The high frequency dynamic magnetic responses of FeCo thin films and structures have been investigated mainly using ferromagnetic resonance (FMR) technique. The FMR resonance condition and linewidth are first derived from the dynamic Landau- Lifshitz-Gilbert equation, followed by a study of the conversion between FMR field and frequency linewidths. It is found that the linewidth conversion relation based on the derivative of resonance condition is only valid for samples with negligible extrinsic linewidth contribution. The dynamic magnetic properties obtained by using FMR measurements of FeCo thin films grown on Si/SiO2 substrates with varying deposition temperatures is then presented. The effective Landé g-factor, extrinsic linewidth, and Gilbert relaxation rate are all found to decrease in magnitude with increasing sample growth temperature from 20oC to about 400–500oC and then on further increase of the growth temperature to increase in magnitude. Samples grown at about 400–450oC display the smallest coercivity, while the smallest value of the Gilbert relaxation rate of about 0.1 GHz is obtained for samples grown at 450–500oC. An almost linear relation between extrinsic linewidth and coercivity is observed, which suggests a positive correlation between magnetic inhomogeneity, coercivity and extrinsic linewidth. Another major discovery in this study is that the Gilbert relaxation decreases with increasing lattice constant, which is ascribed to the degree of structural order in the films. A micromagnetic model is established for an asymmetric trilayer system consisting of two different ferromagnetic (FM) layers separated by thin non-magnetic (NM) layer, treating the magnetization in each FM layer as a macrospin. Based on the model, numerical simulations of magnetization curves and FMR dispersion relations, of both the acoustic mode where magentizations in the two FM layers precess in phase and the optic mode where they precess out-of-phase, have been carried out. The most significant implication from the results is that the coupling strength can be extracted by detecting only the acoustic mode resonances at many different unsaturated magnetic states using broadband FMR technique. Finally, trilayer films of FeCo(100 Å)/NM/FeNi(100 Å) with NM=Ru or Cu were prepared and studied. The thickness of the Ru and Cu spacer was varied from 0 to 50 Å. For the Ru spacer series, the film with 10 Å Ru spacer shows antiferromagnetic coupling while all other films are ferromagnetically coupled. For the Cu spacer trilayers, it is found that all films are ferromagnetically coupled and that films with thin Cu spacer are surprisingly strongly coupled (the coupling constant is 3 erg/cm2 for the sample with 5 Å Cu spacer). The strong coupling strength is qualitatively understood within the framework of a combined effect of Ruderman-Kittel- Kasuya-Yosida interaction and pinhole coupling, which is evidenced by transmission electron microscopy analysis. The magnetic coupling constant decreases exponentially with increasing Cu spacer thickness, without showing an oscillatory thickness dependence. The results have implications for the design of multilayers for spintronic applications.

The morphology, composition, and magnetic properties of NiFe thin films were characterized. Films with thicknesses up to 137 nm were deposited in an RF induction evaporator at high vacuum (10^-8 mbar). Time resolved magneto-optic Kerr effect microscopy (TR-MOKE) was used to measure the Gilbert damping constant, an important dynamic magnetic property with applications to magnetic data storage. The composition of each film was measured with energy-dispersive X-ray (EDX) microscopy and used to determine the weight percent of Ni and Fe in each film. A trend of increased damping with increased thickness was found, in agreement with published results. Magnetic properties and roughness were found to differ significantly from previous films grown in the same vacuum chamber by Rudge, and are attributed to different growth modes produced by differing deposition conditions. However, the weight percent of Ni in each film was found to be inconsistent, deviating by up to 7% from the Ni80Fe20 evaporation source. Inconsistent composition, caused by the inability to control deposition parameters, prevents insight into Gilbert damping from being drawn from the analysis. / Graduate

