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This dissertation presents our theoretical study of fundamental topological properties of ferromagnetic and antiferromagnetic systems, including topological magnetic excitations and topological magnetic textures. In the first part, we explored the topological magnonic phases in various systems with Dzyaloshinskii-Moriya interaction using a linear spin-wave theory. We have calculated the magnonic Chern number, topological phase diagram, and magnon thermal Hall conductivity at low temperature with tunable interactions due to the lattice deformation. In particular, we have investigated the topological phase transitions between distinct topological magnonic phases characterized by magnonic Chern numbers. We have also studied the magnon band topology and magnonic edge states in each topological phase. We found a sign reversal of the thermal Hall conductivity during topological phase transitions. We explicitly demonstrated the correspondence of thermal Hall conductivity with the topological edge states and their propagation directions. In the second part, a magnonic metamaterial in the presence of spatially modulated Dzyaloshinskii-Moriya interaction was theoretically proposed and demonstrated by micromagnetic simulations. By analogy to the fields of photonics, we first established magnonic Snell’s law for spin waves passing through an interface between two media with different dispersion relations due to different Dzyaloshinskii-Moriya interactions. Based on magnonic Snell’s law, we found that spin waves can experience total internal reflection. The critical angle of total internal reflection was strongly dependent on the sign and strength of Dzyaloshinskii-Moriya interaction. Furthermore, spin-wave beam fiber and spin-wave lens were designed by utilizing the artificial magnonic metamaterials with inhomogeneous Dzyaloshinskii-Moriya interactions. In the last part, we studied the impact of spin Hall torque, spin transfer torque, and topological torque on the velocity-current relation of antiferromagnetic skyrmions with the aim of reducing the deformation. Using a combination of micromagnetic simulations and analytical derivations, we demonstrated that the lateral expansion of the antiferromagnetic skyrmion is reminiscent of the well-known Lorentz contraction identified in one-dimensional antiferromagnetic domain walls. We also showed that in the flow regime the lateral expansion is accompanied by a progressive saturation of the skyrmion velocity when driven by spin Hall and topological torques. This saturation occurs at much smaller velocities when driven by the topological torque, while the lateral expansion is reduced, preventing the skyrmion size from diverging at large current densities. Our findings suggested that a compromise must be made between skyrmion velocity and lateral expansion during the device design. In this respect, exploiting the topological torque could lead to better control of the skyrmion velocity in antiferromagnetic racetracks.

This work presents the development of a GaN-based dilute magnetic semiconductor (DMS) by metal organic chemical vapor deposition (MOCVD) that is ferromagnetic at room temperature (RT), electrically conductive, and possesses magnetic properties that can be tuned by n- and p-doping. The transition metal series (TM: Cr, Mn, and Fe) along with the rare earth (RE) element, Gd, was investigated in this work as the magnetic ion source for the DMS. Single- phase and strain-free GaTMN films were obtained. Optical measurements revealed that Mn is a deep acceptor in GaN, while Hall measurements showed that these GaTMN films were semi-insulating, making carrier mediated exchange unlikely. Hysteresis curves were obtained for all the GaTMN films, and by analyzing the effect of n- and p-dopants on the magnetic properties of these films it was determined that the magnetization is due to magnetic clusters. These findings are supported by the investigation of the effect of TM dopants in GaN nanostructures which reveal that TMs enhance nucleation resulting in superparamagnetic nanostructures. Additionally, this work presents the first report on the development of GaGdN by MOCVD providing an alternate route to developing a RT DMS. Room temperature magnetization results revealed that the magnetization strength increases with Gd concentration and can be enhanced by n- and p-doping, with holes being more efficient at stabilizing the ferromagnetic signal. The GaGdN films obtained in this work are single-phase, unstrained, and conductive making them suitable for the development of multifunctional devices that integrate electrical, optical, and magnetic properties.

