnetic domains in individual nanowires. The various properties of are investigated using electrical measurements, magnetic microscopy techniques and micromagnetic simulations. In addition to packing multiple domains in a cylindrical nanowire, this dissertation reports the current assisted motion of domain walls along multisegmented Co/Ni nanowires, which is a fundamental step towards achieving a high density cylindrical nanowire-based data storage device.

京都大学 / 新制・課程博士 / 博士(理学) / 甲第23722号 / 理博第4812号 / 新制||理||1689(附属図書館) / 京都大学大学院理学研究科化学専攻 / (主査)教授 小野 輝男, 教授 寺西 利治, 教授 島川 祐一 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

L'objectif de cette thèse est d'étudier l'effet du champ magnétique Oersted sur le mouvement induit par courant de parois de domaines magnetiques dans des nanobandes de bicouches IrPy. Nous avons optimisé la croissance épitaxiale des couches minces IrPy avec faible rugosité de surface et d'interface, peu de défauts structurels et un faible champ coercitif. Cela peut réduire le piégeage de parois et donc augmenter sa mobilité. Nanobandes polycristallins PtPy préparées par pulvérisation ont également été étudiées pour comparer les résultats avec des échantillons épitaxiés. Une première preuve directe de l'effet du champ Oersted sur la configuration magnétique de nanobandes magnétiques a été donnée par V. Uhlir et al. utilisant des mesures XMCD-PEEM résolues en temps. Ils ont observé une grande inclinaison transversale de l'aimantation du Py et CoFeB dans les nanobandes en tricouchesCoCuPy et CoCuCoFeB. Nous avons observé le changement de chiralité des parois transverses sous champ Oersted avec des impulsions de courant en utilisant la microscopie à force magnétique. Un mouvement de parois stochastique a été observé en raison du piégeage, ce qui donne lieu à une large distribution de vitesses de paroi de domaine. Déplacement de paroi opposé au flux d'électrons et transformations de paroi ont également été observés en raison de Joule chauffage. Les grains de grande taille (comparable à la largeur de bande) dans nos couches minces épitaxiales bi-cristallins par rapport aux échantillons polycristallins (~10nm) peut être la source possible du fort piégeage. Néanmoins, des vitesses de parois maximales très élevées (jusqu'à 700 et 250m/s) pour des densités de courant relativement faible (1.7x1012 et 1x1012 A/m2) ont été observées dans échantillons épitaxiales et pulvérisées respectivement. Ces vitesses sont 2 à 5 fois plus élevées avec des densités de courant similaires ou plus faible que celles observées dans des nanobandes de Py seul, rapportés dans la littérature. Le champ Oersted est peut-être à l'origine de la plus grande efficacité du couple de transfert de spin dans ces bandes en bicouche. Des simulations micromagnétiques réalisées dans notre groupe confirment qu'un champ magnétique transverse appliqué en plus d'un champ longitudinal pour déplacemer la paroi peut stabiliser le cœur d'une paroi vortex au centre de la nanobande, supprimant ainsi l'expulsion de cœur au bord de la nanobande et donc empêchant la transformation de parois vortex. De même, il peut stabiliser les parois transverses, empêchant des transformations. Cela peut conduire à une décalage du seuil de Walker vers des courants plus élevés, résultant en une augmentation de la vitesse de paroi. Des mesures XMCD-PEEM résolue en temps seront réalisées dans un avenir proche pour confirmer l'effet du champ Oersted sur le mouvement de la paroi. / The aim of this thesis is to study the effect of the magnetic Oersted field on current-induced domain wall (DW) motion in IrPy bilayer nanostripes. We optimized the epitaxial growth of IrPy films on sapphire (0001) substrates with less structural defects, small surface and interface roughness and small coercive fields. This was expected to reduce the DW pinning and hence increase the DW mobility. Polycrystalline PtPy nanostripes prepared by sputtering were also studied to compare the results with epitaxial samples. A first direct evidence of the effect of the Oersted field on the magnetic configuration of magnetic nanostripes was given by V. Uhlir et al. using time-resolved XMCD-PEEM measurements. They observed a large tilt of the Py and CoFeB magnetization in the direction transverse to the stripes in CoCuPy and CoCuCoFeB trilayer nanostripes. We observed chirality switching of transverse walls induced by the Oersted field due to current pulses using magnetic force microscopy. DW motion was found to be stochastic due to DW pinning, which results in a distribution of velocities. DW motion opposite to the electron flow and DW transformations were also observed due to Joule heating. The large grain size (comparable to the stripe width) in our epitaxial bi-crystalline films with respect to the polycrystalline samples (~10nm) may be a possible source of pinning. Nevertheless, very high maximum DW velocities (up to 700 and 250m/s) for relatively low current densities (1.7 x1012 and 1 x1012 A/m2) were observed in epitaxial and sputtered samples respectively. These velocities are 2 to 5 times higher with similar or even smaller current densities than observed in single layer Py nanostripes, reported in the literature. The Oersted field may be at the origin of the high efficiency of the spin transfer torque in these bilayer stripes. Micromagnetic simulations performed in our group confirm that when a transverse magnetic field is applied in addition to a longitudinal field along the nanostripe for VW motion, the vortex core can be stabilized in the center of nanostripe, suppressing the core expulsion at the nanostripe edge and hence preventing the VW transformation. Similarly, it can stabilize transverse walls, preventing DW transformations. This can result in a shift of the Walker breakdown to higher fields/currents, resulting in an increase in DW velocity. Time-resolved XMCD-PEEM measurements will be performed in the near future to confirm the effect of the Oersted field on the DW motion.