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 dynamic process of how fast a spintronic device can be switched from one state to another is characterized by the Gilbert damping parameter. It has been found that the Gilbert damping along with other intrinsic properties in permalloy, can be tuned by different dopants and doping concentration. Therefore, a study of intrinsic magnetic properties with emphasis on the dependence of the Gilbert damping parameter from first principles calculations is investigated. It is aimed at to give an insight of the microscopic understanding originated from electronic structure and to provide a guideline in the practical spintronic design. The topic of the present thesis is to investigate, by means of first principle calculations, how the variation of the Gilbert damping parameter depends upon the electronic structure of pure and doped permalloy. We show that the Gilbert damping has a monotonic increase with the doping concentration due to an increasing amount of scattering processes. The dopants of the 5d transition metal give rise to a much larger impact than the 4d, as the spin orbit coupling effect is more pronounced in the heavy elements. Our results are in satistying agreement with experiment. / <p>QC 20150629</p>

Within the exciting current trend to explore novel low-dimensional systems, the possibility to inject pure spin currents in graphene and other two-dimensional crystals has attracted considerable attention in the past few years. The theoretical prediction of large spin relaxation times and experimentally observed mesoscopic-scale spin diffusion lengths places graphene as a promising base system for future spintronics devices. This is due to the unique characteristics intrinsic to the two-dimensional lattice of carbon atoms forming graphene, such as the lack of nuclear spins and weak spin-orbit coupling of the charge carriers. Interestingly for some spintronic applications, the latter can be chemically and physically engineered, with large induced spin-orbit couplings found in functionalized graphene sheets. Understanding spin injection, spin current and spin dynamics in graphene is of a great interest, both from the fundamental and applied points of view. This thesis presents an experimental study of dynamical generation of spin currents in macroscopic graphene sheets by means of spin pumping from the precessing magnetization of an adjacent ferromagnet. The spin pumping characteristics are studied by means of ferromagnetic resonance (FMR) measurements in Permalloy/graphene (Py/Gr) bilayers. Changes in the FMR linewidth induced by the presence of graphene (when compared to studies with only Py films) correspond to an increase in the Gilbert damping in the ferromagnetic layer (proportional to the FMR linewidth) and interpreted as a consequence of spin pumping at the Py/Gr interface driven by the Py magnetization dynamics (i.e., magnetic induced by the microwave stimulus). FMR experiments are performed on different FM/Gr interfaces, completing a set of studies designed to systematically identify and eliminate damping enhancement arising from processes other than spin pumping. Remarkably, a substantial enhancement of the Gilbert damping observed in Py/Gr strips with graphene protruding a few micrometers from the strip sides is univocally associated to spin pumping at the quasi-onedimensional interface between the Py strip edges and graphene. This increase in the FMR linewidth compares with observations in other bilayer systems, in where thick (thicker than the spin diffusion length) layers of heavy metals with strong spin-orbit coupling are employed as the non-magnetic layer, indicating that spin relaxation in chemically grown graphene must be greatly enhanced in order to account for the losses of angular momentum lost by the ferromagnet. The fundamental implications of the results presented in this thesis point to a non-trivial nature of the spin pumping mechanism owing to the two-dimensionality of the non-magnetic layer (i.e., graphene). In addition, a spintronics device designed to interconvert charge and spin currents has been designed. A high-frequency microwave irradiation lock-in modulation technique is employed to detect the small electrical voltages generated by the inverse spin Hall effect (ISHE). As a proof of principle, a successful spin-charge interconversion in Py/Pt-based devices is experimentally demonstrated in this thesis. The challenges associated with the spin-charge interconversion in twodimensional devices are discussed and systematically addressed, and a potential device geometry for measuring the ISHE in Py/Gr-based systems is provided.

