arXiv subjects

V. V. Mazurenko

Publications and source records attributed to V. V. Mazurenko.

At least 55 records · Page 3Linked to original sources

arXiv2014

Twisted magnetic patterns: Exploring the Dzyaloshinskii--Moriya vector

Magnetism - the spontaneous alignment of atomic moments in a material - is driven by quantum-mechanical `exchange' interactions which operate over atomic distances as a result of the fundamental symmetry of electrons. Currently, one of the most active fields of condensed matter physics involves the study of magnetic interactions that cause, or are caused by a twisting of nearby atoms. This can lead to the magnetoelectric effect that couples electric and magnetic properties, and is predicted to play a prominent role in future technology. Here, we discuss the complex relativistic interplay between magnetism and atomic crystal structure in a class of materials called `weak ferromagnets'. The sign of the underpinning Dzyaloshinskii--Moriya interaction has been determined for the first time, by using synchrotron radiation to study iron borate (FeBO3). We present a novel experimental technique based on interference between two x-ray scattering processes (one acts as a reference wave) which we combine with a second unusual approach of turning the atomic antiferromagnetic motif with a small magnetic field. We show that the experimental results provide a clear validation of state-of-the-art theoretical calculations. These experimental and theoretical approaches open up new possibilities for exploring, modelling and exploiting novel magnetic and magnetoelectric materials.

cond-mat.str-el↗

arXiv2013

The electronic structure of the high-symmetry perovskite iridate Ba2IrO4

We report angle-resolved photoemission (ARPES) measurements, density functional and model tight-binding calculations on Ba$_2$IrO$_4$ (Ba-214), an antiferromagnetic ($T_N=230$ K) insulator. Ba-214 does not exhibit the rotational distortion of the IrO$_6$ octahedra that is present in its sister compound Sr$_2$IrO$_4$ (Sr-214), and is therefore an attractive reference material to study the electronic structure of layered iridates. We find that the band structures of Ba-214 and Sr-214 are qualitatively similar, hinting at the predominant role of the spin-orbit interaction in these materials. Temperature-dependent ARPES data show that the energy gap persists well above $T_N$, and favour a Mott over a Slater scenario for this compound.

cond-mat.str-el↗

arXiv2013

Magnetism of sodium superoxide

By combining first-principles electronic-structure calculations with the model Hamiltonian approach, we systematically study the magnetic properties of sodium superoxide (NaO2), originating from interacting superoxide molecules. We show that NaO2 exhibits a rich variety of magnetic properties, which are controlled by relative alignment of the superoxide molecules as well as the state of partially filled antibonding molecular π_g-orbitals. The orbital degeneracy and disorder in the high-temperature pyrite phase gives rise to weak isotropic antiferromagnetic (AFM) interactions between the molecules. The transition to the low-temperature marcasite phase lifts the degeneracy, leading to the orbital order and formation of the quasi-one-dimensional AFM spin chains. Both tendencies are consistent with the behavior of experimental magnetic susceptibility data. Furthermore, we evaluate the magnetic transition temperature and type of the long-range magnetic order in the marcasite phase. We argue that this magnetic order depends on the behavior of weak isotropic as well as anisotropic and Dzyaloshinskii-Moriya exchange interactions between the molecules. Finally, we predict the existence of a multiferroic phase, where the inversion symmetry is broken by the long-range magnetic order, giving rise to substantial ferroelectric polarization.

cond-mat.mtrl-sci↗

arXiv2013

Correlation effects in insulating surface nanostructures

We study the role of static and dynamical Coulomb correlation effects on the electronic and magnetic properties of individual Mn, Fe and Co adatoms deposited on the CuN surface. For these purposes, we construct a realistic Anderson model, solve it by using finite-temperature exact diagonalization method and compare the calculated one-particle spectral functions with the LDA+$U$ densities of states. In contrast to Mn/CuN and Fe/CuN, the cobalt system tends to form the electronic excitations at the Fermi level. Based on the calculated magnetic response functions, the relative relaxation times for the magnetic moments of impurity orbitals are estimated. To study the effect of the dynamical correlations on the exchange interaction in nanoclusters, we solve the two-impurity Anderson model for the Mn dimer on the CuN surface. It is found that the experimental exchange interaction can be well reproduced by employing $U$=3 eV, which is two times smaller than the value used in static mean-field LDA+$U$ calculations. This suggests on important role of dynamical correlations in the interaction between adatoms on a surface.

