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Articles by Peter Grünberg Institut
Spin dynamics of the centrosymmetric skyrmion material GdRu2Si2
Keywords magnetism skyrmions neutron scattering density functional theory Introduction Magnetic skyrmions are nanometer-scale swirling spin textures that have attracted renewed interest because of their potential applications in high-density magnetic memory technologies.1,2 This possibility is based on two properties of skyrmions. First, their nontrivial topology offers them a high degree of protection against external perturbations.
Spin-Dependent $\ensuremath{\pi}{\ensuremath{\pi}}^{*}$ Gap in Graphene on a Magnetic Substrate
We present a detailed analysis of the electronic properties of graphene/Eu/Ni(111). By using angle- and spin-resolved photoemission spectroscopy and ab initio calculations, we show that the intercalation of Eu in the graphene/Ni(111) interface gives rise to a gapped freestanding dispersion of the ππ* Dirac cones at the K¯ point with an additional lifting of the spin degeneracy due to the mixing of graphene and Eu states.
Time-dependent driving and topological protection in the fractional Josephson effect
The control of any type of quantum hardware invariably necessitates time-dependent driving. If the basis depends on the control parameter, the presence of a time-dependent control field yields an extra term in the Schrödinger equation that is often neglected. Here, we examine the effect of this term in a flux-controlled Majorana junction. We show that a time-varying flux gives rise to an electromotive force, which is highly nonlinear when truncating to the junction's low-energy degrees of freedom.
Dzyaloshinskii-Moriya interaction from unquenched orbital angular momentum
Orbitronics is an emerging and fascinating field that explores the utilization of the orbital degree of freedom of electrons for information processing. An increasing number of orbital phenomena are being currently discovered, with spin-orbit coupling mediating the interplay between orbital and spin effects, thus providing a wealth of control mechanisms and device applications.
Spontaneous Chirality Flipping in an Orthogonal Spin-Charge Ordered Topological Magnet
Popular Summary Chirality, a geometrical concept that distinguishes an object from its mirror image, plays a pivotal role in fields as diverse as biology and quantum physics. For quantum materials, the presence of chirality has for decades been proposed as a mechanism underlying a wide range of novel quantum states. While the direction, or “handedness,” of a chiral state can be changed via external fields, spontaneous chirality flipping has not been observed—until now.
Tuning magnetic interactions of Co and $4d$ transition-metal atomic bilayers on Re(0001) via interface engineering
Employing ab initio density functional theory (DFT), we performed a systematic investigation of the electronic structure and the magnetic properties of atomic bilayers composed of a 4d transition-metal layer (Rh, Pd, and Ru) and a Co layer on the Re(0001) surface. Our DFT calculations reveal the influence of the bilayer composition and their stacking sequence on the magnetic ground state and magnetic interactions.
Unified topological characterization of electronic states in spin textures from noncommutative $K$-theory
The nontrivial topology of spin systems such as skyrmions in real space can promote complex electronic states. Here, we provide a general viewpoint at the emergence of topological spectral gaps in spin systems based on the methods of noncommutative K-theory.
Interorbital Cooper pairing at finite energies in Rashba surface states
Multiband effects in hybrid structures provide a rich playground for unconventional superconductivity. We combine two complementary approaches based on density-functional theory (DFT) and effective low-energy model theory in order to investigate the proximity effect in a Rashba surface state in contact with an s-wave superconductor. We discuss these synergistic approaches and combine the effective model and DFT analysis at the example of a Au/Al heterostructure.
Magnetic properties of $4f$ adatoms on graphene: Density functional theory investigations
Rare-earth atoms on top of 2D materials represent an interesting platform with the prospect of tailoring the magnetic anisotropy for practical applications. Here, we investigate the ground state and magnetic properties of selected 4f atoms deposited on a graphene substrate in the framework of the DFT+U approach.
