arXiv subjects

Anders Bergman

Publications and source records attributed to Anders Bergman.

At least 37 records · Page 2Linked to original sources

arXiv2024

Unraveling the connection between high-order magnetic interactions and local-to-global spin Hamiltonian in non-collinear magnetic dimers

A spin Hamiltonian, which characterizes interatomic interactions between spin moments, is highly valuable in predicting and comprehending the magnetic properties of materials. A deeper understanding of the microscopic origin of magnetic interactions can open new pathways toward realizing nanometer-scale systems for future spintronic devices. Here, we explore a method for explicitly calculating interatomic exchange interactions in non-collinear configurations of magnetic materials considering only a bilinear spin Hamiltonian in a local scenario. Based on density-functional theory (DFT) calculations of dimers adsorbed on metallic surfaces, and with a focus on the Dzyaloshinskii-Moriya interaction (DMI) which is essential for stabilizing chiral non-collinear magnetic states, we discuss the interpretation of the DMI when decomposed into microscopic electron and spin densities and currents. We clarify the distinct origins of spin currents induced in the system and their connection to the DMI. In addition, we reveal how non-collinearity affects the usual DMI, which is solely induced by spin-orbit coupling, and DMI-like interactions brought about by non-collinearity. We explain how the dependence of the DMI on the magnetic configuration establishes a connection between high-order magnetic interactions, enabling the transition from a local to a global spin Hamiltonian.

cond-mat.mtrl-sci↗

arXiv2023

Tunable phonon-driven magnon-magnon entanglement at room temperature

We report the existence of entangled steady-states in bipartite quantum magnonic systems at elevated temperatures. We consider dissipative dynamics of two magnon modes in a bipartite antiferromagnet, subjected to interaction with a phonon mode and an external rotating magnetic field. To quantify the bipartite magnon-magnon entanglement, we use entanglement negativity and compute its dependence on temperature and magnetic field. We provide evidence that the coupling between magnon and phonon modes is necessary for the entanglement, and that, for any given phonon frequency and magnon-phonon coupling rate, there are always ranges of the magnetic field amplitudes and frequencies for which magnon-magnon entanglement persists at room temperature.

cond-mat.mes-hall↗

arXiv2023

Magnetism in AV3Sb5 (A = Cs, Rb, K): Complex Landscape of the Dynamical Magnetic Textures

We have investigated the dynamical magnetic properties of the V-based kagome stibnite compounds by combining the ab-initio calculated magnetic parameters of a spin Hamiltonian like inter-site exchange parameters, magnetocrystalline anisotropy and site projected magnetic moments, with full-fledged simulations of atomistic spin-dynamics. Our calculations reveal that in addition to a ferromagnetic order along the [001] direction, the system hosts a complex landscape of magnetic configurations comprised of commensurate and incommensurate spin-spirals along the [010] direction. The presence of such chiral magnetic textures may be the key to solve the mystery about the origin of the experimentally observed inherent breaking of the C6 rotational symmetry- and the time-reversal symmetry.

cond-mat.mtrl-sci↗

arXiv2023

Magnetism in AV3Sb5 (Cs, Rb, K): Origin and Consequences for the Strongly Correlated Phases

The V-based kagome systems AV3Sb5 (A = Cs, Rb and K) are unique by virtue of the intricate interplay of non-trivial electronic structure, topology and intriguing fermiology, rendering them to be a playground of many mutually dependent exotic phases like charge-order and superconductivity. Despite numerous recent studies, the interconnection of magnetism and other complex collective phenomena in these systems has yet not arrived at any conclusion. Using first-principles tools, we demonstrate that their electronic structures, complex fermiologies and phonon dispersions are strongly influenced by the interplay of dynamic electron correlations, non-trivial spin-polarization and spin-orbit coupling. An investigation of the first-principles-derived inter-site magnetic exchanges with the complementary analysis of q-dependence of the electronic response functions and the electron-phonon coupling indicate that the system conforms as a frustrated spin-cluster, where the occurrence of the charge-order phase is intimately related to the mechanism of electron-phonon coupling, rather than the Fermi-surface nesting.

