Explore the vast range of titles available.

Two hundred years ago, Ampère discovered that electric loops in which currents of electrons are generated by a penetrating magnetic field can mutually interact. Here we show that Ampère’s observation can be transferred to the quantum realm of interactions between triangular plaquettes of spins on a lattice, where the electrical currents at the atomic scale are associated with the orbital motion of electrons in response to the non-coplanarity of neighbouring spins playing the role of a magnetic field. The resulting topological orbital moment underlies the relation of the orbital dynamics with the topology of the spin structure. We demonstrate that the interactions of the topological orbital moments with each other and with the spins form a new class of magnetic interactions − topological–chiral interactions − which can dominate over the Dzyaloshinskii–Moriya interaction, thus opening a path for realizing new classes of chiral magnetic materials with three-dimensional magnetization textures such as hopfions. The motion of electrons in a complex magnetic background may generate novel magnetic interactions. Here, Grytsiuk et al. report that a peculiar orbital motion of electrons in response to a non-coplanarity of neighbouring spins leads to a topological orbital moment, which further gives rise to a new class of magnetic interactions.

Using the model of classical Heisenberg exchange and Dzyaloshinskii-Moriya (DM) interaction we show that the ground state of B20 FeGe chiral magnet is a superposition of twisted helical spin-density waves formed by different sublattices of the crystal. Such twisted spin-density waves propagate in the same direction but with different phases and different directions of the rotation axes. We derive an advanced micromagnetic expression describing the exchange and DM interaction for such magnetic structures. By employing first-principles calculations based on density functional theory and using our micromagnetic model we show that the magnitude of the spin-spiral twist in B20 FeGe is of the same order as global spiraling. While the energy difference between the ground state of twisted spirals and the FM state is in good agreement with the experimental results, for the spin spirals without a twist it is smaller by factor three. In addition, we verify our results by employing spin dynamics simulations. This calls for new experiments exploring the ground state properties of B20 chiral magnets.

FeGe in the B20 phase is an experimentally well-studied prototypical chiral magnet exhibiting helical spirals, skyrmion lattices and individual skyrmions with a robust length of 70~nm. While the helical spiral ground state can be verified by first-principles calculations based on density functional theory, this feature size could not be reproduced even approximately. To develop a coherent picture of the discrepancy between experiment and theory, we investigate in this work the magnetic properties of FeGe from first-principles using different electronic-structure methods. We study atomistic as well as micromagnetic parameters describing exchange and Dzyaloshinskii-Moriya interactions, and discuss their subtle dependence on computational, structural, and correlation parameters. In particular, we quantify how these magnetic properties are affected by changes of the lattice parameter, different atomic arrangements, exchange and correlation effects, finite Fermi-function broadening, and momentum-space sampling. In addition, we use the obtained atomistic parameters to determine the corresponding Curie temperature, which agrees well with experiments. Our results indicate that the well-known and well-accepted relation between the micromagnetic parameters and the period of the helical structure, is not valid for FeGe. This calls for new experiments exploring the relation by measuring independently the spin stiffness, the spiralization and the period of the helical spin spiral.

Despite its simplicity and relevance for the description of electronic correlations in solids, the Hubbard model is seldom inarguably realized in real materials. Here, we show that monolayer Nb3Cl8 is an ideal candidate to be described within a single-orbital Hubbard model, constructed within a “molecular” rather than atomic basis set using ab initio constrained random phase approximation calculations. We provide the necessary ingredients to connect experimental reality with ab initio material descriptions and correlated electron theory, which clarifies that monolayer Nb3Cl8 is a Mott insulator with a gap of about 1.4 to 2.0 eV depending on its dielectric environment. Comparisons to an atomistic three-orbital model show that the single-molecular-orbital description is adequate and reliable. We further comment on the electronic and magnetic structure of the compound and show that the Mott insulating state survives in the low-temperature bulk phases of the material featuring distinct experimentally verifiable characteristics.

Introduction. For many modern manufacturing processes, induction heating provides an attractive combination of speed, consistency and control. Multi-inductor (zone) systems with continuous billets feed are the most promising, which keep the billet cross sectional average temperature equal. It allows to avoid overheating at low throughputs and reduces the number of rejected billets. Problem. With zone induction heating systems for metal billets developing it is necessary, at the design stage, to perform a quantitative analysis of the main characteristics of the electrothermal process and provide recommendations for optimal parameters and heating modes selections. Accurate calculations for induction heating systems involve considering the distribution of the magnetic field, current density, and changes of material properties throughout volume of the heated billet. The goal of the work is to develop the numerical model and analyze the coupled electromagnetic and thermal processes in zone induction heating system for metal billets to determine the optimal power ratio of the inductors and choose rational heating modes for the billets. Methodology. The spatiotemporal distribution of the electromagnetic field and temperature throughout the volume of the billet during the induction heating process is described by the system of Maxwell and Fourier equations. For numerical calculations by the finite element method, the COMSOL Multiphysics 6.1 software package was used. All three methods of heat transfer are taken into account – conduction, convection, and radiation. Multiphysics couplings use electromagnetic power dissipation as a heat sources, and the billet material properties are specified by temperature functions. The operation of the inductors’ coils is modeled using the «Multi-Turn Coil» function, which uses a homogenized model. The translational motion of the billet is modeled by using the «Translational Motion» function. Results. The numerical 3D-model of coupled electromagnetic and thermal processes in the zone induction heating system for metal billets has been developed. Modeling was carried out for the design of a four-inductor system with the nominal capacity of 5000 kg/h. Data on the spatial distribution of the electromagnetic and temperature fields in the moving heated steel billet were obtained. Originality. Three-dimensional graphs of electrical conductivity and relative magnetic permeability change inside the moving heated steel billet are presented. Results of the temperature distribution calculations along the length of the steel billet for different inductors power ratios are provided. It is shown how the change in the power distribution of the inductors affects the billet heating parameters. Practical value. Analysis of the obtained data allows to determinate the necessary inductors powers to ensure the required heating mode. The results make it possible to reduce the time and resources required for the development, optimization of the design and improvement of the technological process of zone induction heating for metal billets. References 20, table 1, figures 13.

