High-temperature Magnetization Research Articles

Two-Dimensional Ferromagnetism of aHe3Film: Influence of Weak Frustration

(3)He films adsorbed on the atomically flat surface of graphite provide a model system for the study of two-dimensional magnetism on a triangular lattice. We have made a study of the regime in which the T=0 ground state of the second (3)He layer is a fully polarized ferromagnet. NMR, using broadband SQUID detection, at a range of low fields above the spin-flop transition, and over a wide temperature range 0.3-200 mK, has enabled us to disentangle the influence of sample finite size effects and magnetic field on the spin-wave spectrum. We demonstrate that the spin-wave spectrum is governed by a different effective exchange constant than that determining the high temperature magnetism. This is understood in terms of frustrated atomic ring exchange.

Laboratory and Field Tests of Movable Conduction-Cooled High-Temperature SMES for Power System Stability Enhancement

This paper introduces the first movable conduction-cooled high-temperature superconducting magnetic energy storage (SMES) system developed in China. The SMES is rated at 380 V/35 kJ/7 kW, consisting of the high-temperature magnet confined in a Dewar, the cryogenic unit, the converter, the monitoring and control unit, the container, etc. The proposed SMES can be loaded onto a truck to move to a desired location and put into operation with easy connection. Laboratory and field tests have been carried out to investigate the operational characteristics and to demonstrate the SMES effectiveness on improvements of system voltage stability and on the oscillation damping. Test results indicate that the SMES system has the features of fast response and four-quadrant power operation. The accessories for the movability of the SMES system are well designed. The system is feasible to be used in power systems.

Multiple temperature-induced magnetization reversals in SmCr1−xFexO3 system

The structural and magnetic properties of the SmCr1−xFexO3 (0≤x≤0.5) system have been investigated. Multiple temperature-induced magnetization reversals were observed in x=0.1. The high-temperature magnetization reversal is due to the special spin structure, in which the net canted moment of the Cr-rich regions and the net moment of the Fe–Cr ordered regions orient in opposite direction; while the low temperature one can be ascribed to the spin reorientation. The sample with x=0.5 exhibits the highest compensation temperature. Coexistence of normal and inverse magnetocaloric effects was observed in all doped samples. Potential applications in magnetic refrigeration based constant temperature bath near room temperature (∼286K) have been demonstrated.

Development of a 5T 2G HTS Magnet With a 20-cm-Diameter Bore

REBCO coated conductors known as second-generation (2G) high-temperature superconductor are expected to show high performance in superconducting applications because of their high mechanical strength and high current density in a magnetic field. Recently, there have been reports about fabricating impregnated REBCO coils. However, more characteristic data of REBCO coils with larger diameters are required in order to apply these conductors to practical superconducting applications. In this work, we fabricated a cryo-cooled high-temperature superconducting magnet, which was composed of 24 pancake coils with an inner diameter of 260 mm, using REBCO coated conductors. The total length of REBCO coated conductors was approximately 7.2 km. These conductors were laminated with a copper stabilizer and were fabricated by Fujikura Ltd. using ion-beam-assisted deposition and pulsed laser deposition methods. Before fabricating the magnet, all pancake coils were evaluated their V -I characteristics in liquid nitrogen, in order to confirm the characteristics without degradation. After the fabrication, the magnet was cooled down to 24 K by a Gifford-McMahon cryocooler. We excited the magnet up to 5 T, successfully.

Application of Ultra-Low Field HTS-SQUID NMR/MRI to Contaminant Detection in Food

We constructed an ultra-low field nuclear magnetic resonance (NMR)/magnetic resonance imaging (MRI) system utilizing a high-temperature superconductor (HTS) SQUID and permanent magnet for application of contaminant detection in food and drink. In order to compensate shortage of signal-to-noise ratio of the HTS-SQUID-based system, a pair permanent magnet of 1.1 T was employed for prepolarization field. It was combined with a sample transfer apparatus, which transferred a water sample with contaminant in a 10-ml glass bottle from the magnet to under the SQUID magnetometer within 0.5 s. Feasibility of contaminant detection in drink was demonstrated using the system by measuring NMR and 1-D MRI of water samples including contaminants of stainless steel, aluminum, and polymer. 2-D MRI measurement of a sample with partitioned water by silicone bulk was demonstrated employing the filtered back projection reconstruction method.

