arXiv subjects

S. Mankovsky

Publications and source records attributed to S. Mankovsky.

At least 19 recordsLinked to original sources

arXiv2024

Spin current control of magnetism

Exploring novel strategies to manipulate the order parameter of magnetic materials by electrical means is of great importance, not only for advancing our understanding of fundamental magnetism, but also for unlocking potential practical applications. A well-established concept to date uses gate voltages to control magnetic properties, such as saturation magnetization, magnetic anisotropies, coercive field, Curie temperature and Gilbert damping, by modulating the charge carrier population within a capacitor structure. Note that the induced carriers are non-spin-polarized, so the control via the electric-field is independent of the direction of the magnetization. Here, we show that the magnetocrystalline anisotropy (MCA) of ultrathin Fe films can be reversibly modified by a spin current generated in Pt by the spin Hall effect. The effect decreases with increasing Fe thickness, indicating that the origin of the modification can be traced back to the interface. Uniquely, the change in MCA due to the spin current depends not only on the polarity of the charge current but also on the direction of magnetization, i.e. the change in MCA has opposite sign when the direction of magnetization is reversed. The control of magnetism by the spin current results from the modified exchange splitting of majority- and minority-spin bands, and differs significantly from the manipulation by gate voltages via a capacitor structure, providing a functionality that was previously unavailable and could be useful in advanced spintronic devices.

physics.app-ph↗

arXiv2021

Electric-field control of the exchange interactions

The impact of an applied electric field on the exchange coupling parameters has been investigated based on first-principles electronic structure calculations by means of the KKR Green function method. The calculations have been performed for a Fe film, free-standing and deposited on two different substrates, having 1 monolayer (ML) thickness to minimize the effect of screening of the electric field typical for metallic systems. By comparing the results for the free-standing Fe ML with those for Fe on the various substrates, we could analyze the origin of the field-induced change of the exchange interactions. Compared to the free-standing Fe ML, in particular rather pronounced changes have been found for the Fe/Pt(111) system due to the localized electronic states at the Fe/Pt interface, which are strongly affected by the electric field and which play an important role for the Fe-Fe exchange interactions.

cond-mat.mtrl-sci↗

arXiv2020

Exchange coupling constants at finite temperature

An approach to account for the effect of thermal lattice vibrations when calculating exchange coupling parameters is presented on the basis of the KKR (Korringa-Kohn-Rostoker) Green function method making use of the alloy analogy model. Using several representative systems, it is shown that depending on the material the effect of thermal lattice vibrations can have a significant impact on the isotropic exchange as well as anisotropic Dzyaloshinskii-Moriya interactions (DMI). This should lead in turn to an additional contribution to the temperature dependence of the magnetic properties of solids, which cannot be neglected in the general case. As an example, we discuss such an influence on the critical temperature of various magnetic phase transitions. In particular, in the case of skyrmion hosting materials, a strong impact of lattice vibrations on the DMI is an additional source for temperature dependent skyrmion stability which should be taken into consideration.

cond-mat.mtrl-sci↗

arXiv2020

Magnetic-field-induced FM-AFM metamagnetic transition and strong negative magnetoresistance in Mn$_{1/4}$NbS$_2$ under pressure

Transition metal dichalcogenides (TMDC) stand out with their high chemical stability and the possibility to incorporate a wide range of magnetic species between the layers. The behavior of conduction electrons in such materials intercalated by 3d-elements is closely related to their magnetic properties and can be sensitively controlled by external magnetic fields. Here, we study the magnetotransport properties of NbS$_2$ intercalated with Mn, Mn$_{1/4}$NbS$_2$, demonstrating a complex behavior of the magnetoresistance and of the ordinary and anomalous Hall resistivities. Application of pressure as tuning parameter leads to the drastic changes of the magnetotransport properties of Mn$_{1/4}$NbS$_2$ exhibiting large negative magnetoresistance up to $65 \%$ at 7.1 GPa. First-principles electronic structure calculations indicates pressure-induced transition from ferromagnetic to antiferromagnetic state. Theoretical calculations accounting for the finite temperature magnetic properties of Mn$_{1/4}$NbS$_2$ suggest a field-induced metamagnetic ferromagnetic-antiferromagnetic transition as an origin of the large negative magentoresistance. These results inspire the development of materials for spintronic applications based on intercalated TMDC with a well controllable metamagnetic transition.