Le développement des dispositifs issus de l'électronique de spin nécessite de nouveaux matériaux qui permettent d'injecter de manière efficace le courant polarisé en spin dans des semiconducteurs. Parmi de nombreux matériaux utilisés comme injecteurs de spin, les semiconducteurs ferromagnétiques dilués (DMS), obtenus en dopant des semiconducteurs avec des impuretés magnétiques tels que Mn ou Co, sont considérés comme des candidats potentiels pour l'injection de spin. Ces matériaux dopés deviennent ferromagnétiques tout en conservant leurs propriétés semiconductrices. Par conséquent, ils présentent une similarité d'impédance électrique par rapport aux substrats semiconducteurs, ce qui rend efficace l'injection de courant polarisé en spin dans ces derniers. Dans ce contexte, l'objectif principal de cette thèse consiste à étudier la cinétique de croissance des semiconducteurs ferromagnétiques dilués GeMn. Nous cherchons à déterminer les paramètres clés de la croissance des couches de GeMn, à savoir la température du substrat, et la concentration en Mn. Pour la fabrication de dispositifs électroniques fonctionnels, le challenge crucial est d'obtenir des DMS ayant une température de Curie (TC) bien supérieure à la température ambiante. Nous nous sommes donc concentrés sur la cinétique de formation de la phase nanocolonnaire GeMn possédant une TC au-delà de 400 K. / The development of active spintronic devices requires new materials, which enable to efficiently inject spin-polarized currents into non-magnetic semiconductors. Among numerous materials that can be used as spin injectors, diluted magnetic semiconductors (DMS), obtained by doping standard semiconductors with magnetic impurities, such as Mn or Co, have emerged as potential candidates for spin injection. The materials become ferromagnetic while conserving their semiconducting properties. They exhibit therefore natural impedance match to host semiconductors and are expected to efficiently inject spin-polarized currents into semiconductors. In this context, the main objectives of this thesis work consist in studying the growth kinetics of GeMn-based diluted magnetic semiconductors. We aim at determining the main growth parameters, such as the substrate temperature and the Mn concentration, that govern the growth process of GeMn layers. Since for device applications it is crucial to obtain DMS exhibiting a Curie temperature (TC) well above room temperature, we have focused our attention to the kinetic formation of the GeMn nanocolumn phase, which exhibits a Curie temperature higher than 400 K.

Neste trabalho apresentamos a determinação autoconsistente da constante de interação spin-órbita em heteroestruturas com duas sub-bandas. Como recentemente proposto, ao obter o hamiltoneano de um sistema com duas sub-bandas na aproximação de massa efetiva, constata-se a presença de um acoplamento inter-subbanda que não se anula mesmo em heteroestruturas simétricas. Apresentamos aqui as deduções teóricas que levaram à proposição desse novo acoplamento e mostramos o cálculo autoconsistente da intensidade do acoplamento e a comparamos com a intensidade do acoplamento Rashba, já amplamente estudado. Discutimos o método k.p e a Aproximação da Função Envelope e mostramos a obtenção do modelo de Kane 8x8 para semicondutores com estrutura zincblende. Aplicamos o método do \"folding down\'\' ao hamiltoneano de Kane isolando o setor correspondente à banda de condução. Escrevemos dessa forma um hamiltoneano efetivo para a banda de condução no contexto de um poço quântico com uma barreira. Através da projeção desse hamiltoneano nos dois primeiros estados da parte orbital verifica-se o surgimento de um acoplamento inter-subbanda. Finalmente escrevemos o hamiltoneano efetivo 4x4 que descreve as duas primeiras subbandas de um poço quântico e obtivemos seus autoestados e autoenergias. Finalmente fizemos o cálculo autoconsistente das funções de onda e energias de um gás de elétrons em poços quânticos simples e duplos através da aproximação de Hartree e a partir desses resultados determinamos o valor da constante de acoplamento Rashba e da nova constante inter-subbanda. Entre os resultados obtidos destacam-se o controle elétrico da constante de acoplamento inter-subbanda através de um eletrodo externo e um efeito de renormalização da massa efetiva que pode chegar até 5% em algumas estruturas. / In this work we present the self-consistent determination of the spin-orbit coupling constant in heterostructure with two subbands.As recently proposed, the effective hamiltonian for the conduction band in the effective mass approximation contains an inter-subband spin-orbit coupling which is non-zero even for symmetric heterostructures. We present the theoretical derivation which leads to this proposal and show a selfconsistent determination of the coupling constant. We also compare the magnitude of the new coupling constant with the usual Rashba coupling. Starting with a discussion of the k.p method and the Envelope Function Approximation (EFA) we show the derivation of the 8x8 Kane model for semiconductors with zincblende structure. We then apply the \"folding down\'\' method, isolating the conduction band sector of the EFA hamiltonian. By projecting this hamiltonian in the first two states of the orbital part, we find an effective 4x4 hamiltonian that contains an inter-subband spin orbit coupling. The eingenvalues and eigenvectors of this hamiltonian are shown and, specializing the model for single and double quantum wells, we self-consistently determine the inter-subband and Rashba coupling constants in the Hartree approximation. The results indicate the possibility of electrical control of the coupling constant and show an effective mass renormalization effect that can be up to 5% in some cases.

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.