Optimization of a Mechanical Bridge Joint Structure in a Stacked HTS Conductor

The mechanical bridge joint (bridge-type lap joint) of a stacked high-temperature superconducting conductor has been investigated for a “remountable” or a segment-fabricated high-temperature superconducting magnet. In a previous study, joint resistivities were evaluated experimentally for the bridge joints of single-layer and double-layer stacked GdBCO coated conductors. However, the joint resistivity increased with an increase in the number of layers due to nonuniform contact pressure distribution caused by a gap or misalignment in the joint region. In this study, therefore, we were aiming at the reduction of joint resistance by achieving more uniform contact pressure distribution. In addition, we investigated the effects of temperature while applying pressure to the joint and positioning the joint structure to investigate its application in an actual large-sized magnet. First, we inserted an indium film between the joint surfaces to make contact pressure uniform. Experimental results showed that joint resistivity with the indium film did not depend on the number of layers. In addition, applying force at room temperature was more effective in decreasing joint resistivity than that at 77 K. Finally, we examined the effect of the joint structure with screw bolt tightening. The result showed that the structure of the convex plate has better joint performance than others.

Characteristics of an HTS Pancake Coil in Persistent Current Mode Using Wind-and-Flip Winding Method

It has been well known that a coated conductor (CC), 2G superconducting tape, shows a higher critical current and better performance than the 1G, BSCCO one. However, there has still been no way to make a superconducting joint with CC, so it has been almost impossible to build a CC superconducting magnet in persistent current mode. As a viable solution for this, we have proposed a special winding technology called the Wind-and-Flip method. It shows a new possibility for the generation of a persistent current mode in a magnet with CC. First, we partially cut a wide CC tape into two pieces along the tape except at both end parts like cutting the abdomen of a fish. Then we wind two pancake coils with the cut but connected CC tape. Finally, one of the two coils is turned over, and then the magnetic fields from both coils would have the same direction with each other. In this paper, we made two high-temperature superconducting double pancake windings by the Wind-and-Flip method. One has turn-to-turn insulation layers while the other does not. They both have oxygen-free copper bobbins for the conduction cooling. First, they were magnetized by field cooling within a fine background magnet, and operated in persistent current mode at 77 K. As a result of the experiment, we measured and compared temporal stabilities for both pancake windings to investigate the possible use of these windings for magnetic resonance imaging or nuclear magnetic resonance applications. We have future plans of measuring the temporal stabilities of similar windings under lower temperatures and confirm the possibility of a high-field high-temperature superconducting magnet, which will be fabricated by stacking these pancake coils with larger cold bore size.

Fabrication and Characterization of 3-T/102-mm RT Bore Magnet Using 2nd Generation (2G) HTS Wire With Conducting Cooling Method

A conduction-cooled high-temperature superconducting magnet using 2nd generation HTS wire, which has a room-temperature bore 102 mm in diameter, has been developed and tested up to 3 T with the operating temperature of 20 K. The magnet consists of 22 double pancake coils (DPCs) with an inner diameter of 140 mm and outer diameter of 182 mm. Twenty-two double pancake coils were tested separately at 77 K for checking the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">IV</i> -curve. Selected DPCs were resistively connected by HTS tape (Splice joint), and an assembled magnet coil with the size of 182.5 mm diameter and 242 mm in height was conduction cooled by a two-stage Gifford-McMahon cryo-cooler to 20 K. Current, voltage, and field strength were measured as a function of time with various ramping up and down conditions. The resulting performance data of the assembled magnet agreed well with the expectation from FEM simulation. The aimed field homogeneity of 0.1% in 10 mm diameter sphere volume was proved when operating current was 141.6 A at 20 K with central magnetic field intensity of 2.9975 T by hall sensor. The magnetic flux density at center showed nonlinear dependence with ramping current within the range of 0.05 A/sec ~0.15 A/sec because of charging delay. However, saturated magnetic flux density showed the same value of 2.9975 T regardless of ramping rate.

