V. E. Kireev

National Academy of Sciences of Ukraine, Kievo, Kyiv City, Ukraine

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Publications (16)25.1 Total impact

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    ABSTRACT: We show that a magnetic vortex is the ground state of an array of magnetic particles shaped as a hexagonal fragment of a triangular lattice, even for an small number of particles in the array $N \leq 100$. The vortex core appears and the symmetry of the vortex state changes with the increase of the intrinsic magnetic anisotropy of the particle $\beta$; the further increase of $\beta$ leads to the destruction of the vortex state. Such vortices can be present in arrays as small in size as dozen of nanometers.
    Physical review. B, Condensed matter 01/2013; 87(18). · 3.66 Impact Factor
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    ABSTRACT: We analyzed the ground state of the array of magnetic particles (magnetic dots) which form a two-dimensional triangular lattice, and magnetic moment of which is perpendicular to the plane of the lattice, in the presence of external magnetic field. In the small fields long range dipole-dipole interaction leads to the specific antiferromagnetic order, where two out of six nearest neighbors of the particle have the same direction of magnetization moment and four - the opposite one. It is shown that magnetization process in such array of particles as opposed to the rectangular lattices results from the formation of the magnetized topological defects (dislocations) in the shape of the domain walls.
    01/2012;
  • V. E. Kireev, B. A. Ivanov
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    ABSTRACT: The ground state of cylindrical magnetic samples of different sizes at nonzero applied magnetic field is studied theoretically taking into account the exchange and dipole-dipole interactions at an arbitrary ratio of coupling constants. In addition to the weakly inhomogeneous and standard vortex states well known for the case of the weak dipole-dipole interaction, the vortex states with the complicated structure of the vortex core have been found. The state diagram for these particles has been constructed and analyzed in terms of scaling consideration.
    JETP Letters 10/2011; 94(4). · 1.36 Impact Factor
  • B. A. Ivanov, V. E. Kireev
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    ABSTRACT: For the frustrated triangular lattice of Ising magnetic moments with an antiferromagnetic interaction, which is in a state with two sublattices, a new type of topological defects with zero energy in the approximation of the interaction between only the nearest-neighbors has been found. These defects have a nonzero magnetic moment, and the magnetization in a low field occurs via the formation of a system of such defects. These properties are valid for a 2D superstructure in the form of a triangular lattice of single-domain magnetic particles with perpendicular anisotropy and dipole coupling.
    JETP Letters 01/2009; 90(12):750-753. · 1.36 Impact Factor
  • B. A. Ivanov, V. E. Kireev
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    ABSTRACT: The coherent quantum tunneling effects in antiferromagnets in the presence of a strong external magnetic field parallel to the easy axis have been investigated using the instanton formalism. In a wide field range including the region of the phase spin-flop transition, the tunneling is described by 180° instantons for which the Euclidean action is real and destructive interference is absent. At the transition point, 90° instantons describing the tunneling between the collinear and spin-flop states appear. The Euclidean action decreases, whereas the tunneling probability and tunneling level splitting in both phases increase significantly in the immediate vicinity of the spin-flop transition point. The possibility of observing the coherent tunneling effects for artificial small particles (magnetic dots) made of antiferromagnets is discussed.
    Journal of Experimental and Theoretical Physics 01/2008; 107(3):445-461. · 0.93 Impact Factor
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    B. A. Ivanov, V. E. Kireev
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    ABSTRACT: Quantum (step-like) magnetization curves are studies for a spin pair with antiferromagnetic coupling in the presence of a magnetic field parallel to the easy axis of the magnetic anisotropy. The consideration is done both analytically and numerically for a wide range of the anisotropy constants and spins up to $S \gtrsim 100$. Depending on the origin of the anisotropy (exchange or single-ion), the magnetization curve can demonstrate the jumps more than unity and the concentration of the unit jumps in a narrow range of the field. We also point the region of the problem parameters, where the behavior is quasiclassical for $S = 5$, and where system is substantially quantum in the limit $S \to \infty$. Comment: 5 pages, 5 figures
    JETP Letters 05/2005; · 1.36 Impact Factor
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    B. A. Ivanov, V. E. Kireev
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    ABSTRACT: For a two-sublattice antiferromagnet the Lagrangian is constructed taking into account Berry phase whose form is matched to the quantum-mechanical Heisenberg Hamiltonian. Tunnel effects are analyzed taking into account the crystallographic symmetry and possible types of the Dzyaloshinskii-Moriya interaction. It is shown that, when the real magnetic symmetry and the Dzyaloshinskii-Moriya interaction are taken into consideration, the effects of a destructive instanton interference and the suppression of macroscopic quantum tunneling can play an essential role. It also may lead to a periodic dependence of the ground-state level splitting on the Dzyaloshinskii-Moriya interaction constant; the magnitude of this splitting is calculated.
    02/2005;
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    B. A. Ivanov, V. E. Kireev
