Spintronique

Résumé

La spintronique, électronique de spin ou magnétoélectronique, est une technique qui exploite la propriété quantique du spin des électrons dans le but de stocker des informations. L’article Giant Magnetoresistance of (001)Fe/(001)Cr Magnetic Superlattices publié par Albert Fert et son équipe en 1988 est considéré comme l’acte de naissance de la spintronique. Introduction L'électronique classique repose sur une propriété essentielle d'une particule élémentaire (électron), sa charge électrique. La spintronique permet d'exploiter une propriété supplémentaire de l'électron, la propriété quantique de spin. Elle offre des possibilités d'applications nouvelles, en particulier la réalisation de capteurs de très grande sensibilité qui ont révolutionné le stockage magnétique de l'information. La spintronique a émergé dans les années 1980 après la mise en évidence de l’existence de courants polarisés en spin (suggérés par Mott) dans les métaux ferromagnétiques. Dans ces métaux comme le

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Atomic layer deposition of Ni and Ni80Fe20 for tubular spin-wave nanocavities

Maria Carmen Giordano

Magnetic thin films and magnetic nanostructures have become essential components of modern technological applications. Modern branches of magnetisms focus on spin-charge coupling (spintronics) and the collective excitation of spin waves in magnetically ordered materials (magnonics). In particular, spin waves represent a promising charge-free medium to encode and transmit information in the GHz frequency regime, relevant for microwaves technologies. The ever-increasing demand for compact and portable electronic and telecommunication devices leads to the need for further miniaturization and high integration density of their functional components. These aspects have prompted nanomagnetism to venture the third dimension. This development is also motivated by novel physical phenomena and functionalities predicted to arise from three-dimensional (3D) complex magnetic configurations. In order to advance the research in 3D spintronics and 3D magnonics, it will be essential to have control over the fabrication of 3D nanoarchitectures and to access the new properties of individual nanostructures emerging from new complex 3D spin-textures. These two challenges are addressed in this thesis. Ferromagnetic nanotubes (NTs) represent the ideal 3D system for the study of shape dependent static and dynamic magnetic properties. Their magnetic configurations, as well as the spin wave confinement and propagation inside them, can be engineered by changing their geometrical parameters. First, in order to fabricate 3D magnetic device architectures, we explored atomic layer deposition (ALD). The thickness and quality of the thin films deposited with ALD are irrespective of the geometry of the surface on which they are deposited. This characteristic makes it ideal to fabricate 3D nanomagnets by magnetically coating 3D nanotemplates, e.g. nanowires (NWs) in the case of NTs. In this thesis we propose ALD processes for the conformal coating by means of two ferromagnetic materials conventionally used in planar spintronics and magnonics systems: Ni and permalloy (Py, Ni80Fe20), the second material expected to have a lower spin wave damping. By identifying correlations between ALD process parameters and thin film properties we optimized the materials in terms of conformality, electrical resistivity, anisotropic magnetoresistance (AMR) effect, spin wave damping and, in the case of permalloy, also stoichiometry. The optimized materials were then transferred onto GaAs nanowires templates, obtaining ferromagnetic NTs with hexagonal cross sections. We achieved Ni NTs with the lowest resistivity ever reported in similar ALD-grown Ni systems and a large AMR effect. We observed the lowest spin waves damping for permalloy NTs, exhibithing a value comparable with permalloy thin films achieved with conventional planar deposition techniques. Second, we investigated magnetic states and spin waves confinement in nanotubes experimentally and via micromagnetic simulations. We combined magnetoresistance measurements with Brillouin light scattering spectroscopy (BLS) and micromagnetic simulations to study Ni and Py NTs. BLS measurements, performed while irradiating NTs with microwaves, show that these nanotubes form spin-wave nanocavities which impose discrete wave vectors and confine GHz microwave signals on the nanoscale. Our experimental results show that the confinement can be engineered by changing the NT's geometrical parameters and magnetic states.