cond-mat.str-el↗

arXiv2012

Magnetic structure and ferroelectric activity in orthorhombic YMnO3: relative roles of magnetic symmetry breaking and atomic displacements

We discuss relative roles played by the magnetic inversion symmetry breaking and the ferroelectric (FE) atomic displacements in the multiferroic state of YMnO3. For these purposes we derive a realistic low-energy model, using results of first-principles calculations and experimental parameters of the crystal structure. Then, we solve this model in the Hartree-Fock approximation. We argue that the multiferroic state in YMnO3 has a magnetic origin, and the centrosymmetric Pbnm structure is formally sufficient for explaining details of the noncentrosymmetric magnetic ground state. The relativistic spin-orbit interaction lifts the degeneracy, caused by the frustration of isotropic interactions, and stabilizes a twofold periodic magnetic state, which is similar to the E-state apart from the spin canting. The noncentrosymmetric atomic displacements in the P2_1nm phase reduce the spin canting, but do not change the symmetry of the magnetic state. The effect of the P2_1nm distortion on the FE polarization P_a is twofold: (i) it gives rise to ionic contributions, associated with the Y and O sites; (ii) it affects the electronic polarization, through the change of the spin canting. The relatively small value of P_a, observed in the experiment, is caused by a partial cancelation of the electronic and ionic contributions in the experimental P2_1nm structure. Finally, we theoretically optimize the crystal structure, by using the LSDA+U approach and assuming the collinear E-type alignment. We have found that the agreement with the experimental data in this case is less satisfactory and P_a is largely overestimated. Although the magnetic structure can be formally tuned by varying the Coulomb repulsion U as a parameter, apparently LSDA+U fails to reproduce some fine details of the experimental structure, and the cancelation of different contributions in P_a does not occur.

cond-mat.mtrl-sci↗

arXiv2012

Magnetic structure of hexagonal YMnO3 and LuMnO3 from a microscopic point of view

The aim of this work is to unravel a basic microscopic picture behind complex magnetic properties of hexagonal manganites. For these purposes, we consider two characteristic compounds: YMnO3 and LuMnO3, which form different magnetic structures in the ground state. First, we establish an electronic low-energy model, which describes the behavior of the Mn 3d bands of YMnO3 and LuMnO3, and derive parameters of this model from the first-principles calculations. From the solution of this model, we conclude that, despite strong frustration effects in the hexagonal lattice, the relativistic spin-orbit interactions lift the degeneracy of the magnetic ground state so that the experimentally observed magnetic structures are successfully reproduced by the low-energy model. Then, we analyze this result in terms of interatomic magnetic interactions, which were computed using different approximations (starting from the model Hamiltonian as well as directly from the first-principles electronic structure calculations in the local-spin-density approximation). We argue that the main reason why YMnO3 and LuMnO3 tend to form different magnetic structures is related to the behavior of the single-ion anisotropy, which reflects the directional dependence of the lattice distortion: namely, the expansion and contraction of the Mn-trimers, which take place in YMnO3 and LuMnO3, respectively. On the other hand, the magnetic coupling between the haxagonal planes is controlled by the next-nearest-neighbor interactions, which are less sensitive to the direction of the trimerization. Finally, using the Berry-phase formalism, we evaluate the magnetic-state dependence of the ferroelectric polarization, and discuss potential applications of the latter in magnetoelectric switching phenomena.

cond-mat.mtrl-sci↗

arXiv2012

Orbital-selective conductance of Co adatom on the Pt(111) surface

We propose an orbital-selective model for the transport and magnetic properties of the individual Co impurity deposited on the Pt(111). Using the combination of the Anderson-type Hamiltonian and the Kubo's linear response theory we show that the magnetization and dI/dV spectrum of Co adatom are originated from the 3d states of the different symmetry. A textbook expression for the spin-dependent differential conductance provides a natural connection between magnetic and transport properties of Co/Pt(111). We found that it is possible to detect and to manipulate the different 3d states of the Co adatom by tuning the spin polarization of the tip and tip-impurity distance in STM experiments.

