Regular and chaotic dynamics of a chain of magnetic dipoles with moments of inertia

The nonlinear dynamic modes of a chain of coupled spherical bodies having dipole magnetic moments that are excited by a homogeneous ac magnetic field are studied using numerical analysis. Bifurcation diagrams are constructed and used to find conditions for the presence of several types of regular, chaotic, and quasi-periodic oscillations. The effect of the coupling of dipoles on the excited dynamics of the system is revealed. The specific features of the Poincaré time sections are considered for the cases of synchronous chaos with antiphase synchronization and asynchronous chaos. The spectrum of Lyapunov exponents is calculated for the dynamic modes of an individual dipole.     

Polar nano-rods under shear: From equilibrium to chaos

The orientational dynamics of rod-like particles with permanent (electric or magnetic) dipole moments in a plane Couette shear flow is investigated using mesoscopic relaxation equations combined with a generalized Landau free energy. The free energy contribution due to the coupling between average alignment and dipole orientation is derived on a microscopic basis. Numerical results of the resulting eight-dimensional dynamical system are presented for the case of longitudinal dipoles and thermodynamic conditions where the equilibrium state is a (polar or non-polar) nematic. Solution diagrams reveal presence of a large variety of periodic, transient chaotic, and chaotic dynamic states of the average alignment and dipole moment, respectively, appearing as a function of Deborah number and tumbling parameter. Compared to rods without dipoles we observe a significant preference of out-of-plane kayaking-tumbling states and, generally, a higher sensitivity to the initial conditions including bistability. We also demonstrate that the average (electric) dipole moment characterizing most of the observed states yields electrodynamic (magnetic) fields of measurable strength.     

Equilibrium configurations and transitions between them in annular magnetic nanodipole systems

The equilibrium states of annular systems of magnetic dipoles have been studied by computer simulation. The bistability conditions under which the total magnetic moment of one of equilibrium configurations is zero, while the magnetic moment of another equilibrium configuration lies in the ring plane and is close to the sum of the magnetic moments of dipoles in the system, have been determined. The realization of other equilibrium configurations has also been demonstrated. We analyze transitions between equilibrium configurations by acting on the system by longitudinal and circular static fields, as well as transitions from the configuration with the maximal magnetic moment of the system to the configuration with zero total magnetic moment after the relaxation of oscillatory regimes excited by a varying field.     

Dynamics of the lattice of magnetic nanodipoles with cubic anisotropy

The 5 × 5 square lattices of magnetic dipoles with cubic crystallographic anisotropy were investigated by the computer simulation method. The conditions for implementing the random orientation of lattice configurations, each of which are characterized by a certain response to the influence of an external magnetic pulse, as well as by the established regime of the oscillation of the total magnetic moment under the influence of an alternating field, are revealed. Regular vibration modes with a doubled frequency and quasi-periodic and chaotic modes are detected. The dependence of the system response on the parameters of the magnetic field pulse is studied.     

Response of a magnetic nanoparticle lattice to a magnetic field pulse near the stability boundary

The dynamic response of a system being near the stable equilibrium boundary to an external magnetic field pulse is studied for 2D lattices of magnetic nanoparticles with cubic crystallographic anisotropy. The conditions under which magnetic moment oscillations from individual dipoles propagate to the entire system are revealed. This effect results in the lattice response are significantly larger in the external pulse duration and with an amplitude rather weakly depending on initial conditions and external field parameters, the processes during which the pulse results in reorientation of only individual lattice dipoles.     

Low-frequency complex magnetic susceptibility of magnetic composite microspheres in colloidal dispersion

A procedure is presented to determine the permanent magnetic dipole moment of composite microspheres containing magnetic nanoparticles with a blocked magnetic dipole moment. The composite particles are dispersed in a solvent, and the complex magnetic susceptibility is measured from 0.1 to 1000 Hz using a highly sensitive new setup. Composite particles with a permanent magnetic dipole moment are revealed by a characteristic frequency that corresponds to the Brownian rotation of the microspheres. From measured susceptibility spectra, we calculate the permanent magnetic dipole moment of recently developed cobalt ferrite-doped silica and latex microspheres.     

