arXiv subjects

Yuriy Mokrousov

Publications and source records attributed to Yuriy Mokrousov.

At least 127 records · Page 7Linked to original sources

arXiv2019

Fully spin-polarized nodal loop semimetals in alkaline-metal monochalcogenide monolayers

Topological semimetals in ferromagnetic materials have attracted enormous attention due to the potential applications in spintronics. Using the first-principles density functional theory together with an effective lattice model, here we present a new family of topological semimetals with a fully spin-polarized nodal loop in alkaline-metal monochalcogenide $MX$ ($M$ = Li, Na, K, Rb, Cs; $X$ = S, Se, Te) monolayers. The half-metallic ferromagnetism can be established in $MX$ monolayers, in which one nodal loop formed by two crossing bands with the same spin components is found at the Fermi energy. This nodal loop half-metal survives even when considering the spin-orbit coupling owing to the symmetry protection provided by the $\mathcal{M}_{z}$ mirror plane. The quantum anomalous Hall state and Weyl-like semimetal in this system can be also achieved by rotating the spin from the out-of-plane to the in-plane direction. The $MX$ monolayers hosting rich topological phases thus offer an excellent materials platform for realizing the advanced spintronics concepts.

cond-mat.mtrl-sci↗

arXiv2019

Spin-order dependent anomalous Hall effect and magneto-optical effect in noncollinear antiferromagnets Mn$_{3}X$N ($X$ = Ga, Zn, Ag, and Ni)

Noncollinear antiferromagnets (AFMs) have recently attracted a lot of attention owing to the potential emergence of exotic spin orders on geometrically frustrated lattices, which can be characterized by corresponding spin chiralities. By performing first-principles density functional calculations together with group-theory analysis and tight-binding modelling, here we systematically study the spin-order dependent anomalous Hall effect (AHE) and magneto-optical effect (MOE) in representative noncollinear AFMs Mn$_{3}X$N ($X$ = Ga, Zn, Ag, and Ni). The symmetry-related tensor shape of the intrinsic anomalous Hall conductivity (IAHC) for different spin orders is determined by analyzing the relevant magnetic point groups. We show that while only the ${xy}$ component of the IAHC tensor is nonzero for right-handed spin chirality, all other elements, $σ_{xy}$, $σ_{yz}$, and $σ_{zx}$, are nonvanishing for a state with left-handed spin chirality owing to lowering of the symmetry. Our tight-binding arguments reveal that the magnitude of IAHC relies on the details of the band structure and that $σ_{xy}$ is periodically modulated as the spin rotates in-plane. The IAHC obtained from first principles is found to be rather large, e.g., it amounts to 359 S/cm in Mn$_{3}$AgN. By extending our analysis to finite frequencies, we calculate the optical isotropy [$σ_{xx}(ω)\approxσ_{yy}(ω)\approxσ_{zz}(ω)$] and the magneto-optical anisotropy [$σ_{xy}(ω)\neqσ_{yz}(ω)\neqσ_{zx}(ω)$] of Mn$_{3}X$N. We argue that the spin-order dependent AHE and MOE are indispensable in detecting complex spin structures in noncollinear AFMs.

cond-mat.mtrl-sci↗

arXiv2018

Engineering Chiral and Topological Orbital Magnetism of Domain Walls and Skyrmions

Electrons which are slowly moving through chiral magnetic textures can effectively be described as if they where influenced by electromagnetic fields emerging from the real-space topology. This adiabatic viewpoint has been very successful in predicting physical properties of chiral magnets. Here, based on a rigorous quantum-mechanical approach, we unravel the emergence of chiral and topological orbital magnetism in one- and two-dimensional spin systems. We uncover that the quantized orbital magnetism in the adiabatic limit can be understood as a Landau-Peierls response to the emergent magnetic field. Our central result is that the spin-orbit interaction in interfacial skyrmions and domain walls can be used to tune the orbital magnetism over orders of magnitude by merging the real-space topology with the topology in reciprocal space. Our findings point out the route to experimental engineering of orbital properties of chiral spin systems, thereby paving the way to the field of chiral orbitronics.