EPFL2021

Chargement

Spin and Charge transport in Dirac Materials

Alexander Ronald Hoyer

The digitalization of our world is proceeding with every new technical product placed on the market. By making them smarter, e.g. linking them together and equipping them with sensors, a vast mass of data is collected. Processing and storing this information is particularly challenging as it should be done in real time and, ideally with a handheld device. This requires logic devices to be very fast, low on power consumption and rather small. The latter requirement is leading towards a transition from the classical physics domain to a domain where quantum effects like tunneling and interference start to dominate the behavior. Thus for miniaturization to continue, it is necessary to adapt new techniques that work in the quantum regime. One promising approach to achieve all of this is to exploit the spin degree of freedom an electron possesses, opening up the field of spin electronics (spintronics). As first principle operation of such devices has been demonstrated with silicon, new materials may possess attributes that better suite the requirements of spintronics devices. Graphene, a two dimensional carbon allotrope, is such a candidate as it shows low spin orbit coupling, which theoretical allows high spin life times to be achieved. Furthermore the 2D nature of graphene allows to control and tailor the electronic and spin properties by external means such as proximity and chemical functionalization. On this basis low temperature magnetotransport measurements are performed to explore the effect of functionalization of graphene with 4-nitrophenyl diazonium tetrafluoroborate within this thesis. Basic measurements in Hall bar geometry show an introduction of disorder due to the functionalization. Additional weak localization, a quantum effect that gives information on the phase coherence length of the electrons, is observed after functionalization and used to explore the electronic properties of the tailored graphene. To address the spin properties of graphene its spin bath needs to be controlled and manipulated. This is done within this thesis by all electrical means and requires the fabrication of spin valve structures on graphene. Two strategies to inject spin polarized currents into graphene are pursued within this framework. First spin valve measurements on functionalized graphene are promising regarding the manipulation of spin properties. The generation of the spin polarized currents is still a big issue. Traditionally, ferromagnets are used as leads to fulfill this task. Latest studies show that topological insulators (TI), a new class of materials, could be better suited to fulfill this task as they give rise to fully spin polarized currents. Conceptually these materials possess a band gap that is crossed by spin polarized bands. Crystals of nanometer size of Tin(II)telluride, a potential TI, are grown in a PVD process and electrically contacted to look for fingerprints of these unusual states. Charge and spin transport experiments show the presence of two types of charge carriers and a high intrinsic p-doping. Finally, a new 3^He system is set up within this thesis and a concept for low noise measurements is implemented that integrates a home build signal generation and amplification stage.

EPFL2016

Chargement

With the advent of spintronics, significant research effort is made to realize efficient injection, transport, manipulation, and detection of spin. The material platforms and device architectures suitable for this purpose 3D topological insulators (TIs) are a class of Dirac materials, which possess 2D spin-polarized helical surface states due to their strong SO coupling. This property offers research possibilities in several directions, specifically (i) to gain a deeper understanding of the electronic properties of massless Dirac fermions hosted by the surface states; (ii) to exploit the spin-momentum locking of the surface states towards efficient spin-charge conversion; and (iii) to investigate the interplay of the surface states with magnetic materials, superconductors, and 2D materials that could lead to the discovery of new surface and interface phenomena with exciting technological prospects. The main challenge in the field of 3D TIs is posed by the difficulty to selectively access the surface states in electrical transport experiments. This issue arises from the intrinsic bulk doping, which leads to a contribution of the bulk states to the conductivity. Furthermore, while they are prospective as efficient spin current generators, the efficiency of the electrical detection of spin currents is limited due to pronounced spin relaxation and dephasing. This motivates the realization of 3D TIs with improved properties and to engineer 3D TI-2D material heterostructures, whose interfaces would enable efficient spin-charge manipulation. The present thesis aims at the characterization of various Dirac materials by magnetotransport experiments at low temperatures, as well as the nanofabrication and investigation of spintronics devices based on 3D TIs. In the first experimental part, the electronic properties of four different 3D TIs are investigated - Sb2Te3 and Bi2Te2Se, which are grown in a vapor-solid process, ZrTe5, grown as single crystals in a Czochralski process, and finally the naturally occurring mineral Aleksite. In the second part, Bi2Te2Se is chosen for the fabrication of lateral spin valve devices, in which electrical detection of charge current-induced spin polarization due to the spin-momentum locking of the 2D TI surface states is demonstrated. Spin measurements are performed for devices with different coupling strength between the FM detector and the TI channel. Subsequently, the spin signal is investigated for the first time in the presence of a hBN tunnel barrier. An inversion of the spin signal is observed, which depends on the resistance of the hBN/FM/TI interface. The third experimental part addresses the spin generation properties of Bi2Te2Se in a van der Waals TI/graphene heterostructure, in which the TI acts as a spin injector, while the injected spin propagates within the graphene channel underneath and is detected non-locally by a ferromagnetic electrode on top of the graphene. This is a first-time demonstration of spin injection and detection in a heterostructure, which combines the best properties of two Dirac materials, determined by the presence of SO coupling in the TI and the lack thereof in graphene. These results are examples of emergent phenomena on 2D surfaces and interfaces, which could be further investigated in vertical and lateral heterostructures utilizing the spin Hall, Rashba, or Edelstein effect for efficient spin-charge conversion towards all-spin-based information technology.

EPFL2017

Chargement