Strong Pulsed Magnetic Fields Research Articles

Magnetoelectric Effect in YMn 2 O 5 in Strong Pulsed Magnetic Fields

The electric polarization of single crystal YMn 2 O 5 was investigated in strong pulsed magnetic fields up to H = 250 kOe at low temperatures. It was shown that at T ∼ 40 K magnetic and structural phase transitions occur, with the structural changes taking place improperly (magnetically induced). We have also shown the existence of the structural phase transition at T ∼ 20 K with the onset of spontaneous polarization P b . The quadratic magnetoelectric effect at temperatures T < 40 K was observed (with b-component domination), the magnetoelectric susceptibility below T = 20 K greatly increased for all field orientations. When H Á a the magnetoelectric effect changed its sign which in analogue with EuMn 2 O 5 was attributed to non-commensurate to commensurate structural change. The polarization anomalies in strong magnetic fields were observed with the former being due to complex magnetic structure changes and the process of spontaneous polarization induction by magnetic field.

Manganite Based Strong Magnetic Field Sensors Used for Magnetocumulative Generators

The effects of strong pulsed magnetic field and shock waves on the electrical properties of La 0.67 Ca 0.33 MnO 3 and La 0.7 Sr 0.3 MnO 3 thin films were investigated. It was demonstrated that these films could be used to measure the magnetic fields produced by magnetocumulative generators. Possible ways to design fast magnetic field sensors operating at room temperature are also discussed.

Multi-composite wires for pulsed field coils

“Multi-composite wire” is a new type of strong wire for winding coils to generate very strong-pulsed magnetic fields. The basic concept is filling the free spaces in a bundle of thin conducting wires with reinforcing fibres, and enclosing this composite wire in an insulating sleeve. In this concept, any desired combination of conductor and reinforcement can be used. To insulate the wire and keep the bundle together, a sleeve consisting of a strong insulating fibre is braided around the core. As a proof of principle, multi-composite wires have been made consisting of soft copper in combination with carbon fibres. Glass fibre was employed for the sleeve. The mechanical properties of the wires were determined from tensile and “explo-vessel” tests. The latter allows the investigation of the mechanical properties under conditions as they prevail in a coil. The first experimental wire had up to 88% of the theoretically calculated UTS. Further development is discussed.

Spin transitions in a series of Fe II molecular complexes induced by a strong-pulsed magnetic field

We report on the first comparative investigations of Fe II spin-crossover complexes under high-pulsed magnetic fields near the transition temperature T c. A partial irreversible triggering of the spin conversion is evidenced when the magnetic field is applied at a temperature belonging to the heating branch of the thermal hysteresis. A strong correlation between the strength of the cooperative interactions and the kinetic character of the spin conversion process is demonstrated.

EFFECT OF STRONG PULSED ELECTROMAGNETIC FIELD ON METAL'S SOLIDIFIED STRUCTURE

It was studied by us that new technologies using strong pulsed magnetic field and electric current were applied to refine metal's solidified structure.These technologies improved remarkably the solidified structure of LY12 aluminum alloy and made the solidified grains more equiaxed and apparently finer compared with that of the normal sample,which is of more dendrite grains.The stronger the strength of the pulsed electromagnetic field,the better the improvement effect was.The refinement mechanisms of the new technologies were analyzed theoretically.New phenomena and problems occurred in our experiments were also pointed out in this paper.

Magnetization process of the alternating Heisenberg chains with ferromagnetic and antiferromagnetic interactions formed in (CH 3) 2CHNH 3CuCl 3

We report magnetization measurements in strong pulsed magnetic fields up to 42 T on the gap system (CH 3) 2CHNH 3CuCl 3, in which ferromagnetic-dominant ferromagnetic and antiferromagnetic alternating Heisenberg chains are formed and then a pair of ferromagnetically coupled S= 1 2 spins behaves as a spin of S=1. The magnetization at 1.6 K begins to increase abruptly at 10.5 T, which is slightly lower than the field corresponding to the energy gap, and saturates at 41.2 T, which is considerably higher than that evaluated from 2| J AF|, where J AF is the antiferromagnetic exchange interaction. We point out that one of the origins of the disagreement mentioned above is due to the effect of the interchain exchange interaction.