cond-mat.mtrl-sci↗

arXiv2018

Helical magnetic structure and the anomalous and topological Hall effects in epitaxial B20 Fe$_{1-y}$Co$_y$Ge films

Epitaxial films of the B20-structure alloy Fe$_{1-y}$Co$_y$Ge were grown by molecular beam epitaxy on Si (111) substrates. The magnetization varied smoothly from the bulk-like values of one Bohr magneton per Fe atom for FeGe to zero for non-magnetic CoGe. The chiral lattice structure leads to a Dzyaloshinskii-Moriya interaction (DMI), and the films' helical magnetic ground state was confirmed using polarized neutron reflectometry measurements. The pitch of the spin helix, measured by this method, varies with Co content $y$ and diverges at $y \sim 0.45$. This indicates a zero-crossing of the DMI, which we reproduced in calculations using first principle methods. We also measured the longitudinal and Hall resistivity of our films as a function of magnetic field, temperature, and Co content $y$. The Hall resistivity is expected to contain contributions from the ordinary, anomalous, and topological Hall effects. Both the anomalous and topological Hall resistivities show peaks around $y \sim 0.5$. Our first principles calculations show a peak in the topological Hall constant at this value of $y$, related to the strong spin-polarisation predicted for intermediate values of $y$. Half-metallicity is predicted for $y = 0.6$, consistent with the experimentally observed linear magnetoresistance at this composition. Whilst it is possible to reconcile theory with experiment for the various Hall effects for FeGe, the large topological Hall resistivities for $y \sim 0.5$ are much larger then expected when the very small emergent fields associated with the divergence in the DMI are taken into account.

cond-mat.mtrl-sci↗

arXiv2018

Spin-orbit torques and tunable Dzyaloshinskii-Moriya interaction in Co/Cu/Co trilayers

We study the spin-orbit torques (SOTs) in Co/Cu/Co magnetic trilayers based on first-principles density-functional theory calculations in the case where the applied electric field lies in-plane, i.e., parallel to the interfaces. We assume that the bottom Co layer has a fixed in-plane magnetization, while the top Co layer can be switched. We find that the SOT on the top ferromagnet can be controlled by the bottom ferromagnet because of the nonlocal character of the SOT in this system. As a consequence the SOT is anisotropic, i.e., its magnitude varies with the direction of the applied electric field. We show that the Dzyaloshinskii-Moriya interaction (DMI) in the top layer is anisotropic as well, i.e., the spin-spiral wavelength of spin-spirals in the top layer depends on their in-plane propagation direction. This effect suggests that DMI can be tuned easily in magnetic trilayers via the magnetization direction of the bottom layer. In order to understand the influence of the bottom ferromagnet on the SOTs and the DMI of the top ferromagnet we study these effects in Co/Cu magnetic bilayers for comparison. We find the SOTs and the DMI to be surprisingly large despite the small spin-orbit interaction of Cu.

cond-mat.mes-hall↗

arXiv2017

Higher-dimensional Wannier interpolation for the modern theory of the Dzyaloshinskii-Moriya interaction: Application to Co-based trilayers

We present an advanced first-principles formalism to evaluate the Dzyaloshinskii-Moriya interaction (DMI) in its modern theory as well as Berry curvatures in complex spaces based on a higher-dimensional Wannier interpolation. Our method is applied to the Co-based trilayer systems Ir$_δ$Pt$_{1-δ}$/Co/Pt and Au$_γ$Pt$_{1-γ}$/Co/Pt, where we gain insights into the correlations between the electronic structure and the DMI, and we uncover prominent sign changes of the chiral interaction with the overlayer composition. Beyond the discussed phenomena, the scope of applications of our Wannier-based scheme is particularly broad as it is ideally suited to study efficiently the Hamiltonian evolution under the slow variation of very general parameters.

cond-mat.mtrl-sci↗

arXiv2017

Chiral damping, chiral gyromagnetism and current-induced torques in textured one-dimensional Rashba ferromagnets

