arXiv subjects

Yuriy Mokrousov

Publications and source records attributed to Yuriy Mokrousov.

179 records · Page 10Linked to original sources

arXiv2012

Conductance fingerprints of non-collinear magnetic states in single atom contacts: a first-principles Wannier functions study

We present a first-principles computational scheme for investigating the ballistic transport properties of one-dimensional nanostructures with non-collinear magnetic order. The electronic structure is obtained within density functional theory as implemented in the full-potential linearized augmented plane-wave (FLAPW) method and mapped to a tight-binding like transport Hamiltonian via non-collinear Wannier functions. The conductance is then computed based on the Landauer formula using the Green's function method. As a first application we study the conductance between two ferromagnetic Co monowires terminated by single Mn apex atoms as a function of Mn-Mn separation. We vary the Mn-Mn separation from the contact (about 2.5 to 5 Å) to the far tunneling regime (5 to 10 Å). The magnetization direction of the Co electrodes is chosen either in parallel or antiparallel alignment and we allow for different spin configurations of the two Mn spins. In the tunneling and into the contact regime the conductance is dominated by $s$-$d_{z^2}$-states. In the close contact regime (below 3.5 Å) there is an additional contribution for a parallel magnetization alignment from the $d_{xz}$- and $d_{yz}$-states which give rise to an increase of the magnetoresistance as it is absent for antiparallel magnetization. If we allow the Mn spins to relax a non-collinear spin state is formed close to contact. We demonstrate that the transition from a collinear to such a non-collinear spin structure as the two Mn atoms approach leaves a characteristic fingerprint in the distance-dependent conductance and magnetoresistance of the junction. We explain the effect of the non-collinear spin state on the conductance based on the spin-dependent hybridization between the $d_{xz,yz}$-states of the Mn spins and their coupling to the Co electrodes.

cond-mat.mes-hall↗

arXiv2012

Topological phases of Bi(111) bilayer in an external exchange field

Using first principles methods, we investigate topological phase transitions as a function of exchange field in a Bi(111) bilayer. Evaluation of the spin Chern number for different magnitudes of the exchange field reveals that when the time reversal symmetry is broken by a small exchange field, the system enters the time-reversal broken topological insulator phase, introduced by Yang {\it et al.} in Phys. Rev. Lett. 107, 066602 (2011). After a metallic phase in the intermediate region, the quantum anomalous Hall phase with non-zero Chern number emerges at a sufficiently large exchange field. We analyze the phase diagram from the viewpoint of the evolution of the electronic structure, edge states and transport properties, and demonstrate that different topological phases can be distinguished by the spin-polarization of the edge states as well as spin or charge transverse conductivity.

cond-mat.mtrl-sci↗

arXiv2012

Anisotropy of spin relaxation and transverse transport in metals

Using first principles methods we explore the anisotropy of the spin relaxation and transverse transport properties in bulk metals with respect to the direction of the spin quantization axis in paramagnets or of the spontaneous magnetization in ferromagnets. Owing to the presence of the spin-orbit interaction the orbital and spin character of the Bloch states depends sensitively on the orientation of the spins relative to the crystal axes. This leads to drastic changes in quantities which rely on interband mixing induced by the spin-orbit interaction. The anisotropy is particularly striking for quantities which exhibit spiky and irregular distribution in the Brillouin zone, such as the spin-mixing parameter or the Berry curvature of the electronic states. We demonstrate this for three cases: (i) the Elliott-Yafet spin-relaxation mechanism in paramagnets with structural inversion symmetry; (ii) the intrinsic anomalous Hall effect in ferromagnets; and (iii) the spin Hall effect in paramagnets. We discuss the consequences of the pronounced anisotropic behavior displayed by these properties for spin-polarized transport applications.

