arXiv subjects

Frank Freimuth

Publications and source records attributed to Frank Freimuth.

At least 19 recordsLinked to original sources

arXiv2024

Efficient motion of 90^{\circ} domain walls in Mn_{2}Au via pure optical torques

Discovering alternative ways to drive domain wall (DW) dynamics is crucial for advancing spintronic applications. Here we demonstrate via atomistic spin dynamics simulations that optical torques can efficiently drive 90^{\circ} DWs in the Mn2Au antiferromagnet but their spatial symmetry forbids the motion of 180^{\circ} walls. In the steady-state regime, the kinematics display special relativity signatures accessed for low laser intensities. At velocities higher than the magnonic limit, the DW enters a proliferation regime in which part of its relativistic energy is invested into the nucleation of novel magnetic textures. Our investigation contributes towards the fundamental understanding of opto-magnetic effects, supporting the development of next generation, all-optically controlled antiferromagnetic spintronics.

cond-mat.mes-hall↗

arXiv2023

Bandgaps 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 ($P=1,2,3,\dots$). 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). For example, the quasiparticle renormalization factor $Z$, which is not explicitly considered in KS-DFT, can be included easily. In the 4-pole approximation of the spectral function of the UEG (corresponding to $P=4$) we can reproduce the momentum distribution, the second spectral moment, and the charge response acceptably well, while a treatment of the UEG by KS-DFT reproduces from these properties only the charge response. For weakly and moderately correlated systems we may reproduce the most important aspects of the 4-pole approximation by an optimized two-pole model, which leaves away the low-energy satellite band. From the optimized two-pole model we extract \textit{parameter-free universal} moment potentials for MFbSDFT, which improve the description of the bandgaps in Si, SiC, BN, MgO, CaO, and ZnO significantly.

cond-mat.str-el↗

arXiv2023

Construction of Wannier functions from the spectral moments of correlated electron systems

When the first four spectral moments are considered, spectral features missing in standard Kohn-Sham (KS) density-functional theory (DFT), such as upper and lower Hubbard bands, as well as spectral satellite peaks, can be described, and the bandwidths can be corrected. Therefore, we have devised a \textit{moment-functional based spectral density functional theory} (MFbSDFT) recently. However, many computational tools in theoretical solid state physics, such as the construction of maximally localized Wannier functions (MLWFs), have been developed for KS-DFT and require modifications if they are supposed to be used in MFbSDFT. Here, we show how generalized Wannier functions may be constructed from the first four spectral moment matrices. We call these functions \textit{maximally localized spectral moment Wannier functions} (MLSMWFs). We demonstrate how MLSMWFs may be used to compute the anomalous Hall effect (AHE) in fcc Ni by Wannier interpolation. More generally, MLSMWFs may be computed from the first $2P$ moments ($P=1,2,3,\dots$). Using more than 4 moments opens the perspective of reproducing all spectral features accurately in MFbSDFT.

cond-mat.str-el↗

arXiv2023

Time-domain observation of ballistic orbital-angular-momentum currents with giant relaxation length in tungsten

The emerging field of orbitronics exploits the electron orbital momentum $\textit{L}$. Compared to spin-polarized electrons, $\textit{L}$ may allow magnetic-information transfera with significantly higher density over longer distances in more materials. However, direct experimental observation of $\textit{L}$ currents, their extended propagation lengths and their conversion into charge currents has remained challenging. Here, we optically trigger ultrafast angular-momentum transport in Ni|W|SiO$_2$ thin-film stacks. The resulting terahertz charge-current bursts exhibit a marked delay and width that grow linearly with W thickness. We consistently ascribe these observations to a ballistic $\textit{L}$ current from Ni through W with giant decay length (~80 nm) and low velocity (~0.1 nm/fs). At the W/SiO$_2$ interface, the $\textit{L}$ flow is efficiently converted into a charge current by the inverse orbital Rashba-Edelstein effect, consistent with ab-initio calculations. Our findings establish orbitronic materials with long-distance ballistic $\textit{L}$ transport as possible candidates for future ultrafast devices and an approach to discriminate Hall- and Rashba-Edelstein-like conversion processes.

physics.app-ph↗

arXiv2022

Moment potentials for spectral density functional theory: Exploiting the momentum distribution of the uniform electron gas