We investigate Gilbert damping, spectroscopic gyromagnetic ratio and current-induced torques in the one-dimensional Rashba model with an additional noncollinear magnetic exchange field. We find that the Gilbert damping differs between left-handed and right-handed Néel-type magnetic domain walls due to the combination of spatial inversion asymmetry and spin-orbit interaction (SOI), consistent with recent experimental observations of chiral damping. Additionally, we find that also the spectroscopic $g$ factor differs between left-handed and right-handed Néel-type domain walls, which we call chiral gyromagnetism. We also investigate the gyromagnetic ratio in the Rashba model with collinear magnetization, where we find that scattering corrections to the $g$ factor vanish for zero SOI, become important for finite spin-orbit coupling, and tend to stabilize the gyromagnetic ratio close to its nonrelativistic value.

cond-mat.mes-hall↗

arXiv2017

The relation of the Dzyaloshinskii-Moriya interaction to spin currents and to the spin-orbit field

Starting from the general Berry phase theory of the Dzyaloshinskii-Moriya interaction (DMI) we derive an expression for the linear contribution of the spin-orbit interaction (SOI). Thereby, we show analytically that at the first order in SOI DMI is given by the ground-state spin current. We verify this finding numerically by ab-initio calculations in Mn/W(001) and Co/Pt(111) magnetic bilayers. We show that despite the strong SOI from the 5$d$ heavy metals DMI is well-approximated by the first order in SOI, while the ground-state spin current is not. We decompose the SOI-linear contribution to DMI into two parts. One part has a simple interpretation in terms of the Zeeman interaction between the spin-orbit field and the spin misalignment that electrons acquire in magnetically noncollinear textures. This interpretation provides also an intuitive understanding of the symmetry of DMI on the basis of the spin-orbit field and it explains in a simple way why DMI and ground-state spin currents are related. Moreover, we show that energy currents driven by magnetization dynamics and associated to DMI can be explained by counter-propagating spin currents that carry energy due to their Zeeman interaction with the spin-orbit field. Finally, we discuss options to modify DMI by nonequilibrium spin currents excited by electric fields or light.

cond-mat.mtrl-sci↗

arXiv2017

Chiral and Topological Orbital Magnetism of Spin Textures

Using a semiclassical Green's function formalism, we discover the emergence of chiral and topological orbital magnetism in two-dimensional chiral spin textures by explicitly finding the corrections to the orbital magnetization, proportional to the powers of the gradients of the texture. We show that in the absence of spin-orbit coupling, the resulting orbital moment can be understood as the electronic response to the emergent magnetic field associated with the real-space Berry curvature. By referring to the Rashba model, we demonstrate that by tuning the parameters of surface systems the engineering of emergent orbital magnetism in spin textures can pave the way to novel concepts in orbitronics.

cond-mat.mtrl-sci↗

arXiv2017

Topological antiferromagnetic spintronics: Part of a collection of reviews on antiferromagnetic spintronics

The recent demonstrations of electrical manipulation and detection of antiferromagnetic spins have opened up a chapter in the spintronics story. In this article, we review the emerging research field that is exploring synergies between antiferromagnetic spintronics and topological structures in real and momentum space. Active topics include proposals to realize Majorana fermions in an antiferromagnetic topological superconductors, to control topological protection of Dirac points by manipulating antiferromagnetic order parameters, and to exploit the anomalous and topological Hall effects of zero net moment antiferromagnets. We explain the basic physics concepts behind these proposals, and discuss potential applications of topological antiferromagnetic spintronics.

cond-mat.mes-hall↗

arXiv2017

Mixed Weyl semimetals and dissipationless magnetization control in insulators by spin-orbit torques

