Dynamic simulations of the dipolar driven demagnetization process of magnetic multi-core nanoparticles

In this work, we investigate dynamically the dipolar driven demagnetization process of magnetic multi-core particles by solving the Landau-Lifshitz equation for single-domain particles distributed on a three-dimensional sphere. We analyze the relaxation time in respect to different geometry and material parameters. Further we show that the demagnetization times differ from the behaviour of a single magnetic sphere in the case of low damping. To explain these dynamics nanoparticular systems of different dimensions are investigated. We show that deviations can be attributed to a confinement of the relaxation dynamics to a lower dimensional submanifold of the k-space.     

Relaxation times in cyclotron resonance in metals

The relation between an effective relaxation time τ in cyclotron resonance, mass anisotropy and magnetic field tipping is studied and a true relaxation time τ₀ is extracted by comparison with measurements. It is found that for copper τ has a maximum value in a slightly tipped magnetic field and that in potassium τ decreases monotonically with increasing tip angle. This is explained in terms of the Fermi surfaces of these metals.     

Magneto-transport in the two dimensional Lorentz model

A recently developed theory for the motion of a classical particle in a random array of scatterers is improved and extended to discuss the effects of weak and intermediate magnetic fields. By deriving expressions for the general relaxation kernels it is shown that only the current relaxation kernel is the physical relevant one diverging at the percolation edge. The percolation density and localization length turn out to be independent of the magnetic field. A negative magneto resistance at low scatterer density, a positive magneto resistance at larger density and a non classical Hall coefficient are obtained. For the velocity correlation spectrum a shift of the cyclotron resonance to higher frequency and a new low frequency side peak is predicted.     

On nuclear relaxation in Bloch walls

The influence of the non-linear rotation of the vector of local magnetization of a Bloch wall on the nuclear spin-lattice relaxation, given by the thermal fluctuations of the wall, is studied. On the basis of a model of uniform mode the fluctuations of the transversal hyperfine field, proportional to the square of the wall displacement, are included in the calculation of the relaxation rate. Compared with the linear process, this quadratic effect plays a role in very small particles having thin walls with a small damping coefficient and for a low frequency of the nuclear magnetic resonance. Orientational estimates of the relaxation rate are made for nuclear magnetic resonance in small particles of nickel.In conclusion the author thanks Z. roubek C. Sc. and J. Kaczér C. Sc. for valuable discussions and remarks.     

Microscopic theory of the long-wavelength modes of two-component plasmas and ionic liquids: II. The longitudinal modes

The modification of the damping rate of the sound modes by Coulomb phenomena is demonstrated from first principles. The heat modes of one- and two-component systems of charged particles are shown to differ by a factor c_p/c_v. Microscopic expressions for the interspecies energy and longitudinal momentum relaxation frequencies are provided. The charge relaxation modes are shown to reduce in the limit of weak-coupling to a pair of plasma oscillations occurring slightly below the plasma frequency while being slightly damped even at infinite wavelength. In the opposite limit of strong-coupling the same pair of charge relaxation modes is shown to split into an interspecies momentum relaxation mode and an approximate hydrodynamic diffusion mode. An Einstein relation between the diffusion constant and the electric conductivity is also demonstrated. All expressions are obtained for arbitrary density and coupling.     

Frequency dependences of the velocity and absorption coefficient of sound waves in a magnetic fluid

On the basis of molecular-kinetic theory, frequency-dependent expressions are derived for the velocity and absorption coefficient of sound in a magnetic fluid with allowance for the contribution of structural relaxation in the presence of an external inhomogeneous magnetic field. The asymptotic behavior of the expressions is investigated for both low-and high-frequency limits. A numerical study is carried out by taking as an example a magnetic fluid consisting of kerosene with magnetic particles of Fe₃O₄ suspended in it. The results of the numerical study show that the calculated frequency dependences of the velocity and absorption coefficient of sound in a magnetic fluid are in good agreement with the static experimental data.     

Stochastic resonance in single-domain nanoparticles with cubic anisotropy

The signal-to-noise ratio for magnetic stochastic resonance in a superparamagnetic particle with cubic anisotropy is shown to be strongly dependent on the Larmor precession damping α. This phenomenon is due to the coupling of the relaxation and precession modes and can be used for measuring α. The dependence of the signal-to-noise ratio on α is characteristic of particles with nonaxial anisotropy; so the effect is absent in uniaxial particles.     

