Magnetic control of flexible thermoelectric devices based on macroscopic 3D interconnected nanowire networks

Spin-related effects in thermoelectricity can be used to design more efficient refrigerators and offer novel promising applications for the harvesting of thermal energy. The key challenge is to design structural and compositional magnetic material systems with sufficiently high efficiency and power output for transforming thermal energy into electric energy and vice versa. Here, the fabrication of large-area 3D interconnected Co/Cu nanowire networks is demonstrated, thereby enabling the controlled Peltier cooling of macroscopic electronic components with an external magnetic field. The flexible, macroscopic devices overcome inherent limitations of nanoscale magnetic structures due to insufficient power generation capability that limits the heat management applications. From properly designed experiments, large spin-dependent Seebeck and Peltier coefficients of $-9.4$ $\mu$V/K and $-2.8$ mV at room temperature, respectively. The resulting power factor of Co/Cu nanowire networks at room temperature ($\sim7.5$ mW/K$^2$m) is larger than those of state of the art thermoelectric materials, such as BiTe alloys and the magneto-power factor ratio reaches about 100\% over a wide temperature range. Validation of magnetic control of heat flow achieved by taking advantage of the spin-dependent thermoelectric properties of flexible macroscopic nanowire networks lay the groundwork to design shapeable thermoelectric coolers exploiting the spin degree of freedom.Comment: 11 pages, 7 figure

Neuromorphic weighted sum with magnetic skyrmions

Integrating magnetic skyrmion properties into neuromorphic computing promises advancements in hardware efficiency and computational power. However, a scalable implementation of the weighted sum of neuron signals, a core operation in neural networks, has yet to be demonstrated. In this study, we exploit the non-volatile and particle-like characteristics of magnetic skyrmions, akin to synaptic vesicles and neurotransmitters, to perform this weighted sum operation in a compact, biologically-inspired manner. To this aim, skyrmions are electrically generated in numbers proportional to the input with an efficiency given by a non-volatile weight. These chiral particles are then directed using localized current injections to a location where their presence is quantified through non-perturbative electrical measurements. Our experimental demonstration, currently with two inputs, can be scaled to accommodate multiple inputs and outputs using a crossbar array design, potentially nearing the energy efficiency observed in biological systems.Comment: 12 pages, 5 figure

Magnetic and magneto-transport properties of 3D networks of interconnected magnetic nanowires

Ce mémoire de Master porte sur la synthèse d’échantillons nanostructurés originaux et la caractérisation de leurs propriétés magnétiques. Il s’agit de réseaux tridimensionnels de nanofils magnétiques interconnectés. La fabrication consiste en une électrodéposition assistée par un template en polymère présentant des nanopores croisées. L'électrodéposition est faite à partir d'une cathode métallique déposée par pulvérisation sur une face du template. C'est réseaux de nanofils présentent comme avantage d’être stables mécaniquement et autosupportés après dissolution de la membrane, par rapport aux réseaux de nanofils parallèles étudiés précédemment à l'Université Catholique de Louvain. La connectivité électrique offre également la possibilité de réaliser aisément des mesures de magnéto-transport en retirant localement la cathode métallique. Les échantillons étudiés sont des réseaux de nanofils interconnectés de fer (Fe), nickel (Ni), cobalt (Co), permalloy (NiFe) et d'alliage nickel-cobalt (NiCo), ainsi que des multicouches de cobalt/cuivre (Co/Cu). La relation entre les propriétés structurales ou la composition des nanofils et les propriétés magnétiques et de magnéto-transport a été étudiée. La présence de parois de domaines au interconnections des réseaux est mise en avant par des mesures de magnéto-transport. Les mesures de magnéto-transport ont été faites à différentes températures comprises entre 10 Kelvin et la température ambiante, ce qui souligne la dépendance en température du comportement magnétique et de magnéto-transport des réseaux étudiés. Les échantillons ont été séparés en quatre groupes, En premier, les nanofils présentant des propriétés magnétiques d’origine uniquement magnétostatiques. Cela inclut les nanofils de Ni, Fe et NiFe. En second, les nanofils de Co, présentant des propriétés magnétiques influencées par leurs propriétés structurales. Celles-ci sont contrôlées en faisant varier le pH de la solution électrolytique. Troisièmement, l’effet de la fraction atomique de nickel dans les nanofils de NiCo a été étudié. Un modèle mathématique est présenté pour extraire la valeur de la magnétorésistance anisotropique des mesures de magnéto-transport de ces échantillons complexes. Pour finir, des multicouches de Co/Cu qui présentent une magnétorésistance géante de type CPP ont été réalisés et caractérisés. Les résultats obtenus sont cohérents avec ceux obtenus pour les même matériaux bulk, en films fins ou en réseaux de nanofils parallèles, ce qui confirme l’intérêt de telle nano-architectures et ouvre la porte à de futures recherches.Master [120] : ingénieur civil en chimie et science des matériaux, Université catholique de Louvain, 201

