arXiv subjects

László Szunyogh

Publications and source records attributed to László Szunyogh.

At least 37 records · Page 2Linked to original sources

arXiv2022

Magnetic ground state of supported monatomic Fe chains from first principles

A new computational scheme is presented based on a combination of the conjugate gradient and the Newton-Raphson method to self-consistently minimize the energy within spin-density functional theory, thus to identify the ground state magnetic order of a cluster of magnetic atoms. The applicability of the new \textit{ab initio} optimization method is demonstrated on Fe chains deposited on different metallic substrates. The obtained magnetic ground states of the Fe chains on Rh(111) are analyzed in details and a good comparison is found with those gained from an extended Heisenberg model containing first principles based interaction parameters. Moreover, the effect of the different bilinear spin-spin interactions in the formation of the magnetic ground states is monitored. In case of Fe chains on Nb(110) spin-spiral configurations with opposite rotational sense are found as compared to previous spin-model results which hints on the importance of higher order chiral interactions. The wavelength of the spin-spiral states of Fe chains on Re(0001) was obtained in good agreement with scanning tunneling microscopy experiments.

cond-mat.mes-hall↗

arXiv2022

Current induced switching in Mn2Au from first principles

It is well established that it is possible to switch certain antiferromagnets electrically, yet the interplay of Néel-spin-orbit torques and thermal activation is only poorly understood. Combining ab initio calculations and atomistic spin dynamics simulations we develop a multiscale model to study the current induced switching in Mn2Au. We compute from first principles the strength and direction of the electrically induced magnetic moments, caused by the Rashba--Edelstein effect, and take these into account in atomistic spin dynamics simulations. Our simulations reveal the switching paths as well as the time scales for switching. The size of the induced moments, however, turns out to be insufficient to lead to fully deterministic switching. Instead, we find that a certain degree of thermal activation is required to help overcoming the relevant energy barrier.

cond-mat.mtrl-sci↗

arXiv2022

Reply to 'Comment on "Proper and improper chiral magnetic interactions" '

In our previous Letter [Phys. Rev. B 103, L140408 (2021)], we presented a discussion of the fundamental physical properties of the interactions parameterizing atomistic spin models in connection to first-principles approaches that enable their calculation for a given material. This explained how some of those approaches can apparently lead to magnetic interactions that do not comply with the expected physical properties, such as Dzyaloshinskii-Moriya interactions which are non-chiral and independent of the spin-orbit interaction, and which we consequently termed `improper'. In the preceding Comment [Phys. Rev. B 105, 026401], the authors present arguments based on the distinction between global and local approaches to the mapping of the magnetic energy using first-principles calculations to support their proposed non-chiral Dzyaloshinskii-Moriya interactions and their dismissal of our distinction between `proper' and `improper' magnetic interactions. In this Reply, we identify the missing step in the local approach to the mapping and explain how ignoring this step leads to the identification of magnetic interactions which do not comply with established physical principles and that we have previously termed `improper'.

cond-mat.mtrl-sci↗

arXiv2021

Relativistic first principles theory of Yu--Shiba--Rusinov states applied to an Mn adatom and Mn dimers on Nb(110)

We present a fully relativistic first principles based theoretical approach for the calculation of the spectral properties of magnetic impurities on the surface of a superconducting substrate, providing a material specific framework for the investigation of the Yu--Shiba--Rusinov (YSR) states. By using a suitable orbital decomposition of the local densities of states we discuss in great details the formation of the YSR states for an Mn adatom and for two kinds of Mn dimers placed on the Nb(110) surface and compare our results to recent experimental findings. In case of the adatom we find that the spin-orbit coupling slightly shifts some of the YSR peaks and also the local spin-polarization on the Nb atoms have marginal effects to the their positions. Moreover, by scaling the exchange field on the Mn site we could explain the lack of the $d_{x^2-y^2}$-like YSR state in the spectrum. While our results for a close packed ferromagnetic dimer are in satisfactory agreement with the experimentally observed splitting of the YSR states, in case of an antiferromagnetic dimer we find that the spin-orbit coupling is not sufficiently large to explain the splitting of the YSR states seen in the experiment. Changing the relative orientation of the magnetic moments in this dimer induces splitting of the YSR states and also shifts their energy, leading even to the formation of a zero bias peak in case of the deepest YSR state.