In standard Kohn-Sham (KS) density-functional theory (DFT) the valence band satellites in Ni and Pd are missing, the band widths in Ni and Na are too large, and the formation of lower and upper Hubbard bands in SrVO$_3$ is not described. These spectral features may be corrected by constructing the spectral function from the first four spectral moments, which may be obtained numerically efficiently within a \textit{moment-functional based spectral density functional theory} (MFbSDFT). In order to obtain a suitable potential for the second moment, one may use existing models of the uniform electron gas (UEG). However, models for the third moment of the UEG are not yet available. Therefore, we show that reproducing the momentum distribution of the UEG within the two-pole approximation of the spectral function determines the third moment, when the second moment is given. This allows us to find a model for the second and third moment potentials, which reproduces the experimental spectra of Ni, Pd, Na, and SrVO$_3$, and which is consistent with the second moment of the UEG at low density. Additionally, we describe an efficient algorithm to compute the spectral function from the first $2P$ spectral moment matrices, which paves the way to increasing the accuracy of MFbSDFT by using more than 4 spectral moments.

cond-mat.mtrl-sci↗

arXiv2022

Moment-functional based spectral density-functional theory

We describe a density-functional method which aims at computing the ground state electron density and the spectral function at the same time. One basic ingredient of our method is the construction of the spectral function from the first four spectral moment matrices. The second basic ingredient is the construction of the spectral moment matrices from density functionals. We call our method moment-functional based spectral density-functional theory (MFbSDFT), because it is based on density-functionals for the spectral moments and because it allows us to compute the spectral function. If it is implemented in second variation our method consumes only a fraction more computer time than a standard DFT calculation with the PBE functional. We show that MFbSDFT captures correlation effects such as the valence-band satellite in Ni and the formation of lower and upper Hubbard bands in SrVO$_3$. For the purpose of constructing the spectral function from the first four $N\times N$ spectral moment matrices we describe an efficient algorithm based on the diagonalization of one hermitean $2N\times 2N$ matrix.

cond-mat.mtrl-sci↗

arXiv2022

Controlling 3D spin textures by manipulating sign and amplitude of interlayer DMI with electrical current

The recently discovered interlayer Dzyaloshinskii-Moriya interaction (IL-DMI) in multilayers with perpendicular magnetic anisotropy favors the canting of spins in the in-plane direction and could thus enable new exciting spin textures such as Hopfions in continuous multilayer films. A key requirement is to control the IL-DMI and so in this study, the influence of an electric current on the IL-DMI is investigated by out-of-plane hysteresis loops of the anomalous Hall effect under applied in-plane magnetic fields. The direction of the in-plane field is varied to obtain a full azimuthal dependence, which allows us to quantify the effect on the IL-DMI. We observe a shift in the azimuthal dependence of the IL-DMI with increasing current, which can be understood from the additional in-plane symmetry breaking introduced by the current flow. Using an empirical model of two superimposed cosine functions we demonstrate the presence of a current-induced term that linearly increases the IL-DMI in the direction of current flow. With this, a new easily accessible possibility to manipulate 3D spin textures by current is realized. As most spintronic devices employ spin-transfer or spin-orbit torques to manipulate spin textures, the foundation to implement current-induced IL-DMI into thin-film devices is broadly available.

cond-mat.mes-hall↗

arXiv2022

Construction of the spectral function from non-commuting spectral moment matrices

The LDA+U method is widely used to study the properties of realistic solids with strong electron correlations. One of its main shortcomings is that it does not provide direct access to the temperature dependence of material properties such as the Dzyaloshinskii-Moriya interaction, the anomalous Hall conductivity, and the spin-orbit torque. While the method of spectral moments allows us in principle to compute these quantities directly at finite temperatures, the standard two-pole approximation can be applied only to Hamiltonians that are effectively of single-band type. We do a first step to explore if the method of spectral moments may replace the LDA+U method in first-principles calculations of correlated solids with many bands in cases where the direct assessment of the temperature dependence of equilibrium and response functions is desired: The spectral moments of many-band Hamiltonians of correlated electrons do not commute and therefore they do not possess a system of common eigenvectors. We show that nevertheless the spectral function may be constructed from the spectral moments by solving a system of coupled non-linear equations. Additionally, we show how to compute the anomalous Hall conductivity of correlated electrons from this spectral function. We demonstrate the method for the Hubbard-Rashba model, where the standard two-pole approximation cannot be applied, because spin-orbit interaction (SOI)couples the spin-up and the spin-down bands. In the quest for new quantum states that arise from the combination of SOI and correlation effects, the Hartree-Fock approximation is frequently used to obtain a first approximation for the phase diagram. We propose that using the many-band generalization of the selfconsistent moment method instead of Hartree-Fock in such exploratory model calculations may improve the accuracy significantly, while keeping the computational burden low.

