Thèses sur le sujet « Spintronique THz »
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Kaushalya. « Ultrafast manipulation of magnetization using on-chip THz ». Electronic Thesis or Diss., Université de Lorraine, 2021. http://www.theses.fr/2021LORR0173.
Texte intégralThe need for memory storage devices has skyrocketed over the last few decades especially after the development of the internet. This need has reached enormous heights in the past two years, soon after the pandemic due to COVID-19. Hard disk drives (HDDs) are known to have the potential to meet up with the high-density data storage demands. This thesis deals with one of the major challenges faced within the spintronic community to improve the speed and the energy consumption of memory devices.The speed of operation during the writing of a magnetic bit depends on the magnetization switching mechanism employed. The switching mechanism is itself dependent on the intrinsic magnetic properties of the sample and the externally induced excitation that drives the reversal of the magnetic bit 1. In this thesis, we will focus on the use of spin-orbit torque (SOT) excitations to drive the reversal, which is a relatively new but fast and energy-efficient approach in comparison with other state-of-the-art methods.The typical speed of magnetization reversal using SOTs is in the range of few nanoseconds, far slower than the picosecond-long switching that is possible with charge-based memory devices2. In fact, a record reversal speed with electrical pulses as short as ~200ps was reported by Garello et. al., 3 in 2011 using SOTs. This thesis reports further efforts to speed up the magnetization reversal by almost 2 orders of magnitude by exploiting such SOTs. To this aim, THz electrical pulses were generated via the use Auston photoconductive switches. We demonstrate that a single 6ps wide electrical pulse can induce a SOT to a 1nm thin Co ferromagnetic layer and result in a full magnetization reversal. A systematic study to understand SOTs in the picosecond time regime is also undertaken via using different magnetic nanostructures.In magnetic memory devices, a “read-head” is used to read the stored information in the device. Typically, in spintronic devices, giant magnetoresistance (GMR) or tunnel magnetoresistance (TMR) based read heads are used for such operations. In this thesis, we also report on the attempts of developing a GMR sensor working in the THz regime.To undertake the aforementioned studies, a pump-probe optical and optoelectrical experimental setup has also been built and a detailed report of the same is also provided in the thesis
Hawecker, Jacques. « Terahertz time resolved spectroscopy of Intersubband Polaritons and Spintronic Emitters ». Electronic Thesis or Diss., Sorbonne université, 2021. http://www.theses.fr/2021SORUS101.
Texte intégralThe terahertz (THz) domain provides a rich playground for many practical and fundamental applications, where the low energy of THz photons permits to probe novel light-matter interactions. This work investigates two recent and emerging scientific areas where ultrafast THz spectroscopy can be used as a probe of fundamental phenomena, as well as potentially enabling the conception of new THz sources. In the first case, ultrafast THz spintronics are studied where ultrafast excitations of spintronic heterojunctions result in efficient pulse generation. These structures consist of nanometer thick ferromagnetic - heavy metal junctions, where an optically generated spin-charge in the former is converted to a charge-current in the latter via the Inverse Spin Hall Effect. Beyond these metal-based junctions, ultrafast THz spintronics based on “quantum” materials is also investigated, where THz pulses are generated using quantum phenomena such as the Inverse Edelstein Effect in Topological Insulators, shown to be a promising research direction. The second subject area is focused on THz intersubband polaritons, quasi-particles that emerge from the strong light-matter coupling of a THz photonic cavity and an intersubband transition. Here we are interested in the bosonic nature of the intersubband polaritons, as a long-term aim of realizing a novel THz laser based on Bose-Einstein condensation. In this work, we investigate resonant narrowband pumping of a polariton branch and probe using spectrally broad THz pulses. This shows strong indications of nonlinear effects and potential signatures of scattering processes that could eventually lead to the demonstration of THz polaritonic gain. Finally, to support our work in the above subject areas, technological developments were made in existing THz sources. This included high power THz photoconductive switches using cavities, which permitted the first demonstrations of real time THz imaging with such devices, and high power THz quantum cascade lasers as narrowband laser pumps
Cadiz, Fabian. « Spin dependent electron transport in semiconductors due to the Pauli principle ». Palaiseau, Ecole polytechnique, 2015. https://theses.hal.science/tel-01174645/document.