cond-mat.supr-con↗

arXiv2021

Coexistence of Antiferromagnetism and Superconductivity in Mn/Nb(110)

We report on the structural and magnetic properties of single and double atomic layers of Mn on a clean and unreconstructed Nb(110) substrate. Low-temperature scanning tunneling spectroscopy measurements reveal a proximity-induced superconducting state in the Mn thin films, which are found to grow pseudomorphically on the Nb surface. Spin-polarized scanning tunneling microscopy measurements reveal a c(2x2) antiferromagnetic order in the Mn layers, with an out-of-plane spin-orientation. First-principles density functional theory calculations confirm the experimentally observed magnetic state, which is understood as the consequence of a strong intra- and inter-layer nearest-neighbor antiferromagnetic exchange coupling. These results are expected to be of importance for the design and the investigation of novel superconducting antiferromagnetic spintronic systems.

cond-mat.mes-hall↗

arXiv2021

Electronic and Magnetic Properties of Building Blocks of Mn and Fe Atomic Chains on Nb(110)

We present results for the electronic and magnetic structure of Mn and Fe clusters on Nb(110) surface, focusing on building blocks of atomic chains as possible realizations of topological superconductivity. The magnetic ground states of the atomic dimers and most of the monatomic chains are determined by the nearest-neighbor isotropic interaction. The dependence on the crystallographic direction as well as on the atomic coordination number is analyzed via an orbital decomposition of this isotropic interaction based on the spin-cluster expansion and the difference in the local density of states between ferromagnetic and antiferromagnetic configurations. A spin-spiral ground state is obtained for Fe chains along the [$1\overline{1}0$] direction as a consequence of the frustration of the isotropic interactions. Here, a flat spin-spiral dispersion relation is identified, which can stabilize spin spirals with various wave vectors together with the magnetic anisotropy. This may lead to the observation of spin spirals of different wave vectors and chiralities in longer chains instead of a unique ground state.

cond-mat.mes-hall↗

arXiv2021

Proper and improper chiral magnetic interactions

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. However, in some cases the proposed interactions or their interpretation do not comply with basic principles such as being independent of the magnetic configuration from which they are evaluated, or even obeying time-reversal invariance. In this letter, we present a simple explanation for the origin of these puzzling findings, and point out how to resolve them.

cond-mat.mtrl-sci↗

arXiv2020

Spin reorientation transition in an ultrathin Fe film on W(110) induced by Dzyaloshinsky-Moriya interactions

Controlling the preferred direction of the magnetic moments is essential for the design of spintronic devices based on ultrathin films and heterostructures. As the film thickness or the temperature is increased, the easy anisotropy axis is typically reoriented from an out-of-plane direction preferred by surface and interface energy contributions to an in-plane alignment favored by the volume anisotropy terms. We study the temperature-driven spin reorientation transition in two atomic layers of Fe on W(110) using well-tempered metadynamics simulations based on a spin model parametrized by ab initio calculations and find that the transition only takes place in the presence of the Dzyaloshinsky--Moriya interaction (DMI). This demonstrates that the chiral DMI does not only differentiate between noncollinear spin structures of different rotational senses, but it also influences the magnetic orientation of collinear magnetic configurations.

cond-mat.mtrl-sci↗

arXiv2020

Spin-orbit coupling induced splitting of Yu-Shiba-Rusinov states in antiferromagnetic dimers