cond-mat.str-el↗

arXiv2010

Renormalized spectral function for Co adatom on the Pt(111) surface

The strong Coulomb correlations effects in the electronic structure of magnetic Co adatom on the Pt(111) surface have been investigated. Using a realistic five d-orbital impurity Anderson model at low temperatures with parameters determined from first-principles calculations we found a striking change of the electronic structure in comparison with the LDA results. The spectral function calculated with full rotationally invariant Coulomb interaction is in good agreement with the quasiparticle region of the STM conductance spectrum. Using the calculated spin-spin correlation functions we have analyzed the formation of the magnetic moments of the Co impurity orbitals.

cond-mat.str-el↗

arXiv2010

Correlated band theory of spin and orbital contributions to Dzyaloshinskii-Moriya interactions

A new approach for calculations of Dzyaloshinskii-Moriya interactions in molecules and crystals is proposed. It is based on the exact perturbation expansion of total energy of weak ferromagnets in the canting angle with the only assumption of local Hubbard-type interactions. This scheme leads to a simple and transparent analytical expression for Dzyaloshinskii-Moriya vector with a natural separation into spin and orbital contributions. The main problem was transferred to calculations of effective tight-binding parameters in the properly chosen basis including spin-orbit coupling. Test calculations for La$_2$CuO$_4$ give the value of canting angle in a good agreement with experimental data.

cond-mat.str-el↗

arXiv2010

Parallel exact diagonalization solver for quantum-electron models

We present a parallel computation scheme based on the Arnoldi algorithm for exact diagonalization of quantum-electron models. It contains a selective data transferring method and distributed storage format for efficient computing of the matrix-vector product on distributed computing systems. The performed numerical experiments demonstrated good performance and scalability of our eigenvalue solver. The developed technique has been applied to investigate the electronic properties of Sr2RuO4 at experimental temperatures. The role of the spin-flip term in the electronic Hamiltonian was analyzed.

physics.comp-ph↗

arXiv2009

First-principles investigation of uranium monochalcogenides

We present first-principles investigation of the electronic structure and magnetic properties of uranium monochalcogenides: US, USe, UTe. The calculations were performed by using recently developed LDA+U+SO method in which both Coulomb and spin-orbit interactions have been taken into account in rotationally invariant form. We discuss the problem of choice of the Coulomb interaction value. The calculated [111] easy axes agree with those experimentally observed. The electronic configuration 5$f^3$ was found for all uranium compounds under investigation.

cond-mat.str-el↗

arXiv2009

Metal-insulator transition and magnetism in correlated band insulator: FeSi and Fe$_{1-x}$Co$_{x}$Si

The LDA+DMFT (local density approximation combined with dynamical mean-field theory) computation scheme has been used to study spectral and magnetic properties of FeSi and Fe$_{1-x}$Co$_{x}$Si. Having compared different models we conclude that a correlated band insulator scenario in contrast to Kondo insulator model agrees with FeSi band structure as well as experimental data. Coulomb correlation effects lead to band narrowing of the states near the Fermi level with mass renormalization parameter $m^*\approx 2$ in agreement with the results of angle-resolved photoemission spectroscopy (ARPES). Temperature dependence of spectral functions and magnetic susceptibility calculated in DMFT reproduces transition from nonmagnetic semiconductor to metal with local magnetic moments observed experimentally. Cobalt doping leads to ferromagnetism that has itinerant nature and can be successfully described by LDA+DMFT method.

cond-mat.str-el↗

arXiv2008

First-principles investigation of symmetric and antisymmetric exchange interactions of SrCu$_{2}$(BO$_{3}$)$_{2}$

We report on a first-principles investigation of the electronic structure and of the magnetic properties of the quasi-two-dimensional Mott insulator SrCu$_{2}$(BO$_{3}$)$_{2}$. Based on the hopping integrals and Coulomb interactions calculated with LDA and LSDA+U, we provide a microscopic explanation of the symmetric Heisenberg and antisymmetric Dzyaloshinskii-Moriya exchange integrals of SrCu$_{2}$(BO$_{3}$)$_{2}$. The intra-atomic exchange interaction of oxygen is shown to strongly contribute to the intra-dimer isotropic exchange. The results are in good agreement with those derived from experimental data, both regarding the orientation of the Dzyaloshinskii-Moriya vectors and the magnitude of all exchange integrals. The microscopic analysis is confirmed by the results of Green function's and total energies difference methods.