Research on spintronics has galvanized the design of new devices that exploit the electronic spin in order to augment the performance of current microelectronic technologies. The sucessful implementation of these devices is largely contingent on a quantitative understanding of nonequilibrium magnetism in conducting ferromagnets. This thesis is largely devoted to expanding the microscopic theory of magnetization relaxation and current-induced spin torques in transition metals ferromagnets as well as in (III,Mn)V dilute magnetic semiconductors. We start with two theoretical studies of the Gilbert damping in electric equilibrium, which treat disorder exactly and include atomic-scale spatial inhomogeneities of the exchange field. These studies enable us to critically review the accuracy of the conventional expressions used to evaluate the Gilbert damping in transition metals. We follow by generalizing the calculation of the Gilbert damping to current-carrying steady states. We find that the magnetization relaxation changes in presence of an electric current. We connect this change with the non-adiabatic spin transfer torque parameter, which is an elusive yet potentially important quantity of nonequilibrium magnetism. This connection culminates in a concise analytical expression that will lead to the first ab initio estimates of the non-adiabatic spin transfer torque in real materials. Subsequently we predict that in gyrotropic ferromagnets the magnetic anisotropy can be altered by a dc current. In these systems spin-orbit coupling, broken inversion symmetry and chirality conspire to yield current-induced spin torques even for uniform magnetic textures. We thus demonstrate that a transport current can switch the magnetization of strained (Ga,Mn)As. This thesis concludes with the transfer of some fundamental ideas from nonequilibrium magnetism into the realm of superconductors, which may be viewed as easy-plane ferromagnets in the particle-hole space. We emphasize on the analogies between nonequilibrium magnetism and superconductivity, which have thus far been studied as completely separate disciplines. Our approach foreshadows potentially new effects in superconductors. / text

L'amélioration des techniques de dépôts et l’évolution de la compréhension de la physique de la matière condensée a conduit à la découverte de phénomènes nouveaux en électronique de spin (spintronique). En particulier, le retournement de l’aimantation par couple de transfert de spin et couple spin-orbite, ainsi que le développement de dispositifs basés sur la propagation d’ondes de spin ont fait de l’amortissement magnétique de Gilbert un paramètre central pour les futures technologies de stockage et de traitement de l’information. Dans cette étude, la prédiction de valeurs très faibles d’amortissement dans les alliages d’Heusler demi métaux magnétiques Co2MnZ est expérimentalement observée et directement corrélée à la structure électronique sous-jacente. En effet, en substituant l’élément Z dans des couches minces monocristallines de haute qualité de Co2MnZ (Z= Al, Si, Ga, Ge, Sn, Sb) faites par épitaxie par jet moléculaire, les propriétés électroniques telles que le gap de spin minoritaire, la position du niveau de Fermi et la polarisation en spin peuvent être accordées et leurs conséquences sur la dynamique de l’aimantation sont analysées. Les résultats expérimentaux nous permettent de comprendre la relation existante entre la structure électronique mesurée et la valeur d’amortissement magnétique, ainsi que de les comparer aux calculs ab initio. Les valeurs d’amortissement entre 4.1 x10-4 et 9 x10-4 pour Co2MnSi, Co2MnGe, Co2MnSn et Co2MnSb sont les plus petites valeurs jamais reportées pour des couches conductrices et constituent une preuve expérimentale qui confirme les prédictions théoriques sur ces alliages d’Heusler demi métaux magnétiques. Ensuite, la relation entre l’amortissement magnétique de Gilbert et le temps de désaimantation ultra-rapide induit par pulse laser dans la série d’alliages quaternaires Co2MnSixAl1-x à polarisation en spin variable est étudiée. Cette partie vise à vérifier des modèles théoriques qui essaient d’unifier ces deux quantités vivant sur des échelles de temps différentes. Finalement, les propriétés structurales et magnétiques de super réseaux Mn3Ga/Co2YZ sont étudiées dans le but de combiner un amortissement de Gilbert très faible, un gap de spin minoritaire ainsi que l’aimantation perpendiculaire aux plans des couches, une caractéristique indispensable pour des dispositifs à faible consommation d’énergie. / Improvements in thin film elaboration methods and a deeper understanding of condensed matter physics have led to new exciting phenomena in spin electronics (spintronics). In particular, magnetization reversal by spin-orbit and spin-transfer torque as well as the development of spin waves based devices have placed the Gilbert magnetic damping coefficient as a key parameter for future data storage and information processing technologies. The prediction of ultralow magnetic damping in Co2MnZ Heusler half-metal magnets is explored in this study and the damping response is shown to be linked to the underlying electronic structure. By substitution of the Z element in high quality Co2MnZ (Z=Al, Si, Ga, Ge, Sn and Sb) epitaxial thin films grown by molecular beam epitaxy, electronic properties such as the minority-spin band gap, Fermi energy position in the band gap, and spin polarization can be tuned and the consequences for magnetization dynamics analyzed. Experimental results allow us to directly explore the interplay of spin polarization, spin gap and Fermi energy position, with the magnetic damping obtained in these films (together with predictions from ab initio calculations). The ultralow magnetic damping coefficients measured in the range from 4.1 x10-4 to 9 x10-4 for Co2MnSi, Co2MnGe, Co2MnSn and Co2MnSb are the lowest values ever reported in conductive layers and offer a clear experimental demonstration of theoretical predictions on half metal magnetic Heusler compounds. Then, the relation between the Gilbert damping and the ultrafast demagnetization time in quaternary Co2MnSixAl1-x compounds with a tunable spin polarization is analyzed. This way, it is possible to confront theoretical models unifying those two quantities that live in different timescales. Finally, structural and magnetic properties of Mn3Ga/Co2YZ Heusler superlattices are investigated in order to combine ultralow Gilbert damping coefficient, minority spin band gap and perpendicularly magnetized heterostructures, another requirement for low energy consumption devices. Through the present work, we aim to prove that Heusler compounds provide an excellent playground to study fundamental magnetism and offer a pathway for future materials design.

