0000000000677237
AUTHOR
Stefan Blügel
showing 32 related works from this author
Hybrid quantum anomalous Hall effect at graphene-oxide interfaces
2021
Interfaces are ubiquitous in materials science, and in devices in particular. As device dimensions are constantly shrinking, understanding the physical properties emerging at interfaces is crucial to exploit them for applications, here for spintronics. Using first-principles techniques and Monte Carlo simulations, we investigate the mutual magnetic interaction at the interface between graphene and an antiferromagnetic semiconductor BaMnO3. We find that graphene deeply affects the magnetic state of the substrate, down to several layers below the interface, by inducing an overall magnetic softening, and switching the in-plane magnetic ordering from antiferromagnetic to ferromagnetic. The grap…
Giant Anomalous Nernst Effect in Noncollinear Antiferromagnetic Mn-based Antiperovskite Nitrides
2020
The anomalous Nernst effect (ANE) - the generation of a transverse electric voltage by a longitudinal heat current in conducting ferromagnets or antiferromagnets - is an appealing approach for thermoelectric power generation in spin caloritronics. The ANE in antiferromagnets is particularly convenient for the fabrication of highly efficient and densely integrated thermopiles as lateral configurations of thermoelectric modules increase the coverage of heat source without suffering from the stray fields that are intrinsic to ferromagnets. In this work, using first-principles calculations together with a group theory analysis, we systematically investigate the spin order-dependent ANE in nonco…
Magnonic Weyl states in Cu2OSeO3
2019
Physical review research 2(1), 013063 (2020). doi:10.1103/PhysRevResearch.2.013063
Dynamical and current-induced Dzyaloshinskii-Moriya interaction: Role for damping, gyromagnetism, and current-induced torques in noncollinear magnets
2020
Both applied electric currents and magnetization dynamics modify the Dzyaloshinskii-Moriya interaction (DMI), which we call current-induced DMI (CIDMI) and dynamical DMI (DDMI), respectively. We report a theory of CIDMI and DDMI. The inverse of CIDMI consists in charge pumping by a time-dependent gradient of magnetization ${\ensuremath{\partial}}^{2}\mathbit{M}(\mathbit{r},t)/\ensuremath{\partial}\mathbit{r}\ensuremath{\partial}t$, while the inverse of DDMI describes the torque generated by ${\ensuremath{\partial}}^{2}\mathbit{M}(\mathbit{r},t)/\ensuremath{\partial}\mathbit{r}\ensuremath{\partial}t$. In noncollinear magnets, CIDMI and DDMI depend on the local magnetization direction. The re…
Comparison of first-principles methods to extract magnetic parameters in ultra-thin films: Co/Pt(111)
2019
We compare three distinct computational approaches based on first-principles calculations within density functional theory to explore the magnetic exchange and the Dzyaloshinskii-Moriya interactions (DMI) of a Co monolayer on Pt(111), namely, (i) the method of infinitesimal rotations of magnetic moments based on the Korringa-Kohn-Rostoker (KKR) Green function method, (ii) the generalized Bloch theorem applied to spiraling magnetic structures and (iii) supercell calculations with noncollinear magnetic moments, the latter two being based on the full-potential linearized augmented plane wave (FLAPW) method. In particular, we show that the magnetic interaction parameters entering micromagnetic …
Wannier90 as a community code: new features and applications
2019
Wannier90 is an open-source computer program for calculating maximally-localised Wannier functions (MLWFs) from a set of Bloch states. It is interfaced to many widely used electronic-structure codes thanks to its independence from the basis sets representing these Bloch states. In the past few years the development of Wannier90 has transitioned to a community-driven model; this has resulted in a number of new developments that have been recently released in Wannier90 v3.0. In this article we describe these new functionalities, that include the implementation of new features for wannierisation and disentanglement (symmetry-adapted Wannier functions, selectively-localised Wannier functions, s…
Orbital Rashba effect in a surface-oxidized Cu film
2020
