External Magnetization Research Articles

Comparison of transport and magnetic AC losses in Bi-2223/Ag tapes — the role of superconducting core geometry

AC losses were measured by 4-probe transport method and by external magnetization method in three samples of Bi-2223/Ag tape: a multifilamentary tape with separated filaments, another multifilamentary tape with `bridges' between filaments, and a two-shell tape. The transport losses agreed with those calculated using I c from DC experiment. Magnetization experiments gave indications about the various paths of induced currents. For the tape with well separated filaments the main part of screening current closes inside individual filaments. Additional screening of the whole filamentary zone involves the normal metal matrix, leading to frequency dependent losses. In the case of tape with `bridged' filaments, supercurrents interconnect the filaments into bundles whose screening (and loss) is frequency-independent. Matching the experimental data indicates that a typical bundle was composed of 8 filaments. Magnetic losses of the two-shell tape were explained by a model for magnetization of superconducting wire with elliptical cross-section.

Angle dependence of the ferromagnetic resonance linewidth in easy-axis and easy-plane single crystal hexagonal ferrite disks

Ferromagnetic resonance measurements were made as a function of the static external field angle for c-plane disks of single crystal flux grown manganese substituted barium M-type (Ba-M) and zinc Y-type (Zn-Y) hexagonal ferrites at 50 and 8.8 GHz, respectively. A shorted waveguide technique was used. Analysis of the FMR field versus angle results confirmed the operational assumption of a uniform mode response. For the easy-axis Ba-M disk, the linewidth was 69 Oe when the external field and magnetization vectors were perpendicular to the disk. The linewidth increased to a maximum measured value of 472 Oe when the magnetization was directed 45.4° from the sample normal. For the easy-plane Zn-Y disk, the linewidth had a minimum value of 18 Oe when the field and magnetization vectors were in-plane. The maximum linewidth was 391 Oe when the magnetization was directed 25.2° from the disk normal. The linewidths were larger than predicted for reasonable values of the Landau–Lifshitz damping and showed angle dependences which indicated nonintrinsic contributions to the loss. A modified two magnon scattering calculation based on the model of Sparks, Loudon, and Kittel was used to investigate these linewidth differences. The calculation included anisotropy modifications to the spin wave band for each material. The angle dependences of the excess linewidths show qualitative agreement with the two magnon predictions, with inhomogeneity sizes on the order of 1 and 0.25 μm and volume fractions of 0.01 and 0.005 for the Ba-M and Zn-Y disks, respectively.

The last topological effect. Quantum effects for neutral particles in an external magnetic field

A new topological quantum effect is predicted for interfering neutral particles with internal electric-dipole moment interacting with an external magnetic induction field. Another quantum effect is predicted for interfering neutral particles with internal magnetic moment passing past a line of current.

Numerical evaluation of natural frequency change of a ferromagnetic plate due to magnetic stiffness effect

This paper describes the numerical simulation of the dynamic behavior of a ferromagnetic thin plate vibration in a magnetic field. The behavior of a ferromagnetic cantilevered plate, which is set in the direction of a uniform magnetic field, shows that its natural frequency increases with the external magnetic induction. Two methods were applied to evaluate this phenomenon. The magnetic field of the deformed shape has been analyzed by 3D analysis and 1D approximation. The natural frequencies were evaluated by the energy method in the 3D case and by a transient response in the 1D case. The results show that the natural frequency change can be explained by the magnetic stiffness effect caused by magnetization including its saturation.

Stochastic resonance in Ising systems.

We study by Monte Carlo techniques the evolution of finite two-dimensional Ising systems in oscillating magnetic fields. The phenomenon of stochastic resonance is observed. The characteristic peak obtained for the correlation function between the external field and magnetization, versus the temperature of the system, is studied for various external fields and lattice sizes.

Dynamic behavior of fusion structural components under strong magnetic fields

Abstract This paper describes dynamic behavior of non-ferromagnetic and ferromagnetic plates, and a non-ferromagnetic cylinder under strong magnetic field. The dynamic behavior of a ferromagnetic cantilevered plate, which is set along the uniform magnetic field, shows that its natural frequency increases with external magnetic induction. This means that a magnetic stiffness effect occurs in a ferromagnetic plate due to magnetization. The behaviors of non-ferromagnetic and ferromagnetic plates show that the magnetic viscous damping effect becomes larger almost proportionally with the squared external magnetic induction. A non-ferromagnetic cylinder also shows the magnetic viscous damping effect in a strong magnetic field. It is necessary to consider these magnetic stiffness and viscous damping effects for the structural design of a magnetic fusion reactor.