cond-mat.mtrl-sci↗

arXiv2019

The 2020 Skyrmionics Roadmap

The notion of non-trivial topological winding in condensed matter systems represents a major area of present-day theoretical and experimental research. Magnetic materials offer a versatile platform that is particularly amenable for the exploration of topological spin solitons in real space such as skyrmions. First identified in non-centrosymmetric bulk materials, the rapidly growing zoology of materials systems hosting skyrmions and related topological spin solitons includes bulk compounds, surfaces, thin films, heterostructures, nano-wires and nano-dots. This underscores an exceptional potential for major breakthroughs ranging from fundamental questions to applications as driven by an interdisciplinary exchange of ideas between areas in magnetism which traditionally have been pursued rather independently. The skyrmionics roadmap provides a review of the present state of the art and the wide range of research directions and strategies currently under way. These are, for instance, motivated by the identification of the fundamental structural properties of skyrmions and related textures, processes of nucleation and annihilation in the presence of non-trivial topological winding, an exceptionally efficient coupling to spin currents generating spin transfer torques at tiny current densities, as well as the capability to purpose-design broad-band spin dynamic and logic devices.

cond-mat.str-el↗

arXiv2019

Extension of the standard Heisenberg Hamiltonian to multispin exchange interactions

An extension of the Heisenberg Hamiltonian is discussed, that allows to go beyond the standard bilinear spin Hamiltonian taking into account various contributions due to multispin interactions having both chiral and non-chiral character. The parameters of the extended Hamiltonian are calculated from first principles within the framework of the multiple scattering Green function formalism giving access to an explicit representation of these parameters in real space. The discussions are focused on the chiral interactions, i.e.\ biquadratic and three-spin Dzyaloshinskii-Moriya like vector interactions $\vec{\cal{D}}_{ijij}$ (BDMI) and $\vec{\cal{D}}_{ijkj}$ (TDMI), respectively, as well as the three-spin chiral interaction (TCI) $J_{ijk}$. Although all parameters are driven by spin-orbit coupling (SOC), some differences in their properties are demonstrated by calculations for real materials. In particular it is shown that the three-spin chiral interactions $J_{ijk}$ may be topology as well as SOC induced, while the TDMI is associated only with the SOC. As the magnitude of the chiral interactions can be quite sizable, they can lead to a stabilization of a noncollinear magnetic texture in some materials that is absent when these interactions are neglected.

cond-mat.mtrl-sci↗

arXiv2018

Spin-wave stiffness and micromagnetic exchange interactions expressed by means of the KKR Green function approach

We represent an approach to calculate micromagnetic model parameters such as the tensor of exchange stiffness, Dzyaloshinskii-Moriya interaction (DMI) as well as spin-wave stiffness. The scheme is based on the fully relativistic Korringa-Kohn-Rostoker Green function (KKR-GF) technique and can be seen as a relativistic extension of the work of Lichtenstein {\em et al.} The expression for $D^{z\alpha}$ elements of DMI differ from the expressions for $D^{x\alpha}$ and $D^{y\alpha}$ elements as the former are derived via second-order perturbation term of the energy caused by spin-spiral while the latter are associated with the first-order term. Corresponding numerical results are compared with those obtained using other schemes reported in the literature.

cond-mat.mtrl-sci↗

arXiv2018

Uncovering electron scattering mechanisms in NiFeCoCrMn derived concentrated solid solution and high entropy alloys

Whilst it has long been known that disorder profoundly affects transport properties, recent measurements on a series of solid solution 3d-transition metal alloys reveal two orders of magnitude variations in the residual resistivity. Using ab-initio methods, we demonstrate that, while the carrier density of all alloys is as high as in normal metals, the electron mean-free-path can vary from ~10 {\AA} (strong scattering limit) to ~10$^3$ {\AA} (weak scattering limit). Here, we delineate the underlying electron scattering mechanisms responsible for this disparate behavior. While spin dependent site-diagonal disorder is always dominant, for alloys containing only Fe, Co, and Ni the majority spin channel experiences negligible disorder scattering, thereby providing a short circuit, while for Cr/Mn containing alloys both spin channels experience strong disorder scattering due to an electron filling effect. Unexpectedly, other scattering mechanisms (e.g. displacement scattering) are found to be relatively weak in most cases.

cond-mat.mtrl-sci↗

arXiv2018

Gilbert damping in non-collinear magnetic system

The modification of the magnetization dissipation or Gilbert damping caused by an inhomogeneous magnetic structure and expressed in terms of a wave vector dependent tensor $\underline{\alpha}(\vec{q})$ is investigated by means of linear response theory. A corresponding expression for $\underline{\alpha}(\vec{q})$ in terms of the electronic Green function has been developed giving in particular the leading contributions to the Gilbert damping linear and quadratic in $q$. Numerical results for realistic systems are presented that have been obtained by implementing the scheme within the framework of the fully relativistic KKR (Korringa-Kohn-Rostoker) band structure method. Using the multilayered system (Cu/Fe$_{1-x}$Co$_x$/Pt)$_n$ as an example for systems without inversion symmetry we demonstrate the occurrence of non-vanishing linear contributions. For the alloy system bcc Fe$_{1-x}$Co$_x$ having inversion symmetry, on the other hand, only the quadratic contribution is non-zero. As it is shown, this quadratic contribution does not vanish even if the spin-orbit coupling is suppressed, i.e.\ it is a direct consequence of the non-collinear spin configuration.