Crystals, magnetic and electronic properties of a new ThCr2Si2-type BaMn2Bi2 and K-doped compositions

This is a report on the new 122 ternary transition-metal pnictide of BaMn2Bi2, which is crystallized from bismuth flux. BaMn2Bi2 adopts ThCr2Si2-type structure (I4/mmm) with a = 4.4902(3) {\AA} and c = 14.687(1) {\AA}; it is antiferromagnetic with anisotropic magnetic susceptibility and semiconducting with a band gap of Eg = 6 meV. Heat capacity result confirms the insulating ground state in BaMn2Bi2 with the electronic residual Sommerfeld coefficient of {\gamma} ~ 0. The high temperature magnetization results show that magnetic ordering temperature is TN ~ 400 K. Hole-doping in BaMn2Bi2 via potassium in Ba1-xKxMn2Bi2 results in metallic behavior for x = 0.10(1), 0.32(1) and 0.36(1). With K-doping, more magnetically anisotropic behavior is observed. Although there is a downturn in electrical resistivity and low-field magnetization data below 4 K in > 30%-doped crystals, there is no sign of zero resistance or diamagnetism. This manuscript is a report on new materials of BaMn2Bi2 and Ba1-xKxMn2Bi2 (0 < x < 0.4). Results from powder X-ray diffraction, anisotropic temperature- and field-dependent magnetization, temperature-and field-dependent electrical resistivity, and heat capacity are presented.

High temperature growth, charge distribution and magnetism in Co and Mn co-doped ZnO

Zn1−2xCoxMnxO (x = 0.03 and 0.05) bulk samples were synthesised by a standard high temperature solid state reaction technique at 1,400 °C. Characterization techniques X-ray diffraction, Vibrating Sample Magnetometry and Scanning Electron Microscopy with Energy Dispersive Analysis of X-rays were used to investigate effect of transition metal element Co and Mn incorporated in the system using high temperature technique. The magnetic, structural, electron density distribution and bonding properties have been investigated for the samples prepared in this work. X-ray diffraction analysis of the synthesized samples showed a single phase ZnO wurtzite structure for x = 0.03 and very slight segregation of a spinel phase for x = 0.05. Magnetization measurements indicate no evidence of ferromagnetism in co-doped (Co and Mn) ZnO. The structural and electron density analysis presented in this work is based on the actual concentrations of Co and Mn ions introduced in the matrix which nearly coincide with the nominal concentrations.

Design and Simulation Experiment Study on Reconfigurable Spacecraft Base on Flux Pinned Interface

Magnetic flux pinning takes place between the high-temperature superconductors and magnets which can form non-contacting linkages between the individual components. This interaction is proposed for use in in-orbit assembly. This paper develops a flux pinned revolute joint which fits for the reconfiguration of modular spacecraft and details the reconfiguration mechanisms of the novel joint. Moreover, the process of reconfiguration is simulated via the rigid body dynamics. Furthermore, two flux pinned modules are constucted to confirm the feasibility of the designed flux pinned rovolute joint for reconfiguration on a testbed.

