Pulsed High Magnetic Fields Research Articles

Angular dependence of the resistive upper critical field of the iron-based superconductor Fe1+δ (Te,Se) in high magnetic fields

We have investigated the angular dependence of the upper critical fields (µ0Hc2) of a single crystal of Fe1+δ(Te,Se) in pulsed high magnetic fields up to 52 T. At T = 10 K, the µ0Hc2 increases with increasing θ, which is the angle between the c-axis and the direction of the applied magnetic field. Using the effective-mass model based on the three-dimensional anisotropic Ginzburg-Landau (G-L) theory, we estimate the effective-mass anisotropy ratio γ = (mc* /mab*)1/2 to be 1.6 at 10 K. This result is consistent with that of our previous study on the anisotropy coefficient of the resistive upper critical field, Γ ≡ Hc2ab/Hc2c.

Finite Element Method Analysis of Microfluidic Channel with Integrated Dielectrophoresis Electrodes for Biological Cell Permeabilization and Manipulation

The microfluidic channel with a planar inductive microcoil for the cell membrane permeabilization and the integrated planar electrodes for cell dielectrophoretic manipulation is proposed and analyzed in the study. The analyzed setup is based on the dielectrophoretic entrapment of the biological cell followed by membrane permeabilization using high pulsed magnetic field. The finite element method analysis of the DEP force and the generated pulsed magnetic field is performed. Based on finite element method analysis the potential applications of the setup in the fields of drug delivery, biomedicine and biotechnology are discussed.

Soft Ferrite Used as Thermal-Magnetic Conversion Intermedium in the Flux Pumping Technology

Recent progress in material science has proved that high-temperature superconductors, such as bulk melt-processed yttrium barium copper oxide (YBCO) single domains, have a great potential to trap significant magnetic fields. In this paper, we will describe a novel method of YBCO magnetization that only requires the applied field to be at the level of a permanent magnet. Instead of applying a pulsed high magnetic field on the YBCO, a thermally actuated material (TAM), such as Mg <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.15</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.15</sub> Zn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.7</sub> Ti <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.04</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.96</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> , has been used as an intermedium to create a travelling magnetic field by changing the local temperature so that the local permeability is changed to build up the magnetization of the YBCO gradually after multiple pumping cycles. It is well known that the relative permeability of ferrite is a function of temperature and its electromagnetic properties can be greatly changed by adding dopants such as Mg or Ti; therefore, it is considered to be the most promising TAM for future flux pumping technology. Ferrite samples were fabricated by means of the conventional ceramic method with different dopants. Zinc and iron oxides were used as raw materials. The samples were sintered at 1100 °C, 1200 °C , and 1300 °C . The relative permeability of the samples was measured at temperatures ranging from 77 to 300 K. This work investigates the variation of the magnetic properties of ferrites with different heat treatments and doping elements and gives a smart insight into finding better ferrites suitable for flux pumping technology.

Magnetic Structure of Phase II inU(Ru0.96Rh0.04)2Si2Determined by Neutron Diffraction under Pulsed High Magnetic Fields

We report neutron diffraction measurements on U(Ru(0.96)Rh(0.04))(2)Si(2) single crystal under pulsed high magnetic fields up to 30 T applied along the tetragonal c axis. The high-field experiments revealed that the field-induced phase II above 26 T corresponds to a commensurate up-up-down ferrimagnetic structure characterized by the wave vector q=(2/3,0,0) with the magnetic moments parallel to the c axis, which naturally explains the one-third magnetization plateau and the substantially changed Fermi surface in phase II. This a-axis modulated magnetic structure indicates that the phase II near the hidden order phase is closely related to the characteristic incommensurate magnetic fluctuations at Q(1)=(0.6,0,0) in the pure system URu(2)Si(2), in contrast to the pressure-induced antiferromagnetic order at Q(0)=(1,0,0).

The Precise Measurements of Coercivity Fields of Nd-Fe Ferro-Magnets in Pulsed High Magnetic Fields up to 20T

The measurements of the coercive force (Hc) of magnets are very difficult to determine as a function of the true effective fields in samples with arbitrary shapes. In this study, we tried to obtain the exact coercive force, M-H curve in high and high pulsed magnetic fields up to 20T by using a double-core pick-up. The values of Hc were found over-estimated more than 20% for the Nd-Fe magnets in comparison with those announced by the company, which coincided well with the numerical simulations by using JMAG. The two samples cut along easy and hard axis showed the same saturation magnetizations, reflected by the isotropic microstructure of the raw material.