cond-mat.str-el↗

arXiv2023

Coupled spin-lattice dynamics from the tight-binding electronic structure

We developed a method which performs the coupled adiabatic spin and lattice dynamics based on the tight-binding electronic structure model, where the intrinsic magnetic field and ionic forces are calculated from the converged self-consistent electronic structure at every time step. By doing so, this method allows us to explore limits where the physics described by a parameterized spin-lattice Hamiltonian is no longer accurate. We demonstrate how the lattice dynamics is strongly influenced by the underlying magnetic configuration, where disorder is able to induce significant lattice distortions. The presented method requires significantly less computational resources than ab initio methods, such as time-dependent density functional theory (TD-DFT). Compared to parameterized Hamiltonian-based methods, it also describes more accurately the dynamics of the coupled spin and lattice degrees of freedom, which becomes important outside of the regime of small lattice and spin fluctuations.

cond-mat.mtrl-sci↗

arXiv2023

Unveiling the impact of temperature on magnon diffuse scattering detection in the transmission electron microscope

Magnon diffuse scattering (MDS) signals could be studied with high spatial resolution in scanning transmission electron microscopy (STEM), thanks to recent technological progress in electron energy loss spectroscopy. However, detecting MDS signals in STEM is challenging due to their overlap with stronger thermal diffuse scattering (TDS) signals. In bcc Fe at 300 K, MDS signals greater than or comparable to TDS signals occur under the central Bragg disk, into a currently inaccesible energy-loss region. Therefore, to detect MDS in STEM, it is necessary to find conditions in which TDS and MDS signals can be separated. Temperature may be a key factor due to the distinct thermal signatures of magnon and phonon signals. In this work, we present a study on the effects of temperature on MDS and TDS in bcc Fe -- considering a detector outside the central Bragg disk and a fixed convergent electron probe -- using the frozen phonon and frozen magnon multislice methods. Our study reveals that neglecting the effects of atomic vibrations causes the MDS signal to grow approximately linearly up to the Curie temperature of Fe, after which it exhibits less variation. The MDS signal displays an alternating behavior due to dynamical diffraction, instead of increasing monotonically as a function of thickness. Including the effects of atomic vibrations through a complex atomic electrostatic potential causes the linear growth of the MDS signal to change to a non-linear behavior that exhibits a predominant peak for a sample of thickness 16.072 nm at 1100 K. In contrast, the TDS signal grows more linearly than the MDS signal but still exhibits appreciable dynamical diffraction effects. An analysis of the signal-to-noise ratio (SNR) shows that the MDS signal can be a statistically significant contribution to the total scattering intensity under realizable measurement conditions and acquisition times.

cond-mat.mtrl-sci↗

arXiv2023

Metaheuristic conditional neural network for harvesting skyrmionic metastable states

We present a metaheuristic conditional neural-network-based method aimed at identifying physically interesting metastable states in a potential energy surface of high rugosity. To demonstrate how this method works, we identify and analyze spin textures with topological charge $Q$ ranging from 1 to $-13$ (where antiskyrmions have $Q<0$) in the Pd/Fe/Ir(111) system, which we model using a classical atomistic spin Hamiltonian based on parameters computed from density functional theory. To facilitate the harvest of relevant spin textures, we make use of the newly developed Segment Anything Model (SAM). Spin textures with $Q$ ranging from $-3$ to $-6$ are further analyzed using finite-temperature spin-dynamics simulations. We observe that for temperatures up to around 20\,K, lifetimes longer than 200\,ps are predicted, and that when these textures decay, new topological spin textures are formed. We also find that the relative stability of the spin textures depend linearly on the topological charge, but only when comparing the most stable antiskyrmions for each topological charge. In general, the number of holes (i.e., non-self-intersecting curves that define closed domain walls in the structure) in the spin texture is an important predictor of stability -- the more holes, the less stable is the texture. Methods for systematic identification and characterization of complex metastable skyrmionic textures -- such as the one demonstrated here -- are highly relevant for advancements in the field of topological spintronics.