Texte intégralThis thesis is concerned with transport of photoinjected minority spin-polarized electrons in doped semiconductors, as a function of both the density and the temperature of the injected electron gas. In p-GaAs thin films, charge and spin transport is investigated theoretically and experimentally by using a novel polarized microphotoluminescence (µPL) technique which consists in imaging the spatially-resolved PL intensity and polarization under a tightly-focused circularly-polarized CW laser excitation. Study of the experimental profiles at low concentration and under an applied electric field shows that the minority electron mobility is mainly determined by the electron temperature instead of the majority hole statistics, introducing a puzzling piece to the current understanding of scattering processes in semiconductors. At higher densities, this experimental technique has allowed us to explore a novel charge-spin coupling mechanism which modifies electron transport. Under degeneracy of the electron gas (high concentration, low temperature), a dip at the centre of the spin polarization profile appears with a polarization maximum at a distance of about r= 2 µm from the excitation. This counterintuitive result reveals that photoelectron diffusion depends on spin, as a direct consequence of the Pauli principle which causes in general a concentration dependence of the spin stiffness. This results in a novel spin filter effect in an homogeneous material. The other effects which may modify spin transport in a degenerate electron gas are thermoelectric spin currrents (spin Soret currents) and ambipolar coupling with holes. A comparison of the data with a numerical solution of the coupled diffusion equations reveals that ambipolar diffusion increases the steady-state photo-electron density at the centre and therefore the amplitude of the degeneracy-induced spin-dependent diffusion, while the contribution of the spin Soret current is negligible. Coulomb spin drag and bandgap renormalization are negligible due to electrostatic screening by the hole gas. It is expected for degeneracy to have larger effects in confined systems, such as quantum wells, where both the spin stiffness and the mobility can have a much strong spin dependence
Endichi, Asmaa. « Thin films based on Gadolinium applied to the magnetic refrigeration ». Electronic Thesis or Diss., Université de Lorraine, 2020. http://www.theses.fr/2020LORR0150.
Texte intégralThe search for materials with a giant magnetocaloric effect in a massive state and at a temperature close to ambient temperature is of great interest and is mainly obtained by varying the composition of the materials. However, the first-order transition in these materials exhibits considerable thermal hysteresis, making them difficult to handle in applications for refrigerators operating cyclically. Much effort has been made in recent years to reduce this hysteresis, but the performance obtained with these massive materials does not meet the requirements of efficient magnetic refrigeration. Magnetocaloric materials have been largely unexplored on the nanoscale. However, nanostructuring is a well-known and used approach to disrupt the developed structure-property relationships, hence the interest in manufacturing new nanoscale materials. This will improve their magnetic and magnetocaloric characteristics by varying the size and shape. On the other hand, the magnetocaloric effect in magnetic thin layers is particularly interesting for micro-refrigeration. It is therefore important to study the magnetocaloric properties of materials in the form of thin layers in order to eliminate thermal hysteresis. In this sense, few studies have been done to show the potential of thin film materials for magnetic refrigeration and magnetic properties (saturation magnetization, variation of magnetic entropy and relative cooling ratio ...) measured so far limited remains. In this thesis project, we studied metallic gadolinium, which is the preferred choice as a magnetic refrigerant for most prototypes of active magnetic regenerator (AMR) in the form of a thin layer. The magnetocaloric (MCE) and electrocaloric (ECE) properties of the manufactured gadolinium films (Si / Ta / Gd (100 nm) / Pt (3nm)) are measured, in order to obtain more information on the physics behind the interesting electronic and magnetic properties of this material we demonstrate the magneto-caloric effect of the thin film Gd by measuring the electrical transport of the resistance. Thus, during this thesis, the electrical and especially magnetic behaviors of LaCr2Si2C and multiferroics TbMn2O5 are described using the ab-initio method, in order to broaden our understanding of the electronic, magnetic and therefore magnetocaloric characteristics of these compounds based on rare earth. The development of thin layers for magnetic refrigeration was carried out in the materials science research laboratory with the nanomagnetism and spin electronics team at the Jean Lamour Institute in Nancy and the theoretical calculations are made in the material laboratory condensed and interdisciplinary sciences at the Faculty of Sciences of Rabat
Duong, Quang ha. « Electrical control of the electron spin dynamics in [111]-oriented GaAs/AGaAs quantum wells ». Thesis, Toulouse, INSA, 2013. http://www.theses.fr/2013ISAT0006/document.