cond-mat.other↗

arXiv2022

Evidence of Magnon-Mediated Orbital Magnetism in a Quasi-2D Topological Magnon Insulator

We explore spin dynamics in Cu(1,3-bdc), a quasi-2D topological magnon insulator. The results show that the thermal evolution of Landé $g$-factor ($g$) is anisotropic: $g_\textrm{in-plane}$ reduces while $g_\textrm{out-plane}$ increases with increasing temperature $T$. Moreover, the anisotropy of the $g$-factor ($Δg$) and the anisotropy of saturation magnetization ($ΔM_\textrm{s}$) are correlated below 4 K, but they diverge above 4 K. We show that the electronic orbital moment contributes to the $g$ anisotropy at lower $T$, while the topological orbital moment induced by thermally excited spin chirality dictates the $g$ anisotropy at higher $T$. Our work suggests an interplay among topology, spin chirality, and orbital magnetism in Cu(1,3-bdc).

physics.app-ph↗

arXiv2022

Driving spin chirality by electron dynamics in laser-excited antiferromagnets

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 for understanding various optical experiments on magnetic materials. Our findings open a new perspective at the interaction of complex chiral magnetism with light.

cond-mat.mes-hall↗

arXiv2021

Effect of magnons on the temperature dependence and anisotropy of spin-orbit torque

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 may become larger than the non-magnonic SOT.

cond-mat.mes-hall↗

arXiv2021

Theory of unidirectional magnetoresistance and nonlinear Hall effect

We study the unidirectional magnetoresistance (UMR) and the nonlinear Hall effect (NLHE) in the ferromagnetic Rashba model. For this purpose we derive expressions to describe the response of the electric current quadratic in the applied electric field. We compare two different formalisms, namely the standard Keldysh nonequilibrium formalism and the Moyal-Keldysh formalism, to derive the nonlinear conductivities of UMR and NLHE. We find that both formalisms lead to identical numerical results when applied to the ferromagnetic Rashba model. The UMR and the NLHE nonlinear conductivities tend to be comparable in magnitude according to our calculations. Additionally, their dependencies on the Rashba parameter and on the quasiparticle broadening are similar. The nonlinear zero-frequency response considered here is several orders of magnitude higher than the one at optical frequencies that describes the photocurrent generation in the ferromagnetic Rashba model. Additionally, we compare our Keldysh nonequilibrium expression in the independent-particle approximation to literature expressions of the UMR that have been obtained within the constant relaxation time approximation of the Boltzmann formalism. We find that both formalisms converge to the same analytical formula in the limit of infinite relaxation time. However, remarkably, we find that the Boltzmann result does not correspond to the intraband term of the Keldysh expression. Instead, the Boltzmann result corresponds to the sum of the intraband term and an interband term that can be brought into the form of an effective intraband term due to the f-sum rule.

cond-mat.mtrl-sci↗

arXiv2021

Spin-orbit torques in strained PtMnSb from first principles

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. We find that the dependence on the strain is much smaller for the higher-order terms than for the lowest order terms. In order to judge the sensitivity of the SOT on the exchange correlation potential we compute the SOT in both GGA and LDA. We find that the higher-order terms depend significantly on the exchange-correlation potential, while the lowest order terms are insensitive to it. Since the higher-order terms are small in comparison to the lowest order terms the total SOT is insensitive to the exchange correlation potential in strained PtMnSb.

cond-mat.mtrl-sci↗

arXiv2021

Laser-induced torques in metallic antiferromagnets

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 used to add SOI only during the Wannier interpolation, which we call Wannier interpolation of SOI (WISOI). This technique allows us to perform the Wannier interpolation conveniently for many magnetization directions from a single set of Wannier functions.