physics.comp-ph↗

arXiv2023

Uncovering the role of the light-induced reduction of the inter-atomic exchange in the ultrafast demagnetization of Fe, Co, and Ni

Time resolved measurements of the linear magneto-optical Kerr rotation reveal that, in the {\it 3d} ferromagnets Fe, Co and Ni, the amplitude of the demagnetization increases linearly with the fluence of the light. We rationalize this phenomenon as being linearly driven by the increase in temperature, the electron-phonon and electron-magnon scattering, and a reduction of the inter-atomic exchange. The amplitude of the latter phenomenon, which until the present study has been widely overlooked, is obtained through \textit{ab initio} density functional theory calculations, and is argued to be the principal source of demagnetization in Ni and Co, while still contributing largely in Fe.

cond-mat.mtrl-sci↗

arXiv2023

Transmon probe for quantum characteristics of magnons in antiferromagnets

The detection of magnons and their quantum properties, especially in antiferromagnetic (AFM) materials, is a substantial step to realize many ambitious advances in the study of nanomagnetism and the development of energy efficient quantum technologies. The recent development of hybrid systems based on superconducting circuits provides the possibility of engineering quantum sensors that exploit different degrees of freedom. Here, we examine the magnon-photon-transmon hybridisation based on bipartite AFM materials, which gives rise to an effective coupling between a transmon qubit and magnons in a bipartite AFM. We demonstrate how magnetically invisible magnon modes, their chiralities and quantum properties such as nonlocality and two-mode magnon entanglement in bipartite AFMs can be characterized through the Rabi frequency of the superconducting transmon qubit.

quant-ph↗

arXiv2023

Adiabatic magnon spectra with and without constraining field: Benchmark against an exact magnon spectrum

The spectrum of magnon excitations in magnetic materials can be obtained exactly from the transverse dynamic magnetic susceptibility, which is however in practice numerically expensive. Many ab initio approaches therefore consider instead the adiabatic magnon spectrum, which assumes a separation of time scales of magnons and electronic excitations. There exist two alternative implementations for adiabatic magnon spectra: one based on the magnetic force theorem (MFT) and the other with a constraining field that enforces static non-collinear spin configurations. We benchmark both implementations against the exact magnon spectrum of an exactly solvable mean-field model. While both adiabatic methods are equally valid in the low magnon energy and strong Stoner coupling limits, we find that the constraining field method performs better than the MFT in both the cases of strong Stoner coupling and high magnon energies,while the MFT performs better for combined weak coupling and low magnon energies.

cond-mat.mtrl-sci↗

arXiv2022

Influence of non-local damping on magnon properties of ferromagnets

We study the influence of non-local damping on magnon properties of Fe, Co, Ni and Fe$_{1-x}$Co$_{x}$ ($x=30\%,50\%$) alloys. The Gilbert damping parameter is typically considered as a local scalar both in experiment and in theoretical modelling. However, recent works have revealed that Gilbert damping is a non-local quantity that allows for energy dissipation between atomic sites. With the Gilbert damping parameters calculated from a state-of-the-art real-space electronic structure method, magnon lifetimes are evaluated from spin dynamics and linear response, where a good agreement is found between these two methods. It is found that non-local damping affects the magnon lifetimes in different ways depending on the system. Specifically, we find that in Fe, Co, and Ni the non-local damping decreases the magnon lifetimes, while in $\rm Fe_{70}Co_{30}$ and Fe$_{50}$Co$_{50}$ an opposite, non-local damping effect is observed, and our data show that it is much stronger in the former.