钫原子磁偶极超精细结构常数及其同位素的磁偶极矩的理论计算

应用基于B样条基组的相对论耦合簇理论方法,计算了~(212)Fr原子的n S (n=7—12), n P (n=7—12)和n D (n=6—11)态的磁偶极超精细结构常数.与精确实验值的比较说明这套理论方法能精确计算出磁偶极超精细结构常数,其中7P态的磁偶极超精细常数的理论值与实验值之间的差异小于1%.在忽略场移效应对Fr原子7P态超精细结构常数的影响下,通过结合实验值进一步定出了~(207-213,220-228)Fr核磁偶极矩μ,这些值与已有的测量值具有非常好的一致性.本文报道了12S, n P (n=9—12)和n D (n=10—11)态的磁偶极超精细结构常数.     

Dephasing of local vibrations of a two-dimensional dipole system

Anharmonically coupled, high-and low-frequency branches of vibrational excitations of a two-dimensional dipole system on a substrate are studied. It is shown that independent low-frequency resonance modes characterized by a two-dimensional wave vector belonging to the first Brillouin zone of the dipole lattice arise as a result of the harmonic interaction of phonons in the dipole system and in the substrate. The positions and widths of the spectral lines of the high-frequency local vibrations, which in the general case depend on the dispersion laws and lifetimes of the resonance modes, are calculated for low-temperature orientational states of degenerate dipoles on triangular and square lattices. Fiz. Tverd. Tela (St. Petersburg) 40, 152–155 (January 1998)     

Vortex structures in planar lattices of magnetic dipoles in the presence of exchange coupling

Vortex equilibrium states of planar square lattices of magnetic dipoles in the presence of the exchange interaction have been studied. It has been shown that the vortex equilibrium configurations differ in the position of the vortex center and, correspondingly, in the magnitude and direction of the total magnetic moment of the system. In the case of the position of the vortex center in the center of the array, the total magnetic moment of the system is zero. The vortex center moves in the direction perpendicular to the field under the action of the external planar magnetic field on the system. Thus, the transitions between different equilibrium vortex configurations are implemented and the magnetic moment of the system of dipoles is controlled.     

Vortex–antivortex states in nanostructured superconductor–ferromagnet hybrids

The formation of vortex–antivortex states in a superconducting film with a square array of magnetic dipoles magnetized perpendicularly to the film is investigated in the framework of the time-dependent Ginzburg–Landau equations. It is shown that a possible route to obtain equilibrium states is obtained following an experimentally realizable field-cooling procedure. The states thus obtained demonstrate a rich variety of phases, depending on magnetic moment intensity and dipole array-to-superconducting film distance. For instance, in the region of the phase diagram where each dipoles is able to generate N = 2 vortex–antivortex pairs, the antivortices induced by the negative stray fields of the dipoles undergo two transitions before ultimately merging into doubly-quantized giant antivortices. For N = 4, a state consisting on a three-quanta giant vortex below each dipole and an interstitial vortex–antivortex molecule was observed. Such state is thermodynamically stable and is induced by the fourfold symmetry of the dipole array, similar to symmetry-induced vortex–antivortex molecules found in mesoscopic superconductors.     

Extraction of hyperfine anomalies without precise values of the nuclear magnetic dipole moment

A new method for extracting the hyperfine anomaly from experimental hyperfine structure constants is suggested. Instead of independent high-precision measurements of the nuclear magnetic dipole moment, precise measurements of magnetic dipole hyperfine interaction constants for two atomic states and a theoretical analysis can be used. This can lead to determination of hyperfine anomaly for radioactive isotopes where the nuclear magnetic dipole moment is not known with high accuracy. Received: 17 February 1998     

Lamellar-like structures in ferrofluids placed in strong magnetic fields

We consider a ferrofluid system consisting of magnetic particles interacting with a magnetic dipole–dipole interaction. We study the strong magnetic field regime where all magnetic dipoles are completely polarized in the direction of the magnetic field. We introduce a lattice gas model that serves to describe space ordering phenomena in such systems. It is found that, within mean field theory, this model predicts a second order phase transition to a phase with inhomogeneous lamellar-like ordering below a certain critical temperature.     