Ultrafast dynamical processes in magnetic systems have become the subject of intense research during the last two decades, initiated by the pioneering discovery of femtosecond laser-induced demagnetization in nickel. In this thesis, we develop theory for fast and ultrafast magnetization dynamics. In particular, we build relativistic theory to explain the magnetization dynamics observed at short timescales in pump-probe magneto-optical experiments and compute from first-principles the coherent laser-induced magnetization. In the developed relativistic theory, we start from the fundamental Dirac-Kohn-Sham equation that includes all relativistic effects related to spin and orbital magnetism as well as the magnetic exchange interaction and any external electromagnetic field. As it describes both particle and antiparticle, a separation between them is sought because we focus on low-energy excitations within the particle system. Doing so, we derive the extended Pauli Hamiltonian that captures all relativistic contributions in first order; the most significant one is the full spin-orbit interaction (gauge invariant and Hermitian). Noteworthy, we find that this relativistic framework explains a wide range of dynamical magnetic phenomena. To mention, (i) we show that the phenomenological Landau-Lifshitz-Gilbert equation of spin dynamics can be rigorously obtained from the Dirac-Kohn-Sham equation and we derive an exact expression for the tensorial Gilbert damping. (ii) We derive, from the gauge-invariant part of the spin-orbit interaction, the existence of a relativistic interaction that linearly couples the angular momentum of the electromagnetic field and the electron spin. We show this spin-photon interaction to provide the previously unknown origin of the angular magneto-electric coupling, to explain coherent ultrafast magnetism, and to lead to a new torque, the optical spin-orbit torque. (iii) We derive a definite description of magnetic inertia (spin nutation) in ultrafast magnetization dynamics and show that it is a higher-order spin-orbit effect. (iv) We develop a unified theory of magnetization dynamics that includes spin currents and show that the nonrelativistic spin currents naturally lead to the current-induced spin-transfer torques, whereas the relativistic spin currents lead to spin-orbit torques. (v) Using the relativistic framework together with ab initio magneto-optical calculations we show that relativistic laser-induced spin-flip transitions do not explain the measured large laser-induced demagnetization. Employing the ab initio relativistic framework, we calculate the amount of magnetization that can be imparted in a material by means of circularly polarized light – the so-called inverse Faraday effect. We show the existence of both spin and orbital induced magnetizations, which surprisingly reveal a different behavior. We establish that the laser-induced magnetization is antisymmetric in the light’s helicity for nonmagnets, antiferromagnets and paramagnets; however, it is only asymmetric for ferromagnets.

No description available.