Pulsed High Magnetic Fields Research Articles

A 3He Cryostat for Scientific Measurements in Pulsed High Magnetic Fields

A top loading 3He cryostat has been developed for scientific experiments with a 60 T pulsed magnetic field facility at Wuhan National High Magnetic Field Center. The cryostat consists of a 4He bath cryostat, a 3He insert and a closed circulation system for 3He gas handling. To eliminate the eddy current heating during the pulse, the tail of the 3He insert with a vacuum space at the bottom is made from fiberglass tubing coated with epoxy. The 3He bath is separated from the 4He bath with the vacuum space. The 4He bath cryostat provides cooling power to condense 3He gas by a neck tube on top of the tail. Experimental results have shown that the sample can be cooled down to 385 mK and kept cold for more than 150 second by one-shot cooling, which is sufficiently long for an experiment in a pulsed high magnetic field.

Mechanism of high pulsed magnetic field treatment of the plasticity of aluminum matrix composites

For aluminum matrix composite, the introduced particles will strengthen the matrix, but as the obstacles, the heterogeneous particles will hinder the dislocation movement, generate uneven material structure, and may become a source of stress concentration. Therefore, they are detrimental severely to the elongation and plasticity of composite. It is known that dislocations exhibit a paramagnetic behavior because they contain paramagnetic centers including localized electrons, holes, triplet excitons, ion radicals, etc. The initial radical pair of the dislocation-obstacle S (spin angular momentum) = ± 1/2 is in a singlet state, and the total spin of the radical pair is 0 and in the antiparallel spin direction, offsetting a magnetism of the radical pair. The magnetic field can change the spin direction from singlet state to triplet state. In the triplet state the electron spin is 1 and in the same spin direction. A strong bond of the dislocation-obstacle is formed only in the singlet state when the spins of the two electrons are antiparallel. So an obstacle is able to pin a dislocation only if the radical pair is in the singlet state. Under the condition of high pulsed magnetic field treatment (HPMFT) the conversion of electronic spin will be a fundamental cause of dislocation motion along a glide plane. The movement of pinned dislocations will change the material microstructure and influence the performance of material. By comparing the microstructural evolutions and the residual stresses of samples subjected to HPMFT with different values of magnetic induced density (B), the positive influence of magnetoplastic effect on the plasticity of aluminum matrix composite is investigated in this paper. The results show that the dislocation density is significantly increased when B changes from 2 T to 4 T. When B=4 T the dislocation density is enhanced by 3.1 times compared with that of the sample without HPMFT. Moreover, the residual stress is reduced apparently from 41 MPa (B=0) to -1 MPa (B=3 T). In the view of atomic scale, the high magnetic field leads to a magnetoplastic effect which contributes to the dislocation movement and promotes the dislocation depinning, thereafter, the number of movable dislocations increases up. From the viewing of the internal structure of composite, the magnetic field accelerates the releasing rate of internal stress and lowers the residual stress in material, which is beneficial to improving the plasticity of aluminum matrix composite.

Optical Detection of Magnetic Orders in HgCr2O4 Frustrated Spin Magnet under Pulsed High Magnetic Fields

A magneto-optical survey was conducted for HgCr$_2$O$_4$ powder samples under pulsed high magnetic fields of up to 55 T. Intensity changes in magnetic fields observed for the exciton-magnon-phonon optical transition spectra coincide well with those of magnetization, lattice distortion from X-ray diffraction, and electron-magnetic resonances. The last-ordered phase was detected prior to the fully polarized magnetic phase, similarly to the other chromium spinel oxide, ZnCr$_2$O$_4$ and CdCr$_2$O$_4$.

Modelling the Cell Transmembrane Potential Dependence on the Structure of the Pulsed Magnetic Field Coils

During high power pulsed magnetic field treatment of biological samples the cells are subjected to both the high magnetic and induced electric fields. The extent of the influence of each treatment component is poorly studied. The work presents the finite element method analysis of pulsed inductive coils that are used for generation of pulsed magnetic and induced electric fields. The simulated coils, electrical parameters and the output characteristics are evaluated in respect to the induced cell transmembrane potential. The model of the Jurkat T lymphocyte cells is introduced in the analysis. The study includes finite element method analysis of four solenoid coils with different structure and inductance in the range of 2.8 μH to 62 μH. Pulsed magnetic field amplitudes up to 5 T are investigated in this work. DOI: http://dx.doi.org/10.5755/j01.eee.20.8.8432

Influence of high magnetic field on the luminescence of Eu3+-doped glass ceramics