Dynamic magnetization reversal in dipole systems

The possibility of magnetization reversal in two different systems of magnetic dipoles by preliminary excitation of certain oscillatory regimes is demonstrated using numerical analysis of bistable states of these systems. Cyclic magnetization reversal of the systems is executed by sequential alternation of the frequency of the applied ac field. The interaction of two ring-shaped dipole systems is analyzed and peculiarities of the change in the total magnetic moments induced by this interaction are revealed.     

Dynamic orientation transitions in a system of magnetic dipoles

Bistable states induced by a varying magnetic field in a system of two, three, and four coupled spherical bodies possessing dipole magnetic moments are studied numerically. Dynamic transitions between low-amplitude regimes differing in the direction of vibrational axis are considered. Possibilities of controlling bistable states by varying magnetic field parameters are indicated.     

Dynamics of a magnetic moment of finite dipolar lattices in the AC field

Dynamic regimes of a magnetic moment of 2 × 2, 3 × 3, and 4 × 4 square dipole lattices in the linearly and circularly polarized ac magnetic field and the perpendicular static field have been investigated. The possibility of regular and chaotic precession dynamics of the magnetic moment of the lattices has been demonstrated. A shift of the main magnetic resonance owing to the dipole–dipole interaction and additional resonances have been revealed. Quasiperiodic regimes and bistability states with a metastable chaotic attractor have been discovered.     

Noise-induced enhancement of regular oscillations of a system of magnetic dipoles

The dynamics of a system consisting of two bodies or a chain of bodies possessing dipole magnetic moments and moments of inertia is studied on the basis of numerical analysis. In constructing the parametric bifurcation diagram, the conditions for oscillatory modes with high noise sensitivity are determined. Features of this dynamics of coupled magnetic moments are considered, and spectral characteristics of these modes are calculated.     

Temperature dependence of the magnetic susceptibility of magnetic dispersed nanosystems

The temperature dependence of the complex magnetic susceptibility of different magnetic nano-colloids and finely dispersed magnetite powder is studied. The results are explained with allowance for the magnetic moment relaxation in single-domain particles and phase transitions in a system of interacting dipoles.     

Spectral tuning by selective enhancement of electric and magnetic dipole emission

We demonstrate that magnetic dipole transitions provide an additional degree of freedom for engineering emission spectra. Without the need for a high-quality optical cavity, we show how a simple gold mirror can strongly tune the emission of trivalent europium. We exploit the differing field symmetries of electric and magnetic dipoles to selectively direct the majority of emission through each of three major transitions (centered at 590, 620, and 700 nm), and present a model that accurately predicts this tuning from the local electric and magnetic density of optical states.     

Equilibrium states and dynamics of the dipole moment of square arrays of dipoles

The dipole lattices in the form of 3 × 3–8 × 8 dipole square arrays have been considered. It has been shown that, after turning off the external field orienting dipoles along the edges of the array, two types of equilibrium configurations of dipole moments can exist depending on the size of the system, namely, the state symmetric with respect to the diagonal of the array and the state with the total dipole moment directed along the edges of the array. It has been found that there are differences in these types of configurations with respect to the alternating-field-induced oscillation modes of the total dipole moment of the system. The dependence of the oscillation modes on the direction of linear polarization of the alternating field has been investigated.     

Surface wave excitation in solids by magnetic dipoles

The paper studies the interaction of magnetic dipole moments with magnetoplasma (in semiconductors) and magnetostatic (in ferromagnets) surface waves. Expressions for energy losses of such dipoles by wave excitation are obtained; experimental realizability of mobile dipole moments are discussed; and estimates are presented. The said interaction is shown to be suitable to detect surface helicons in semiconductors.