arXiv subjects

B. A. Ivanov

Publications and source records attributed to B. A. Ivanov.

At least 37 records · Page 2Linked to original sources

arXiv2013

Exchange relaxation as the mechanism of ultrafast spin reorientation in two-sublattice ferrimagnets

In the exchange approximation, an exact solution is obtained for the sublattice magnetizations evolution in a two-sublattice ferrimagnet. Nonlinear regimes of spin dynamics are found that include both the longitudinal and precessional evolution of the sublattice magnetizations, with the account taken of the exchange relaxation. In particular, those regimes describe the spin switching observed in the GdFeCo alloy under the influence of a femtosecond laser pulse.

cond-mat.str-el↗

arXiv2013

Inertial longitudinal magnetization reversal for non-Heisenberg ferromagnets

We analyze theoretically the novel pathway of ultrafast spin dynamics for ferromagnets with high enough single-ion anisotropy (non-Heisenberg ferromagnets). This longitudinal spin dynamics includes the coupled oscillations of the modulus of the magnetization together with the quadrupolar spin variables, which are expressed through quantum expectation values of operators bilinear on the spin components. Even for a simple single-element ferromagnet, such a dynamics can lead to an inertial magnetization reversal under the action of an ultrashort laser pulse.

cond-mat.str-el↗

arXiv2013

Dynamics and relaxation in spin nematics

We study dynamics and relaxation of elementary excitations (magnons) in the spin nematic (quadrupole ordered) phase of S=1 magnets. We develop a general phenomenological theory of spin dynamics and relaxation for spin-1 systems. Results of the phenomenological approach are compared to those obtained by microscopic calculations for the specific S=1 model with isotropic bilinear and biquadratic exchange interactions. This model exhibits a rich behavior depending on the ratio of bilinear and biquadratic exchange constants, including several points with an enhanced symmetry. It is shown that symmetry plays an important role in relaxation. Particularly, at the SU(3) ferromagnetic point the magnon damping $Γ$ depends on its wavevector k as $Γ\propto k^{4}$, while a deviation from the high-symmetry point changes the behavior of the leading term to $Γ\propto k^{2}$. We point out a similarity between the behavior of magnon relaxation in spin nematics to that in an isotropic ferromagnet.

cond-mat.str-el↗

arXiv2013

Vortex ground state for small arrays of magnetic particles with dipole coupling

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 $β$; the further increase of $β$ leads to the destruction of the vortex state. Such vortices can be present in arrays as small in size as dozen of nanometers.

cond-mat.str-el↗

arXiv2012

Ground states and magnetization process for an triangular lattice array of magnetic dots with perpendicular anisotropy

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.

cond-mat.mtrl-sci↗

arXiv2011

Core-Core Dynamics in Spin Vortex Pairs

We investigate magnetic nano-pillars, in which two thin ferromagnetic nanoparticles are separated by a nanometer thin nonmagnetic spacer and can be set into stable spin vortex-pair configurations. The 16 ground states of the vortex-pair system are characterized by parallel or antiparallel chirality and parallel or antiparallel core-core alignment. We detect and differentiate these individual vortex-pair states experimentally and analyze their dynamics analytically and numerically. Of particular interest is the limit of strong core-core coupling, which we find can dominate the spin dynamics in the system. We observe that the 0.2 GHz gyrational resonance modes of the individual vortices are replaced with 2-6 GHz range collective rotational and vibrational core-core resonances in the configurations where the cores form a bound pair. These results demonstrate new opportunities in producing and manipulating spin states on the nanoscale and may prove useful for new types of ultra-dense storage devices where the information is stored as multiple vortex-core configurations.

cond-mat.mes-hall↗

arXiv2011

Two-Dimensional Propagation of a Photoinduced Spin Wave Packet

We report the two-dimensional propagation of photoinduced spin wave packets in Bi-doped rare-earth iron garnet. Spin waves were excited nonthermally and impulsively by a circularly polarized light pulse via the inverse Faraday effect. Space- and time resolved spin waves were detected with a magneto-optical pump-probe technique. We investigated propagation in two directions, parallel and perpendicular to the magnetic field. Backward volume magnetostatic waves (BVMSWs) were detected in both directions. The frequency of BVMSWs depends on the propagation direction. The experimental results agreed well with the dispersion relation of BVMSWs.