Upgrade of a vacuum arc ion source using a strong pulsed magnetic field

A pulsed magnetic field of up to 10 kG was incorporated into a vacuum arc ion source. The field was established by a small coil surrounding the arc discharge region, powered by either an additional power supply (capacitor bank) or by the arc power supply (arc current and coil current in series). This addition has led to a number of improvements in source performance: The mean charge state of the metal ions produced was enhanced by a factor of up to 2, for 30 different cathode materials from carbon to bismuth; hybrid metal/gaseous ion beams could be generated when an additional gas (nitrogen, oxygen, or argon) was admitted into the source, with gaseous ion fraction as high as 50%; triggering of the source could be done by a very long lifetime gaseous predischarge technique. We also report on the use of a wire mesh to stabilize the plasma emission surface at the extractor as a means for achieving a flat beam current characteristic as a function of extraction voltage.

Magnet laboratory facilities worldwide—An update

Generation of high magnetic fields for materials research and fundamental science requires specialized laboratories of considerable size. In this paper we provide an overview of facilities available worldwide for very strong static and pulsed magnetic fields.

Comparison of homogeneous and heterogeneous tissue models for coil optimization in neural stimulation

In stimulation of peripheral nerves by strong pulsed magnetic fields, it is desirable to produce a well‐focused activating function. Previous analyses and an experiment have indicated that quadruple coils provide many advantages. An analytical solution has been obtained for a homogeneous cylindrical volume conductor, and a finite difference numerical solution for a realistic heterogeneous model of a human forearm. In both models it has been assumed that nerves are straight and long, and therefore the activating function is an induced field derivative in the neuron direction. The magnitudes of the activating function for various coil geometries are compared for the two models.

Analytical expressions for the optimised radial distribution of internal reinforcement in high magnetic field coils

Analytical expressions are obtained for the optimal radial distribution of the internal reinforcement in coils for strong pulsed magnetic fields. The internal reinforcement in these coils is distributed such that the stress level is the same throughout the coil. Predictions of the performance of layer by layer reinforced coils are derived on basis of the analytical results, i.e., there is a universal relation between the amount of reinforcement and the maximum magnetic field. >

Effects of strong pulsed magnetic fields on the cardiac activity of an open chest dog.

The heart of an open chest dog was stimulated by strong magnetic fields which were damped sinusoidal pulses with the one-cycle period of 1.47 ms. Stimulation effects were detected by electrocardiograph (ECG) and arterial blood pressure as a function of the strength of the field, the triggering point in the cardiac cycle, and the position of a stimulating coil. The threshold for arrhythmias was a minimum for the stimuli triggered at the apex of the T wave and on the P wave in the ECG. Premature ventricular and premature atrial contractions occurred according to whether the coil was placed over the ventricles or the atria. Ventricular defibrillation can not be attained by the magnetic stimulus with the flux density of 9.2 T which was the maximum field used.

Effects of a strong pulsed magnetic field on the proliferation of tumour cells in vitro

The effect of a pulsed magnetic field was studied on the proliferation of HeLa cells in culture. The uniform magnetic field was pulsed (square wave, 0,8 Hz). Its intensity was 0.18 T at the maximum. HeLa cells were cultured in either flasks or multiple-well micro plates with an initial concentration of 10 5 cell/ml. Exposure to the field led to a 15% decrease of cell proliferation measured 24 h after exposure.

The use of strong-pulsed magnetic fields to weld metals

The success of pulsed power production (the generation of strong-pulsed magnetic fields and electrical currents) suggests some new possibilities for the development of progressive industrial technology. Magnetic fields with strengths of hundreds of kiloersteds create a high-energy density (H2/8~) and can be employed for pulsed high-speed methods of processing materials. The first successful technology application of strong-pulsed magnetic fields involved the high-speed deformation of thin-walled blanks made of metals with a high electrical conductivity: stamping, pressing, calibration, welding, etc. These operations are carried out for low parameters of the magnetic field (amplitude H m = 100-200 kOe, frequency f = i020 kHz). Studies have shown that the energy of a pulsed magnetic field can also be used for more complex technological operations, such as welding, which require a higher rate of deformation of the pieces being welded, as well as higher amplitudes of the magnetic field [i].