cond-mat.mtrl-sci↗

arXiv2017

Composition-dependent magnetic response properties of Mn$_{1-x}$Fe$_x$Ge alloys

The composition-dependent behavior of the Dzyaloshinskii-Moriya interaction (DMI), the spin-orbit torque (SOT), as well as anomalous and spin Hall conductivities of Mn$_{1-x}$Fe$_x$Ge alloys have been investigated by first-principles calculations using the relativistic multiple scattering Korringa-Kohn-Rostoker (KKR) formalism. The $D_{\rm xx}$ component of the DMI exhibits a strong dependence on the Fe concentration, changing sign at $x \approx 0.85$ in line with previous theoretical calculations as well as with experimental results demonstrating the change of spin helicity at $x \approx 0.8$. A corresponding behavior with a sign change at $x \approx 0.5$ is predicted also for the Fermi sea contribution to the SOT, as this is closely related to the DMI. In the case of anomalous and spin Hall effects it is shown that the calculated Fermi sea contributions are rather small and the composition-dependent behavior of these effects are determined mainly by the electronic states at the Fermi level. The spin-orbit-induced scattering mechanisms responsible for both these effects suggest a common origin of the minimum of the AHE and the sign change of the SHE conductivities.

cond-mat.mtrl-sci↗

arXiv2017

An accurate scheme to calculate the interatomic Dzyaloshinskii-Moriya interaction parameters

An new and accurate scheme to calculate the interatomic Dzyaloshinskii-Moriya interaction (DMI) parameters is presented, which is based on the fully relativistic Korringa-Kohn-Rostoker Green function (KKR-GF) technique. Corresponding numerical results are compared with those obtained using other schemes reported in the literature. The differences found can be attributed primarily to the different reference states used in the various approaches. In addition an expression for the DMI parameters formulated for a micromagnetic model Hamiltonian is presented that provides a connection to the DMI parameters calculated for atomistic Hamiltonians. This formulation also allows the discussion of the DMI in terms of specific features of the electronic band structure.

cond-mat.mtrl-sci↗

arXiv2016

Magnetism and ultra-fast magnetization dynamics of Co and CoMn alloys at finite temperature

Temperature-dependent magnetic experiments like pump-probe measurements generated by a pulsed laser have become a crucial technique for switching the magnetization in the picosecond time scale. Apart from having practical implications on the magnetic storage technology, the research field of ultrafast magnetization poses also fundamental physical questions. To correctly describe the time evolution of the atomic magnetic moments under the influence of a temperature-dependent laser pulse, it remains crucial to know if the magnetic material under investigation has magnetic excitation spectrum that is more or less dependent on the magnetic configuration, e.g. as reflected by the temperature dependence of the exchange interactions. In this article, we demonstrate from first-principles theory that the magnetic excitation spectra in Co with fcc, bcc and hcp structures are nearly identical in a wide range of non-collinear magnetic configurations. This is a curious result of a balance between the size of the magnetic moments and the strength of the Heisenberg exchange interactions, that in themselves vary with configuration, but put together in an effective spin Hamiltonian results in a configuration independent effective model. We have used such a Hamiltonian, together with ab-initio calculated damping parameters, to investigate the magnon dispersion relationship as well as the ultrafast magnetisation dynamics of Co and Co-rich CoMn alloys.

cond-mat.mtrl-sci↗

arXiv2016

Electronic and magnetic properties of the 2H-NbS$_2$ intercalated by 3d transition metal atoms

The electronic structure and magnetic properties of the 2H-NbS$_2$ compound intercalated by Cr, Mn and Fe, have been investigated by means of the Korringa-Kohn-Rostoker (KKR) method. The calculations demonstrate easy plane magneto-crystalline anisotropy (MCA) of Cr$_{1/3}$NbS$_2$ monotonously decreasing towards the Curie temperature in line with the experimental results. The modification of the electronic structure results in a change of the easy axis from in-plane to out-of-plane. It is shown, that for Cr$_{1/3}$NbS$_2$ and Mn$_{1/3}$NbS$_2$ the in-plane MCA and Dzyaloshinskii-Moriya interactions results in a helimagnetic structure along the hexagonal $c$ axis, following the experimental observations. The negative exchange interactions in the Fe$_{1/3}$NbS$_2$ compound results in a non-collinear frustrated magnetic structure if the MCA is not taken into account. It is shown, however, that a strong MCA along the hexagonal $c$ axis leads to a magnetic ordering referred to as an ordering of the third kind, which was observed experimentally.