3D interconnected magnetic nanowire networks

Track-etched polymer membranes with crossed nanochannels have been revealed suitable as templates for the electrodeposition of interconnected magnetic nanofiber networks with controlled morphology, material composition and nano-architecture. Three-dimensional networks of nanowires, core-shell nanocables, nanotubes and multilayered nanowires have been successfully fabricated. The interconnected structure provides the mechanical stability and electrical connectivity to the self-supported three-dimensional nano-architectures, while the polycarbonate template provides flexibility to the system. In addition, the local removing of the cathode enables a two-probe design suitable for electric and thermoelectric measurements, with the electric current flow restricted along the nanofiber segments. The tunability of the magnetic, magneto-transport and thermoelectric properties of various three-dimensional interconnected magnetic nanofiber networks has been demonstrated. The unique architecture of the three-dimensional nanofiber network system has been found suitable for a wide range of applications such as three-dimensional magnetic sensoring, magnetic devices with controlled anisotropy and microwave absorption properties, light and planar flexible thermoelectric modules for active cooling of hotspots and spin-caloritronic devices.(FSA - Sciences de l'ingénieur) -- UCL, 202

Skyrmion-based compact neuromorphic computing devices

Electrical control of magnetic skyrmions, which are topologically stable particle-like spin textures, holds promises for emerging memory and computing applications such as racetrack memory, logic devices and neuromorphic computing. Among the main advantages of magnetic skyrmions are their stability and non-volatility at room temperature, the low energy requirement for the motion, their sub-microscopic size and particle-like behavior. On the other hand, to perform neuromorphic computing with electrical devices, an architecture integrating both artificial neurons and synapses is required. The neurons operate a weighted sum and a non-linear activation function and the synapses store the synaptic weight with non-volatile tunability. To date, all existing proposal of electrical neuromorphic computing devices use artificial synapses and neurons which are physically separated. Therefore, macroscopic connections between synapses and neurons are required, which limits the miniaturization. In this study, we propose to use fully electrical control of skyrmions in compact devices made of magnetic multilayers to implement locally the main neuromorphic computing operations. The devices are based on the electrical control of a number of magnetic skyrmions injected into a detection zone as the weighted sum of the current inputs and the synaptic weights, and the non-linear electrical detection of their number. Using electrical current pulses with controlled current density and time duration, we show how it is possible to nucleate at room temperature a desired number of magnetic skyrmions and move them at a desired velocity within micron-wide tracks. We show that skyrmions moving within a Hall cross can be detected in real-time electrically by the Anomalous Hall effect, while these skyrmions can also be tracked by magneto-optic Kerr effect (MOKE) microscopy. Moreover, we also investigate how the tuning of the magnetic anisotropy and Dzyaloshinskii-Moriya interaction of the magnetic multilayer stack using electrical field manipulation with ionic liquid gating can be used to induce non-volatile modification of the nucleation and motion of the magnetic skyrmions in the devices. The long-term objective is to use all these basic knobs in order to perform neuromorphic functions using the assets of magnetic skyrmions