A new spectrometric method,using a magneto-optical effect,to study magnetic liquids

We present an original polarimetric method to study the magnetic relaxation of ferrofluids. The Brownian relaxation of three ferrofluids with Fe₂CoO₄ particles dispersed in water, dibutyl phthalate and diethylene glycol is presented and compared with the theoretical model of Debye adapted to the magnetism. The results show that this system has a good sensitivity even at low volume concentration of particles and under low ac magnetic field. The analysis of the relaxation parameters leads to a good estimation of particles diameter and put in evidence a distribution of relaxation time.     

Characterization and Suppression Techniques for Degree of Radiation Damping in Inversion Recovery Measurements

Radiation damping is a phenomenon in which transverse nuclear magnetization couples with the current in a coil used in nuclear magnetic resonance experiments. This results in an additional magnetic field that increases the relaxation pathway for the magnetization, which then relaxes back to equilibrium more quickly. Radiation damping has been shown to affect longitudinal relaxation time (T₁) measurement in inversion recovery experiments. In this work, we demonstrate that the extent of radiation damping depends upon the T₁ of the sample. Radiation damping difference spectroscopy is used to characterize the severity of radiation damping, while gradient inversion recovery is used for radiation damping suppression in T₁ measurements. At field strength of 9.4 T, for the radiation damping characteristic time (T_rd) of 50 ms, these investigations show non-negligible radiation damping effects for T₁ values greater than T_rd, with severe distortions for T₁ longer than about 150 ms, showing reasonable agreement with the predicted T_rd. We also report a discrepancy between published expressions for the characteristic radiation damping time.     

Relaxation of Translational Energy in Binary Mixtures of Dilute Gases Composed of Hard Spheres

Analytical expressions for the relaxation time τ necessary for the equilibration of translational energies of components in binary mixtures of dilute gases composed of hard spheres are derived. An expression for the collision number Z describing the mean number of collisions of one particle during the relaxation time is also given. The derivation has been performed for the very beginning of the relaxation and we choose the initial translational energy of the sort which is in excess to be equal to zero. Computer results obtained by a direct simulation method of solution of the Boltzmann equation do not only confirm these results but also show them to be valid for a relatively long part of the equilibration.     

Stress dependence of the domain wall dynamics in the adiabatic regime

In this work, we determine the domain wall velocity in the low field region and study the domain dynamics in as-cast and annealed bi-stable amorphous glass-covered Fe_77.5Si_7.5B₁₅ microwires. In particular, from the relation between the domain wall velocity and magnetic field in the adiabatic regime, the power-law critical exponent β, the critical field H₀ and the domain wall damping η were obtained. It has been verified that the main source of domain wall damping is the eddy current and spin relaxation, both with a strong relation with the magnetoelastic energy. This energy term is changed by the axial applied stress, which, by its time, modifies the damping mechanisms. It was also verified that the domain wall damping terms present different behavior at low (mainly eddy currents) and high applied stress (spin relaxation).     

Bistability effects of the brownian motion in periodic potentials

In the stationary state the equation of motion for particles moving in a periodic potential has two solutions, a locked one and a running one, for low and intermediate damping constants and for suitable external forces. The effect of an additional Langevin force to this bistable behaviour is investigated. For finite noise strength, the mobility depends continuously on the external force, whereas in the limit of vanishing strength of the noise force one gets a sharp transition between the locked and the running solution at a critical external force. This critical force is calculated exactly in the low friction limit and approximately for intermediate friction constants. Furthermore the temperature dependence for various forces including the critical one is shown in the low friction limit.     

Distributions of switching times of single-domain particles using a time quantified Monte Carlo method

Using a time quantified Monte Carlo scheme we performed simulations of the switching time distribution of single mono-domain particles in the Stoner–Wohlfarth approximation. We considered uniaxial anisotropy and different conditions for the external applied field. The results obtained show the switching time distribution can be well described by two relaxation times, either when the applied field is parallel to the easy axis or for an oblique external field and a larger damping constant. We found that in the low barrier limit these relaxation times are in very good agreement with analytical results obtained from solutions of the Fokker–Planck equation related to this problem. When the damping is small and the applied field is oblique the shape of the distribution curves shows several peaks and resonance effects.     