Electron mass anisotropy of BaZrO3 doped YBCO thin films in pulsed magnetic fields up to 30 T

The high anisotropy of the critical current density in the high-temperature superconductor YBa2Cu3O6+x can be compensated by changing the pinning landscape, for example, through doping with BaZrO3. We measure the change due to BaZrO3 doping in the effective electron mass anisotropy, γ, by resistive measurements of the upper critical field in pulsed high magnetic fields. It is found that the angular dependence of the upper critical field follows the Blatter scaling up to 30 T but the irreversibility field does not. We also report a significant reduction in the effective electron mass anisotropy from 6.0 to 3.4 by BaZrO3 doping.

In vivo pulsed magneto-motive ultrasound imaging using high-performance magnetoactive contrast nanoagents.

Previously, pulsed magneto-motive ultrasound (pMMUS) imaging has been introduced as a contrast-agent-assisted ultrasound-based imaging modality capable of visualizing biological events at the cellular and molecular level. In pMMUS imaging, a high intensity pulsed magnetic field is used to excite cells or tissue labeled with magnetic nanoparticles. Then, ultrasound (US) imaging is used to monitor the mechanical response of the tissue to an externally applied magnetic field (i.e., tissue displacement). Signal to noise ratio (SNR) in pMMUS imaging can be improved by using superparamagnetic nanoparticles with larger saturation magnetization. Metal-doped magnetic nanoparticles with enhanced tunable nanomagnetism are suitable candidates to improve the SNR and, therefore, sensitivity of pMMUS imaging, which is essential for in vivo pMMUS imaging. In this study, we demonstrate the capability of pMMUS imaging to identify the presence and distribution of zinc-doped iron oxide nanoparticles in live nude mice bearing A431 (human epithelial carcinoma) xenograft tumors.

The influence of high magnetic field on electric-dipole emission spectra of Eu3+ in different single crystals

In this work, the effect of pulsed high magnetic field on the integral intensity of the electric-dipole emission spectrum of Eu3+ in YVO4:Eu3+ and GdVO4:Eu3+ single crystals was studied and the results showed magnetic field induced luminescence suppression in YVO4:Eu3+ and luminescence expansion in GdVO4:Eu3+. The single crystals were prepared by the optical floating zone method. The strong effect of the magnetic field on the splitting and emission of 5D0–7F2 and 5D0–7F4 transitions in Eu3+ was investigated. The integral luminescence intensities of Eu3+ are proportional to the magnetic field at higher than 15 T. For GdVO4:Eu3+, the integral intensity increases with the strengthening of magnetic field, and is completely opposite to that for YVO4:Eu3+. In this study, the dependence of luminescence integral intensity on the magnetic field was proposed to be the result of the structural change induced by high magnetic field which changes the symmetry of Eu3+ ions in single crystals.

Hole density of ferromagnetic semiconductor (Ga, Mn)As studied via pulsed high magnetic field

Transport measurements of as-grown and annealed (Ga,Mn)As samples were conducted under pulsed high magnetic field up to 40T. Assuming that the magnetization follows modified Brillouin function, we obtained the hole densities of the (Ga,Mn)As samples at various temperatures, which generally increase with temperature and become two times larger after annealing process. The hole density determined by electrical transport measurement at low magnetic field up to 1.5T is found to be underestimated.

Transport Properties of the Iron-Oxypnictide Superconductor PrFeAsO1−y in High Magnetic Fields

We report the resistively determined upper critical field μ0Hc2 of the iron-oxypnictide superconductor PrFeAsO1−y (y ∼ 0.15), which exhibits superconductivity at Tc = 44 K The resistivity ρ(H,T) was measured with a typical four-probe method in static magnetic fields of up to 14 T and in pulsed high magnetic fields of up to 52 T With increasing magnetic fields, the superconducting transition width of the ρ(T) curve for H ‖ c becomes broader than that for H ‖ ab. This behavior is likely to be due to dissipation associated with thermally activated vortex motion. The μ0Hc2(T) curves for both H ‖ ab and H ‖ c exhibit a pronounced upward curvature below Tc and are very different from the conventional one-band Werthamer-Helfand-Hohenberg behavior. This result suggests that the iron-oxypnictide superconductor is a multiband system. We demonstrate the results of the two-band analysis for μ0Hc2(T) and discuss the anisotropy of μ0Hc2 on some kinds of iron-based superconductors.