Making flexible spin caloritronic devices with interconnected nanowire networks

Spin caloritronics has recently emerged from the combination of spintronics and thermoelectricity. Here, we show that flexible,macroscopic spin caloritronic devices based on large-area interconnectedmagnetic nanowire networks can be used to enable controlled Peltier cooling of macroscopic electronic components with an external magnetic field. We experimentally demonstrate that three-dimensional CoNi/Cu multilayered nanowire networks exhibit an extremely high, magnetically modulated thermoelectric power factor up to 7.5 mW/K2m and large spin-dependent Seebeck and Peltier coefficients of −11.5 mV/K and −3.45 mV at room temperature, respectively. Our investigation reveals the possibility of performing efficient magnetic control of heat flux for thermal management of electronic devices and constitutes a simple and cost-effective pathway for fabrication of large-scale flexible and shapeable thermoelectric coolers exploiting the spin degree of freedom

Magnetic control of flexible thermoelectric devices based on macroscopic 3D interconnected nanowire networks

Spin-related effects in thermoelectricity can be used to design more efficient refrigerators and offer novel promising applications for the harvesting of thermal energy. The key challenge is to design structural and compositional magnetic material systems with sufficiently high efficiency and power output for transforming thermal energy into electric energy and vice versa. Here, the fabrication of large-area 3D interconnected Co/Cu nanowire networks is demonstrated, thereby enabling the controlled Peltier cooling of macroscopic electronic components with an external magnetic field. The flexible, macroscopic devices overcome inherent limitations of nanoscale magnetic structures due to insufficient power generation capability that limits the heat management applications. From properly designed experiments, large spin-dependent Seebeck and Peltier coefficients of −9.4μV/K and −2.8mV at room temperature, respectively. The resulting power factor of Co/Cu nanowire networks at room temperature (∼7.5mW/K2m) is larger than those of state of the art thermoelectric materials, such as BiTe alloys and the magneto-power factor ratio reaches about 100\% over a wide temperature range. Validation of magnetic control of heat flow achieved by taking advantage of the spin-dependent thermoelectric properties of flexible macroscopic nanowire networks lay the groundwork to design shapeable thermoelectric coolers exploiting the spin degree of freedom

Flexible thermoelectrics based on 3D interconnected magnetic nanowire networks

3D networks of ferromagnetic (FM) nanowires fabricated by direct electrodeposition into the crossed nanopores of polymer templates are effective thermoelectric materials. The interconnected nanowire networks are formed from pure metals, alloys, and FM/Cu multilayers. Giant magneto-Seebeck effects in multilayer nanowires allow the determination of key parameters in spin caloritronics such as spin-dependent Seebeck coefficients and can be exploited to design flexible thermoelectric switches with optimal magnetic field-induced control of the sign and magnitude of the thermoelectric power output. Homogeneous nanowire arrays exhibit extremely high thermoelectric power factors that make them effective materials as flexible active Peltier coolers for thermal management of hot spots

Flexible thermoelectric films based on interconnected magnetic nanowire networks

Recently, there has been increasing interest in the fabrication of flexible thermoelectric devices capable of cooling or recovering waste heat from hot surfaces with complex geometries. This paper reviews recent developments on three-dimensional networks of interconnected ferromagnetic nanowires, which offer new perspectives for the fabrication of flexible thermoelectric modules. The nanowire arrays are fabricated by direct electrodeposition into the crossed nanopores of polymeric templates. This low-cost, easy and reliable method allows control over the geometry, composition and morphology of the nanowire array. Here we report measured thermoelectric characteristics as a function of temperature and magnetic field of nanowire networks formed from pure metals (Co, Fe, Ni), alloys (NiCo, NiFe and NiCr) and FM/Cu multilayers (with FM = Co, Co50Ni50 and Ni80Fe20). Homogeneous nanowire arrays have high thermoelectric power factors, almost as high as their bulk constituents, and allow for positive and negative Seebeck coefficient values. These high thermoelectric power factors are essentially maintained in multilayer nanowires which also exhibit high magnetic modulability of electrical resistivity and Seebeck coefficient. This has been exploited in newly designed flexible thermoelectric switches that allow switching from an ‘off’ state with zero thermoelectric output voltage to an ‘on’ state that can be easily measured by applying or removing a magnetic field. Overall, these results are a first step towards the development of flexible thermoelectric modules that use waste heat to power thermally activated sensors and logic devices