Magnons, as the most elementary excitations of magnetic materials, have recently emerged as a prominent tool in electrical and thermal manipulation and transport of spin, and magnonics as a field is considered as one of the pillars of modern spintronics. On the other hand, orbitronics, which exploits the orbital degree of freedom of electrons rather than their spin, emerges as a powerful platform in efficient design of currents and redistribution of angular momentum in structurally complex materials. Here, we uncover a way to bridge the worlds of magnonics and electronic orbital magnetism, which originates in the fundamental coupling of scalar spin chirality, inherent to magnons, to the orbital degree of freedom in solids. We show that this can result in efficient generation and transport of electronic orbital angular momentum by magnons, thus opening the road to combining the functionalities of magnonics and orbitronics to their mutual benefit in the realm of spintronics applications. The chiral spin texture hosted by Kagome lattices is emerging as a prominent playground for investigating exotic phenomena related to topological quantum phases. Here the authors utilize a tight-binding approach to unveil the existence of spontaneous interactions capable of bridging the gap between magnonics and spin-orbitronics.

Recent experiments in twisted bilayer transition-metal dichalcogenides have revealed a variety of strongly correlated phenomena. To theoretically explore their origin, we combine here ab initio calculations with correlated model approaches to describe and study many-body effects in twisted bilayer WSe\\(_2\\) under pressure. We find that the interlayer distance is a key factor for the electronic structure, as it tunes the relative energetic positions between the K and the \\(\\Gamma\\) valleys of the valence band maximum of the untwisted bilayer. As a result, applying uniaxial pressure to a twisted bilayer induces a charge-transfer from the K valley to the flat bands in the \\(\\Gamma\\) valley. Upon Wannierizing moiré bands from both valleys, we establish the relevant tight-binding model parameters and calculate the effective interaction strengths using the constrained random phase approximation. With this, we approximate the interacting pressure-doping phase diagram of WSe\\(_2\\) moiré bilayers using self-consistent mean field theory. Our results establish twisted bilayer WSe\\(_2\\) as a platform that allows the direct pressure-tuning of different correlated phases, ranging from Mott insulators, charge-valley-transfer insulators to Kondo lattice-like systems.

Recent experiments in twisted bilayer transition-metal dichalcogenides have revealed a variety of strongly correlated phenomena. To theoretically explore their origin, we combine here ab initio calculations with correlated model approaches to describe and study many-body effects in twisted bilayer WSe\\(_2\\) under pressure. We find that the interlayer distance is a key factor for the electronic structure, as it tunes the relative energetic positions between the K and the \\(\\Gamma\\) valleys of the valence band maximum of the untwisted bilayer. As a result, applying uniaxial pressure to a twisted bilayer induces a charge-transfer from the K valley to the flat bands in the \\(\\Gamma\\) valley. Upon Wannierizing moiré bands from both valleys, we establish the relevant tight-binding model parameters and calculate the effective interaction strengths using the constrained random phase approximation. With this, we approximate the interacting pressure-doping phase diagram of WSe\\(_2\\) moiré bilayers using self-consistent mean field theory. Our results establish twisted bilayer WSe\\(_2\\) as a platform that allows the direct pressure-tuning of different correlated phases, ranging from Mott insulators, charge-valley-transfer insulators to Kondo lattice-like systems.

B20-type compounds, such as MnSi and FeGe, host helimagnetic and skyrmion phases at the mesoscale, which are canonically explained by the combination of ferromagnetic isotropic interactions with weaker chiral Dzyaloshinskii-Moriya ones. Mysteriously, MnGe evades this paradigm as it displays a noncollinear magnetic state at a much shorter nanometer scale. Here we show that the length scale and volume-dependent magnetic properties of MnGe stem from purely isotropic exchange interactions, generally obtained in the paramagnetic state. Our approach is validated by comparing MnGe with the canonical B20-helimagnet FeGe. The free energy of MnGe is calculated, from which we show how triple-q magnetic states can stabilize by adding higher-order interactions.

Fundamental research on two-dimensional (2D) magnetic systems based on van der Waals materials has been gaining traction rapidly since their recent discovery. With the increase of recent knowledge, it has become clear that such materials have also a strong potential for applications in devices that combine magnetism with electronics, optics, and nanomechanics. Nonetheless, many challenges still lay ahead. Several fundamental aspects of 2D magnetic materials are still unknown or poorly understood, such as their often-complicated electronic structure, optical properties, and magnetization dynamics, and their magnon spectrum. To elucidate their properties and facilitate integration in devices, advanced characterization techniques and theoretical frameworks need to be developed or adapted. Moreover, developing synthesis methods which increase critical temperatures and achieve large-scale, high-quality homogeneous thin films is crucial before these materials can be used for real-world applications. Therefore, the field of 2D magnetic materials provides many challenges and opportunities for the discovery and exploration of new phenomena, as well as the development of new applications. This Roadmap presents the background, challenges, and potential research directions for various relevant topics in the field on the fundamentals, synthesis, characterization, and applications. We hope that this work can provide a strong starting point for young researchers in the field and provide a general overview of the key challenges for more experienced researchers.