Magnetic atoms coupled to the Cooper pairs of a superconductor induce Yu-Shiba-Rusinov states (in short Shiba states). In the presence of sufficiently strong spin-orbit coupling, the bands formed by hybridization of the Shiba states in ensembles of such atoms can support low-dimensional topological superconductivity with Majorana bound states localized on the ensembles' edges. Yet, the role of spin-orbit coupling for the hybridization of Shiba states in dimers of magnetic atoms, the building blocks for such systems, is largely unexplored. Here, we reveal the evolution of hybridized multi-orbital Shiba states from a single Mn adatom to artificially constructed ferromagnetically and antiferromagnetically coupled Mn dimers placed on a Nb(110) surface. Upon dimer formation, the atomic Shiba orbitals split for both types of magnetic alignment. Our theoretical calculations attribute the unexpected splitting in antiferromagnetic dimers to spin-orbit coupling and broken inversion symmetry at the surface. Our observations point out the relevance of previously unconsidered factors on the formation of Shiba bands and their topological classification.

cond-mat.supr-con↗

arXiv2020

High-resolution tunneling spin transport characteristics of topologically distinct magnetic skyrmionic textures from theoretical calculations

High-resolution tunneling electron spin transport properties (longitudinal spin current (LSC) and spin transfer torque (STT) maps) of topologically distinct real-space magnetic skyrmionic textures are reported by employing a 3D-WKB combined scalar charge and vector spin transport theory in the framework of spin-polarized scanning tunneling microscopy (SP-STM). For our theoretical investigation metastable skyrmionic spin structures with various topological charges ($Q=-3,-2,-1,0,1,2$) in the (Pt$_{0.95}$Ir$_{0.05}$)/Fe/Pd(111) ultrathin magnetic film are considered. Using an out-of-plane magnetized SP-STM tip it is found that the maps of the LSC vectors acting on the spins of the magnetic textures and all STT vector components exhibit the same topology as the skyrmionic objects. In contrast, an in-plane magnetized tip generally does not result in spin transport vector maps that are topologically equivalent to the underlying spin structure, except for the LSC vectors acting on the spins of the skyrmionic textures at a specific relation between the spin polarizations of the sample and the tip. The magnitudes of the spin transport vector quantities exhibit close relations to charge current SP-STM images irrespectively of the skyrmionic topologies. Moreover, we find that the STT efficiency (torque/current ratio) acting on the spins of the skyrmions can reach large values up to $\sim$25 meV/$μ$A ($\sim$0.97 $h/e$) above the rim of the magnetic objects, but it considerably varies between large and small values depending on the lateral position of the SP-STM tip above the topological spin textures. A simple expression for the STT efficiency is introduced to explain its variation. Our calculated spin transport vectors can be used for the investigation of spin-polarized tunneling-current-induced spin dynamics of topologically distinct surface magnetic skyrmionic textures.

cond-mat.mtrl-sci↗

arXiv2019

Non-collinear antiferromagnetic states in Ru-based Heusler compounds induced by biquadratic coupling

We investigate the magnetic properties of Ru$_{2}$Mn$Z$ ($Z$ = Sn, Sb, Ge, Si) chemically ordered full Heusler compounds for zero as well as finite temperatures. Based on first principles calculations we derive the interatomic isotropic bilinear and biquadratic couplings between Mn atoms from the paramagnetic state. We find frustrated isotropic couplings for all compounds and in case of $Z$ = Si and Sb a nearest-neighbor biquadratic coupling that favors perpendicular alignment between the Mn spins. By using an extended classical Heisenberg model in combination with spin dynamics simulations we obtain the magnetic equilibrium states. From these simulations we conclude that the biquadratic coupling, in combination with the frustrated isotropic interactions, leads to non-collinear magnetic ground states in the Ru$_{2}$MnSi and Ru$_{2}$MnSb compounds. In particular, for these alloys we find two distinct, non-collinear ground states which are energetically equivalent and can be identified as $3-q$ and $4-q$ states on a frustrated fcc lattice. Investigating the thermal stability of the non-collinear phase we find that in case of Ru$_{2}$MnSi the multiple$-q$ phase undergoes a transition to the single$-q$ phase, while in case of Ru$_{2}$MnSb the corresponding transition is not obtained due to the larger magnitude of the nearest-neighbor biquadratic coupling.