Advanced Applications of High Temperature Magnetics

In recent years, high temperature magnetic structures have been developed and used for inductors and transformers in high temperature applications ranging from power electronics to wireless telemetry systems. Research in the high temperature magnetics field has led to the development of more advanced magnetic structures that can enable diverse applications ranging from regulators to amplifiers, with far reaching implications for the high temperature electronics community. Current high temperature electronics have shown potential in lab and rig tests, but high temperature electronics systems suffer from the relatively limited lifetime of the semiconductor devices themselves. The advanced magnetics discussed in this paper can be designed to have extreme lifetime capabilities even at elevated temperatures, and as such can have an immediate impact on the implementation of true field deployable high temperature electronic systems. Aerospace, power generation, and automotive industries may especially benefit from this technology, as significant advances in health monitoring and active engine control will be enabled by these advanced magnetic structures. A theoretical understanding of these advanced magnetic structures is necessary for initial design and feasibility, while the true development and implementation of this technology depends on state of the art high temperature packaging approaches. By combining high temperature, grain-oriented magnetic materials along with high temperature packaging processes, APEI, Inc. has created advanced high temperature magnetic systems that indicate the technology described in this paper is a viable one, with applications across a wide range of high temperature electronics systems.

A nanocrystalline Sm–Co compound for high-temperature permanent magnets

The inherently high magnetic anisotropy and nanoscale grain size in a Sm5Co19 compound result in an intrinsic coercivity far higher than those of known Sm-Co compounds prior to orientation treatment. The combination of ultrahigh intrinsic coercivity, high Curie temperature and low coercivity temperature coefficient of nanocrystalline Sm5Co19 as a single phase material shows it to be a very promising compound to develop outstanding high-temperature permanent magnets.

Study on Flux Pinned Vertical Force between High-Temperature Superconductor and Permanent Magnet

The flux pinned force attracts our eyes, where the levitating system is widely investigated for the transportable application. Although plenty of experiments can lead to many significant consequences, the mechanism has been not understood completely since the enormously complicated behavior and quantum effect involve. In this paper, in order to calculate the flux pinned force analytically, the image-dipole model is adopted and improved and the simple and precise expression of magnetic strength is utilized, so the flux pinned vertical force can be calculated in analytical form. In addition, the experimental data in case of zero field-cooling and field-cooling experiment can be collected in our experimental table and the comparison is completed between the precise and simple formula and the experimental data, the precise version of expression fro flux pinned vertical force can correspond to the experimental data well.

Thermal stability of surface modified Sm2Co17-type high temperature magnets

The oxidation behavior and magnetic loss of Sm(CobalFe0.1Cu0.1Zr0.033)6.93 magnets modified with nickel coating were systematically studied. The research reveals the laws of oxidation kinetics and magnetic properties of the Ni-coated and uncoated magnets oxidized at 500°C in air. The results show that the mass gain of the uncoated magnets is 4.02mg/cm2 for 100h oxidation and 8.48mg/cm2 for 500h oxidation. As for the coated magnets oxidized at 500°C for 100h and 500h, the mass gain is only 0.06mg/cm2 and 0.22mg/cm2, respectively. The loss of remanence Br, coercivity Hcb and maximum energy product (BH)max of the coated magnets oxidized at 500°C for different time were compared with those of the uncoated magnets. The magnetic properties of the coated magnets are stable at both room temperature and 500°C, while the magnetic properties of the uncoated magnets decrease drastically. The results of scanning electron microscope and X-ray diffraction reveal the phase constitution of ordered surface oxidation zones, showing that the interdiffusion between Ni coating and the magnet is slight and Ni coating can isolate oxygen invasion and hinder Sm volatilization. These results indicate that nickel coating can protect and stabilize Sm2Co17-type high temperature magnet effectively at 500°C.

Superparamagnetism in Gd-doped GaN induced by Ga-vacancy clustering

On the basis of the energetics and magnetic exchange interactions obtained from large-scale first-principles electronic structure calculations, a gallium-vacancy clustering model is proposed for the origin of colossal magnetic moments and high-temperature magnetism in gadolinium-doped gallium nitride. Monte Carlo simulations of the growth coupled to simulations of the magnetic moments show that the clustered geometry of vacancies plays a crucial role in establishing the existence of two distinct temperature regimes, observed experimentally and corresponding respectively to alignment of moments only within each cluster at high temperatures and globally at low temperatures.