cond-mat.mes-hall↗

arXiv2011

Spectral dependence of photoinduced spin precession in DyFeO3

Spin precession was nonthermally induced by an ultrashort laser pulse in orthoferrite DyFeO3 with a pump-probe technique. Both circularly and linearly polarized pulses led to spin precessions; these phenomena are interpreted as the inverse Faraday effect and the inverse Cotton-Mouton effect, respectively. For both cases, the same mode of spin precession was excited; the precession frequencies and polarization were the same, but the phases of oscillations were different. We have shown theoretically and experimentally that the analysis of phases can distinguish between these two mechanisms. We have demonstrated experimentally that in the visible region, the inverse Faraday effect was dominant, whereas the inverse Cotton-Mouton effect became relatively prominent in the near-infrared region.

cond-mat.str-el↗

arXiv2010

Magnetic vortex as a ground state for micron-scale antiferromagnetic samples

Here we consider micron-sized samples with any axisymmetric body shape and made with a canted antiferromagnet, like hematite or iron borate. We find that its ground state can be a magnetic vortex with a topologically non-trivial distribution of the sublattice magnetization $\vec{l}$ and planar coreless vortex-like structure for the net magnetization $\vec{M}$. For antiferromagnetic samples in the vortex state, in addition to low-frequency modes, we find high-frequency modes with frequencies over the range of hundreds of gigahertz, including a mode localized in a region of radius $\sim$ 30--40 nm near the vortex core.

cond-mat.mtrl-sci↗

arXiv2010

Spin Oscillations in Antiferromagnetic NiO Triggered by Circularly Polarized Light

Coherent spin oscillations were non-thermally induced by circularly polarized pulses in fully compensated antiferromagnetic NiO. This effect is attributed to an entirely new mechanism of the action, on the spins, of the effective magnetic field generated by an inverse Faraday effect. The novelty of this mechanism is that spin oscillations are driven by the time derivative of the effective magnetic field acting even on "pure" antiferromagnets with zero net magnetic moment in the ground state. The measured frequencies (1.07 THz and 140 GHz) of the spin oscillations correspond to the out-of-plane and in-plane modes of antiferromagnetic magnons.

cond-mat.str-el↗

arXiv2010

Collective modes for an array of magnetic dots with perpendicular magnetization

The dispersion relations of collective oscillations of the magnetic moment of magnetic dots arranged in square-planar arrays and having magnetic moments perpendicular to the array plane are calculated. The presence of the external magnetic field perpendicular to the plane of array, as well as the uniaxial anisotropy for single dot are taken into account. The ferromagnetic state with all the magnetic moments parallel, and chessboard antiferromagnetic state are considered. The dispersion relation yields information about the stability of different states of the array. There is a critical magnetic field below which the ferromagnetic state is unstable. The antiferromagnetic state is stable for small enough magnetic fields. The dispersion relation is non-analytic as the value of the wave vector approaches zero. Non-trivial Van Hove anomalies are also found for both ferromagnetic and antiferromagnetic states.

cond-mat.mtrl-sci↗

arXiv2010

Surface waves versus negative refractive index in layered superconductors

We predict a new branch of surface Josephson plasma waves (SJPWs) in layered superconductors for frequencies higher than the Josephson plasma frequency. In this frequency range, the permittivity tensor components along and transverse to the layers have different signs, which is usually associated with negative refraction. However, for these frequencies, the bulk Josephson plasma waves cannot be matched with the incident and reflected waves in the vacuum, and, instead of the negative-refractive properties, abnormal surface modes appear within the frequency band expected for bulk modes. We also discuss the excitation of high-frequency SJPWs by means of the attenuated-total-reflection method.

cond-mat.supr-con↗

arXiv2009

Spin nematic state for a spin S=3/2 isotropic non-Heisenberg magnet

$S=3/2$ system with general isotropic nearest-neighbor exchange within a mean-field approximation possesses a magnetically ordered ferromagnetic state and antiferromagnetic state, and two different spin nematic states, with zero spin expectation values. Both spin nematic phases display complicated symmetry break, including standard rotational break described by the vector-director $\vec {u}$ and specific symmetry break with respect to the time reversal. The break of time reversal is determined by non-trivial quantum averages cubic over the spin components and can be described by unit "pseudospin" vector $\vec σ$. The vectors $\vec σ$ on different sites are parallel for a nematic state, and $\vec σ$'s are antiparallel for different sublattices for an antinematic phase.