Production of a pair by a polarized photon in a uniform constant electromagnetic field

The total probability of production of an electron-positron pair by a polarized photon in a constant uniform electromagnetic field of an arbitrary configuration is determined using the imaginary part of the diagonalized polarization operator. Approximate expressions are derived for this probability in four ranges of photon energy. In the high-energy range, the corrections to the standard semiclassical approximation are calculated. In the range of intermediate energies, in which this approximation is inapplicable, the probability of the process is calculated using the steepest descent method. It is shown that in the range of photon energies higher than the pair production threshold in a magnetic field, a weak electric field removes root divergences in the probability of production of the particles at the Landau levels. For relatively low photon energies, a low-energy approximation is developed. At such energies, the effect of the electric field on the process is decisive, while the effect of the magnetic field is associated with its interaction with the magnetic moment of the particles being produced. Such an interaction is manifested, in particular, in the difference in the probabilities of production of a pair by an external field for scalar and spinor particles.     

Viscous flow of magnetic vortices and rapid relaxation of magnetization in YBaCuO ceramics

The viscous flow of magnetic vortices in granular YBaCuO ceramics of various granule sizes is investigated using the microwave absorption method (f=80 MHz) in the range of varying magnetic fields H=±1500 Oe and of temperatures T=77 to 95 K. The conditions for the formation and melting of the vortex lattice, as well as the transition of the high-temperature superconductor (HTSC) state from the irreversibility region to the region of the viscous flow of magnetic vortices, are considered. The rapid magnetic relaxation times in the range τ=72 to 111 ms are determined from the magnetization relaxation in the HTSC materials under investigation depending on the granule size d, which varies from 1 to 30 μm. The experimental results on the variation of the relaxation time τ from 77 to 120 ms in a ceramic exposed to fast neutrons with various fluences (Φ=10¹⁶ to 10¹⁹ cm⁻²) are considered. The depinning process is analyzed; it occurs in the form of a thermally assisted flux flow in the range of low activation energies. __________ Translated from Fizika Tverdogo Tela, Vol. 43, No. 6, 2001, pp. 968–973. Original Russian Text Copyright ? 2001 by Chashchin.     

Theoretical limit of the minimal magnetization switching field and the optimal field pulse for Stoner particles

The theoretical limit of the minimal magnetization switching field and the optimal field pulse design for uniaxial Stoner particles are investigated. Two results are obtained. One is the existence of a theoretical limit of the smallest magnetic field out of all possible designs. It is shown that the limit is proportional to the damping constant in the weak damping regime and approaches the Stoner-Wohlfarth (SW) limit at large damping. For a realistic damping constant, this limit is more than 10 times smaller than that of so-called precessional magnetization reversal under a noncollinear static field. The other is on the optimal field pulse design: if the magnitude of a magnetic field does not change, but its direction can vary during a reversal process, there is an optimal design that gives the shortest switching time. The switching time depends on the field magnitude, damping constant, and magnetic anisotropy.     

Glassy vortex dynamics induced by a random array of magnetic particles above a superconductor

The magnetic relaxation of a Nb film covered with a random array of permalloy particles has been studied using various procedures. When the sample undergoes a field-cooled process, the magnetic relaxation becomes logarithmic in time. The relaxation rate is nearly temperature independent at low temperature and characteristic glassy dynamics-aging and memory effects-are observed. These results are interpreted as the consequence of pinning by the statistical variation of the number of nanoparticles within the area of a vortex core.     

Damping mechanism of the relaxation oscillations in a gas mixture fast flow laser

It is shown that the exchange of perturbations of moving active medium components, such as a CO₂-N₂ laser working mixture, results in the damping of relaxation oscillations and stabilization of stationary lasing. Analytical expressions for frequencies and increments of relaxation oscillations and their self-excitation threshold are obtained using the characteristics of stationary lasing.     

Damping of Magnetorheological Elastomers

利用改装后的力磁耦合动态测试系统实验研究了磁流变弹性体在磁场下的阻尼性能. 实验得出了外加磁场的磁感应强度、基体的本征阻尼、铁粉含量、动态应变以及激励频率对磁流变弹性体阻尼的影响规律. 发现了磁流变弹性体的阻尼在很大程度上是由基体和颗粒之间的界面滑移所决定的. 而且该界面滑移不同于一般复合材料,它会受到外加磁场的影响..