Rare earth (RE) doped materials have been widely exploited as the intriguing electronic configuration of RE ions offers diverse functionalities from optics to magnetism. However, the coupling of magnetism with photoluminescence (PL) in such materials has been rarely reported in spite of its fundamental significance. In the present paper, the effect of high pulsed magnetic field on the photoluminescence intensity of Eu3+-doped nano-glass-ceramics has been investigated. In our experiment, Eu-doped oxyfluoride glass and glass ceramic were prepared by the conventional melt-quenching process and controlled heat treatment. The results demonstrate that the integrated PL intensity of Eu3+ decreases with the enhancement of magnetic field, which can be interpreted in terms of cooperation effect of Zeeman splitting and magnetic field induced change in site symmetry. Furthermore, as a result of Zeeman splitting, both blue and red shift in the emission peaks of Eu3+ can be observed, and this effect becomes more prominent with the increase of magnetic field. Possible mechanisms associated with the observed magneto-optical behaviors are suggested. The results of the present paper may open a new gate for modulation of luminescence by magnetic field and remote optical detection of magnetic field.

Successive Magnetic Transitions of Ca2CoSi2O7in High Magnetic Fields

Magnetic and dielectric properties of \aa kermanite Ca$_2$CoSi$_2$O$_7$ single crystals were investigated in pulsed high magnetic fields. In magnetic fields along the $c$ axis, this material shows a magnetization plateau in a wide range of field below the saturation. Magnetization processes for fields along the $a$ and $b$ axes show multiple anomalies but different traces to each other, indicating the breaking of four-fold symmetry. Measurements of the magnetoelectric effects exhibit the changes in electric polarization according to the changes in the spin system. The experimentally determined quadratic magnetoelectric tensor is consistent with that expected in the crystal symmetry of orthorhombic $P2_12_12$.

Possible restoration of superconductivity in the quasi-one-dimensional conductorLi0.9Mo6O17in pulsed high magnetic fieldsH≃100T

We present a theoretical study of restoration of superconductivity in the form of the triplet reentrant superconducting phase in a quasi-one-dimensional (Q1D) conductor. Substitution of known band and superconducting parameters of the presumably triplet Q1D superconductor Li$_{0.9}$Mo$_6$O$_{17}$ into our theoretical equations shows that such restoration can happen in feasibly high non-destructive pulsed magnetic field of the order of $H \simeq 100 \ T$. We investigate in detail how small inclinations of a direction of a magnetic field from its best experimental geometry decrease superconducting transition temperature of the reentrant phase, which is important for its possible experimental discovery.

Room temperature ferromagnetic Cr–Ni codoped ZnO diluted magnetic semiconductors synthesized by hydrothermal method under high pulsed magnetic field

Pulse magnetic field-assisted hydrothermal method was used for the preparation of Cr–Ni codoped ZnO diluted magnetic semiconductors. XRD analysis reveals that all the samples have hexagonal wurtzite structure. HRTEM, EDS measurements and XPS results ensure that the divalent Cr and Ni ions have incorporated in the wurtzite host matrix without any detectable impurity phase formed. M–H and ZFC/FC curves of the samples reveal the enhancement of ferromagnetism resulted from the magnetic field processing. XPS measurement and Raman scattering spectra indicate that the content of oxygen vacancies in the sample increases with pulse magnetic field processing. According to the bound magnetic polaron model, this may be the reason for the sample with magnetic field processing having better ferromagnetism than that without field processing.

Development of a High-Stability Flat-Top Pulsed Magnetic Field Facility

A high stability flat-top pulsed magnetic field has both high field strength and high field stability simultaneously, which can be used for the high precision scientific experiments. In this paper, we present such a high stability flat-top pulsed magnetic field facility developed in the Wuhan National High Magnetic Field Center. A passive bypass circuit is developed to regulate current up to tens of thousands amperes, as well as a 25 T/200 ms flat-top pulsed magnetic field with a ripple of 250 ppm (0.025%). The main circuit scheme of the facility as well as the preliminary experimental results will be presented in this paper.

Irreversible magnetoporation of micro-organisms in high pulsed magnetic fields.

Electroporation is an appealing way of stimulating living cells, which causes permanent or temporary nanoporosities in the structure of the biological objects. However, the technique has a disadvantage such as a requirement of contact between the electrodes and the cell medium. In this review, a methodology of contactless treatment of the biological objects using pulsed magnetic fields is proposed. The eukaryotic micro-organisms Achlya americana and Saprolegnia diclina have been used in the study and magnetic fields up to 7 T were applied, which caused effects similar to irreversible electroporation resulting in the death of the species. The proposed technique is applicable for different types of the biological cells or micro-organisms and possibly can be used in the area of cancer, antifungal treatment and other biotechnological fields.