cond-mat.str-el↗

arXiv2009

Magnetic vortex as a ground state for sub-micron antiferromagnetic particles

For submicron particles shaped as any axisymmetric body and made with standard canted antiferromagnet like hematite or iron borate, the ground state may comprise of a magnetic vortex with topologically non-trivial distribution of the sublattice magnetization $\vec{l}$ and planar coreless vortex-like structure for the net magnetization $\vec{M}$. For antiferromagnetic particles in the vortex state, in addition to low-frequency modes, there are high frequency modes with frequencies over the range of hundreds gigahertz, including a mode localized in the region of the radius 30-40 nm near a vortex core.

cond-mat.mtrl-sci↗

arXiv2009

Stable topological textures in a classical 2D Heisenberg model

We show that stable localized topological soliton textures (skyrmions) with $π_2$ topological charge $ν\geq 1$ exist in a classical 2D Heisenberg model of a ferromagnet with uniaxial anisotropy. For this model the soliton exist only if the number of bound magnons exceeds some threshold value $N_{\rm cr}$ depending on $ν$ and the effective anisotropy constant $K_{\rm eff}$. We define soliton phase diagram as the dependence of threshold energies and bound magnons number on anisotropy constant. The phase boundary lines are monotonous for both $ν=1$ and $ν>2$, while the solitons with $ν=2$ reveal peculiar nonmonotonous behavior, determining the transition regime from low to high topological charges. In particular, the soliton energy per topological charge (topological energy density) achieves a minimum neither for $ν=1$ nor high charges, but rather for intermediate values $ν=2$ or $ν=3$.

cond-mat.stat-mech↗

arXiv2008

Large temperature dependence of the Casimir force at the metal-insulator transition

The dependence of the Casimir force on material properties is important for both future applications and to gain further insight on its fundamental aspects. Here we derive a general theory of the Casimir force for low-conducting compounds, or poor metals. For distances in the micrometer range, a large variety of such materials is described by universal equations containing a few parameters: the effective plasma frequency, dissipation rate of the free carriers, and electric permittivity in the infrared range. This theory can also describe inhomogeneous composite materials containing small regions with different conductivity. The Casimir force for mechanical systems involving samples made with compounds that have a metal-insulator transition shows an abrupt large temperature dependence of the Casimir force within the transition region, where metallic and dielectric phases coexist.

cond-mat.mes-hall↗

arXiv2007

Chirality tunneling and quantum dynamics for domain walls in mesoscopic ferromagnets

We studied the quantum dynamics of ferromagnetic domain walls (topological kink-type solitons) in one dimensional ferromagnetic spin chains. We show that the tunneling probability does not depend on the number of spins in a domain wall; thus, this probability can be large even for a domain wall containing a large number of spins. We also predict that there is a strong interplay between the tunneling of a wall from one lattice site to another (tunneling of the kink coordinate) and the tunneling of the kink topological charge (so-called chirality). Both of these elementary processes are suppressed for kinks in one-dimensional ferromagnets with half-integer spin. The dispersion law (i.e., the domain wall energy versus momentum) is essentially different for chains with either integer or half-integer spins. The predicted quantum effects could be observed for mesoscopic magnetic structures, e.g., chains of magnetic clusters, large-spin molecules, or nanosize magnetic dots.

cond-mat.mes-hall↗

arXiv2007

Solitons in isotropic antiferromagnets: beyond a sigma model

Isotropic antiferromagnets shows a reach variety of magnetic solitons with non-trivial static and dynamic properties. One-dimensional soliton elementary excitations have a periodic dispersion law. For two-dimensional case, planar antiferromagnetic vortices having non-singular macroscopic core with the saturated magnetic moment are present. The dynamic properties of these planar antiferromagnetic vortex are characterized by presence of a gyroforce

cond-mat.other↗