The pressure chamber in a strong pulsed magnetic field: application to magnetic measurements

We describe the set-up for magnetization measurements in a pulsed magnetic field (17T) under hydrostatic pressure (15 kbar) generated in helium gas. The pressure chamber is situated inside the pulse solenoid. The design, performance and limitations of the apparatus are presented, using simple calculations for the diffusion of a pulsed magnetic field into the metallic chamber body.

Strong pulsed magnetic fields combined with very low temperatures

New types of pulsed field coils have been developed: optimized reinforcement by fibre composites has yielded ultimate peak fields of 67T in a 12 mm bore and 58T in a 20 mm bore, and a coil made with a newly developed cable of pearlitic steel wires around a copper core has reached 55T in a 17 mm bore. The temperature range of pulsed field experiments has been extended into the 400 mK range by means of top-loading 3He cryostats. Experiments with the fractional quantum Hall effect have shown that sample heating due to eddy currents can be kept within acceptable limits but it becomes critical for a pulse duration of 10 ms or less.

Far-infrared and strong pulsed magnetic fields

Experiments with FIR radiation in strong pulsed magnetic fields are reviewed. Different techniques for the generation of pulsed magnetic fields are described, mainly with capacitor discharges in the field ranges 20–60 T (nondestructive) and 100–1500 T (destructive). The requirements for sources and detectors in pulsed field experiments are briefly discussed. As examples for experiments, we give paramagnetic resonances in antiferromagnets and high temperature superconductors, cyclotron resonances in semiconductors, and ferromagnetic resonances in metals.

Low-temperature strengthening of structural alloys under the action of strong magnetic fields

Results are presented for an experimental study of the effect of strong constant and pulsed magnetic fields on the mechanical properties and deformation features of steels 03Kh20N16AG6, 03KM3AG19, and 12Kh19N10T at temperatures of 293-4.2 K. Relationships are presented connecting the level of critical stresses with magnetic field parameters and the possibility is substantiated of using the low-temperature strengthening effect for steels in determining permissible stresses for structural elements subjected to the effect of strong magnetic fields.

Magnetostriction in high pulsed magnetic fields on a single crystal of Nd 2Fe 14B

Magnetostriction measurements on a single crystal of the tetragonal intermetallic compound Nd 2Fe 14B, between T ⫋ 5 and 300 K, in strong pulsed magnetic fields up to 15 T are reported. The measurements have been performed along the main symmetry directions. All magnetostriction measurements show weak anomalies at around the spin reorientation phase transition temperature T SR = 135 K. For fields applied in the basal plane, anomalies in the isotherm curves are observed. Such anomalies are related to first order magnetization processes (FOMP). For fields applied along the tetragonal c-axis a linear dependence of the strain is observed, in agreement with a model prediction for d-band magnetism.

Deformation of copper under the action of strong magnetic fields and cryogenic temperatures

Results are presented for an experimental study of the effect of a strong constant and pulsed magnetic field on the mechanical characteristics and deformation features of copper M1 at 293–4.2 K. Dependences have been obtained for the increase in specimen strain on the intensity of a pulsed magnetic field and flow stress, and possibilities have been studied for increasing the effectiveness of pulsedmagnetic deformation of metals. A calculation relationship is suggested for determining the critical stress level under strong skin-effect conditions of a magnetic field.

Magnetization process of an S=1 linear-chain Heisenberg antiferromagnet.

Magnetization measurements in strong pulsed magnetic fields on single-crystal samples of the S=1 linear-chain Heisenberg antiferromagnet Ni(${\mathrm{C}}_{2}$${\mathrm{H}}_{8}$${\mathrm{H}}_{2}$${)}_{2}$${\mathrm{NO}}_{2}$(${\mathrm{ClO}}_{4}$) are reported. The magnetization is very small at the low-field region and begins to increase sharply at a finite field for all of the three principal axes. This is evidence for the existence of the Haldane gap in this compound.