51 research outputs found
Kinetic and Transport Equations for Localized Excitations in Sine-Gordon Model
We analyze the kinetic behavior of localized excitations - solitons,
breathers and phonons - in Sine-Gordon model. Collision integrals for all type
of localized excitation collision processes are constructed, and the kinetic
equations are derived. We analyze the kinetic behavior of localized excitations
- solitons, breathers and phonons - in Sine-Gordon model. Collision integrals
for all type of localized excitation collision processes are constructed, and
the kinetic equations are derived. We prove that the entropy production in the
system of localized excitations takes place only in the case of inhomogeneous
distribution of these excitations in real and phase spaces. We derive transport
equations for soliton and breather densities, temperatures and mean velocities
i.e. show that collisions of localized excitations lead to creation of
diffusion, thermoconductivity and intrinsic friction processes. The diffusion
coefficients for solitons and breathers, describing the diffusion processes in
real and phase spaces, are calculated. It is shown that diffusion processes in
real space are much faster than the diffusion processes in phase space.Comment: 23 pages, latex, no figure
Spin-flop transition in uniaxial antiferromagnets: magnetic phases, reorientation effects, multidomain states
The classical spin-flop is the field-driven first-order reorientation
transition in easy-axis antiferromagnets. A comprehensive phenomenological
theory of easy-axis antiferromagnets displaying spin-flops is developed. It is
shown how the hierarchy of magnetic coupling strengths in these
antiferromagnets causes a strongly pronounced two-scale character in their
magnetic phase structure. In contrast to the major part of the magnetic phase
diagram, these antiferromagnets near the spin-flop region are described by an
effective model akin to uniaxial ferromagnets. For a consistent theoretical
description both higher-order anisotropy contributions and dipolar stray-fields
have to be taken into account near the spin-flop. In particular,
thermodynamically stable multidomain states exist in the spin-flop region,
owing to the phase coexistence at this first-order transition. For this region,
equilibrium spin-configurations and parameters of the multidomain states are
derived as functions of the external magnetic field. The components of the
magnetic susceptibility tensor are calculated for homogeneous and multidomain
states in the vicinity of the spin-flop. The remarkable anomalies in these
measurable quantities provide an efficient method to investigate magnetic
states and to determine materials parameters in bulk and confined
antiferromagnets, as well as in nanoscale synthetic antiferromagnets. The
method is demonstrated for experimental data on the magnetic properties near
the spin-flop region in the orthorhombic layered antiferromagnet
(C_2H_5NH_3)_2CuCl_4.Comment: (15 pages, 12 figures; 2nd version: improved notation and figures,
correction of various typos
High antiferromagnetic domain wall velocity induced by Néel spin-orbit torques
We demonstrate the possibility to drive an antiferromagnetic domain wall at high velocities by fieldlike Néel spin-orbit torques. Such torques arise from current-induced local fields that alternate their orientation on each sublattice of the antiferromagnet and whose orientation depends primarily on the current direction, giving them their fieldlike character. The domain wall velocities that can be achieved by this mechanism are 2 orders of magnitude greater than the ones in ferromagnets. This arises from the efficiency of the staggered spin-orbit fields to couple to the order parameter and from the exchange-enhanced phenomena in
antiferromagnetic texture dynamics, which leads to a low domain wall effective mass and the absence of a Walker breakdown limit. In addition, because of its nature, the staggered spin-orbit field can lift the degeneracy between two 180° rotated states in a collinear antiferromagnet, and it provides a force that can move such walls and control the switching of the states
Thermodynamically self-consistent non-stochastic micromagnetic model for the ferromagnetic state
In this work, a self-consistent thermodynamic approach to micromagnetism is
presented. The magnetic degrees of freedom are modeled using the
Landau-Lifshitz-Baryakhtar theory, that separates the different contributions
to the magnetic damping, and thereby allows them to be coupled to the electron
and phonon systems in a self-consistent way. We show that this model can
quantitatively reproduce ultrafast magnetization dynamics in Nickel.Comment: 5 pages, 3 figure
Symmetry and the macroscopic dynamics of antiferromagnetic materials in the presence of spin-polarized current
Antiferromagnetic (AFM) materials with zero or vanishingly small macroscopic
magnetization are nowadays the constituent elements of spintronic devices.