Soft-X-ray Magnetic Circular Dichroism under Pulsed High Magnetic Fields at EuM4,5Edges of Mixed Valence Compound EuNi2(Si0.18Ge0.82)2

By applying our new experimental technique of X-ray absorption spectroscopy (XAS) and its X-ray magnetic circular dichroism (XMCD) in the soft-X-ray region under high pulsed magnetic fields of up to 21.3 T, we measured the XAS and XMCD spectra at the Eu M 4,5 edges of EuNi 2 (Si 0.18 Ge 0.82 ) 2 . The measured spectra are analyzed by taking into account the atomic multiplet structures to clarify the mixed valence properties, which are caused by the hybridization between Eu 2+ and Eu 3+ configurations. We obtain the field dependence of the average valence number from the XAS spectra and that of the magnetizations of Eu 2+ and Eu 3+ components separately using the spin and orbital sum rules from the XMCD spectra. These results are in good agreement with theoretical ones calculated with a modified single-impurity Anderson model. The present study reveals the key to solving a puzzling problem that was posed in previous XAS and XMCD experiments in the hard-X-ray region.

A Dual-Capacitors Type Energy Recovery Power System for Repetitive Pulsed High Magnetic Fields

As a flexible and convenient tool, a repetitive pulsed high magnetic field (RPHMF) would be employed for scientific research and industrial applications. A novel RPHMF system design adopting a dual-capacitors type energy recovery power system is introduced in this paper. The energy stored in the magnet can be fed back to the capacitor by a choke coil and a resonant capacitor while the energy dissipated in the discharge will be replenished to the capacitor through a high frequency resonant capacitor charging power system (CCPS). The main advantages of the design are as followed: first, the energy feedback make the system more efficient; second, during the whole process there is no reverse voltage on the metalized film capacitors, improving the energy storage capacitors’ service lifetime and reliability remarkably; finally, convenience can be brought to the high frequency CCPS’s application. In this paper, theoretic analysis of RPHMF system is described and an experimental device with a bitter magnet as the load is built to test the design for its verification. A 1.2 Hz, 8 T repetitive pulsed high magnetic field is generated. Experimental results show that there is no reverse voltage on the energy storage capacitors in the whole process. The factors influencing the efficiency and frequency of the system are analyzed in detail.

Transport Properties of the Iron-based Superconductor SrFe2(As,P)2 in High Magnetic Fields

We have investigated the resistive upper critical field (μ 0 H c2) of the iron-based superconductor SrFe2(As1−x P x )2 (x=0.35, T c∼29 K) in pulsed high magnetic fields of up to 52 T. For H∥ab, $\mu_{0}H_{\mathrm{c}2}^{ab}(T)$ exhibits the Werthamer-Helfand-Hohenberg (WHH) like behavior. While, for H∥c, $\mu_{0}H_{\mathrm {c}2}^{c}(T)$ is almost linear against T and exhibits a slightly upward curvature near T c. The anisotropy of μ 0 H c2 decreases from 2.5 near T c to ∼1.2 at T=0 K monotonically. Similar isotropic behavior of μ 0 H c2 at low temperatures has also been observed in the Fe1+δ (Te,Se). We demonstrate the results of the two-band analysis for μ 0 H c2(T) on the present sample and discuss the anisotropy of μ 0 H c2 by comparing with those in some kinds of iron-based superconductors.

Design and Realization of the Control and Measurement System of the Long Pulsed High Magnetic Field Facility Supplied by Battery

A Control and Measurement System (CMS) is designed to ensure the reliable operation in the long pulsed high magnetic field facility supplied by lead-acid batteries. The CMS is mainly composed of a Programmable Logic Controller (PLC), a fault monitor and protection circuit, a signal processing and data acquisition unit, a local triggering sequence generator and the main control program. The system architecture and kernel parts of the CMS are analyzed and described in detail. The results prove that the designed CMS could perform efficiently and reliably.