Texte intégralWe have studied the electron spin dynamics in <111>-oriented GaAs/AlGaAs quantum wells grown on <111>-substrate by time-resolved photoluminescence spectroscopy. By applying an external electric field about 50 kV/cm along growth direction, we observed the spectacular increase of electron spin which can attain values greater than 30 ns. This phenomenon comes from the electrical control of spin-orbit interaction in conduction band that make the Rashba term compensate exactly with the Dresselhaus term. The cancellation effect of these two terms results in the suppression of electron spin relaxation induced by D'yakonov-Perelmechanism which is dominant in undoped quantum wells and at the temperatures greater than 50K. The measurement under an external transverse magnetic field (Voigt configuration) demonstrates that the spin relaxation times in three spatial directions are also controlled simultaneously by electric field. The "total" control of electron spin relaxation can only be observed in <111>-oriented quantum wells. Finally, we also develop the model to interpret the experimental measurement of spin relaxation anisotropy depending on electric field in <111>-oriented quantum wells
Lesne, Edouard. « Non-Equilibrium Spin Accumulation Phenomena at the LaAlO3/SrTiO3(001) Quasi-Two-Dimensional Electron System ». Thesis, Paris 6, 2015. http://www.theses.fr/2015PA066417/document.
Texte intégralWe investigated the generation, manipulation, and detection of non-equilibrium spin accumulation in the nonmagnetic LaAlO3/SrTiO3 (LAO/STO) oxide heterostructure, which is the host of a quasi-two-dimensional electron system (q2DES). In electrical tunneling spin injection experiments, we made use of the (three-terminal) Hanle effect to probe the magnitude of spin accumulation at Co/LAO/STO interfaces. We report on large amplification effects of the spin signal, ascribed to spin-conserving sequential tunneling processes via localized electronic states of enhanced spin lifetimes. A substantial modulation of the spin signal, by electrostatic field-effect, evidences the successful generation of spin accumulation inside the q2DES. We further resorted to ferromagnetic resonance experiments in a cavity to adiabatically pump a spin current from a permalloy layer toward the LAO/STO interface. We find that the generated spin current is converted into a sizeable planar charge current within the q2DES. This is attributed to an inverse Edelstein effect deriving from a Rashba-like spin-orbit interaction, both of which are efficiently modulated by electrostatic field-effect. Hence, our findings expand the general field of interest from planar charge transport to the exploration of spin-dependent phenomena in a prototypical nonmagnetic conducting oxide channel. Additionally, we have also demonstrated that the critical thickness threshold for the onset of a q2DES at LAO/STO interfaces can be reduced to a single unit cell of LAO when resorting to various metal capping layers. It opens up a new field of investigation to tentatively identify the potential mechanisms driving the formation of the q2DES
Lacoste, Bertrand. « Mastering the influence of thermal fluctuations on the magnetization switching dynamics of spintronic devices ». Thesis, Grenoble, 2013. http://www.theses.fr/2013GRENY039/document.