cond-mat.mtrl-sci↗

arXiv2022

Adiabatic spin dynamics and effective exchange interactions from constrained tight-binding electronic structure theory: Beyond the Heisenberg regime

We consider an implementation of the adiabatic spin dynamics approach in a tight-binding description of the electronic structure. The adiabatic approximation for spin-degrees of freedom assumes that the faster electronic degrees of freedom are always in a quasi-equilibrium state, which significantly reduces the numerical complexity in comparison to the full electron dynamics. Non-collinear magnetic configurations are stabilized by a constraining field, which allows to directly obtain the effective magnetic field from the negative of the constraining field. While the dynamics are shown to conserve energy, we demonstrate that adiabatic spin dynamics does not conserve the total spin angular momentum when the lengths of the magnetic moments are allowed to change, which is confirmed by numerical simulations. Furthermore, we develop a method to extract an effective two-spin exchange interaction from the energy curvature tensor of non-collinear states, which we calculate at each time step of the numerical simulations. We demonstrate the effect of non-collinearity on this effective exchange and limitations due to multi-spin interactions in strongly non-collinear configurations beyond the regime where the Heisenberg model is valid. The relevance of the results are discussed with respect to experimental pump-probe experiments that follow the ultra-fast dynamics of magnetism.

cond-mat.mtrl-sci↗

arXiv2022

Tuning skyrmions in B20 compounds by 4d and 5d doping

Skyrmion stabilization in novel magnetic systems with the B20 crystal structure is reported here, primarily based on theoretical results. The focus is on the effect of alloying on the 3d sublattice of the B20 structure by substitution of heavier 4d and 5d elements, with the ambition to tune the spin-orbit coupling and its influence on magnetic interactions. State-of-the-art methods based on density functional theory are used to calculate both isotropic and anisotropic exchange interactions. Significant enhancement of the Dzyaloshinskii-Moriya interaction is reported for 5d-doped FeSi and CoSi, accompanied by a large modification of the spin stiffness and spiralization. Micromagnetic simulations coupled to atomistic spin-dynamics and ab initio magnetic interactions reveal a helical ground state and field-induced skyrmions for all these systems. Especially small skyrmions $\sim$50 nm are predicted for Co$_{0.75}$Os$_{0.25}$Si, compared to $\sim$148 nm for Fe$_{0.75}$Co$_{0.25}$Si. Convex-hull analysis suggests that all B20 compounds considered here are structurally stable at elevated temperatures and should be possible to synthesize. This prediction is confirmed experimentally by synthesis and structural analysis of the Ru-doped CoSi systems discussed here, both in powder and in single-crystal forms.

cond-mat.mtrl-sci↗

arXiv2022

Entanglement duality in spin-spin interactions

We examine entanglement of thermal states for spin-12 dimers in external magnetic fields. Entanglement transition in the temperature-magnetic field plane demonstrates a duality in spin-spin interactions. This identifies dual categories of symmetric and anti-symmetric dimers with each category classified into toric entanglement classes. The entanglement transition line is preserved from each toric entanglement class to its dual toric class. The toric classification is an indication of topological nature of the entanglement.

quant-ph↗

arXiv2022

Ab-initio study of the electronic structure and magnetic properties of Ce$_2$Fe$_{17}$

The Ce$_2$Fe$_{17}$ intermetallic compound has been studied intensely for several decades; its low-temperature state is reported experimentally either as ferromagnetic or antiferromagnetic by different authors, with a measured ordering temperature ranging within a hundred Kelvin. The existing theoretical investigations overestimate the experimental total magnetic moment of Ce$_2$Fe$_{17}$ by 20-40 % and predict a ferromagnetic ground state. By means of first-principle electronic structure calculations, we show that the total magnetic moment of Ce$_2$Fe$_{17}$ can be reproduced within the Local Density Approximation while functionals based on the Generalized Gradient Approximation fail. Atomistic spin dynamics simulations are shown to capture the change in the magnetic state of Ce$_2$Fe$_{17}$ with temperature, and closely replicate the reported helical structure that appears in some of the experimental investigations.