cond-mat.mes-hall↗

arXiv2012

One-dimensional ballistic transport with FLAPW Wannier functions

We present an implementation of the ballistic Landauer-Büttiker transport scheme in one-dimensional systems based on density functional theory (DFT) calculations within the full-potential linearized augmented plane-wave (FLAPW) method. In order to calculate the conductance within the Green's function method we map the electronic structure from the extended states of the FLAPW calculation to Wannier functions which constitute a minimal localized basis set. Our approach benefits from the high accuracy of the underlying FLAPW calculations allowing us to address the complex interplay of structure, magnetism, and spin-orbit coupling and is ideally suited to study spin-dependent electronic transport in one-dimensional magnetic nanostructures. To illustrate our approach we study ballistic electron transport in non-magnetic Pt monowires with a single stretched bond including spin-orbit coupling, and in ferromagnetic Co monowires with different collinear magnetic alignment of the electrodes with the purpose of analysing the magnetoresistance when going from tunneling to the contact regime. We further investigate spin-orbit scattering due to an impurity atom. We consider two configurations: a Co atom in a Pt monowire and vice versa. In both cases, the spin-orbit induced band mixing leads to a change of the conductance upon switching the magnetization direction from along the chain axis to perpendicular to it. The main contribution stems from ballistic spin-scattering for the magnetic Co impurity in the non-magnetic Pt monowire and for the Pt scatterer in the magnetic Co monowire from the band formed from states with $d_{xy}$ and $d_{x^2-y^2}$ orbital symmetry. We quantify this effect by calculating the ballistic anisotropic magnetoresistance which displays values up to as much as 7% for ballistic spin-scattering and gigantic values of around 100% for the Pt impurity in the Co wire.

cond-mat.mes-hall↗

arXiv2012

Information transfer by vector spin chirality in finite magnetic chains

Vector spin chirality is one of the fundamental characteristics of complex magnets. For a one-dimensional spin-spiral state it can be interpreted as the handedness, or rotational sense of the spiral. Here, using spin-polarized scanning tunneling microscopy, we demonstrate the occurrence of an atomic-scale spin-spiral in finite individual bi-atomic Fe chains on the (5x1)-Ir(001) surface. We show that the broken inversion symmetry at the surface promotes one direction of the vector spin chirality, leading to a unique rotational sense of the spiral in all chains. Correspondingly, changes in the spin direction of one chain end can be probed tens of nanometers away, suggesting a new way of transmitting information about the state of magnetic objects on the nanoscale.

cond-mat.mes-hall↗

arXiv2011

Modelling impurity-assisted chain creation in noble-metal break junctions

In this work we present the generalization of the model for chain formation in break-junctions, introduced by Thiess {\it et al.} [\onlinecite{Alex08}], to zigzag transition-metal chains with $s$ and $p$ impurities. We apply this extended model to study the producibility trends for noble-metal chains with impurities, often present in break junction experiments, namely, Cu, Ag and Au chains with H, C, O and N adatoms. Providing the material-specific parameters for our model from systematic full-potential linearized augmented plane-wave first-principles calculations, we find that the presence of such impurities crucially affects the binding properties of the noble-metal chains. We reveal that both, the impurity-induced bond strengthening and the formation of zigzag bonds, can lead to a significantly enhanced probability for chain formation in break junctions.

cond-mat.mes-hall↗

arXiv2011

Ab Initio Theory of Scattering-Independent Anomalous Hall Effect

We report on first-principles calculations of the side-jump contribution to the anomalous Hall conductivity (AHC) directly from the electronic structure of a perfect crystal. We implemented our approach for a short-range scattering disorder model within the density functional theory and computed the full scattering-independent AHC in elemental bcc Fe, hcp Co, fcc Ni, and L1o FePd and FePt alloys. The full AHC thus calculated agrees systematically with experiment to a degree unattainable so far, correctly capturing the previously missing elements of side-jump contributions, hence paving the way to a truly predictive theory of the anomalous Hall effect and turning it from a characterization tool to a probing tool of multi-band complex electronic band structures.

cond-mat.mes-hall↗

arXiv2011

Electrically tunable quantum anomalous Hall effect in 5d transition-metal adatoms on graphene

The combination of the unique properties of graphene with spin polarization and magnetism for the design of new spintronic concepts and devices has been hampered by the small Coulomb interaction and the tiny spin-orbit coupling of carbon in pristine graphene. Such device concepts would take advantage of the control of the spin degree of freedom utilizing the widely available electric fields in electronics or of topological transport mechanisms such as the conjectured quantum anomalous Hall effect. Here we show, using first-principles methods, that 5d transition-metal (TM) adatoms deposited on graphene display remarkable magnetic properties. All considered TM adatoms possess significant spin moments with colossal magnetocrystalline anisotropy energies as large as 50 meV per TM atom. We reveal that the magneto-electric response of deposited TM atoms is extremely strong and in some cases offers even the possibility to switch the spontaneous magnetization direction by a moderate external electric field. We predict that an electrically tunable quantum anomalous Hall effect can be observed in this type of hybrid materials.

