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Electrical detection and nucleation of a magnetic skyrmion in a magnetic tunnel junction observed via operando magnetic microscopy
No full textMagnetic skyrmions are topological spin textures which are envisioned as nanometer scale information carriers in magnetic memory and logic devices. The recent demonstrations of room temperature skyrmions and their current induced manipulation in ultrathin films were first steps toward the realization of such devices. However, important challenges remain regarding the electrical detection and the low power nucleation of skyrmions, which are required for the read and write operations. Here, we demonstrate, using operando magnetic microscopy experiments, the electrical detection of a single magnetic skyrmion in a magnetic tunnel junction MTJ and its nucleation and annihilation by gate voltage via voltage control of magnetic anisotropy. The nucleated skyrmion can be manipulated by both gate voltages and external magnetic fields, leading to tunable intermediate resistance states. Our results unambiguously demonstrate the readout and voltage controlled write operations in a single MTJ device, which is a major milestone for low power skyrmion based technologie
Observation by SANS and PNR of pure N el type domain wall profiles and skyrmion suppression below room temperature in magnetic [Pt CoFeB Ru]10 multilayers
Get PDFWe report investigations of the magnetic textures in periodic multilayers [Pt 1 amp; 8201;nm CoFeB 0.8 amp; 8201;nm Ru 1.4 amp; 8201;nm ]10 using polarised neutron reflectometry PNR and small angle neutron scattering SANS . The multilayers are known to host skyrmions stabilized by Dzyaloshinskii Moriya interactions induced by broken inversion symmetry and spin orbit coupling at the asymmetric interfaces. From depth dependent PNR measurements, we observed well defined structural features and obtained the layer resolved magnetization profiles. The in plane magnetization of the CoFeB layers calculated from fitting of the PNR profiles is found to be in excellent agreement with magnetometry data. Using SANS as a bulk probe of the entire multilayer, we observe long period magnetic stripe domains and skyrmion ensembles with full orientational disorder at room temperature. No sign of skyrmions is found below 250 K, which we suggest is due to an increase of an effective magnetic anisotropy in the CoFeB layer on cooling that suppresses skyrmion stability. Using polarised SANS at room temperature, we prove the existence of pure N el type windings in both stripe domain and skyrmion regimes. No Bloch type winding admixture, i.e. an indication for hybrid windings, is detected within the measurement sensitivity, in good agreement with expectations according to our micromagnetic modelling of the multilayers. Our findings using neutron techniques provide valuable microscopic insights into the rich magnetic behavior of skyrmion hosting multilayers, which are essential for the advancement of future skyrmion based spintronic device
Effects of disorder on the magnetic properties of the Heusler alloy V2FeAl
Get PDFMagnetic properties of multicomponent alloys depend sensitively on the degree of atomic order on the different crystallographic sites. Here we report the contrast between the magnetic properties of bulk and thin film samples of the Heusler alloy V2FeAl. Arc melted bulk ingots show no site preference of the elements A2 structure , whereas magnetron sputtered thin film samples display a degree of atomic ordering with a tendency towards XA type order. Electronic structure calculations favour ferrimagnetic XA type order, and the effect of different pairwise atomic disorder on the element specific and net magnetic moments are evaluated to reproduce experimental observations. XA type thin films with iron moment of 1.24 mu B determined by X ray magnetic circular dichroism are in agreement with calculation, but the measured net moment of 1.0 mu B per formula unit and average vanadium moment are smaller than expected from calculations. The measured Curie temperature is approximately 500 K. Films with a higher degree of disorder have a lower TC, close to 300 K, with a net moment of only 0.1 mu B at low temperature. The large calculated vanadium moments are destroyed by partial disorder on 4d vanadium sites. By contrast, the arc melted and annealed bulk alloy with a fullydisordered A2 structure shows no spontaneous magnetization at any temperature; it is a Pauli paramagnet with dimensionless susceptibility xv 2.95 x 10
Jellyfish A modular code for wave function based electron dynamics simulations and visualizations on traditional and quantum compute architectures