cond-mat.mtrl-sci↗

arXiv2021

Spin-lattice couplings in two-dimensional CrI$_3$ from first-principles study

Since thermal fluctuations become more important as dimensions shrink, it is expected that low-dimensional magnets are more sensitive to lattice distortions and phonons than bulk systems are. Here we present a fully relativistic first-principles study on the spin-lattice coupling, i.e. how the magnetic interactions depend on local lattice distortions, of the prototypical two-dimensional ferromagnet CrI$_3$. We extract an effective measure of the spin-lattice coupling in CrI$_3$ which is up to ten times larger than what is found for bcc Fe. The magnetic exchange interactions, including Heisenberg and relativistic Dzyaloshinskii-Moriya interactions, are sensitive both to the in-plane motion of Cr atoms and out-of-plane motion of ligand atoms. We find that significant magnetic pair interactions change sign from ferromagnetic (FM) to anti-ferromagnetic (AFM) for atomic displacements larger than 0.16 Å. We explain the observed strong spin-lattice coupling by analyzing the orbital decomposition of isotropic exchange interactions, involving different crystal-field-split Cr$-3d$ orbitals. The competition between the AFM t$_{2g}$ - t$_{2g}$ and FM t$_{2g}$ - e$_{g}$ contributions depends on the bond angle formed by Cr and I atoms as well as Cr-Cr distance. In particular, if a Cr atom is displaced, the FM-AFM sign change when the I-Cr-I bond angle approaches 90$^\circ$. The obtained spin-lattice coupling constants, along with the microscopic orbital analysis can act as a guiding principle for further studies of the thermodynamic properties and combined magnon-phonon excitations in two-dimensional magnets.

cond-mat.mtrl-sci↗

arXiv2021

Thermally-induced magnetic order from glassiness in elemental neodymium

Temperature in thermodynamics is synonymous with disorder, and responsible for ultimately destroying ordered phases. Here, we show an unusual magnetic transition where, with increasing the temperature of elemental neodymium, long-range multi-Q magnetic order emerges from a self-induced spin glass. Using temperature-dependent spin-polarized scanning tunneling microscopy, we characterize the local Q order in the spin-Q glass phase and quantify the emergence of long-range multi-Q order with increasing temperature. We develop two distinct analysis tools, which enable the quantification of the glass transition temperature, based on measured spatially-dependent magnetization. We compare these observations with atomic spin dynamics simulations, which reproduce the qualitative observation of a phase transition from a low-temperature spin glass phase to an intermediate ordered multi-Q phase. These simulations trace the origin of the unexpected high temperature order in weakened frustration driven by temperature-dependent sublattice correlations. These findings constitute an example of order from disorder and provide a rich platform to study magnetization dynamics in a self-induced spin glass.

cond-mat.mtrl-sci↗

arXiv2021

Magnon-magnon entanglement and its detection in a microwave cavity

Quantum magnonics is an emerging research field, with great potential for applications in magnon based hybrid systems and quantum information processing. Quantum correlation, such as entanglement, is a central resource in many quantum information protocols that naturally comes about in any study toward quantum technologies. This applies also to quantum magnonics. Here, we investigate antiferromagnets in which sublattices with ferromagnetic interactions can have two different magnon modes, and we show how this may lead to experimentally detectable bipartite continuous variable magnon-magnon entanglement. The entanglement can be fully characterized via a single squeezing parameter, or, equivalently, entanglement parameter. The clear relation between the entanglement parameter and the Einstein, Podolsky, and Rosen (EPR) function of the ground state opens up for experimental observation of magnon-magnon continuous variable entanglement and EPR non-locality. We propose a practical experimental realization to detect the EPR function of the ground state, in a setting that relies on magnon-photon interaction in a microwave cavity.

quant-ph↗