A roadmap for the design of four-terminal spin valves and the extraction of spin diffusion length

Graphene is a promising substrate for future spintronics devices owing to its remarkable electronic mobility and low spin-orbit coupling. Hanle precession in spin valve devices is commonly used to evaluate the spin diffusion and spin lifetime properties. In this work, we demonstrate that this method is no longer accurate when the distance between inner and outer electrodes is smaller than six times the spin diffusion length, leading to errors as large as 50% for the calculations of the spin figures of merit of graphene. We suggest simple but efficient approaches to circumvent this limitation by addressing a revised version of the Hanle fit function. Complementarily, we provide clear guidelines for the design of four-terminal spin valves able to yield flawless estimations of the spin lifetime and the spin diffusion coefficient.Comment: 7 pages, 5 figure

Microscale Metasurfaces for On-Chip Magnetic Flux Concentration

Magnetic metamaterials have demonstrated promising perspectives to improve the efficiency of magnetic flux concentrators. In this work, the effects of downscaling these devices for on-chip integration is investigated. The influence of the non-linear magnetic response of the ferromagnetic components, their magnetic irreversibility, the formation of magnetic domains, as well as the effects of geometry and size of the devices are scrutinized. The results demonstrate that the implementation of metasurfaces at the microscale opens up new technological possibilities for enhancing the performance of magnetic field detectors and remotely charging small electric devices, thus paving the way toward new approaches in information and communication technologies.</p

Tunable domino effect of thermomagnetic instabilities in superconducting films with multiply-connected topological structures

peer reviewedAbstract Topology is a crucial ingredient for understanding the physical properties of superconductors. Magnetic field crowds to adopt the form of topologically-protected quantum flux lines which can lose this property when moving at high velocities. These extreme conditions can be realized when superconductors undergo a thermomagnetic instability for which the sample topology come also into play. In this work, utilizing the magneto-optical imaging technique, we experimentally study magnetic flux avalanches in superconducting films with multiply-connected geometries, including single and double rings. We observe a domino effect in which avalanches triggered at the outer ring, stimulate avalanches at the inner ring thus impairing the expected magnetic shielding resulting from the outer ring and gap. We implement numerical simulations in order to gain more insight into the underlying physical mechanism and demonstrate that such event is not caused by the heat conduction, but mainly attributed to the local current distribution variation near the preceding flux avalanche in the outer ring, which in turn has a ripple effect on the local magnetic field profile in the gap. Furthermore, we find that the domino effect of thermomagnetic instabilities can be switched on/off by the environmental temperature and the gap width between the concentric rings. These findings provide new insights on the thermomagnetic instability in superconducting devices with complex topological structures, such as the superconductor–insulator–superconductor multilayer structures of superconducting radio-frequency cavities

Tunable Perpendicular Magnetoresistive Sensor Driven by Shape and Substrate Induced Magnetic Anisotropy

Control of magnetization reversal processes is a key issue for the implementation of magnetic materials in technological applications. The modulation of shape magnetic anisotropy in nanowire structures with a high aspect ratio is an efficient way to tune sharp in-plane magnetic switching. However, control of fast magnetization reversal processes induced by perpendicular magnetic fields is much more challenging. Here, tunable sharp magnetoresistance changes, triggered by out-of-plane magnetic fields, are demonstrated in thin permalloy strips grown on LaAlO3 single crystal substrates. Micromagnetic simulations are used to evaluate the resistance changes of the strips at different applied field values and directions and correlate them with the magnetic domain distribution. The experimentally observed sharp magnetic switching, tailored by the shape anisotropy of the strips, is properly accounted for by numerical simulations when considering a substrate-induced uniaxial magnetic anisotropy. These results are promising for the design of magnetic sensors and other advanced magnetoresistive devices working with perpendicular magnetic fields by using simple structures.The authors acknowledge financial support from Spanish Ministry of Science and Innovation MCIN/ AEI /10.13039/501100011033/ through the “Severo Ochoa” Programme for Centres of Excellence in CEX2019-000917-S, HTSUPERFUN PID2021-124680OB-I00 funded by MCIN/AEI/10.13039/501100011033 and FEDER Una manera de hacer Europa, the Catalan Government with Grant 2017-SGR-1519, the EU COST action SUPERQUMAP CA21144, Fonds de la Recherche Scientifique - FNRS under the grants PDR T.0204.21 and CDR J.0176.22, and EraNet-CHIST-ERA R.8003.21, PCI2021-122028-2A and PCI2021-122083- 2A financed by MCIN/AEI/10.13039/501100011033 adn Unión Europea NextGenerationEU/PRTR. The authors also acknowledge the Scientific Services at ICMAB and the UAB PhD program in Materials Science. The authors thank J. Jazquez for fruitful discussions.With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe

Static and dynamic properties of selected micromagnetic devices

Magnetic properties of matter are essential for a wide range of current and future technologies, especially in the domain of microelectronics for which spintronics is largely involved in the next generations of devices. This dissertation, composed of two distinct parts, presents an original research work on microscopic magnetic devices allowing the control and detection of static magnetic fields (part I), and the transmission and sensing of pure spin currents (part II). Firstly, the development of metasurfaces composed of a concentric arrangement of micrometer-wide ferromagnetic petals and allowing the magnetic flux concentration is investigated. Micromagnetic simulations demonstrate the importance of the magnetic domains configuration on the linear response of the device. In the operating regime of the device, a concentrated magnetic field around two times the external field is predicted irrespective of the in-plane applied field direction. The experimental proof-of-concept is demonstrated with 60 nm-thick permalloy structures. The concentration gain is obtained by optically tracking the magnetic vortex at the center of a permalloy disk sensor, using Kerr microscopy. The second study presented in this thesis focuses on the sharp magnetoresistance changes, triggered by out-of-plane magnetic fields, probed in thin permalloy strips grown on monocrystalline lanthanum aluminate substrates. Micromagnetic simulations are used to evaluate the resistance changes of the strips at different applied field values and directions and correlate them with the magnetic domain distribution. The experimentally observed sharp magnetic switching, tailored by the shape anisotropy of the strips, is properly accounted for by the numerical simulations when considering an important substrate-induced uniaxial magnetic anisotropy with a main direction sligthly tilted from the out-of-plane direction. The second part of this thesis is devoted to non-local spin-valves made of ferromagnetic tunnel junctions and implemented for electron spin injection, transport and detection of pure spin currents. We first demonstrate that the non-linear electrical transport occurring in tunnel junctions may lead to a spin-to-charge conversion efficiency larger than 10 times the spin polarization of the tunnel barrier when the latter is under a bias voltage of a few millivolts. The underlying mechanisms are attributed to the tunnel-barrier deformation and the conduction-band shift resulting from a change of the applied voltage. An approximated analytical expression predicting the detector spin sensitivity is suggested. Calculations performed for different barrier shapes show that this enhancement is present in oxide barriers as well as in Schottky-tunnel barriers, and that it depends on the intensity of the spin accumulation generated in the channel. Moreover, although reduced at high temperatures, the spin signal remains superior to the value predicted by the linear model. Finally, we demonstrate that the Hanle precession method as conventionally applied is no longer accurate when the distance between the inner and outer electrodes becomes smaller than 6 times the spin diffusion length, leading to errors as large as 50% for the calculation of the spin figures of merit. We suggest simple but efficient approaches to circumvent this limitation by addressing a revised version of the Hanle fit function and by proposing a refined fabrication process for four-terminal non-local spin valves

Magnetic observation of micro and nanostructures

Comparaison of magnetic force macroscopy and magneto-optical imaging. These two techniques available in in liège allow for observation of magnetic field on scales ranging from few nanometer to several millimeters

Injection d'un courant polarisé en spin dans un dispositif en Germanium : étude théorique des apsects géométriques

We present a two-dimensional (2D) model of spin injection/extraction from a ferromagnetic (FM) germanide Mn5Ge3 into Ge in a geometry similar to real devices. Our model addresses the challenges of describing current line distributions, barrier interface nonlinearity (thermionic emission and tunnel effect), the formation of a depletion region and the influence of the detection method (spin valves, Hanle precession). Results show the impact of confinement effects due to a scaling-down of injector electrode width, channel thickness and channel length. We highlight the importance of current crowding and depletion area on the spin injection ratio and spin lifetime and we determine the existence of an optimal current density for efficient spin-injection. Finally, we demonstrate that the spin diffusion length is impacted by current lines distribution and intensity, limiting the coherence of spin transport.Quantum magnetic tweezers for manipulating charge and spi