Texte intégralSpin-transfer torque magnetic random-access memory (STTRAM) are very promising non-volatile and enduring memories to replace charged-based RAM. However, in conventional in-plane or out-of-plane STTRAM technologies, the switching time is limited to about 10~ns because the reversal process is stochastic i.e. it is triggered by thermal fluctuations. In order to render the reversal deterministic and faster, an approach consists in adding to the magnetic tunnel junction (MTJ) stack another spin-polarizing layer whose magnetization is orthogonal to that of the MTJ reference layer. We particularly investigated the case where a perpendicular polarizer is added to an in-plane magnetized tunnel junction. The STT from the perpendicular polarizer initiates the reversal, but it also creates oscillations of the resistance between its two extremal values. This behavior is usually interesting to realize STT nano-oscillators (STO). In this thesis, the dynamics of the system comprising an in-plane free layer, an in-plane reference layer and a perpendicular polarizer is studied both experimentally and theoretically (analytically and by simulations) in the framework of the macrospin approximation. For a single layer free layer oscillating due to the STT of the perpendicular polarizer, an accurate description of the oscillations is presented, in which the anisotropy field, the applied field and the in-plane STT are treated as perturbations. In the particular case of a synthetic ferrimagnetic (SyF) free layer, analytical expressions of the critical currents and of the oscillations equation of motion are computed and compared to simulations. These results are used to determine the phase diagram of the complete system. The in-plane anisotropy field is found to play a dramatic role, which is confirmed by experimental data from real-time measurements on MgO-based nano-pillars. It is shown that the cell aspect ratio can be used to tune the relative influence of the STT from the in-plane reference layer and from the out-of-plane polarizer. This allows achieving well controlled sub-nanosecond switching in STTRAM
Huang, Tianxun. « A study about the behavior and mechanism of all-optical switching ». Electronic Thesis or Diss., Université de Lorraine, 2023. http://www.theses.fr/2023LORR0054.
Texte intégralTo meet the future needs of high density, low power consumption, and fast rate of magnetic storage technology, it is one of the urgent tasks in the field of spintronics to develop a new method of magnetization manipulation with shorter magnetization reversal time and lower energy consumption. Ultrashort pulsed laser technology offers a new way to manipulate spins in femtosecond timescale, sparking great research interest in both academia and industry. Two methods of controlling magnetization by laser, all-optical helicity-dependent switching (AO-HDS) and all-optical helicity-independent switching (AO-HIS), are discovered recently and raise numerous discussion on their mechanisms, behaviors and applications. However, the origin of two phenomena is still largely debated, which will be the main task of this thesis. A Co/Pt multilayered stack exhibiting AO-HDS phenomenon is employed to study the mechanism of AO-HDS. The film is fabricated to a 10x10 um^2 magnetic square on a Hall bar and its switching behavior is observed optically and electrically at different timescale. The switching of this magnetic unit can be demonstrated with ten consecutive circularly polarized laser pulses. The spin dynamics of AO-HDS can be understood in terms of the magnetic domain thermal nucleation and domain wall propagation driven bythermal gradient. For the past years, AO-HIS has never been observed in other rare-earth transition-metal alloys except when the rare-earth is Gd. To study the speciality of Gd, a complete series of GdRCo (R represents Tb, Dy or Ho) alloys is grown and investigated, it is demonstrated that AO-HIS can be observed when the composition of R is as low as 1.5% near the compensation point of ferrimagnet. State diagrams describing the key parameters depending on the element concentrations and spin dynamics in various samples are studied, providing some suggestion on the origin of AO-HIS and its engineering application in the future
Verdierre, Gaétan. « Development of new material systems for the ferroelectric control of Rashba spin-orbit coupling and spin-charge interconversion ». Electronic Thesis or Diss., université Paris-Saclay, 2024. http://www.theses.fr/2024UPASP018.