cond-mat.mtrl-sci↗

arXiv2011

Anisotropic intrinsic anomalous Hall effect in ordered 3dPt alloys

By performing first principles calculations we investigate the intrinsic anomalous Hall conductivity (AHC) and its anisotropy in ordered L1o FePt, CoPt and NiPt ferromagnets, and their intermediate alloys. We demonstrate that the AHC in this family of compounds depends strongly on the direction of the magnetization in the crystal. We predict that such pronounced orientational dependence in combination with the general decreasing trend of the AHC when going from FePt to NiPt leads to a sign change of the AHC upon rotating the magnetization direction in the crystal of CoPt alloy. We also suggest that for a range of concentration x in Co(x)Ni(1-x)Pt alloy it is possible to achieve a complete quenching of the anomalous Hall current for a certain direction of the magnetization in the crystal. By analyzing the spin-resolved AHC in 3dPt alloys we endeavor to relate the overall trend of the AHC in these compounds to the changes in their densities of d-states around the Fermi energy upon varying the atomic number. Moreover, we show the generality of the phenomenon of anisotropic anomalous Hall effect by demonstrating its occurrence within the three-band tight-binding model.

cond-mat.mtrl-sci↗

arXiv2011

Non-collinear magnetism in freestanding and supported monatomic Mn chains

Using first-principles calculations, we study the occurrence of non-collinear magnetic order in monatomic Mn chains. First, we focus on freestanding Mn chains and demonstrate that they exhibit a pronounced non-collinear ground state in a large range of interatomic distances between atoms in the chain. By artificially varying the atomic number of Mn we investigate how the magnetic ground state is influenced by alloying the Mn chains with Fe and Cr. With increasing number of 3d-electrons we find a smooth transition in the magnetic phase space starting from an antiferromagnetic state for pure Cr chains through a regime of non-collinear ground states for Mn-rich chains to a ferromagnetic solution approaching the limit of pure Fe chains. Second, we investigate the magnetism in supported Mn chains on the (110)-surfaces of Cu, Pd, and Ag. We show that even a weak chain-surface hybridization is sufficient to dramatically change the magnetic coupling in the chain. Nevertheless, while we observe that Mn chains are antiferromagnetic on Pd(110), a weak non-collinear magnetic order survives for Mn chains on Cu(110) and Ag(110) a few meV in energy below the antiferromagnetic solution. We explain the sensitive dependence of the exchange interaction in Mn chains on the interatomic distance, chemical composition, and their environment based on the competition between the ferromagnetic double exchange and the antiferromagnetic kinetic exchange mechanism. Finally, we perform simulations which predict that the non-collinear magnetic order of Mn chains on Cu(110) and Ag(110) could be experimentally verified by spin-polarized scanning tunneling microscopy.

cond-mat.mes-hall↗

arXiv2011

The role of spin-flip transitions in the anomalous Hall effect of FePt alloy

We carry out first-principles calculations which demonstrate the importance of the non-spin-conserving part of the spin-orbit interaction for the intrinsic anomalous Hall conductivity of ordered FePt alloys. The impact of this interaction is strongly reduced if Pt is replaced by the lighter isoelectronic element Pd. An analysis of the interband transitions responsible for the anomalous velocity reveals that spin-flip transitions occur not only at avoided band crossings near the Fermi level, but also between well-separated pairs of bands with similar dispersions. We also predict a strong anisotropy in the anomalous Hall conductivity of FePt caused entirely by low-frequency spin-flip transitions.

cond-mat.mes-hall↗

arXiv2010

Anisotropic spin Hall effect from first principles

We report on first principles calculations of the anisotropy of the intrinsic spin Hall conductivity (SHC) in nonmagnetic hcp metals and in antiferromagnetic Cr. For most of the metals of this study we find large anisotropies. We derive the general relation between the SHC vector and the direction of spin polarization and discuss its consequences for hcp metals. Especially, it is predicted that for systems where the SHC changes sign due to the anisotropy the spin Hall effect may be tuned such that the spin polarization is parallel either to the electric field or to the spin current.