Reliable and energy efficient magnetization switching by electrically-induced spin-orbit torques is of crucial technological relevance for spintronic devices implementing memory and logic functionality. Here we predict that the strength of spin-orbit torques and the related Dzyaloshinskii-Moriya interaction in topologically non-trivial magnetic insulators can exceed by far that of conventional metallic magnets. In analogy to the quantum anomalous Hall effect, we explain this extraordinary response in absence of longitudinal currents as a hallmark of magnetic monopoles in the electronic structure of systems that are interpreted most naturally within the framework of mixed Weyl semimetals. We thereby launch the effect of spin-orbit torque into the field of topology and reveal its crucial role in mediating the topological phase transitions arising due to the complex interplay between magnetization direction and momentum-space topology. The concepts presented here may be exploited to understand and utilize magneto-electric coupling phenomena in insulating ferromagnets and antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2017

Geometrical contributions to the exchange constants: Free electrons with spin-orbit interaction

Using thermal quantum field theory we derive an expression for the exchange constant that resembles Fukuyama's formula for the orbital magnetic susceptibility (OMS). Guided by this formal analogy between the exchange constant and OMS we identify a contribution to the exchange constant that arises from the geometrical properties of the band structure in mixed phase space. We compute the exchange constants for free electrons and show that the geometrical contribution is generally important. Our formalism allows us to study the exchange constants in the presence of spin-orbit interaction (SOI). Thereby, we find sizable differences between the exchange constants of helical and cycloidal spin spirals. Furthermore, we discuss how to calculate the exchange constants based on a gauge-field approach in the case of the Rashba model with an additional exchange splitting and show that the exchange constants obtained from this gauge-field approach are in perfect agreement with those obtained from the quantum field theoretical method.

cond-mat.mes-hall↗

arXiv2017

Charge pumping driven by the laser-induced dynamics of the exchange splitting

We show that electric currents are induced in inversion asymmetric ferromagnets if the exchange splitting varies in time after excitation by laser pulses. We interpret this phenomenon as the magnetic variant of the inverse Edelstein effect. Based on \textit{ab initio} calculations we determine the size of this effect in Co/Pt(111) and Mn/W(001) magnetic bilayer systems and investigate its dependence on the magnetization direction. The comparison of our theoretical results to experiments measuring the THz signal emitted after laser excitation suggests that ultrafast demagnetization in 3$d$ transition metal ferromagnets is dominated by transverse spin fluctuations rather than by a reduction of the exchange splitting.

cond-mat.mtrl-sci↗

arXiv2017

Topological Nodal-line Semimetals in Two Dimensions with time-reversal symmetry breaking

Topological nodal-line semimetals (TNLSs) exhibit exotic physical phenomena due to a one-dimensional (1D) band touching line, rather than discrete (Dirac or Weyl) points. While so far proposed two-dimensional (2D) TNLSs possess closed nodal lines (NLs) only when spin-orbit coupling (SOC) is neglected, here using Na$_3$Bi trilayers as an example, we show that 2D TNLSs can been obtained from topological (crystalline) insulators (TI/TCI) by time-reversal symmetry breaking even in the presence of SOC. We further reveal that these obtained NLs are protected by crystalline mirror symmetry, while a mirror symmetry breaking perturbation opens a full gap thus giving rise to a phase transition from 2D TNLS to a quantum anomalous Hall insulator (QAHI). We thereby uncover a close correlation between various topological phases. Remarkably, a strong spin Hall effect, important for transport applications, is predicted in 2D TNLS. Finally, a Na$_2$CrBi trilayer is proposed to realize the 2D TNLS without extrinsic magnetic field. Our work not only proposes a new strategy for realizing 2D TNLSs with truely closed NLs, but also reveals potential applications of TNLS in spintronics.

cond-mat.mtrl-sci↗

arXiv2017

Two-dimensional topological nodal line semimetal in layered $X_2Y$ ($X$ = Ca, Sr, and Ba; $Y$ = As, Sb, and Bi)