Texte intégralWhile classical spintronics has traditionally relied on ferromagnetic metals as spin generators and spin detectors, spin-orbit effects now make it possible to efficiently generate and detect spin currents from charge currents in architectures free from ferromagnets.However, existing materials able to achieve this spin/charge interconversion do not possess the non-volatile character of ferromagnets required for non-volatile data storage and processing. In the work presented in this thesis, we tried to develop material systems in which it would be possible to combine the advantages of Rashba-type spin-orbit coupling (RSOC) for efficient interconversion between spin and charge currents with another family of ferroic systems, ferroelectrics. RSOC appears in systems with broken inversion symmetry, and is stronger in the presence of heavy elements. Ferroelectrics intrinsically break inversion symmetry and could thus harbour RSOC.As a bonus, switching the polarization of the ferroelectric with an electric field should switch the sign of the RSOC and thus the polarity with which the material would interconvert spin into charge via the Edelstein effect. In addition, ferroelectrics can accumulate or deplete very large carrier densities in adjacent materials and thus generate interfacial electric fields that would depends on the ferroelectric polarization direction. This would result in an interfacial RSOC that should also be tunable, allowing for a non-volatile electrical control of spin/charge interconversion.In the framework of this thesis, we investigated the growth and characterisation of two types of ferroelectric thin film heterostructures which should present switchable RSOC: (i) a single phase material, SrBiO₃, that we predicted to become ferroelectric with switchable RSOC at compressivestrain; (ii) an interface system combining the novel family of ferroelectrics, the wurtzites (Al,Sc)N, with BiSb, a topological insulator whose spin-charge interconversion properties could be modulated through the means of ferroelectricity
Talatchian, Philippe. « Bio-inspired computing leveraging the synchronization of magnetic nano-oscillators ». Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLS008/document.
Texte intégralSpin-torque nano-oscillators are non-linear, nano-scale, low power consumption, tunable magnetic microwave oscillators which are promising candidates for building large networks of coupled oscillators. Those can be used as building blocks for neuromorphic hardware which requires high-density networks of neuron-like complex processing units coupled by tunable connections. The neuromorphic approach allows to overcome the limitation of nowadays computers and to reduce their energy consumption. Indeed, in order to perform cognitive tasks as voice recognition or image recognition, the brain is much more efficient in terms of energy consumption. Due to the large number of required neurons (100 billions), a neuromorphic chip requires very small oscillators such as spin-torque nano-oscillators to emulate neurons. Recently a first demonstration of neuromorphic computing with a single spin-torque nano-oscillator was established, allowing spoken digit recognition with state of the art performance. However, to realize more complex cognitive tasks, it is still necessary to demonstrate a very important property of neural networks: learning an iterative process through which a neural network can be trained using an initial fraction of the inputs and then adjusting internal parameters to improve its recognition or classification performance. One difficulty is that training networks of coupled nano-oscillators requires tuning the coupling between them. Here, through the high frequency tunability of spin-torque nano-oscillators, we demonstrate experimentally the learning ability of coupled nano-oscillators to classify spoken vowels with a recognition rate of 88%. To realize this classification task, we took inspiration from the synchronization of rhythmic activity of biological neurons and we leveraged the synchronization of spin-torque nano-oscillators to external microwave stimuli. The high experimental recognition rates stem from the weak-coupling regime and the high tunability of spin-torque nano-oscillators. Finally, in order to realize more difficult cognitive tasks requiring large neural networks, we show numerically that arrays of hundreds of spin-torque nano-oscillators can be designed with the constraints of standard nano-fabrication techniques
Lesne, Edouard. « Non-Equilibrium Spin Accumulation Phenomena at the LaAlO3/SrTiO3(001) Quasi-Two-Dimensional Electron System ». Electronic Thesis or Diss., Paris 6, 2015. https://accesdistant.sorbonne-universite.fr/login?url=https://theses-intra.sorbonne-universite.fr/2015PA066417.pdf.