cond-mat.mtrl-sci↗

arXiv2019

Metadynamics study of the temperature dependence of magnetic anisotropy and spin-reorientation transitions in ultrathin films

We employ metadynamics simulations to calculate the free energy landscape of thin ferromagnetic films and perform a systematic study of the temperature dependence of magnetic anisotropy and of the spin-reorientation transitions. By using a simple spin model we recover the well-known power-law behavior of the magnetic anisotropy energy against magnetization and present a rather detailed analysis of the spin-reorientation transitions in ultrathin films. Based on tensorial exchange interactions and anisotropy parameters derived from first-principles calculations we perform simulations for Fe double layers deposited on Au(001) and W(110). In case of Fe$_2$W(110) our simulations display an out-of-plane to in-plane spin-reorientation transition in agreement with experiments.

cond-mat.mes-hall↗

arXiv2019

Weak ferromagnetism in hexagonal Mn3Z (Z=Sn, Ge, Ga) alloys

We present combined spin model and first principles electronic structure calculations to study the weak ferromagnetism in bulk Mn$_3$Z (Z=Sn, Ge, Ga) compounds. The spin model parameters were determined from a spin-cluster expansion technique based on the relativistic disordered local moment formalism implemented in the screened Korringa--Kohn--Rostoker method. We describe the magnetic ground state of the system within a three-sublattice model and investigate the formation of the weak ferromagnetic states in terms of the relevant model parameters. First, we give a group-theoretical argument how the point-group symmetry of the lattice leads to the formation of weak ferromagnetic states. Then we study the ground states of the classical spin model and derive analytical expressions for the weak ferromagnetic distortions by recovering the main results of the group-theoretical analysis. As a third approach we obtain the weak ferromagnetic ground states from self-consistent density functional calculations and compare our results with previous first principles calculations and with available experimental data. In particular, we demonstrate that the orbital moments follow a decomposition predicted by group theory. For a deeper understanding of the formation of weak ferromagnetism we selectively trace the effect of the spin-orbit coupling at the Mn and Z sites. In addition, for the case of Mn$_3$Ga, we gain information on the role of the induced moment of Ga from constrained local density functional calculations.

physics.comp-ph↗

arXiv2019

Magnetic structure of monatomic Fe chains on Re(0001): emergence of chiral multi-spin interactions

We present results of first-principles calculations of the magnetic properties of Fe chains deposited on the Re(0001) surface. By increasing the length of the chain, a transition is found from an almost collinear antiferromagnetic state for a 5-atom-long chain to a spin spiral state with the rotational plane slightly tilted from the surface of the substrate for the 15-atom-long chain. It is shown that a classical spin model derived from the ab initio calculations containing only two-spin interactions supports opposite chirality of the spin spiral compared to a direct optimization of the spin configuration within the ab initio method. The differences between the results of the two methods can be understood by introducing chiral four-spin interactions in the spin model.

cond-mat.mes-hall↗

arXiv2019

Exchange interactions from a nonorthogonal basis set: from bulk ferromagnets to the magnetism in low-dimensional graphene systems

We present a computational method to determine the exchange constants in isotropic spin models. The method uses the Hamiltonian and overlap matrices computed from density functional schemes that are based on nonorthogonal basis sets. We demonstrate that the new method as implemented in the SIESTA code reproduces the Heisenberg interactions of simple metallic bulk ferromagnets as obtained from former well--established computational approaches. Then we address $sp$ magnetism in graphene nanostructures. For fluorinated graphene we obtain exchange interactions in fairly good agreement with previous calculations using maximally localized Wannier functions and we confirm the theoretical prediction of a 120$^\circ$ Néel state. Associated with the magnetic edge-states of a zigzag graphene nanoribbon we find rapidly decaying exchange interactions, however, with an unconventional distance dependence of $\exp(-\sqrt{r/δ})$. We show that the stiffness constant derived from the exchange interactions is consistent with previous estimate based on total energy differences of twisted spin configurations. We highlight that our method is an efficient tool for the analysis of novel hybrid nano-structures where metallic and organic components are integrated to form exotic magnetic patterns.