In topological semimetals the Dirac points can form zero-dimensional and one-dimensional manifolds, as predicted for Dirac/Weyl semimetals and topological nodal line semimetals, respectively. Here, based on first-principles calculations, we predict a topological nodal line semimetal phase in the two-dimensional compounds $X_2Y$ ($X$=Ca, Sr, and Ba; $Y$=As, Sb, and Bi) in the absence of spin-orbit coupling (SOC) with a band inversion at the M point. The mirror symmetry as well as the electrostatic interaction, that can be engineered via strain, are responsible for the nontrivial phase. In addition, we demonstrate that the exotic edge states can be also obtained without and with SOC although a tiny gap appears at the nodal line for the bulk states when SOC is included.

cond-mat.mtrl-sci↗

arXiv2017

Non-local field-like spin-orbit torques in Rashba systems: an ab-initio study of Ag$_{2}$Bi/Ag/Fe film

We investigate from first principles the field-like spin-orbit torques (SOTs) in a Ag$_{2}$Bi-terminated Ag(111) film grown on ferromagnetic Fe(110). We find that a large part of the SOT arises from the spin-orbit interaction (SOI) in the Ag$_{2}$Bi layer far away from the Fe layers. These results clearly hint at a long range spin transfer in the direction perpendicular to the film that does not originate in the spin Hall effect. In order to bring evidence of the non-local character of the computed SOT, we show that the torque acting on the Fe layers can be engineered by the introduction of Bi vacancies in the Ag$_{2}$Bi layer. Overall, we find a drastic dependence of the SOT on the disorder type, which we explain by a complex interplay of different contributions to the SOT in the Brillouin zone.

cond-mat.mtrl-sci↗

arXiv2017

Robust Dual Topological Character with Spin-Valley Polarization in a Monolayer of the Dirac Semimetal Na$_3$Bi

Topological materials with both insulating and semimetal phases can be protected by crystalline (e.g. mirror) symmetry. The insulating phase, called topological crystalline insulator (TCI), has been intensively investigated and observed in three-dimensional materials. However, the predicted two-dimensional (2D) materials with TCI phase are explored much less than 3D TCIs and 2D topological insulator, while so far considered 2D TCIs almost exclusively possess a square lattice structure with the mirror Chern number $\mathcal C_{M} =-2$. Here, we predict theoretically that hexagonal monolayer of Dirac semimetal Na$_3$Bi is a 2D TCI with a mirror Chern number $\mathcal C_{M} =-1$. The large nontrivial gap of 0.31 eV is tunable and can be made much larger via strain engineering while the topological phases are robust against strain, indicating a high possibility for room-temperature observation of quantized conductance. In addition, a nonzero spin Chern number $\mathcal C_{S} =-1$ is obtained, indicating the coexistence of 2D topological insulator and 2D TCI, i.e. the dual topological character. Remarkably, a spin-valley polarization is revealed in Na$_3$Bi monolayer due to the breaking of crystal inversion symmetry. The dual topological character is further explicitly confirmed via unusual edge states' behavior under corresponding symmetry breaking.

cond-mat.mtrl-sci↗

arXiv2017

Topological spin Hall effect in antiferromagnetic skyrmions

The topological Hall effect (THE), as one of the primary manifestations of non-trivial topology of chiral skyrmions, is traditionally used to detect the emergence of skyrmion lattices with locally ferromagnetic order. In this work we demonstrate that the appearance of non-trivial two-dimensional chiral textures with locally {\it anti}-ferromagnetic order can be detected through the spin version of the THE $-$ the topological spin Hall effect (TSHE). Utilizing the semiclassical formalism, here used to combine chiral antiferromagnetic textures with a density functional theory description of the collinear, degenerate electronic structure, we follow the real-space real-time evolution of electronic SU(2) wavepackets in an external electric field to demonstrate the emergence of sizeable transverse pure spin current in synthetic antiferromagnets of the Fe/Cu/Fe trilayer type. We further unravel the extreme sensitivity of the TSHE to the details of the electronic structure, suggesting that the magnitude and sign of the TSHE in transition-metal synthetic antiferromagnets can be engineered by tuning such parameters as the thickness or band filling. Besides being an important step in our understanding of the topological properties of ever more complex skyrmionic systems, our results bear great potential in stimulating the discovery of antiferromagnetic skyrmions.

cond-mat.mtrl-sci↗