Suppression of magnetic relaxation by a transverse alternating magnetic field

The evolution of the spatial distribution of the magnetic induction in a superconductor after the action of the alternating magnetic field perpendicular to the trapped magnetic flux has been analyzed. The observed stabilization of the magnetic induction profile is attributed to the increase in the pinning force, so that the screening current density becomes subcritical. The last statement is corroborated by direct measurements.     

The heating effect of magnetic fluids in an alternating magnetic field

The heating mechanism and influencing factors of magnetite particles in a 63 kHz alternating magnetic field and 7 kA/m were studied. The results from in vivo heating experiments suggest that magnetite particles can generate enough energy to heat tumor tissue and perform effective hyperthermia. A novel model for predicting power losses has been proposed.     

Mechanisms of dissipation in a Josephson medium based on a high-temperature superconductor in a magnetic field

This paper reports on the results of an investigation into the influence of magnetic fields (0–60 kOe) on the temperature dependences of the electrical resistance R(T) of the Y_3/4Lu_1/4Ba₂Cu₃O₇ + CuO composites. The structure of these composites is considered to be a network of tunnel-type Josephson junctions in which a nonsuperconducting component (CuO) forms boundaries (barriers) between high-temperature superconducting crystallites. The temperature dependence R(T) of the composites has two steps characteristic of granular superconductors: (i) an abrupt change in the electrical resistance at the critical temperature of high-temperature superconducting crystallites and (ii) a smooth transition to the superconducting state under the influence of the boundaries between the crystallites. The experimental dependences R(T) are analyzed within the Ambegaokar-Halperin model of thermal fluctuations in Josephson junctions and the flux creep model. An increase in the magnetic field leads to a crossover from the Ambegaokar-Halperin mechanism to the flux creep mechanism. The temperature dependences R(T) in the range of weak magnetic fields (from 0 to 10² Oe) are adequately described by the relationship following from the Ambegaokar-Halperin model. In the range of strong magnetic fields (from 10³ to 6 × 10⁴ Oe), the dissipation obeys the Arrhenius law R ～ exp(?U(H)/T)], which is characteristic of the flux creep model with a temperature-independent pinning energy U(H). The effective Josephson coupling energies and the pinning energies corresponding to the Ambegaokar-Halperin and flux creep mechanisms are determined.     

Dominant influence of the compression effect of a magnetic flux in the intergranular medium of a granular high-temperature superconductor on dissipation processes in an external magnetic field

Experiments have been presented that demonstrate the effect of the compression of a magnetic flux in grain boundaries of a granular high-temperature superconductor in an external magnetic field on the dissipation processes. The compression of the magnetic flux is associated with the diamagnetic behavior of superconducting grains and the existence of a Josephson medium in grain boundaries. Under these conditions, grain boundaries are in an effective magnetic field that depends on the magnetic state (magnetization) of the superconducting grains. Based on the analysis of experimental data (dependences of the electrical resistance R and magnetization on the magnetic field H and temperature T, as well as current-voltage characteristics), the conclusion has been drawn that it is the temperature evolution of the effective magnetic field in the intergranular medium which primarily determines the behavior of the dependences R(T) in weak external magnetic fields of no more than ～10³ Oe. This should be taken into account in the interpretation of experiments on the magnetoresistance effect in granular high-temperature superconductors in terms of different theories. The conclusion drawn here also implies a significant correction of the previously obtained results.     

Pinning of planar vortices and magnetic field penetration in a three-dimensional Josephson medium

The behavior of planar (laminar) vortices in a three-dimensional, ordered Josephson medium as a function of the parameter I, which is proportional to the critical junction current and the cell size, is investigated with allowance for pinning due to the cellular structure of the medium. The minimum possible distances between two isolated vortices are calculated. A system of vortices formed in a sample in a monotonically increasing external magnetic field is analyzed. The minimum distance from the outermost vortex to the nearest neighbor is proportional to I ^−1.1. For I⩽1.3 each vortex contains a single flux quantum Φ₀, and the distance between them does not decrease in closer proximity to the boundary but remains approximately constant, implying that the magnetic field does not depend on the coordinate in the region penetrated by vortices. These facts contradict the generally accepted Bean model. The sample magnetization curve has a form typical of type II superconductors. Allowance for pinning raises the critical field H _c and induces a sudden jump in the curve at H=H _c. Zh. Tekh. Fiz. 67, 38–46 (September 1997)     

Specific features of relaxation of an alternating flux in YBCO plates

It is shown that the relaxation of a magnetic flux in superconductors is significantly affected by the flux configuration and the sample shape. In particular, variation in the magnetic flux configuration in YBCO plates makes it possible not only to change the relaxation rate but also significantly delay the flux relaxation.     