Origin of the Giant Spin-Detection Efficiency in Tunnel-Barrier-Based Electrical Spin Detectors

Efficient conversion of a spin signal into an electric voltage in mainstream semiconductors is one of the grand challenges of spintronics. This process is commonly achieved via a ferromagnetic tunnel barrier, where nonlinear electric transport occurs. In this work, we demonstrate that nonlinearity may lead to a spin-to-charge conversion efficiency larger than 10 times the spin polarization of the tunnel barrier when the latter is under a bias of a few millivolts. We identify the underlying mechanisms responsible for this remarkably efficient spin detection as the tunnel-barrier deformation and the conduction-band shift resulting from a change of applied voltage. In addition, we derive an approximate analytical expression for the detector spin sensitivity P_det(V). Calculations performed for different barrier shapes show that this enhancement is present in oxide barriers as well as in Schottky-tunnel barriers, even if the dominant mechanisms differ with the barrier type. Moreover, although the spin signal is reduced at high temperatures, it remains superior to the value predicted by the linear model. Our findings shed light onto the interpretation and understanding of electrical spin-detection experiments and open paths to optimizing the performance of spin-transport devices

Origine de la non-linéarité du l'efficacité de la détection de spin dans les junctions tunnel sous tension

Efficient conversion of a spin signal into a electrical voltage in mainstream semiconductors is one of the main challenge of spintronics. It is commonly achieved via a ferromagnetic tunnel barrier where non-linear transport occurs. In this work, we demonstrate that non-linearity may lead to a spin-to-charge conversion efficiency larger than 10 times the spin polarization of the tunnel barrier when the latter is under bias of a few mV. We determine that the underlying mechanisms responsible for this huge spin detection efficiency are the tunnel barrier deformation and conduction band shift resulting from a change of applied voltage. Calculations are achieved for different barrier shape showing that this enhancement is present in oxide barrier as well as in Schottky tunnel barrier. Moreover, we show that the spin signal is reduced at higher temperature but remains superior to the value predicted by the linear model. We assume that our study of the mechanisms governing the non-linear spin detection will improve the interpretation and understanding of electrical spin detection experiment and bring new possibility to optimize spin transport devices.La détection d'un signal de spin via sa conversion en un signal électrique est l'un des trois défis majeurs de la spintronique appliquée aux semiconducteurs. Cette conversion est généralement réalisée à l'aide d'une barrier tunnel ferromagnétique où un transport non-linéaire des porteurs a lieu. Dans ce travail, il est démontré que le transport non-linéaire perturbe la conversion du signal de spin en signal de charge, menant à une augmentation de l'efficacité de la détection supérieure à 10 fois la polarisation de conductance de spin de la barrière tunnel, lorsque une tension de quelques mV est appliquée à cette dernière. Nous avons déterminé que l'importante augmentation de l'efficacité de la détection est due à la déformation de la barrière tunnel ainsi que la déplacement de la bande de conduction du semiconducteur lorsqu'un potentiel est appliqué aux bornes de la jonction. Des simulations ont été réalisées pour différentes formes de barrières tunnel, montrant que l'augmentation de l'efficacité de détection est aussi bien présente dans des barrière tunnel formée par un isolant que dans des barrière Schottky. De plus, nous démontrons que la conversion du signal de spin en une différence de potentiel se réduit avec la température mais reste cependant toujours supérieure à la valeur prédite par le modèle linéaire

Remote teaching experience : the case study of a course in experimental physics with practical laboratory sessions

In this talk, we will describe how a third-year bachelor course in experimental physics with practical laboratory sessions was transformed from a traditional face-to-face experience to a fully remote teaching mode. The objective is to provide an assessment of the key aspects that ensured the pedagogical continuity as well as the level of engagement of the students. We will also highlight a few details about the platform that was extensively used for the transition, especially for carrying out the final exam of the course