However, possibility to use them as active elements that show nontrivial
controllable magnetic dynamics is still discussible. In the present paper we
extend the theory [A.F.Andreev, V.I.Marchenko, Sov. Phys. --- Uspekhi, 23
(1980), 21] of macroscopic dynamics in AFMs for the cases typical for
spin-valve devices. In particular, we consider the solid-like magnetic dynamics
of AFMs with strong exchange coupling in the presence of spin-polarized current
and give an expression for the current-induced Rayleigh dissipation function in
terms of the rotation vector for different types %generalized potential of
AFMs. Basing on the analysis of linearized equations of motion we predict the
current-induced reorientation and AFM resonance, and found the values of
critical currents in terms of AFMR frequencies and damping constants. We show
the possibility of current-induced spin-diode effect and second-harmonic
generation in AF layer. The proposed approach is generalized for the
description of current-related phenomena in inhomogeneous AFMs.Comment: 10 pages, 3 figures, to be submitted to PR
Magnetic Phase Transitions in the NdFe_3(BO_3)_4 multiferroic
Low temperature studies of the behavior of the sound velocity and attenuation
of acoustic modes have been performed on a single crystal NdFe_3(BO_3)_4.
Transitions of the magnetic subsystem to the antiferromagnetically ordered
state at T_N \approx 30.6 K have been revealed in the temperature behavior of
the elastic characteristics. The features in the temperature behavior of
elastic characteristics of the neodymium ferroborate and its behavior in the
external magnetic field, applied in the basic plane of the crystal, permit us
to suppose that the transition to an incommensurate spiral phase is realized in
the system. This phase transition behaves as the first order one. H-T phase
diagrams for the cases H \parallel a and H \parallel b have been constructed.
The phenomenological theory, which explains observed features, has been
developed
Magnetoelectric effects in single crystals of the cubic ferrimagnetic helimagnet Cu2OSeO3
We present magnetodielectric measurements in single crystals of the cubic
spin-1/2 compound CuOSeO. A magnetic field-induced electric
polarization () and a finite magnetocapacitance (MC) is observed at
the onset of the magnetically ordered state ( K). Both and
MC are explored in considerable detail as a function of temperature (T),
applied field , and relative field orientations with respect to the
crystallographic axes. The magnetodielectric data show a number of anomalies
which signal magnetic phase transitions, and allow to map out the phase diagram
of the system in the -T plane. Below the 3up-1down collinear ferrimagnetic
phase, we find two additional magnetic phases. We demonstrate that these are
related to the field-driven evolution of a long-period helical phase, which is
stabilized by the chiral Dzyalozinskii-Moriya term D \vec{M}
\cdot(\bs{\nabla}\times\vec{M}) that is present in this non-centrosymmetric
compound. We also present a phenomenological Landau-Ginzburg theory for the
ME effect, which is in excellent agreement with experimental data, and
shows three novel features: (i) the polarization has a uniform as
well as a long-wavelength spatial component that is given by the pitch of the
magnetic helices, (ii) the uniform component of points along the
vector , and (iii) its strength is proportional to
, where is the longitudinal
and is the transverse (and spiraling) component of the magnetic
ordering. Hence, the field dependence of P provides a clear signature of the
evolution of a conical helix under a magnetic field. A similar phenomenological
theory is discussed for the MC
Non-linear dynamics and two-dimensional solitons for spin ferromagnets with biquadratic exchange
We develop a consistent semiclassical theory of spin dynamics for an
isotropic ferromagnet with a spin taking into consideration both
bilinear and biquadratic over spin operators exchange interaction. For such
non-Heisenberg magnets, a peculiar class of spin oscillations and waves, for
which the quantum spin expectation value does
not change it direction, but changes in length, is presented. Such
``longitudinal'' excitations do not exist in regular magnets, dynamics of which
are described in terms of the Landau-Lifshitz equation or by means of the spin
Heisenberg Hamiltonian. We demonstrate the presence of non-linear uniform
oscillations and waves, as well as self-localized dynamical excitations
(solitons) with finite energy. A possibility of excitation of such oscillations
by ultrafast laser pulse is discussed.Comment: 11 pages, 7 figures, MikTE
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