Texte intégralWe investigated the generation, manipulation, and detection of non-equilibrium spin accumulation in the nonmagnetic LaAlO3/SrTiO3 (LAO/STO) oxide heterostructure, which is the host of a quasi-two-dimensional electron system (q2DES). In electrical tunneling spin injection experiments, we made use of the (three-terminal) Hanle effect to probe the magnitude of spin accumulation at Co/LAO/STO interfaces. We report on large amplification effects of the spin signal, ascribed to spin-conserving sequential tunneling processes via localized electronic states of enhanced spin lifetimes. A substantial modulation of the spin signal, by electrostatic field-effect, evidences the successful generation of spin accumulation inside the q2DES. We further resorted to ferromagnetic resonance experiments in a cavity to adiabatically pump a spin current from a permalloy layer toward the LAO/STO interface. We find that the generated spin current is converted into a sizeable planar charge current within the q2DES. This is attributed to an inverse Edelstein effect deriving from a Rashba-like spin-orbit interaction, both of which are efficiently modulated by electrostatic field-effect. Hence, our findings expand the general field of interest from planar charge transport to the exploration of spin-dependent phenomena in a prototypical nonmagnetic conducting oxide channel. Additionally, we have also demonstrated that the critical thickness threshold for the onset of a q2DES at LAO/STO interfaces can be reduced to a single unit cell of LAO when resorting to various metal capping layers. It opens up a new field of investigation to tentatively identify the potential mechanisms driving the formation of the q2DES
Halisdemir, Ufuk. « Probing the impact of structural defects on spin dependent tunneling using photons ». Thesis, Strasbourg, 2016. http://www.theses.fr/2016STRAE018/document.
Texte intégralThe study of the impact of defects on the electrical properties of semiconductors played a crucial role in the revolution of information technologies in the middle of the 20th century. Up to now, the race to miniaturization allowed to meet the increasing demand in terms of processing power. However, this strategy is predicted to encounter physical limits impossible to overcome and new approaches are necessary. Within this new research paradigm, oxide based electronic devices are promising candidates to fabricate new multifunctional devices. The importance of defects on the nominal properties of oxides is not acknowledged as much as it is in the field of semiconductors. Our research project revolved around two primary objectives, the first one aimed to explicitly identify the impact of specific defects on the properties of oxide-based electronic devices. The second one aimed to actually take advantage of properties induced by defects for optoelectronic applications
Ferraro, Filippo Jacopo. « Magnetic anisotropies and exchange bias in ultrathin cobalt layers for the tunnel anisotropic magnetoresistance ». Thesis, Université Grenoble Alpes (ComUE), 2015. http://www.theses.fr/2015GREAY086/document.
Texte intégralIn the context of studying magnetic and spintronics phenomena occurring at the nanoscale, we investigated several aspects of Pt/Co/AlOx asymmetric structures. One of the objectives of this thesis was the control of the oxidation and the tailoring of the magnetic properties of these multilayers. We combined structural (X-Ray Reflectivity), transport (Anomalous Hall Effect) and magnetic measurements (VSM-SQUID), to study the interplay of magnetic and interfacial effects. One objective was to analyze the role that few monolayers (MLs) of CoO (which can form when overoxidizing the Al layer), could have on the properties of the stack. We used a wedge deposition techniques to control the oxidation on a subnanometer scale. We established that few MLs of CoO largely affect the total anisotropy of the stack. To further investigate the impact of the CoO, we engineered ultrathin Co(0.6nm)/CoO(0.6nm) bilayers. We performed field cooled measurements on this system and we found a large exchange bias anisotropy. These results indicate that the CoO keeps a large anisotropy even in the ML regime, help to rule out some of the models proposed to explain the exchange bias effect and imply that the usually neglected CoO presence must be considered in the energy balance of the system. We build perpendicular Tunneling Anisotropic MagnetoResistance (TAMR) devices based on the Pt/Co/AlOx structure. The TAMR is a relatively new spintronics effect in which the rotation of the magnetization in a single magnetic electrode (combined with the Spin-Orbit Coupling) can cause a change of the tunnel probability, which manifests as a magnetoresistance effect. We demonstrated that a careful control of the interface oxidation is crucial for the TAMR effect. The large induced magnetic anisotropy allowed us to achieve enhanced TAMR values compared to similar Pt/Co/AlOx structures
Denawi, Hassan. « Electronic and magnetic properties of polymer chains exploiting the reactivity of zwitterionic quinone with transition metal atoms ». Electronic Thesis or Diss., Aix-Marseille, 2019. http://theses.univ-amu.fr.lama.univ-amu.fr/191010_DENAWI_493r256nufumz934umq262qn_TH.pdf.