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    ABSTRACT: We consider an effect of a strong magnetic field on the ground state and macroscopic coherent tunneling in small antiferromagnetic particles with uniaxial and biaxial single-ion anisotropy. We find several tunneling regimes that depend on the direction of the magnetic field with respect to the anisotropy axes. For the case of a purely uniaxial symmetry and the field directed along the easy axis, an exact instanton solution with two different scales in imaginary time is constructed. For a rhombic anisotropy the effect of the field strongly depends on its orientation: with the field increasing, the tunneling rate increases or decreases for the field parallel to the easy or medium axis, respectively. The analytical results are complemented by numerical simulations. Comment: 11 pages, 6 figures
    Physical Review B 04/2004; · 3.66 Impact Factor
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    V. E. Kireev, B. A. Ivanov
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    ABSTRACT: We investigate analytically and numerically the ground and metastable states for easy-plane Heisenberg magnets with single-ion surface anisotropy and disk geometry. The configurations with two half-vortices at the opposite points of the border are shown to be preferable for strong anisotropy. We propose a simple analytical description of the spin configurations for all values of a surface anisotropy. The effects of lattice pinning leads to appearance of a set of metastable configurations. Comment: 10 pages, 7 figures; submitted to Phys. Rev. B
    Physical Review B 11/2002; · 3.66 Impact Factor
  • B. A. Ivanov, V. E. Kireev
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    ABSTRACT: For a two-sublattice antiferromagnet, the Lagrangian is constructed taking into account Berry’s phase whose form is matched with the quantum-mechanical Heisenberg Hamiltonian. Tunnel effects are analyzed taking into account the crystallographic symmetry and possible types of Dzyaloshinski interaction. It is shown that, when the real magnetic symmetry and the Dzyaloshinski interaction are taken into consideration, the effects of destructive instanton interference and the suppression of macroscopic quantum tunneling may come into play. This may lead to a periodic dependence of the ground-state level splitting on the Dzyaloshinski interaction constant; the magnitude of this splitting is calculated.
    Journal of Experimental and Theoretical Physics 02/2002; 94(2):270-282. · 0.93 Impact Factor
  • B. A. Ivanov, V. E. Kireev
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    ABSTRACT: A screw dislocation perpendicular to layers in layered antiferromagnets with a ferromagnetic exchange interaction of spins in the atomic planes and an antiferromagnetic interaction between planes gives rise to nonsingular disclinations with a ferromagnetic core.
    JETP Letters 01/2001; 73(4):188-191. · 1.36 Impact Factor
  • B. A. Ivanov, V. E. Kireev, A. Merkulov
    Materials Science Forum 01/2001;
  • B. A. Ivanov, V. E. Kireev
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    ABSTRACT: An exact instanton solution describing macroscopic quantum tunneling for a small antiferromagnetic particle with uncompensated spin and biaxial quadratic anisotropy is constructed. The solution is valid for any relation between anisotropy parameters and relative value of uncompensated spin. The obtained solution is used for calculating the tunneling amplitude taking into account the pre-exponential factor. The amplitude is characterized by a nonanalytic dependence on the ratio of small parameters of the problem, viz., anisotropy in the basal plane and the value of uncompensated spin.
    Low Temperature Physics 12/1999; 25(12). · 0.88 Impact Factor
  • B. A. Ivanov, V. E. Kireev
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    ABSTRACT: A new macroscopic quantum tunneling effect is predicted for a particle of an uncompensated ferritin-type antiferromagnet in a noncollinear phase induced by a strong magnetic field.
    JETP Letters 02/1999; 69(5):398-403. · 1.36 Impact Factor
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    B. A. Ivanov, A. K. Kolezhuk, V. E. Kireev
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    ABSTRACT: We consider a domain wall in the mesoscopic quasi-one-dimensional sample (wire or stripe) of weakly anisotropic two-sublattice antiferromagnet, and estimate the probability of tunneling between two domain wall states with different chirality. Topological effects forbid tunneling for the systems with half-integer spin S of magnetic atoms which consist of odd number of chains N. External magnetic field yields an additional contribution to the Berry phase, resulting in the appearance of two different tunnel splittings in any experimental setup involving a mixture of odd and even N, and in oscillating field dependence of the tunneling rate with the period proportional to 1/N. Comment: 4 pages + 2 figures, references corrected
    Physical Review B 03/1998; · 3.66 Impact Factor
  • B. A. Ivanov, V. E. Kireev, V. P. Voronov
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    ABSTRACT: The spin structure of magnetic disclination, viz., spin inhomogeneity manifested in antiferromagnets in the presence of a dislocation, is analyzed. The analysis is carried out on the basis of the discrete model as well as in the specially constructed generalized continual theory based on the local introduction of the antiferromagnetism vector l and taking into account the possibility of a change in the length of this vector. In the solution without singularities constructed for disclination, the modulus of the antiferromagnetism vector vanishes at the center of the disclination. In the discrete model, the disclination energy depends significantly on the arrangement of spins near the core and on the type of their interaction. With the proposed model of spin arrangement, the results of numerical analysis based on the discrete and continual models are in good agreement. It is shown that planar disclinations are stable to the emergence of spins from the easy plane.
    Low Temperature Physics 08/1997; 23(8). · 0.88 Impact Factor