Band gaps of insulators from moment-functional-based spectral density functional theory
Within the method of spectral moments it is possible to construct the spectral function of a many-electron system from the first 2P spectral moments (P=1,2,3,⋯). The case P=1 corresponds to standard Kohn-Sham density functional theory (KS-DFT). Taking P>1 allows us to consider additional important properties of the uniform electron gas (UEG) in the construction of suitable moment potentials for moment-functional-based spectral density functional theory (MFbSDFT).
${\mathrm{CrTe}}_{2}$ as a two-dimensional material for topological magnetism in complex heterobilayers
The discovery of two-dimensional (2D) van der Waals magnetic materials and their heterostructures provided an exciting platform for emerging phenomena with intriguing implications in information technology.
Long-Range Orbital Torque by Momentum-Space Hotspots
Abstract While it is often assumed that the orbital response is suppressed and short ranged due to strong crystal field potential and orbital quenching, we show that the orbital response can be remarkably long ranged in ferromagnets. In a bilayer consisting of a nonmagnet and a ferromagnet, spin injection from the interface results in spin accumulation and torque in the ferromagnet, which rapidly oscillate and decay by spin dephasing.
Microscopic theory of spin relaxation of a single Fe adatom coupled to substrate vibrations
Abstract Understanding the spin-relaxation mechanism of single adatoms is an essential step towards creating atomic magnetic memory bits or even qubits. Here, we present an essentially parameter-free theory by combining ab initio electronic and vibrational properties with the many-body nature of atomic states.
Stability of hopfions in bulk magnets with competing exchange interactions
Abstract Magnetic hopfions are string-like three-dimensional topological solitons, characterised by the Hopf number. They serve as a fundamental prototype for three-dimensional magnetic quasiparticles and are an inspiration for novel device concepts in the field of spintronics. Based on a micromagnetic model and without considering temperature, the existence of such hopfions has been predicted in certain magnets with competing exchange interactions.
Crucial role of Fe in determining the hard magnetic properties of ${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$
Abstract Nd2Fe14B's unsurpassed, hard magnetic properties for a wide range of temperatures result from a combination of a large volume magnetization from Fe and a strong single-ion anisotropy from Nd. Here, using finite temperature first-principles calculations, we focus on the other crucial roles played by the Fe atoms in maintaining the magnetic order on the Nd sublattices, and hence the large magnetic anisotropy, and directly generating significant uniaxial anisotropy at high temperatures.
Hidden interplay of current-induced spin and orbital torques in bulk ${\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$
Abstract Low crystal symmetry of magnetic van der Waals materials naturally promotes spin-orbital complexity unachievable in common magnetic materials used for spin-orbit torque switching. Here, using first-principles methods, we demonstrate that an interplay of spin and orbital degrees of freedom has a profound impact on spin-orbit torques in the prototypical van der Waals ferromagnet Fe3GeTe2.
Laser-induced charge and spin photocurrents at the ${\mathrm{BiAg}}_{2}$ surface: A first-principles benchmark
Abstract Here, we report first-principles calculations of laser-induced photocurrents at the surface of a prototype Rashba system. By referring to Keldysh nonequilibrium formalism combined with the Wannier interpolation scheme, we perform first-principles electronic structure calculations of a prototype BiAg2 surface alloy, which is a well-known material realization of the Rashba model. In addition to the nonmagnetic ground state situation, we also study the case of in-plane magnetized BiAg2.
Engineering spin-orbit effects and Berry curvature by deposition of a monolayer of Eu on ${\mathrm{WSe}}_{2}$
COVID-19 has impacted many institutions and organizations around the world, disrupting the progress of research. Through this difficult time APS and the Physical Review editorial office are fully equipped and actively working to support researchers by continuing to carry out all editorial and peer-review functions and publish research in the journals as well as minimizing disruption to journal access.
Ab initio calculation of the magnetic Gibbs free energy of materials using magnetically constrained supercells
Abstract We present a first-principles approach for the computation of the magnetic Gibbs free energy of materials using magnetically constrained supercell calculations. Our approach is based on an adiabatic approximation of slowly varying local moment orientations, the so-called finite-temperature disordered local moment picture. It describes magnetic phase transitions and how electronic and/or magnetostructural mechanisms generate a discontinuous (first-order) character.