Get PDFUltrafast electron dynamics have made rapid progress in the last few years. With Jellyfish, we now introduce a program suite that enables to perform the entire workflow of an electron dynamics simulation. The modular program architecture offers a flexible combination of different propagators, Hamiltonians, basis sets, and more. Jellyfish can be operated by a graphical user interface, which makes it easy to get started for nonspecialist users and gives experienced users a clear overview of the entire functionality. The temporal evolution of a wave function can currently be executed in the time dependent configuration interaction method TDCI formalism, however, a plugin system facilitates the expansion to other methods and tools without requiring in depth knowledge of the program. Currently developed plugins allow to include results from conventional electronic structure calculations as well as the usage and extension of quantum compute algorithms for electron dynamics. We present the capabilities of Jellyfish on three examples to showcase the simulation and analysis of light driven correlated electron dynamics. The implemented visualization of various densities enables an efficient and detailed analysis for the long standing quest of the electron hole pair formatio
Controlled boron content in lightly B doped single crystal diamond films by variation of methane concentration
Get PDFObtaining desirable electrical properties from B doped single crystal diamond SCD films hinges on precise control of boron incorporation into the crystal lattice structure. In this study, the impact of methane concentration during plasma deposition on boron incorporation of lightly B doped SCD films is investigated. SCD layers are grown successively by microwave plasma enhanced chemical vapor deposition CVD at different methane to hydrogen concentrations 1 , 2 , and 3 , with residual boron atoms present in the CVD reactor. An increase in methane concentration leads to surface defects such as unepitaxial crystallites and pyramidal hillocks. The charge carrier mobility, electrical conductivity, and boron content of samples are evaluated and discussed. The temperature dependent mobility is analyzed through theoretical modeling, revealing dominant scattering mechanisms at different temperatures. At 300 K, the maximum hole mobility reached 1200 cm2 V s for the 1 methane concentration sample, transitioning to hopping conduction at lower temperatures. An increase in boron doping level with rising methane concentration is detected by Fourier transform infrared spectroscopy, cathodoluminescence spectroscopy, Hall effect, and X ray photoelectron spectroscopy measurements. These findings highlight the potential of methane concentration in plasma feedgas to control boron concentration in CVD diamond and open avenues for crafting efficient high power electronic applications using p type SCD film
Thermal synthesis of electron deficient oxygen species on crystalline IrO2
Get PDFWater splitting is a promising technology in the path towards complete renewable energy within the hydrogen economy but overcoming the sluggishness of the oxygen evolution reaction OER is a major challenge. Iridium based oxides remain the most attractive materials for the OER under acidic conditions since they offer the combination of activity and stability. Gaining knowledge about how these materials have such an ability is of great interest to develop improved electrocatalysts for the OER. Among the different iridium based oxides the materials with high concentrations of electron deficient oxygen OI amp; 8722; have been shown to have higher OER activity, however, they also have high dissolution rates, seemingly due to the presence or formation of IrIII species. In contrast, rutile type IrO2, which does not contain IrIII species, has high dissolution resistance but the OER activity remains comparatively low as only low coverages of OI amp; 8722; species are formed under OER. The apparent link between OI amp; 8722; and IrIII species that comes from these observations has yet to be proven. In this work, using ab initio thermodynamics and in situ X ray photoelectron and absorption spectroscopy we show that the same electrophilic OI amp; 8722; species that appear on Ir based oxides under OER can be formed on IrIV amp; 948; by mild thermal oxidation of rutile type IrO2, without the presence IrIII specie
Facile and Green Synthesis of Well Defined Nanocrystal Oxygen Evolution Catalysts by Rational Crystallization Regulation