cond-mat.str-el↗

arXiv2008

Weak ferromagnetism in Mn nanochains on the CuN surface

We investigate electronic and magnetic structures of the Mn chains supported on the CuN surface using first-principle LSDA and LDA+U calculations. The isotropic exchange integrals and anisotropic Dzyaloshinskii-Moriya interactions between Mn atoms are calculated using Green function formalism. It is shown that the account of lattice relaxation and on-site Coulomb interaction are important for accurate description of magnetic properties of the investigated nanosystems. We predict a weak ferromagnetism phenomenon in the Mn antiferromagnetic nanochains on the CuN surface. The value of a net magnetic moment and direction of spin canting are calculated. We show that some experimental features may be explained using anisotropic exchange interactions.

cond-mat.mtrl-sci↗

arXiv2007

Wannier functions and exchange integrals: The example of LiCu$_{2}$O$_{2}$

Starting from a single band Hubbard model in the Wannier function basis, we revisit the problem of the ligand contribution to exchange and derive explicit formulae for the exchange integrals in metal oxide compounds in terms of atomic parameters that can be calculated with constrained LDA and LDA+U. The analysis is applied to the investigation of the isotropic exchange interactions of LiCu$_{2}$O$_{2}$, a compound where the Cu-O-Cu angle of the dominant exchange path is close to 90$^{\circ}$. Our results show that the magnetic moments are localized in Wannier orbitals which have strong contribution from oxygen atomic orbitals, leading to exchange integrals that considerably differ from the estimates based on kinetic exchange only. Using LSDA+U approach, we also perform a direct {\it ab-initio} determination of the exchange integrals LiCu$_{2}$O$_{2}$. The results agree well with those obtained from the Wannier function approach, a clear indication that this modelization captures the essential physics of exchange. A comparison with experimental results is also included, with the conclusion that a very precise determination of the Wannier function is crucial to reach quantitative estimates.

cond-mat.str-el↗

arXiv2006

The semiconductor-to-ferromagnetic-metal transition in FeSb2

We propose FeSb2 to be a nearly ferromagnetic small gap semiconductor, hence a direct analog of FeSi. We find that despite different compositions and crystal structures, in the local density approximation with on-site Coulomb repulsion correction (LDA+U) method magnetic and semiconducting solutions for U=2.6 eV are energetically degenerate similar to the case of FeSi. For both FeSb2 and FeSi (FeSi1-xGex alloys) the underlying transition mechanism allows one to switch from a small gap semiconductor to a ferromagnetic metal with magnetic moment 1 muB per Fe ion with external magnetic field.

cond-mat.str-el↗

arXiv2005

Electronic Structure and Exchange Interactions of Na$_{2}$V$_{3}$O$_{7}$

We have performed first-principle calculations of the electronic structure and exchange couplings for the nanotube compound Na$_{2}$V$_{3}$O$_{7}$ using the LDA+U approach. Our results show that while the intra-ring exchange interactions are mainly antiferromagnetic, the inter-ring couplings are {\it ferromagnetic}. We argue that this is a consequence of the strong hybridization between filled and vacant 3d vanadium orbitals due to the low symmetry of Na$_{2}$V$_{3}$O$_{7}$, which results into strong - and often dominant - ferromagnetic contributions to the total exchange interaction between vanadium atoms. A comparison with results of previous works is included.

cond-mat.str-el↗

arXiv2004

Weak ferromagnetism in antiferromagnets: Fe$_{2}$O$_{3}$ and La$_{2}$CuO$_{4}$

The problem of weak ferromagnetism in antiferromagnets due to canting of magnetic moments was treated using Green's function technique. At first the eigenvalues and eigenfunctions of the electronic Hamiltonian corresponding to collinear magnetic configuration are calculated which are then used to determine first and second variations of the total energy as a function of the magnetic moments canting angle. Spin-orbit coupling is taken into account via perturbation theory. The results of calculations are used to determine an effective spin Hamiltonian. This Hamiltonian can be mapped on conventional spin Hamiltonian that allows to determine parameters of isotropic and anisotropic (Dzyaloshinskii-Moriya) exchange interactions. The method was applied to the typical antiferromagnets with weak ferromagnetism Fe$_{2}$O$_{3}$ and La$_{2}$CuO$_{4}$. The obtained directions and values of the magnetic moments canting angles are in a reasonable agreement with experimental data.

cond-mat.str-el↗