Předkládaná dizertační práce pojednává o problematice pohybu doménových stěn (DS) vyvolaného spinově polarizovaným proudem v magnetických nanodrátech na bázi spinového ventilu NiFe/Cu/Co. Jedná se o tzv. efekt přenosu spinového momentu. Multivrstevnatý systém NiFe/Cu/Co, kde se doménová stěna pohybuje ve vrstvě NiFe, vykazuje velmi vysokou účinnost přenosu spinového momentu, což bylo v literatuře potvrzeno na základě magnetotransportních měření. Tato práce má za cíl pozorovat stav DS během jejich pohybu, pomocí fotoelektronové mikroskopie kombinované s kruhovým magnetickým dichroismem. Tato technika využívá synchrotronové záření, které svým časovým rozlišením umožňuje sledovat dynamickou odezvu magnetizace na elektrický proud. Podstatnou částí řešení byla optimizace růstu vrstev NiFe/Cu/Co kvůli snížení magnetické dipolární interakce mezi vrstvami. V práci je také řešen způsob přípravy nanodrátů litografickými metodami. Byly provedeny dva módy měření: i) kvazistatický, tj. pozorování DS před a po injekci proudu do nanodrátu a ii) dynamické měření, kde je DS sledována během působení proudového pulzu. S využitím kvazistatickém módu byla vypracována rozsáhlá statistika pohybu DS: i) byly naměřeny jejich vysoké rychlosti přesahující 600 m/s za působení průměrné proudové hustoty nutné k posuvu doménové stěny - 5x10^11 A/m^2; ii) DS jsou v systému NiFe/Cu/Co velmi silně zachycovány dipolární interakcí mezi NiFe a Co způsobenou nehomogenitou krystalové struktury ve vrstvě Co. V dynamickém módu bylo odhaleno, že působením Oerstedovského pole kolmého na nanodráty v rovině vzorku se magnetizace ve vrstvě NiFe silně natáčí. Tento efekt přispívá k vysokým rychlostem DS pozorovaných v nanodrátech NiFe/Cu/Co.

Neuromorphic computing applications could be made faster and more power efficient by emulating the function of a biological synapse. Non-conventional spintronic devices have been proposed that demonstrate synaptic behavior through domain wall (DW) driving. In this work, current induced domain wall motion has been studied through micromagnetic simulations. We investigate the synaptic behavior of a head to head domain wall driven by a spin polarized current in permalloy (Py) nanostrips with shape anisotropy, where triangular notches have been modeled to account for edge roughness and provide pinning sites for the domain wall. We seek optimal material parameters to keep the critical current density for driving the domain wall at order 1011 A/m2.