cond-mat.mtrl-sci↗

arXiv2010

The rise of spin-flip transitions in the anomalous Hall effect of FePt alloy

We carry out first-principles calculations which demonstrate the importance of spin-flip transitions for the intrinsic anomalous Hall conductivity of ordered FePt alloys. We show the such transitions get enhanced by large spin-orbit coupling of Pt atoms, becoming negligible when Pt is replaced by lighter isoelectronic Pd. We find that spin-flip transitions in FePt originate not only from conventional band anticrossings at the Fermi level, but also from transitions between well-separated pairs of bands with similar dispersions. We also predict a strong anisotropy in the anomalous Hall conductivity of FePt, which comes from spin-flip transitions entirely, and investigate the influence of disorder on it.

cond-mat.mes-hall↗

arXiv2010

Competing magnetic anisotropies in atomic-scale junctions

Using first-principles calculations, we study the magnetism of 5d transition-metal atomic junctions including structural relaxations and spin-orbit coupling. Upon stretching monatomic chains of W, Ir, and Pt suspended between two leads, we find the development of strong magnetism and large values of the magnetocrystalline anisotropy energy (MAE) of up to 30 meV per chain atom. We predict that switches of the easy magnetization axis of the nanocontacts upon elongation should be observable by ballistic anisotropic magnetoresistance measurements. Due to the different local symmetry, the contributions to the MAE of the central chain atoms and chain atoms in the vicinity of the leads can have opposite signs which reduces the total MAE. We demonstrate that this effect occurs independent of the chain length or geometry of the electrodes.

cond-mat.mes-hall↗

arXiv2009

Magnetically hindered chain formation in transition-metal break junctions

Based on first-principles calculations, we demonstrate that magnetism impedes the formation of long chains in break junctions. We find a distinct softening of the binding energy of atomic chains due to the creation of magnetic moments that crucially reduces the probability of successful chain formation. Thereby, we are able to explain the long standing puzzle why most of the transition-metals do not assemble as long chains in break junctions and provide thus an indirect evidence that in general suspended atomic chains in transition-metal break junctions are magnetic.

cond-mat.mes-hall↗

arXiv2009

Structurally-driven magnetic state transition of biatomic Fe chains on Ir(001)

Using first-principles calculations, we demonstrate that the magnetic exchange interaction and the magnetocrystalline anisotropy of biatomic Fe chains grown in the trenches of the 5x1 reconstructed Ir(001) surface depend sensitively on the atomic arrangement of the Fe atoms. Two structural configurations have been considered which are suggested from recent experiments. They differ by the local symmetry and the spacing between the two strands of the biatomic Fe chain. Since both configurations are very close in total energy they may coexist in experiment. We have investigated collinear ferro- and antiferromagnetic solutions as well as a collinear state with two moments in one direction and one in the opposite direction (up-down-up-state). For the structure with a small interchain spacing, there is a strong exchange interaction between the strands and the ferromagnetic state is energetically favorable. In the structure with larger spacing, the two strands are magnetically nearly decoupled and exhibit antiferromagnetic order along the chain. In both cases, due to hybridization with the Ir substrate the exchange interaction along the chain axis is relatively small compared to freestanding biatomic iron chains. The easy magnetization axis of the Fe chains also switches with the structural configuration and is out-of-plane for the ferromagnetic chains with small spacing and along the chain axis for the antiferromagnetic chains with large spacing between the two strands. Calculated scanning tunneling microscopy images and spectra suggest the possibility to experimentally distinguish between the two structural and magnetic configurations.

cond-mat.mes-hall↗

arXiv2009

Orientation Dependence of the Intrinsic Anomalous Hall Effect in hcp Cobalt

We carry out first-principles calculations of the dependence of the intrinsic anomalous Hall conductivity of hcp Co on the magnetization direction. The Hall conductivity decreases smoothly from 481 S/cm to 116 S/cm as the magnetization is tilted from the c-axis to the ab-plane. This factor-of-four reduction compares well with measurements on single crystals, while the angular average of 226 S/cm is in excellent agreement with the value of 205 S/cm measured in polycrystalline films. The strong anisotropy of the anomalous Hall conductivity is a consequence of spin-orbit induced changes in the electron states near the Fermi level as the magnetization is rotated.

cond-mat.mtrl-sci↗