Recent experimental observation of an unexpectedly large current-induced spin-orbit torque in surface oxidized Cu on top of a ferromagnet pointed to a possibly prominent role of the orbital Rashba effect (ORE) in this system. Here, we use first principles methods to investigate the ORE in a system of oxygen monolayer deposited on top of a Cu(111) film. We show that surface oxidization of the Cu film leads to a gigantic enhancement of the ORE near the Fermi energy. The resulting chiral orbital texture in the momentum space is exceptionally strong, reaching as much as $\ensuremath{\sim}0.5\ensuremath{\hbar}$ in magnitude. We find that resonant hybridization between O $p$ states and Cu $d$ sta…
Spin-order dependent anomalous Hall effect and magneto-optical effect in the noncollinear antiferromagnets Mn3XN with X=Ga , Zn, Ag, or Ni
2019
The anomalous Hall effect (AHE) and the magneto-optical effect (MOE) are two prominent manifestations of time-reversal symmetry breaking in magnetic materials. Noncollinear antiferromagnets (AFMs) have recently attracted a lot of attention owing to the potential emergence of exotic spin orders on geometrically frustrated lattices, which can be characterized by corresponding spin chiralities. By performing first-principles density functional calculations together with group-theory analysis and tight-binding modeling, here we systematically study the spin-order dependent AHE and MOE in representative noncollinear AFMs ${\mathrm{Mn}}_{3}X\mathrm{N}\phantom{\rule{4pt}{0ex}}(X=\mathrm{Ga}$, Zn, …
Topological magneto-optical effects and their quantization in noncoplanar antiferromagnets
2018
Reflecting the fundamental interactions of polarized light with magnetic matter, magneto-optical effects are well known since more than a century. The emergence of these phenomena is commonly attributed to the interplay between exchange splitting and spin-orbit coupling in the electronic structure of magnets. Using theoretical arguments, we demonstrate that topological magneto-optical effects can arise in noncoplanar antiferromagnets due to the finite scalar spin chirality, without any reference to exchange splitting or spin-orbit coupling. We propose spectral integrals of certain magneto-optical quantities that uncover the unique topological nature of the discovered effect. We also find th…
Tailoring the anomalous Hall effect of SrRuO$_3$ thin films by strain: a first principles study
2021
Motivated by the recently observed unconventional Hall effect in ultra-thin films of ferromagnetic SrRuO$_3$ (SRO) we investigate the effect of strain-induced oxygen octahedral distortion in the electronic structure and anomalous Hall response of the SRO ultra-thin films by virtue of density functional theory calculations. Our findings reveal that the ferromagnetic SRO films grown on SrTiO$_3$ (in-plane strain of $-$0.47$\%$) have an orthorhombic (both tilting and rotation) distorted structure and with an increasing amount of substrate-induced compressive strain the octahedral tilting angle is found to be suppressed gradually, with SRO films grown on NdGaO$_3$ (in-plane strain of $-$1.7$\%$…
Charge and spin photocurrents in the Rashba model
2017
In metallic noncentrosymmetric crystals and at surfaces the response of spin currents and charge currents to applied electric fields contains contributions that are second order in the electric field, which are forbidden by symmetry in centrosymmetric systems. Thereby, photocurrents and spin photocurrents can be generated in inversion asymmetric metals by the application of femtosecond laser pulses. We study the laser-induced charge current in the ferromagnetic Rashba model with in-plane magnetization and find that this \textit{magnetic photogalvanic effect} can be tuned to be comparable in size to the laser-induced photocurrents measured experimentally in magnetic bilayer systems such as C…
Chiral Hall Effect in Noncollinear Magnets from a Cyclic Cohomology Approach
2020
We demonstrate the emergence of an anomalous Hall effect in chiral magnetic textures which is neither proportional to the net magnetization nor to the well-known emergent magnetic field that is responsible for the topological Hall effect. Instead, it appears already at linear order in the gradients of the magnetization texture and exists for one-dimensional magnetic textures such as domain walls and spin spirals. It receives a natural interpretation in the language of Alain Connes' noncommutative geometry. We show that this chiral Hall effect resembles the familiar topological Hall effect in essential properties while its phenomenology is distinctly different. Our findings make the reinterp…
Theory of Current-Induced Angular Momentum Transfer Dynamics in Spin-Orbit Coupled Systems.