Texte intégralUsing the VASP code (Vienna Ab initio Simulation Package) several first principle calculations have been performed to study metal-organic monolayers as free-standing layers and on metallic substrates. The first principles calculations are based on spin-polarized density functional theory (DFT) with the spin polarized gradient approximation with Hubbard term U (SGGA+U) with explicit treatment of the strong electron correlation in the incompletely filled d-shell of the transition metal ions. We study polymers of transition metals (TM) with zwitterionic quinone (ZQ) molecules as one dimensional (1D) chains, two-dimensional (2D) arrangements, or adsorbed on metallic substrates. From the ab-initio calculations we predict the Fe-ZQ zwitterionic quinoidal polymer chains to be one-dimensional spin cross-over compounds. The calculations determine the atomic positions, the magnetic couplings and the electronic structure. We investigate the electronic and magnetic structure of a recently synthesized two-dimensional (2D) arrangement of polymer chains based on Fe atoms and zwitterionic quinone on different metallic substrates (Au(110), Ag(111), Cu(110) and Cu(111)). The adsorption of the Fe atoms and zwitterionic quinone on the surfaces was studied via SGGA+U and the free-standing isolated polymer chain, the 2D arrangement and the adsorbed polymers have been calculated. Furthermore, all the series of 3d TM-ZQ chains has been studied, as well as many 4d and 5d TM. Promising properties show also alternating chains of FeV-ZQ
Verlhac, Benjamin. « Atomic-scale spin-sensing with a single molecule at the apex of a scanning tunneling microscope ». Thesis, Strasbourg, 2019. http://www.theses.fr/2019STRAE007/document.
Texte intégralThe study presented in this manuscript is part of the field of surface magnetism, which has undergone major developments in recent years thanks to the scanning tunneling microscope (STM). It aims to show that a single molecule, nickelocene [Ni(C5H5)2], can be attached to the tip of a STM to produce a magnetic probe-tip, which, in the context of magnetic imaging, has undeniable advantages compared to conventional tips. Unlike other molecular systems studied with STM, we show that the magnetic properties of nickelocene in the gas phase are preserved in the presence of a metal, even when the molecule is attached to the tip of a STM. We present three remarkable results with this molecular probe-tip: 1) We show that we can control the spin of nickelocene, activating at will a Kondo effect; 2) We monitor the spin states of nickelocene by producing electrically-driven excitations, which we can easily identify through the molecular conductance. These states are sensitive to the magnetic environment surrounding nickelocene; 3) We use these states to probe surface magnetism. We show that by magnetically coupling the molecular probe tip with single atoms, either isolated or in a ferromagnetic surface, we can measure their spin polarization, as well as the nickelocene-atom exchange coupling. By monitoring this coupling it is possible to obtain a magnetic contrast in the STM images with atomic-scale resolution
She, Diana. « Molecular beam epitaxy growth of the BiSb/MnGa heterosrtuctures for the charge current to spin current conversion study ». Electronic Thesis or Diss., université Paris-Saclay, 2024. http://www.theses.fr/2024UPASP003.