Bitter coil design methodology for electromagnetic pulse metal processing techniques

Electromagnetic pulse metal processing techniques (EPMPT) such as welding, forming and cutting have proven to be an effective solution to specific manufacturing problems. A high pulse magnetic field coil is a critical part of these technologies and its design is a challenging task. This paper describes a Bitter coil design using a newly developed methodology for a simplified analytical calculation of the coil and complementary finite element models (FE) of different complexity. Based on the methodology a Belgian Welding Institute (BWI) Bitter coil has been designed and tested by means of short circuit experiments, impedance and B-field measurements. A good agreement between the calculated and the experimental design parameters was found.

Neutron Diffractions in Pulsed High Magnetic Fields

In frustrated magnets, varieties of exotic states appear under high magnetic fields. Examples are found as complex magnetic phase diagram in multi-ferroics, a quantum phase transition in strongly correlated 5f intermetallics and quantum magnetic plateau in low-dimensional magnets. The most important basis to understand such exotic phases is the determination of their magnetic structures. Neutron diffraction is the unique and the direct method for this purpose. However, experiments in strong magnetic fields have been quite limited for technical difficulties. We have developed the techniques to combine pulsed high magnetic fields with neutron diffractions. The use of pulsed magnets have broken the technical limit of 17.5 T achieved by superconducting magnets and enabled us to access the extremely high magnetic fields of 30-40 T. At present, we have four different lines of pulsed magnet devices for practical experiments as follows. 1. Compact potable 30 T horizontal solenoid magnet system combined with an orange cryostat, 2. Compact 40 T horizontal solenoid with a custom cryostat, 3. Compact 25 T vertical split-pair magnet with a custom cryostat, 4. Middle size horizontal 50 T solenoid magnet with a custom cryostat. The temperature as low as 1.7 K is possible for all of those systems. The compact 30 T system is used in ILL, SNS, ISIS and JRR3. Upgraded 40 T version is installed in ISIS and ILL. The split-pair and the middle size system are used in J-PARC. In reactor sources, a very week magnetic peak can be detected for low background. A continuous field variation of the peak intensity is monitored by means of time-resolved technique. In spallation sources, multiple Bragg-peaks are captured by using the white beam Laue method. It is very useful to trace the change of magnetic wave vectors. We report the overview of the recent activities examining magnetic phase diagrams of multi-ferroic compounds and URu2Si2 with a multi-step meta-magnetic transition.

Unusual doping effect of non-magnetic ion on magnetic properties of CuFe1−xGaxO2

The structural and magnetic properties of nonmagnetic Ga3+ ion doped CuFe1−xGaxO2 (x=0, 0.02, 0.03, and 0.05) single crystal samples have been investigated. In pulsed high magnetic fields, the field-induced multi-step transitions were observed in all the samples. Compared with pure CuFeO2, the transition temperatures, critical magnetic fields decrease and the magnetic hysteresis of the doped samples become small, which may result from the partial release of the spin frustration and the changes of the magnetic coupling both inter- and intra-planes due to the Ga3+ dopant. The magnetization measurements show an abnormal dilution behavior, especially in a lower temperature region, the magnetic moment was enhanced due to the nonmagnetic Ga3+ ion doping, the enhancement becomes more obviously in the sample with the Ga3+ doping level of x=0.03. These results may connected with the substitution of nonmagnetic Ga3+ ions destroying the stability of ground state and affecting the stability of the ferroelectricity incommensurate phase. Based on the experimental results, a super-cell model and their magnetic diagram were assumed.

Влияние электромагнитной обработки наполнителей на физико-химические свойства клеевых композиций для изготовления фанеры

During this work the reduction of emissions of free formaldehyde of the cured adhesive composition for the manufacture of environmentally friendly plywood is identified, through the introduction of pre-activated sorbent of fillers into its composition, as well as reducing internal stresses in the adhesive layer of the cured binder. The resulting effects are related with the use of adhesive formulations of different sorbents fillers subjected to preactivation in various electromagnetic fields (fields over high frequency and weak pulsed magnetic fields).

Contactless dielectrophoretic manipulation of biological cells using pulsed magnetic fields

Contactless method for manipulation of polar or polarisable micro and nanoscale particles based on the dielectrophoresis force exerted by the induced electric field in high pulsed magnetic field is presented in this study. Finite element method analysis of the magnetic and resulting induced electric fields is carried out. The structure of the magnetic field generator that was based on a controlled frequency spark gap, and the structure of the coil that was used as a load are described. Experimental data showing positive dielectrophoresis on the Jurkat T-lymphoblasts is presented. The study discusses further developments of the technique, its limitations and possible applications.