Crystal-Field Splitting of the Bright Eu3+ Ions in YPO4 Micro-crystals Detected by Zeeman Splitting in Pulsed High Magnetic Fields

The crystal-field splitting levels in YPO4 Eu3+ micro-crystals are investigated by the Zeeman splitting in photoluminescence spectrum. X-ray diffraction pattern and microscopic images indicate small tetragonal nano-particles attaching on the surfaces of the mainly hexagonal micro-crystals. The peaks correspond to the transitions from 5D0 excited state to 7F1 and 7F2 ground states are split in pulsed high magnetic fields, crystal-field splitting levels associated with different magnetic moments are clarified, which are completely different from that was supposed by the X-ray, microscopy, and zero-field photoluminescence results.

Nuclear magnetic resonance apparatus for pulsed high magnetic fields

A nuclear magnetic resonance apparatus for experiments in pulsed high magnetic fields is described. The magnetic field pulses created together with various magnet coils determine the requirements such an apparatus has to fulfill to be operated successfully in pulsed fields. Independent of the chosen coil it is desirable to operate the entire experiment at the highest possible bandwidth such that a correspondingly large temporal fraction of the magnetic field pulse can be used to probe a given sample. Our apparatus offers a bandwidth of up to 20 MHz and has been tested successfully at the Hochfeld-Magnetlabor Dresden, even in a very fast dual coil magnet that has produced a peak field of 94.2 T. Using a medium-sized single coil with a significantly slower dependence, it is possible to perform advanced multi-pulse nuclear magnetic resonance experiments. As an example we discuss a Carr-Purcell spin echo sequence at a field of 62 T.

Isothermal switching of perpendicular exchange bias by pulsed high magnetic field

Isothermal switching of a perpendicular exchange bias by a strong pulsed magnetic field has been investigated using a Pt/Co/α-Cr2O3 thin film system. The switching of the perpendicular exchange bias is accompanied by the spin reversal of interfacial uncompensated antiferromagnetic Cr spins. We have also demonstrated that the switching of the exchange bias is reversible by changing the pulsed magnetic field direction. The mechanism of the demonstrated switching is discussed from the viewpoint of the spin flop transition of the α-Cr2O3 layer.

The Design of Cryogen-Free Cryostat for Pulsed High Magnetic Field Based on GM Cryocooler

Based on a small GM cryocooler, a cryogen-free insert cryostat for physics experiment in a pulsed high magnetic field has been designed. The GM cryocooler is used as the cooling source. The sample is cooled by liquid helium for the low temperatures and by helium gas for the high temperatures. The inner diameter of the cryostat is 8 mm and the outer diameter of the tail is 16 mm, thus it can be inserted into a 60 T pulsed magnet. Experimental results show that the temperature can be controlled from 1.5 K to 300 K easily. In the sample region, stability of the temperature is obtained: ±0.1 K at 20 K-300 K, ±0.2 K at 5 K-20 K and ±0.02K at 1.5 K-5 K. The system may be used for electrical transport and magnetization measurements in a pulsed high magnetic field.

Spectral Domain Magneto-Optical Magnetometry

In this work we present a new concept for measuring high intensity pulsed magnetic fields (h (t)) through the spectral analysis of spin excitations in a saturated yttrium-iron garnet (YIG) thin film grown on a gadolinium gallium garnet (GGG) substrate. The spectrum of spin excitations, generated in the sample by picosecond h (t), was determined with the help of a magnetooptical (MO) Faraday probe in the guided wave regime of lightwave propagation. This technique, compared to the standard MO cells, allows one to avoid saturation effects because the amplitude of h (t) is measured in the frequency domain, and allows us to realize the analysis in real time taking advantage of strong a MO signal.

Design of a Novel Pulsed Power System for Repetitive Pulsed High Magnetic Fields

A repetitive pulsed high magnetic field is a useful tool for scientific research and industrial applications. In this paper, a novel repetitive pulsed power system (RPPS) adopted to generate repetitive pulsed high magnetic fields (RPHMF) at the Wuhan National High Magnetic Field Center (WHMFC) is designed. The capacitors serve as the energy storage elements in the pulsed power system, and the key issue of the RPHMF system is to decrease the loss and to charge the capacitors quickly, safely and efficiently. For this purpose, a new discharge topology is used to feed the energy stored in the magnet back to the capacitor by an additional choke coil and a resonant capacitor. The energy lost during each pulse will be replenished to the capacitor through a high frequency series-parallel resonant capacitor charging power supply (CCPS).