Texte intégralRecently, topological insulators (TI) have attracted much attention with their promising prop-erties potentially useful for the emerging memory technologies, as magnetic random access memory (MRAM). Indeed, TI/ferromagnetic (FM) systems may drastically reduce the writing current using the spin-orbit torque (SOT) switching as a writing method. This advantage stems from the utilization of the spin-polarized topological surface states of the TIs. In this work, we were aiming to grow TI/FM bilayer heterostructures by molecular beam epitaxy (MBE), to perfrom comprehensive structural, magnetic and electronic characterization, and to study their charge-to-spin interconversion properties.We developped a high-quality Bi ₁₋ ₓSbₓ (TI)/ Mnₓ Ga₁₋ₓ (FM)//GaAs(001) bilayer heterostrucure by MBE, despite the difference in crystal symmetries. The FM Mnₓ Ga₁₋ₓ thin films exhibit perpendicular magnetic anisotropy, low coercive field, high Curie temperature, square hysteresis cycle, and sufficiently high resistivity. This ensured that a major portion of the electrical current flows through the Bi ₁₋ ₓSbₓ layer during the SOT experiments. The SOT measurements suggest that bulk states of Bi ₁₋ ₓSbₓ predominantly contributed to the transport properties. However, we demostrated the presence of topological surface states with ARPES. So the bulk and surface states may coexist. Additionally, the preliminary results of terahertz spectroscopy showed the efficient spin-to-charge conversion in the studied bilayer, showing promising potential. Consequently, our findings indicate that Bi ₁₋ ₓSbₓ holds significant promise for spintronic devices
Wang, Hangtian. « Interfacial Engineering of the Magnetism in 2D Magnets, Topological Insulators, and Their Heterostructures ». Electronic Thesis or Diss., Université de Lorraine, 2023. http://www.theses.fr/2023LORR0206.
Texte intégralWith the critical node of integrated circuits (IC) entering the 1 nm stage, traditional three-dimensional materials cannot maintain their original physical properties, and thus cannot meet the needs of IC manufacturing processes. Meanwhile, the shrinking line width also introduces an inevitable increase in static power consumption. Therefore, researching new materials and new technologies to break through the "Size Wall" and "Power Wall" has become a crucial direction in the IC industry. As a new member of the two-dimensional (2D) material family, the 2D magnets can maintain its long-range magnetic order at the atomic scale with its physical properties easily controlled by external stimuli, which provides an ideal platform for the high-density and low-power spintronic devices. However, due to the dimensional effect, 2D magnetism cannot exist at high temperatures. Although several methods can enhance the Curie temperature (Tc) of 2D magnets (such as doping, ion intercalation, or laser pumping), they are far from easy-controllability and high-efficiency. More importantly, the widely-used preparation method via mechanical exfoliation abandons the merit of 2D interfacial effect, which was proved to be an important approach to efficient 2D magnetic manipulation. Therefore, studying the interfacial effect in epitaxial 2D magnets is regarded as a key field to achieving large-scale, high-Tc, easy-controlling, and stable 2D ferromagnetic order. Topological insulator (TI) is another 2D material with strong spin-orbital coupling. The topology-protected surface states provided TI with numerous fascinates spin-related effects, such as spin-momentum locking, spin exchange effect, etc., which makes this material a potential candidate to fabricate effective spintronic devices. In addition, the TI can be integrated with 2D magnets to form a 2D heterostructure, in which not only the magnetism can be enhanced via the interfacial effect, but also the spin-related properties of the heterostructure can be manipulated due to the advantages of these two materials
Davesne, Vincent. « Organic spintronics : an investigation on spin-crossover complexes from isolated molecules to the device ». Phd thesis, Université de Strasbourg, 2013. http://tel.archives-ouvertes.fr/tel-01062266.
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