Observation of the Orbital Rashba-Edelstein Magnetoresistance
Abstract We report the observation of magnetoresistance (MR) that could originate from the orbital angular momentum (OAM) transport in a permalloy (Py)/oxidized Cu (Cu*) heterostructure: the orbital Rashba-Edelstein magnetoresistance. The angular dependence of the MR depends on the relative angle between the induced OAM and the magnetization in a similar fashion as the spin Hall magnetoresistance.
Chiral response of spin-spiral states as the origin of chiral transport fingerprints of spin textures
Abstract The transport properties of nontrivial spin textures are coming under closer scrutiny as the amount of experimental data and theoretical simulations is increasing. To extend the commonly accepted yet simplifying and approximate picture of transport effects taking place in systems with spatially varying magnetization, it is important to understand the transport properties of building blocks for spin textures—the homochiral spin-spiral states.
General connection between time-local and time-nonlocal perturbation expansions
Abstract There exist two canonical approaches to describe open quantum systems by a time-evolution equation: the Nakajima-Zwanzig quantum master equation, featuring a time-nonlocal memory kernel K, and the time-convolutionless equation with a time-local generator G. These key quantities have recently been shown to be connected by an exact fixed-point relation [Phys. Rev. X 11, 021041 (2021)].
Proper and improper chiral magnetic interactions
Abstract Atomistic spin models are of great value for predicting and understanding the magnetic properties of real materials, and extensions of the existing models open routes to new physics and potential applications. The Dzyaloshinskii-Moriya interaction is the prototype for chiral magnetic interactions, and several recent works have uncovered or proposed various types of generalized chiral interactions.
Magnetic skyrmions, chiral kinks, and holomorphic functions
Abstract We present a novel approach to understanding the extraordinary diversity of magnetic skyrmion solutions. Our approach combines an original classification scheme with efficient analytical and numerical methods. We introduce the concept of chiral kinks to account for regions of disfavored chirality in spin textures, and classify two-dimensional magnetic skyrmions in terms of closed domain walls carrying such chiral kinks.
Coupled quasimonopoles in chiral magnets
Abstract Magnetic singularities, also known as magnetic monopoles or Bloch points, represent intriguing phenomena in nanomagnetism. We show that a pair of coupled Bloch points—a dipole string—may appear as a stable state in cubic chiral magnets. Analysis of the thermodynamic stability of such objects in the interior of crystals and in geometrically confined systems is presented.
Defect-implantation for the all-electrical detection of non-collinear spin-textures
Abstract The viability of past, current and future devices for information technology hinges on their sensitivity to the presence of impurities. The latter can reshape extrinsic Hall effects or the efficiency of magnetoresistance effects, essential for spintronics, and lead to resistivity anomalies, the so-called Kondo effect.
Systematic derivation of realistic spin models for beyond-Heisenberg solids
Abstract We present a systematic derivation of effective lattice spin Hamiltonians derived from a rotationally invariant multiorbital Hubbard model including a term ensuring Hund's rule coupling. The Hamiltonians are derived down-folding the fermionic degrees of freedom of the Hubbard model into the proper low-energy spin sector using Löwdin partitioning, which will be outlined in detail for the case of two sites and two orbitals at each site.
Ab initio analysis of magnetic properties of the prototype B20 chiral magnet FeGe
Abstract 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.
Ab initio phase diagrams of Hf-O, Zr-O and Y-O: a comparative study
We present phase diagrams of binary oxides, Hf–O, Zr–O and Y–O, obtained by ab initio evolutionary simulations, in order to explore possible metastable crystalline suboxide structures which could be quenched during the electroforming processes within the conductive filaments in stoichiometric HfO , ZrO and Y O host materials, in resistive switching devices.
Tailor-made currents
Spin currents in magnetic trilayers exhibit an unexpected spin polarization that facilitates current-induced switching of magnetization.
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