Viscous and Joulean power losses in liquid-metal sliding electrical contacts with finite electrically conducting electrodes

Designing high performance liquid-metal sliding electrical contacts for homopolar machinery requires a precise knowledge of the magnitudes of the viscous and Joulean losses under various operating conditions. The liquid metal, which is confined to a channel between a rotor and stator, is subjected to a large external magnetic induction while transporting current. Significant power losses can occur in these devices. The geometry and electrical conductivity of the channel walls have a significant effect on these losses. In past theoretical work, copper electrodes were generally treated as perfect electrical conductors as compared to liquid metals. Calculations based on this perfectly conducting electrode approximation predicted unrealistically high power losses. In the present study, the effects of electrodes with finite conductivity on both the viscous dissipation and Joulean heating are explored. Numerical results are presented for both radial and axial magnetic inductions. It is found that the magnetic induction orientation has a significant impact on the losses. The results of this type of analysis can be used to minimize the power losses in the design of liquid-metal sliding electrical contacts.

The green function method in the theory of nuclear and electron spin polarization. II. The first approximation and its application in the CIDEP theory

The present paper deals with a further development of the method of calculating nuclear and electron spin polarization based on the Green function formalism specifying the character of the relative motion in a pair of radical centers. In the first approximation with respect to the exchange interaction influence the calculations of the desired quantities are reduced to matrix operations. A theory of chemical polarization of electrons serves to illustrate the method. Calculations of this effect in micellar radical pairs are performed. It is shown that quantum oscillations are possible here depending on the magnitude of the external magnetic field induction.

Magnetooptical detection of standing longitudinal hypersound waves excited by uniform FMR mode

The ferromagnetic resonance (FMR) of Bi-containing LPE garnet film on a GGG substrate has been investigated at frequencies of 0.4-1.2 GHz. FMR magnetooptical detection has been used to measure the magnetization precession amplitude. It was found that a strong interaction between precessing magnetization in the garnet film and longitudinal standing elastic waves in the film/substrate structure is possible only if external magnetic field is inclined on a small angle with respect to the film plate. Such elastic resonances lead to a fine structure of the FMR line. The dependence of the efficiency of this effect upon external magnetic field direction and magnetization orientation was studied experimentally and theoretically. >

Physical and constructional principles for high-Tc superconducting magnets

Abstract The development of high temperature superconductors offers on the one hand a new possibility for building magnets due to their extremely high upper critical magnetic field, but represents on the other hand rigorous limitations because of their relatively low critical current and rigid and brittle mechanical properties. Among the practicable methods of magnet manufacture the simplest one is to use ceramic spirals with conventional electrical contacts to convey the current. As a second possibility, solenoids of traditional type can be realized, based on cut, electrically contacting rings. Both of these types can be supplied by conventional d.c. electrical treatment. These magnets can, however, create fields of only low intensity and the enormous power dissipation in the contact resistivities causes almost unsolvable cooling problems. In order to avoid these difficulties we have elaborated a new procedure for building high-Tc superconducting magnets, which seems to be more useful from the point of view of applications. They are constructed of individual rings and energized by a contactless external magnetic induction method. Subseqently, the persistent self current provides a magnetic field for every ring. Stacking the rings on top of each other allows the magnetic field strength of the solenoid-like arrangement to be increased, although the small critical current and the thickness of the rings limit its value. Further improvements may be looked for using thick film- or thin layer systems, the high critical current densities of which guarantee a radical increase in the magnetic field. Among the superconducting materials, mainly the YBaCuO and Bi(Pb)SrCaCuO compounds are worthy of consideration.

The effect of viscous forces in the flux-line lattice of high-Tc superconductors on the shape of magnetic hysteresis loops

The half-height M of magnetic hysteresis loops in superconductors (directly related to the induced critical current) is larger for larger rates of sweep, B=dBext/dt, of the external magnetic induction, Bext. If the flux-line (FL) motion is determined only by thermally activated flux creep then the induced magnetization M, increases linearly with the logarithm of B. In this paper, the author analyses another mechanism affecting the distribution of FL due to viscous forces. This mechanism yields a linear increase of M with B. Measurements on YBaCuO single crystals initially show a close to logarithmic dependence (corresponding to the dominant flux creep mechanism of FL motion), but the dependence turns towards higher values of M at higher speeds of FL motion. This deviation can be partly attributed to viscous forces in a moving FL lattice.

Thermally activated penetration of magnetic flux through a surface barrier in high- Tc superconductors

Thermally assisted penetration of magnetic flux through the surface barrier governs the behavior of the penetration field and the magnetization in HTSC samples with weak volume pinning. The energy barrier for flux penetration through an ideal surface is estimated as a function of external magnetic field and magnetic induction within the sample for both individual and collective penetration regimes.

Thermopower anisotropy in magnetic films of Co and Ni plated in presence of an external magnetic induction

Electroplated films of cobalt and nickel were grown on copper substrate both in absence and presence of external magnetic induction ranging from 25 to 72 gauss. The latter films showed a magnetic behaviour and an anisotropy in thermopower with respect to the direction of magnetisation.