cond-mat.mtrl-sci↗

arXiv2016

The temperature dependence of FeRh's transport properties

The finite-temperature transport properties of FeRh compounds are investigated by first-principles Density Functional Theory-based calculations. The focus is on the behavior of the longitudinal resistivity with rising temperature, which exhibits an abrupt decrease at the metamagnetic transition point, $T = T_m$ between ferro- and antiferromagnetic phases. A detailed electronic structure investigation for $T \geq 0$ K explains this feature and demonstrates the important role of (i) the difference of the electronic structure at the Fermi level between the two magnetically ordered states and (ii) the different degree of thermally induced magnetic disorder in the vicinity of $T_m$, giving different contributions to the resistivity. To support these conclusions, we also describe the temperature dependence of the spin-orbit induced anomalous Hall resistivity and Gilbert damping parameter. For the various response quantities considered the impact of thermal lattice vibrations and spin fluctuations on their temperature dependence is investigated in detail. Comparison with corresponding experimental data finds in general a very good agreement.

cond-mat.mtrl-sci↗

arXiv2016

Non-magnetic ground state of Ni adatoms on Te-terminated bismuth chalcogenide topological insulators

We report on the quenching of single Ni adatom moments on Te-terminated Bi2Te2Se and Bi2Te3 topological insulator surfaces. The effect becomes manifested as a missing X-ray magnetic circular dichroism for resonant L3,2 transitions into partially filled Ni 3d states of occupancy n_d = 9.2. On the basis of a comparative study of Ni and Fe using scanning tunneling microscopy and ab initio calculations we are able to relate the element specific moment formation to a local Stoner criterion. While Fe adatoms form large spin moments of m_s = 2.54 mu_B with out-of-plane anisotropy due to a sufficiently large density of states at the Fermi energy, Ni remains well below an effective Stoner threshold for local moment formation. With the Fermi level remaining in the bulk band gap after adatom deposition, non-magnetic Ni and preferentially out-of-plane oriented magnetic Fe with similar structural properties on Bi2Te2Se surfaces constitute a perfect platform to study off-on effects of time-reversal symmetry breaking on topological surface states.

cond-mat.mtrl-sci↗

arXiv2015

Fully relativistic multiple scattering calculations for general potentials

The formal basis for fully relativistic Korringa-Kohn-Rostoker (KKR) or multiple scattering calculations for the electronic Green function in case of a general potential is discussed. Simple criteria are given to identify situations that require to distinguish between right and left hand side solutions to the Dirac equation when setting up the electronic Green function. In addition various technical aspects of an implementation of the relativistic KKR for general local and non-local potentials will be discussed.

cond-mat.other↗

arXiv2015

Finite temperature magnetism of FeRh compounds

The temperature dependent stability of the magnetic phases of FeRh were investigated by means of total energy calculations with magnetic disorder treated within the uncompensated disordered local moment (uDLM) approach. In addition, Monte Carlo simulations based on the extended Heisenberg model have been performed, using exchange coupling parameters obtained rom first principles. The crucial role and interplay of two factors in the metamagnetic transition in FeRh has been revealed, namely the dependence of the Fe-Fe exchange coupling parameters on the temperature-governed degree of magnetic disorder in the system and the stabilizing nature of the induced magnetic moment on Rh-sites. An important observation is the temperature dependence of these two competing factors.

cond-mat.mtrl-sci↗

arXiv2015

Dzyaloshinskii-Moriya interactions and magnetic texture in the Fe films deposited on transition-metal dichalcogenides

The magnetic properties of materials based on two-dimensional transition-metal dichalcogenides (TMDC) have been investigated by means of first-principles DFT calculations, namely Fe-intercalated bulk Fe$_{1/4}$TaS$_2$ compounds as well as TMDC monolayers with deposited Fe films. Changing the structure and the composition of systems consisting of Fe overlayers on top of a TMDC monolayers resulted in considerable variations of their physical properties. For the considered systems the Dzyaloshinskii-Moriya (DM) interaction has been determined and used for the subsequent investigation of their magnetic structure using Monte Carlo simulations. Rather strong DM interactions as well as large $D/J$ ratios have been obtained in some of these materials, which can lead to the formation of skyrmionic structures varying with the strength of the applied external magnetic field.

cond-mat.mtrl-sci↗