2020
Motivated by the importance of understanding competing mechanisms to current-induced spin-orbit torque in complex magnets, we develop a unified theory of current-induced spin-orbital coupled dynamics. The theory describes angular momentum transfer between different degrees of freedom in solids, e.g., the electron orbital and spin, the crystal lattice, and the magnetic order parameter. Based on the continuity equations for the spin and orbital angular momenta, we derive equations of motion that relate spin and orbital current fluxes and torques describing the transfer of angular momentum between different degrees of freedom. We then propose a classification scheme for the mechanisms of the c…
Effect of magnons on the temperature dependence and anisotropy of spin-orbit torque
2020
We investigate the influence of magnons on the temperature-dependence and the anisotropy of the spin-orbit torque (SOT). For this purpose we use 3rd order perturbation theory in the framework of the Keldysh formalism in order to derive suitable equations to compute the magnonic SOT. We find several contributions to the magnonic SOT, which depend differently on the spin-wave stiffness $\mathcal{A}$ and on the temperature $T$, with the dominating contribution scaling like $T^{2}/\mathcal{A}^{2}$. Based on this formalism we compute the magnonic SOT in the ferromagnetic Rashba model. For large Rashba parameters the magnonic SOT is strongly anisotropic and for small quasiparticle broadening it m…
Mixed topological semimetals driven by orbital complexity in two-dimensional ferromagnets
2018
The concepts of Weyl fermions and topological semimetals emerging in three-dimensional momentum space are extensively explored owing to the vast variety of exotic properties that they give rise to. On the other hand, very little is known about semimetallic states emerging in two-dimensional magnetic materials, which present the foundation for both present and future information technology. Here, we demonstrate that including the magnetization direction into the topological analysis allows for a natural classification of topological semimetallic states that manifest in two-dimensional ferromagnets as a result of the interplay between spin-orbit and exchange interactions. We explore the emerg…
Laser-induced torques in spin spirals
2020
We investigate laser-induced torques in magnetically non-collinear ferromagnets with a spin-spiral magnetic structure using \textit{ab-initio} calculations. Since spin-spirals may be used to approximate the magnetization gradients locally in domain walls and skyrmions, our method may be used to obtain the laser-induced torques in such objects from a multiscale approach. Employing the generalized Bloch-theorem we obtain the electronic structure computationally efficiently. We employ our method to assess the laser-induced torques in bcc Fe, hcp Co, and L$_{1}0$ FePt when a spin-spiral magnetic structure is imposed. We find that the laser-induced torques in these magnetically noncollinear syst…
Ab initio analysis of magnetic properties of the prototype B20 chiral magnet FeGe
2019
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 di…
Laser-induced torques in metallic antiferromagnets
2021
We study the laser-induced torques in the antiferromagnet (AFM) Mn$_2$Au. We find that even linearly polarized light may induce laser-induced torques in Mn$_2$Au, i.e., the light does not have to be circularly polarized. The laser-induced torques in Mn$_2$Au are comparable in magnitude to those in the ferromagnets Fe, Co and FePt at optical frequencies. We also compute the laser-induced torques at terahertz (THz) frequencies and compare them to the spin-orbit torques (SOTs) excited by THz laser-pulses. We find the SOTs to be dominant at THz frequencies for the laser-field strengths used in experiments. Additionally, we show that the matrix elements of the spin-orbit interaction (SOI) can be…
Spin-orbit torques and tunable Dzyaloshinskii-Moriya interaction in Co/Cu/Co trilayers
2017
We study the spin-orbit torques (SOTs) in Co/Cu/Co magnetic trilayers based on first-principles density-functional theory calculations in the case where the applied electric field lies in-plane, i.e., parallel to the interfaces. We assume that the bottom Co layer has a fixed in-plane magnetization, while the top Co layer can be switched. We find that the SOT on the top ferromagnet can be controlled by the bottom ferromagnet because of the nonlocal character of the SOT in this system. As a consequence the SOT is anisotropic, i.e., its magnitude varies with the direction of the applied electric field. We show that the Dzyaloshinskii-Moriya interaction (DMI) in the top layer is anisotropic as …