No full textThe development of catalysts for an economical and efficient oxygen evolution reaction OER is critical for clean and sustainable energy storage and conversion. Nickel iron based NiFe nanostructures are widely investigated as active OER catalysts and especially shape controlled nanocrystals exhibit optimized surface structure and electronic properties. However, the structural control from amorphous to well defined crystals is usually time consuming and requires multiple stages. Here, a universal two step precipitation hydrothermal approach is reported to prepare a series of NiFe based nanocrystals e.g., hydroxides, sulfides, and molybdates from amorphous precipitates. Their morphology and evolution of atomic and electronic structure during this process are studied using conclusive microscopy and spectroscopy techniques. The short term, additive free, and low cost method allows for the control of the crystallinity of the materials and facilitates the generation of nanosheets, nanorods, or nano octahedra with excellent water oxidation activity. The NiFe based crystalline catalysts exhibit slightly compromised initial activity but more robust long term stability than their amorphous counterparts during electrochemical operation. This facile, reliable, and universal synthesis method is promising in strategies for fabricating NiFe based nanostructures as efficient and economically valuable OER electrocatalyst
Tuning the Interfacial Electronic Structure of MoS2 by Adsorption of Cobalt Phthalocyanine Derivatives
Get PDFWe investigate the interfacial electronic structure of n type bulk MoS2 upon the adsorption of CoPc and CoPcF16 monolayers and few layers using advanced spectroscopic techniques. These include X ray photoelectron spectroscopy XPS , X ray absorption spectroscopy XAS , angle resolved photoelectron spectroscopy ARPES , and ultraviolet photoelectron spectroscopy UPS . Our findings indicate that the adsorption of CoPc enhances the degree of n doping at the interface with MoS2. In contrast, CoPcF16 acts as an electron acceptor and results in a nearly intrinsic position of the Fermi level of MoS2. Furthermore, we note the formation of an induced gap state near the valence band maximum for monolayer CoPcF16 on MoS2. These observations underscore the potential to fine tune the interfacial electronic properties of transition metal dichalcogenides through molecular functionalization for application in optoelectronic device
A Unique Wide Spacing Fence Type Superstructure for Robust High Voltage O3 Type Sodium Layered Cathode
No full textEnhancing the energy density of layered oxide cathode materials is of great significance for realizing high performance sodium ion batteries and promoting their commercial application. Lattice oxygen redox at high voltage usually enables a high capacity and energy density. But the structural degradation, severe voltage decay, and the resultant poor cycling performance caused by irreversible oxygen release seriously restrict the practical application. Herein we introduce a novel fence type superstructure 2a 3a type supercell into O3 type layered cathode material Na0.9Li0.1Ni0.3Mn0.3Ti0.3O2 and achieve a stable cycling performance at a high voltage of 4.4 V. The fence type superstructure effectively inhibits the formation of the vacancy clusters resulting from out of plane Li migration and in plane transition metal migration at high voltage due to the wide d spacing, thereby significantly reducing the irreversible release of lattice oxygen and greatly stabilizing the crystal structure. The cathode exhibits a high energy density of 545 Wh kg amp; 8722;1, a high rate capability 112.8 mAh g 1 at 5C and a high cycling stability 85.8 200 cycles with a high initial capacity of 148.6 mAh g 1 at 1C accompanied by negligible voltage attenuation 98.5 200 cycles . This strategy provides a distinct spacing effect of superstructure to design stable high voltage layered cathode materials for Na ion batterie
Influence of Conductive Additives and Binder on the Impedance of Lithium Ion Battery Electrodes Effect of an Inhomogeneous Distribution
Get PDFThe conductive additive and binder domain CBD is an essential component of lithium ion battery electrodes. It enhances the electrical connectivity and mechanical stability within the solid electrode matrix. Migration of the binder during electrode drying can lead to an inhomogeneous distribution of the CBD, impeding transport of lithium ions into the electrodes, and diminishing the electronic pathways between solid particles and the current collector. This is especially prominent in thick electrodes at high drying rates. Therefore, we investigate the effect of a non uniform CBD distribution on the electrochemical performance of NMC622 electrodes via microstructure resolved three dimensional 3D simulations on virtual electrodes, based on tomographic image data, and compare them with experimental results. The valuable information derived by combining microstructure resolved models with electrochemical impedance spectroscopy measurements on symmetric cells under blocking electrolyte conditions is used to characterize the lithium ion transport in the electrode pore space, including the contributions of the CBD. The effect of this inhomogeneity on electrode performance is then gauged via galvanostatic discharge simulations under changing discharge currents and for varying electrode densities. Through our work, we demonstrate the significance of the CBD distribution and enable predictive simulations for future battery desig