cond-mat.mtrl-sci↗

arXiv2021

Roadmap of spin-orbit torques

Spin-orbit torque (SOT) is an emerging technology that enables the efficient manipulation of spintronic devices. The initial processes of interest in SOTs involved electric fields, spin-orbit coupling, conduction electron spins and magnetization. More recently interest has grown to include a variety of other processes that include phonons, magnons, or heat. Over the past decade, many materials have been explored to achieve a larger SOT efficiency. Recently, holistic design to maximize the performance of SOT devices has extended material research from a nonmagnetic layer to a magnetic layer. The rapid development of SOT has spurred a variety of SOT-based applications. In this Roadmap paper, we first review the theories of SOTs by introducing the various mechanisms thought to generate or control SOTs, such as the spin Hall effect, the Rashba-Edelstein effect, the orbital Hall effect, thermal gradients, magnons, and strain effects. Then, we discuss the materials that enable these effects, including metals, metallic alloys, topological insulators, two-dimensional materials, and complex oxides. We also discuss the important roles in SOT devices of different types of magnetic layers. Afterward, we discuss device applications utilizing SOTs. We discuss and compare three-terminal and two-terminal SOT-magnetoresistive random-access memories (MRAMs); we mention various schemes to eliminate the need for an external field. We provide technological application considerations for SOT-MRAM and give perspectives on SOT-based neuromorphic devices and circuits. In addition to SOT-MRAM, we present SOT-based spintronic terahertz generators, nano-oscillators, and domain wall and skyrmion racetrack memories. This paper aims to achieve a comprehensive review of SOT theory, materials, and applications, guiding future SOT development in both the academic and industrial sectors.

cond-mat.mes-hall↗

arXiv2021

Faster chiral versus collinear magnetic order recovery after optical excitation revealed by femtosecond XUV scattering

While chiral spin structures stabilized by Dzyaloshinskii-Moriya interaction (DMI) are candidates as novel information carriers, their dynamics on the fs-ps timescale is little known. Since with the bulk Heisenberg exchange and the interfacial DMI two distinct exchange mechanisms are at play, the ultra-fast dynamics of the chiral order needs to be ascertained and compared to the dynamics of the conventional collinear order. Using an XUV free-electron laser we determine the fs-ps temporal evolution of the chiral order in domain walls in a magnetic thin film sample by an IR pump - X-ray magnetic scattering probe experiment. Upon demagnetisation we observe that the dichroic (CL-CR) signal connected with the chiral order correlator $m_z m_x$ in the domain walls recovers significantly faster than the (CL+CR) sum signal representing the average collinear domain magnetisation $m_z^2 + m_x^2$. We explore possible explanations based on spin structure dynamics and reduced transversal magnetisation fluctuations inside the domain walls and find that the latter can explain the experimental data leading to different dynamics for collinear magnetic order and chiral magnetic order.

cond-mat.mtrl-sci↗

arXiv2021

Direct and inverse spin-orbit torques in antiferromagnetic and ferromagnetic FeRh/W(001)

We use \textit{ab-initio} calculations to investigate spin-orbit torques (SOTs) in FeRh(001) deposited on W(100). Since FeRh undergoes a ferromagnetic-antiferromagnetic phase transition close to room temperature, we consider both phases of FeRh. In the antiferromagnetic case we find that the effective magnetic field of the even torque is staggered and therefore ideal to induce magnetization dynamics or to switch the antiferromagnet (AFM). At the antiferromagnetic resonance the inverse SOT induces a current density, which can be determined from the SOT. In the ferromagnetic case our calculations predict both even and odd components of the SOT, which can also be used to describe the ac and dc currents induced at the ferromagnetic resonance. For comparison we compute the SOTs in the c($2\times 2$) AFM state of Fe/W(001).

cond-mat.mtrl-sci↗

arXiv2021

Charge and spin photocurrents in the Rashba model

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 Co/Pt. Additionally, we show that femtosecond laser pulses excite strong spin currents in the nonmagnetic Rashba model when the Rashba parameter is large.

cond-mat.mes-hall↗