Interplay of Dzyaloshinskii-Moriya and Kitaev interactions for magnonic properties of Heisenberg-Kitaev honeycomb ferromagnets
2020
The properties of Kitaev materials are attracting ever increasing attention owing to their exotic properties. In realistic two-dimensional materials, Kitaev interaction is often accompanied by the Dzyloshinskii-Moriya interaction, which poses a challenge of distinguishing their magnitude separately. In this work, we demonstrate that it can be done by accessing magnonic transport properties. By studying honeycomb ferromagnets exhibiting Dzyaloshinskii-Moriya and Kitaev interactions simultaneously, we reveal non-trivial magnonic topological properties accompanied by intricate magnonic transport characteristics as given by thermal Hall and magnon Nernst effects. We also investigate the effect …
Mixed topology ring states for Hall effect and orbital magnetism in skyrmions of Weyl semimetals
2020
Skyrmion lattices as a novel type of chiral spin states are attracting increasing attention, owing to their peculiar properties stemming from real-space topological properties. At the same time, the properties of magnetic Weyl semimetals with complex $k$-space topology are moving into the focus of research in spintronics. We consider the Hall transport properties and orbital magnetism of skyrmion lattices imprinted in topological semimetals, by employing a minimal model of a 2D mixed Weyl semimetal which, as a function of the magnetization direction, exhibits two Chern insulator phases separated by a Weyl state for an an in-plane magnetization direction. We find that while the orbital magne…
Distinct magnetotransport and orbital fingerprints of chiral bobbers
2019
While chiral magnetic skyrmions have been attracting significant attention in the past years, recently, a new type of a chiral particle emerging in thin films $-$ a chiral bobber $-$ has been theoretically predicted and experimentally observed. Here, based on theoretical arguments, we provide a clear pathway to utilizing chiral bobbers for the purposes of future spintronics by uncovering that these novel chiral states possess inherent transport fingerprints that allow for their unambiguous electrical detection in systems comprising several types of chiral states. We reveal that unique transport and orbital characteristics of bobbers root in the non-trivial magnetization distribution in the …
Driving spin chirality by electron dynamics in laser-excited antiferromagnets
2020
Optical generation of complex spin textures is one of the most exciting challenges of modern spintronics. Here, we uncover a distinct physical mechanism for imprinting spin chirality into collinear magnets with short laser pulses. By simultaneously treating the laser-ignited evolution of electronic structure and magnetic order, we show that their intertwined dynamics can result in an emergence of quasi-stable chiral states. We find that laser-driven chirality does not require any auxiliary external fields or intrinsic spin-orbit interaction to exist, and it can survive on the time scale of nanoseconds even in the presence of thermal fluctuations, which makes the uncovered mechanism relevant…
Dzyaloshinskii-Moriya Interaction and Hall Effects in the Skyrmion Phase ofMn1−xFexGe
2015
We carry out density functional theory calculations which demonstrate that the electron dynamics in the Skyrmion phase of Fe-rich Mn_{1-x}Fe_{x}Ge alloys is governed by Berry phase physics. We observe that the magnitude of the Dzyaloshinskii-Moriya interaction directly related to the mixed space-momentum Berry phases, changes sign and magnitude with concentration x in direct correlation with the data of Shibata et al. [Nat. Nanotechnol. 8, 723 (2013)]. The computed anomalous and topological Hall effects in FeGe are also in good agreement with available experiments. We further develop a simple tight-binding model able to explain these findings. Finally, we show that the adiabatic Berry phase…
Spin-orbit torques in strained PtMnSb from first principles
2021
We compute spin-orbit torques (SOTs) in strained PtMnSb from first principles. We consider both tetragonal strain and shear strain. We find a strong linear dependence of the field-like SOTs on these strains, while the antidamping SOT is only moderately sensitive to shear strain and even insensitive to tetragonal strain. We also study the dependence of the SOT on the magnetization direction. In order to obtain analytical expressions suitable for fitting our numerical \textit{ab-initio} results we derive a general expansion of the SOT in terms of all response tensors that are allowed by crystal symmetry. Our expansion includes also higher-order terms beyond the usually considered lowest order…