The effect of an oblique magnetic field on the superparamagnetic relaxation time

The effect of a constant magnetic field, applied at an angle ψ to the easy axis of magnetization, on the Néel relaxation time τ of a single domain ferromagnetic particle with uniaxial anisotropy is investigated by calculating the smallest non-vanishing eigenvalue of the appropriate Fokker-Planck equation as a function of ψ. The curve of λ₁ versus ψ is symmetric about ψ = 90°. Thus the maximum decrease in τ occurs at ψ = 45° with maximum increase to a value exceeding that at ψ = 0, at ψ = 90°, the ψ = 0 value being again attained at ψ = 180°. The results are shown to be consistent with the behaviour predicted by the Kramers theory of the rate of escape of particles over potential barriers. This theory when applied to the potential barriers for the ψ = 90° orientation for rotation in space yields a simple approximate formula for the escape rate which is in reasonable agreement with the exact λ₁ calculated from the Fokker-Planck equation.     

Relaxation dynamics of superconducting Josephson cubits in a strong alternating field

The quantum trajectory method has been applied to study the influence of noise on the level populations of a cubit in separate realizations of the experiment and to follow the transition to the averaged dynamics obtained by multiple measurements of the cubit state. As an example of applying the developed method, the influence of noise on the interference pattern appearing in amplitude spectroscopy due to the Landau-Zener transitions in an alternating field has been analyzed. The influence of the number of repeated measurements and fluctuations in the phase of the exciting pulse during formation of the response of a cubit to the external field has been studied, which made it possible to interpret recent experiments from the viewpoint of single realizations and averaged dynamics.     

On possibility of the spontaneous magnetic flux in a Josephson junction containing magnetic impurities

The Josephson junction containing localized magnetic moments in a dielectric layer between two superconductors is considered. Conditions are studied under which the phase difference between superconductors in the state with energy minimum is equal to π (such a junction we call π-junction). In addition we consider “one-dimensional” Josephson junction one part (2) of which is π-junction, the other (1) being the usual Josephson junction ( 0 - junction). Conditions are found under which in such a system there is a spontaneous vortex with the centre at the boundary between the parts 1 and 2 and magnetic flux associated with this vortex. The vortex appears by second order phase transition as temperature decreases for T_c.     

Josephson effect through an isotropic magnetic molecule

We study the Josephson effect through a quantum dot magnet whose spin is isotropic and which is coupled to the dot electron spin via exchange coupling. We calculate the Andreev levels and the supercurrent and examine the intertwined effect of the exchange coupling, Kondo correlation, and superconductivity. The former suppresses Kondo correlations, which triggers phase transitions from the 0 to the pi state, but strong antiferromagnetic coupling restores the 0 state. The asymmetric phase diagram in the exchange coupling suggests that the coupling sign could be determined in experiments.     

Electroviscous effect in an alternating electric field

Experimental dependence of the relative dynamic viscosities of liquid dielectrics on the applied alternating voltage amplitude and frequency is presented. The voltage frequency was varied from 20 Hz to 2 kHz. The variation of viscosity can be attributed to a change in the liquid structure, with ion-molecule complexes centered at the ions produced as a result of charge injection from an electrode. A change in the injection regime leads to a change in viscosity.     

Origin of the magnetomechanical effect in an alternating field

The paper describes further experiments on the internal friction of nickel in an alternating magnetic field. It was found that the effect is not caused by macroscopic eddy currents but by microscopic eddy currents connected with changes in domain structure, which macroscopically have a reversible character and which appear both in the region of wall displacements and in the region of the rotation of magnetization vectors. The anelastic character of the effect is confirmed by measuring the dependence of the effect on the frequency of the field and on torsional oscillations; the relaxation time of the effect is determined by the circular frequency of the alternating magnetic field. The different aspects of the effect are discussed and a possible way of calculation is indicated.

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The autor wishes to express his gratitude to Dr. Wotruba, Dr. L. paek and others for valuable discussions.     

Approximate calculation of the magnetic moment of an elliptic-profile cylinder in a uniform alternating magnetic field

Equations are derived for the magnetic moment of a cylinder of elliptic profile magnetized by a uniform alternating magnetic field. The problem is solved by the approximate Neumann-Gel'bukh method. A correction which permits the use of the Neumann-Gel'bukh method for small ¦k¦a >- 1 is proposed for calculating the magnetic moment of a cylinder with a permeability = 1. Approximate expressions are found for the magnetic moment of a circular cylinder and of a sphere having permeability of = 1. The calculation error does not exceed 17%.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 12, No. 7, pp. 18–23, July, 1969.     

Hysteresis in the behavior of a 3D ordered Josephson medium in magnetic field at a small pinning parameter

Using the approach based on analyzing variations in the configuration in the direction of the decreasing Gibbs potential, the magnetization curve for a three-dimensional ordered Josephson medium upon magnetic field cycling has been calculated for the case of a small pinning parameter. It has been demonstrated that, at any turning point, the hysteresis loop is part of some universal curve that is strictly periodic along both axes. The existence of a universal curve and its periodicity have been explained by analyzing the vortex configurations.     

导磁导电球体在均匀谐变磁场中产生的磁场分布规律

从求解导磁导电各向同性球体的赫兹矢量π*出发,推导出了导磁导电球体置于均匀谐变低频弱磁场中球体产生的磁场分布规律和球频率函数.