Modulation of spike frequencies by varying the ambient magnetic field and magnetite candidates in bees ( Tapis mellifera)

1. 1. Spontaneous activity was recorded from neurons in the second abdominal ganglion of bees. 2. 2. Thirty per cent intensity modulations of the horizontal component of the background magnetic field provoked changes in the firing pattern of single neurons. 3. 3. Two classes of neurons were distinguished and confirmed by statistical analysis. 4. 4. Electron dense material in hairs and in or near the cutex may be single domain (SD) and superparamagnetic (SPM) magnetite. 5. 5. A hypothesis is proposed for magnetoreception. Magnetite would act as an amplifier of the external magnetic induction changes. 6. 6. The amplified magnetic field would influence neuronal elements only in restricted regions near the magnetite.

Electrodeposition and electrodissolution of copper with a magnetic field: dynamic and stationary investigations

The electrodeposition and electrodissolution of copper by the potentiostatic method is made with an applied external magnetic induction for both low and high overpotential. It is shown that this system is in accordance with the magnetoelectrochemical mass transport rule which has already been demonstrated for the reversible mass transport controlled redox system. The cathodic and anodic kinetic parameters are determined by stationary and dynamic methods and results are compared with the literature ones.

Evidence for a new magnetic order-order transition in MnAs1 − xPx Crystals

The ac-susceptibility χ of MnAs 1 − x P x -crystals with x = 0.08 and 0.09 has been measured as a function of the temperature (4.2 K < T < 300 K) and the external magnetic induction (0 < B> < 7 T). A minimum-maximum structure of χ ( B) at intermediate temperatures 50 K < T < 230 K is interpreted as a sequence of three magnetic phases (fm-spincompensated-fm). While χ of the high field fm phase shows magnetic saturation, the initial susceptibility of the low field fm phase shows a Rayleigh behaviour.

Possibility of contactless measurement of free charge carrier mobility in semiconductors by the uhf resonator method

A contactless method for measuring free charge carrier mobility in semiconductors using a uhf resonator is described. The method is based on the dependence of the Q of a quasistatic uhf resonator containing the semiconductor specimen on external magnetic field induction. Simple relationships are obtained which allow low determination of the magnetoresistive mobility from the change in Q of the external portion of the resonator under the action of a weak magnetic field.

The production of ion conics by oblique double layers

Magnetized test ions are subjected to acceleration through a numerically simulated oblique double layer in order to determine whether they emerge with velocity vectors aligned with or oblique to the ambient magnetic field. A criterion for oblique alignment, depending on the double‐layer parameters and on the external magnetization, is obtained. When it is applied to observed and theoretical auroral double layers, this criterion predicts that accelerated heavy ions will be substantially less magnetic field aligned than will accelerated hydrogen ions, thus suggesting auroral double layers as a source of high‐energy ion conies. Test particle simulations are also used to investigate the perpendicular heating of ions at low altitudes by the electric fields associated with moving auroral arcs. The rapid motion of small‐scale structures in the arcs is suggested as a source of low‐energy conical ion distributions, and the slow drifts of the entire arc forms are inferred to heat ionospheric ions.

Behavior of magnetic particles in hamster lungs: estimates of clearance and cytoplasmic motility.

Ferrimagnetic particles suspended in saline were instilled intratracheally into the lungs of Syrian golden hamsters. The particles were magnetized and aligned by applying an external magnetic field. Upon removal of the external field, the particles produced a remanent magnetic field from the lungs which decayed due to random misalignment of the particles (relaxation). Magnetization and relaxation measurements were performed immediately after instillation, then repeatedly during the first 24 h, and finally at intervals of several days up to 30 days after the instillation. The size of the initial remanent magnetic field immediately following each external magnetization is a measure of the amount of iron oxide in the lungs. It decreased with time, reflecting particle clearance. The rate of relaxation increased steeply during the first 12 h after the instillation and decreased slowly between the 5th and 30th day. Changes in the location of particles from extracellular to intracellular sites and movements from ectoplasmic to endoplasmic sites within cells may be responsible for the observed changes in relaxation rates with time.

The effect of annealing conditions on the magneto-mechanical properties of Fe-B-Si amorphous ribbons

The effect of annealing conditions on the effective magneto-mechanical coupling factor, Young’s modulus at constant external magnetic field and constant induction (cH and cB), the magnetic anisotropy and the incremental permeability are reported of Fe80B20, Fe80B15Si5 and Fe80B10Si10. The effect of transverse field annealing (parallel to the ribbon width) on keff is relatively small in F80B20 and Fe80B10Si10. It results in keff values ranging from 0.5 to 0.6 and from 0.4 to 0.55, respectively. In Fe80B15Si5 field annealing causes a marked increase in keff reaching a maximum value of 0.86 at a bias field of 60 A/m after annealing at a temperature of 350° C for 2 min. The relative change in cH then reaches a maximum as well, arriving at the extremely large value of (csatH−cminH)/cminH = 4.5 (’’ΔE effect’’). These values are higher than previously reported for Fe78B12Si10, which were at that time the highest known1,2.