Analysis and Experiment of Battery Bank Power Supply System for Long Pulse Helical Magnet in WHMFC

Traditionally, pulse capacitor and pulse generator power supply are frequently used to energize pulse high magnet up to 100 T, but they are not the ideal power supply for long pulse and low ripple high magnetic field so that they cannot meet higher requirement of science research and wider application in industry. In Wuhan National High Magnetic Field Center (WHMFC), a new type power supply system which uses lead-acid batteries has been developed to output more energy and longer pulse than capacitor bank, and also output more steady voltage and smaller current ripple than pulse generator using ac/dc convertor. The battery power supply is a new application that valve-regulated lead-acid battery (VRLA) is used in high-rate discharge with pulse current 2 kA/2 s or peak 2.5 kA/1 s, and it is not unnecessary to analyze the changes of internal resistance and main factors which influence the battery life under the high current discharge. Based on the simulation analysis of the battery bank system, the structure and parameters including charge and discharge circuit of battery bank which can output maximum 35 kA/1 s under 770 V are designed. The tests of battery bank discharge, current sharing among the 15 parallel branches, and experiment of the helical magnet 32 T/1.5 s as battery bank outputs 34.5 kA/0.8 s are presented.

Short and Long Pulse High Magnetic Field Facility at the Wuhan National High Magnetic Field Center

The pulsed high magnetic field facility funded by the Chinese National Development and Reformation Committee has been developed at the Wuhan National High Magnetic Field Center (WHMFC). Magnets of short pulse, long pulse and the combination of both with bore sizes from 12 to 34 mm have been developed and are operational for electric transport, magnetization, magneto-optics and electron spin resonance at temperatures in the range from 100 mK to 350 K. The power supplies for these magnets consist of a capacitor bank with 12 modules of 1 MJ/25 kV each and 2 modules of 0.8 MJ/25 kV each, a 100 MVA/100 MJ flywheel pulse generator and a 771 V/180 kAh battery bank. A dual-coil magnet driven by the capacitor banks has successfully generated 86.3 T field with a total pulse duration over 350 ms in a 12-mm bore. A dual-coil long pulse magnet wound from soft copper wire energized by the flywheel generator produces 50 T peak field with a 100 ms flat-top and a total 1.1 s pulse duration in a 22 mm bore. A battery bank driven long pulse magnet composed by the series connection of two nested coils generates 32 T peak field with the pulse duration of 1.5 s in a 21-mm bore.

Influence of High Magnetic Field on β-a Phase Transformation in TC4 Titanium Alloy

In the TC4 titanium alloy subject to high pulsed magnetic field, the phase transformation from α to β occurs. When the magnetic induced intensity increased from 2T, 3T to 4T with constant 30 pulses, the percent of α phase adds up gradually. When the magnetic induced intensity is 4T the volume fraction of α phase amounts up to 59%. It is ascertained that magnetic field has taken a positive influence on the phase transformation because of the variation of inner energy induced by magnetic field.

High magnetic field phase diagram in electron-doped manganites La(0.4)Ca(0.6)Mn(1-y)Cr(y)O3.

We report the charge-order to ferromagnetic phase transition induced by pulsed high magnetic field and impurity doping effects in manganites La0.4Ca0.6(Mn1−yCry)O3 (0 ≤ y ≤ 0.2). Significant charge-order suppression and ferromagnetic tendency upon the Cr3+-doping are evidenced, and three different ground states are identified, namely the charge-order state, the phase separated state, and the spin-glass like state. Phase diagram in the H-y plane at 4.2 K is determined by the high magnetic field study, in which the charge-order and ferromagnetic phase boundary is clearly figured out. The critical magnetic field for melting the charge-order phase of La0.4Ca0.6MnO3 is revealed to reach up to 46 T at 4.2 K. Interestingly, distinct responses of the three states to the high magnetic field are observed, indicating the special physics regarding the charge order melting process in each state. The mechanism of the doping induced charge-order suppression and ferromagnetism promotion can be understood by the competition between the antiferromagnetic interaction of Cr-Mn and local enhancement of electron hopping by Cr3+.

Use of the Magnetic Saw Effect for Manufacturing

This paper describes a novel technique for making fine cuts in metal plates and foils. The process is based on a variant of the magnetic saw effect, a magnetic instability associated with high pulsed magnetic fields. When not controlled, the magnetic saw effect can lead to failure in high-field pulsed conductors. By limiting energy and pulse duration, the authors have produced controlled cuts with kerfs as fine as 0.01 mm. These promising results motivated a systematic study of the parameters that govern the quality and reproducibility of magnetic saw cuts. This paper provides an overview of the technique, of some key experiments, and of the practical issues associated with using pulsed magnetic fields for manufacturing.