Helical magnetic structure and the anomalous and topological Hall effects in epitaxial B20 Fe$_{1-y}$Co$_y$Ge films
2018
Epitaxial films of the B20-structure compound Fe1−yCoyGe were grown by molecular beam epitaxy on Si (111) substrates. The magnetization varied smoothly from the bulklike values of one Bohr magneton per Fe atom for FeGe to zero for nonmagnetic CoGe. The chiral lattice structure leads to a Dzyaloshinskii-Moriya interaction (DMI), and the films' helical magnetic ground state was confirmed using polarized neutron reflectometry measurements. The pitch of the spin helix, measured by this method, varies with Co content y and diverges at y∼0.45. This indicates a zero crossing of the DMI, which we reproduced in calculations using first-principles methods. We also measured the longitudinal and Hall r…
Spin caloric transport from density-functional theory
2019
Spin caloric transport refers to the coupling of heat with spin transport. Its applications primarily concern the generation of spin currents and control of magnetisation by temperature gradients for information technology, known by the synonym spin caloritronics. Within the framework of ab initio theory, new tools are being developed to provide an additional understanding of these phenomena in realistic materials, accounting for the complexity of the electronic structure without adjustable parameters. Here, we review this progress, summarising the principles of the density-functional-based approaches in the field and presenting a number of application highlights. Our discussion includes th…
Imprinting and driving electronic orbital magnetism using magnons
2020
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 orb…
Orbital Nernst Effect of Magnons
2019
In the past, magnons have been shown to mediate thermal transport of spin in various systems. Here, we reveal that the fundamental coupling of scalar spin chirality, inherent to magnons, to the electronic degrees of freedom in the system can result in the generation of sizeable orbital magnetization and thermal transport of orbital angular momentum. We demonstrate the emergence of the latter phenomenon of orbital Nernst effect by referring to the spin-wave Hamiltonian of kagome ferromagnets, predicting that in a wide range of systems the transverse current of orbital angular momentum carried by magnons in response to an applied temperature gradient can overshadow the accompanying spin curre…
Spin-order dependent anomalous Hall effect and magneto-optical effect in noncollinear antiferromagnets Mn$_{3}X$N ($X$ = Ga, Zn, Ag, and Ni)
2019
Noncollinear antiferromagnets (AFMs) have recently attracted a lot of attention owing to the potential emergence of exotic spin orders on geometrically frustrated lattices, which can be characterized by corresponding spin chiralities. By performing first-principles density functional calculations together with group-theory analysis and tight-binding modelling, here we systematically study the spin-order dependent anomalous Hall effect (AHE) and magneto-optical effect (MOE) in representative noncollinear AFMs Mn$_{3}X$N ($X$ = Ga, Zn, Ag, and Ni). The symmetry-related tensor shape of the intrinsic anomalous Hall conductivity (IAHC) for different spin orders is determined by analyzing the rel…
The chiral Hall effect of magnetic skyrmions from a cyclic cohomology approach
2019
We demonstrate the emergence of an anomalous Hall effect in chiral magnetic textures which is neither proportional to the net magnetization nor to the well-known emergent magnetic field that is responsible for the topological Hall effect. Instead, it appears already at linear order in the gradients of the magnetization texture and exists for one-dimensional magnetic textures such as domain walls and spin spirals. It receives a natural interpretation in the language of Alain Connes' noncommutative geometry. We show that this chiral Hall effect resembles the familiar topological Hall effect in essential properties while its phenomenology is distinctly different. Our findings make the re-inter…
Topological-chiral magnetic interactions driven by emergent orbital magnetism
2019
Two hundred years ago, Andr\'e-Marie Amp\`ere discovered that electric loops in which currents of electrons are generated by a penetrating magnetic field can interact with each other. Here we show that Amp\`ere'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 a peculiar type of 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 …