cond-mat.mes-hall↗

arXiv2018

Magnetic Excitations in Non-Collinear Antiferromagnetic Weyl Semimetal $\mathsf{Mn_{3}Sn}$

$\mathsf{Mn_{3}Sn}$ has recently attracted considerable attention as a magnetic Weyl semimetal exhibiting concomitant transport anomalies at room temperature. The topology of the electronic bands, their relation to the magnetic ground state and their nonzero Berry curvature lie at the heart of the problem. The examination of the full magnetic Hamiltonian reveals otherwise hidden aspects of these unusual physical properties. Here, we report the full spin wave spectra of $\mathsf{Mn_{3}Sn}$ measured over a wide momentum - energy range by the inelastic neutron scattering technique. Using a linear spin wave theory, we determine a suitable magnetic Hamiltonian which not only explains the experimental results but also stabilizes the low-temperature helical phase, consistent with our DFT calculations. The effect of this helical ordering on topological band structures is further examined using a tight-binding method, which confirms the elimination of Weyl points in the helical phase. Our work provides a rare example of the intimate coupling between the electronic and spin degrees of freedom for a magnetic Weyl semimetal system.

cond-mat.str-el↗

arXiv2018

Theory of high-resolution tunneling spin transport on a magnetic skyrmion

Tunneling spin transport characteristics of a magnetic skyrmion are described theoretically in magnetic scanning tunneling microscopy (STM). The spin-polarized charge current in STM (SP-STM) and tunneling spin transport vector quantities, the longitudinal spin current and the spin transfer torque, are calculated in high spatial resolution within the same theoretical framework. A connection between the conventional charge current SP-STM image contrasts and the magnitudes of the spin transport vectors is demonstrated that enables the estimation of tunneling spin transport properties based on experimentally measured SP-STM images. A considerable tunability of the spin transport vectors by the involved spin polarizations is also highlighted. These possibilities and the combined theory of tunneling charge and vector spin transport pave the way for gaining deep insight into electric-current-induced tunneling spin transport properties in SP-STM and to the related dynamics of complex magnetic textures at surfaces.

cond-mat.mtrl-sci↗

arXiv2017

Spin-polarized scanning tunneling microscopy characteristics of skyrmionic spin structures exhibiting various topologies

The correct identification of topological magnetic objects in experiments is an important issue. In the present work we report on the characterization of metastable skyrmionic spin structures with various topological charges ($Q=-3,-2,-1,0,+1,+2$) in the (Pt$_{1-x}$Ir$_{x}$)Fe/Pd(111) ultrathin magnetic film by performing spin-polarized scanning tunneling microscopy (SP-STM) calculations. We find that an out-of-plane magnetized tip already results in distinguished SP-STM contrasts for the different skyrmionic structures corresponding to their symmetries. Our work also establishes an understanding of the relationship between in-plane SP-STM contrasts and skyrmionic topologies through an investigation of the variation of the in-plane angle between the spins along the perimeter of the structures, which can be characterized by the local vorticity or linear density of the winding number. For spin structures exhibiting a uniform sign of the local vorticity throughout the whole skyrmionic area, we demonstrate that (i) $|Q|$ can be determined from a single SP-STM image taken by any in-plane magnetized tip, and (ii) an in-plane tip magnetization rotation provides the sign of $Q$ independently of the sign of the effective spin polarization in the tunnel junction. We also discuss cases where the local vorticity is changing sign. Finally, by increasing the Ir content of the PtIr overlayer, we find an appearing secondary outer ring in-plane SP-STM contrast that is indicative of attractive skyrmions or antiskyrmions.

cond-mat.mtrl-sci↗