High temperature magnetic properties of SmCo5/α-Fe(Co) bulk nanocomposite magnets

To find alternative high temperature magnets containing no heavy rare earths for power applications, SmCo5/Fe bulk nanocomposite magnets with enhanced energy density and high thermal stability have been produced by using a ball-milling plus warm-compaction route. Up to 30% of the Fe soft magnetic phase has been added to the composites with grain size &amp;lt;20 nm distributed homogenously in the matrix of the SmCo5 hard magnetic phase. It was observed that the microstructure does not change with temperature up to 500 °C. It is also observed that the thermal stability of bulk nanocomposite samples is closely related to bulk density. Energy products above 11 MGOe have been obtained at 300 °C in fully dense bulk SmCo5/Fe nanocomposite magnets, which is 65% higher than that of a single-phase counterpart at the same temperature.

Sol-gel-Derived $\hbox{Al}_{2}\hbox{O}_{3}{-} \hbox{SiO}_{2}$ Composite Coating for Electrical Insulation in HTS Magnet Technology

Electrically insulating Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thin coatings have been deposited on long-length 316 stainless steel (SS) tape using a reel-to-reel continuous solgel dip coating process for cowinding insulation into YBCO pancake coils, a high-temperature superconductor magnet technology. Coatings with a thickness of ~ 5 μm are achieved after just two dips with a tape withdrawal speed of ~ 8 mm/s (0.5 m/min) and a calcination at 700 °C. The coatings are measured to have a room-temperature breakdown voltage of ~ 200 V, corresponding to a dc dielectric strength of about 40 MV/m. Consequently, this process has low cost and high throughput and produces a thin electrical insulation with excellent thermal, dielectric, and mechanical properties. A new technique has been developed in the coating process to mitigate Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> particulate aggregation and coating buildup along the edges of the SS tape.

Effect of divalent cation substitution in the magnetoplumbite structured BaFe12O19 system

Synthesis of magnetically ordered barium hexaferrite powders and the adjustment of magnetic properties for perpendicular magnetic recording media are realized through substitution of divalent cation (Ca) in the BaFe12O19 system. The Ca2+ substituted Ba1−xCaxFe12O19 (where x = 0.05, 0.1, 0.15 and 0.2) compounds have been prepared through solid state reaction technique. The powder X-ray diffraction analysis reveals that all the prepared compounds crystallized in magnetoplumbite hexagonal structure and the flat hexagonal platelet morphology of the crystallites was identified through scanning electron microscopy. The formation of magnetoplumbite structured Ba1−xCaxFe12O19 system due to mechanical activation was supported by micro-Raman measurements. Both pure and Ca substituted BaFe12O19 compounds exhibit sharp intense peaks which reveals defect free environment in the crystal lattice. From the room temperature magnetization studies, it was observed that the saturation magnetization (MS) and remanent magnetization (MR) values drastically decreases for the Ba0.95Ca0.05Fe12O19 compound which may be due to the existence of spin canting effect and leads to the reduction of super exchange fields. The increase in MS and MR values for the Ba0.9Ca0.1Fe12O19 and Ba0.85Ca0.15Fe12O19 compounds could be attributed to the enhanced hyperfine fields at 12k and 2b sites due to the strengthening of Fe3+–O–Fe3+ super exchange interactions. A large reduction in the coercivity value from 3,090 to 1,548 Gauss may be attributed to the fall in magneto crystalline anisotropy. The high temperature magnetization studies infer that while increasing substitution level of Ca in the BaFe12O19 system results in decreasing trend in Curie temperature. The room temperature dielectric measurement shows that the incorporation of Ca2+ in the BaFe12O19 system results with increase in the dielectric constant and this case substantiates the space charge polarization. The magnetically ordered BaFe particulate having higher saturation and low coercivity values with superior magnetic and dielectric behaviour exhibiting the possibility for future high-recording-density storage products.

Field-dependent anomalies in the thermodynamic and transport properties of RuSr2GdCu2O8—Evidence for a spin-flop transition

Thermopower and magnetization studies of epitaxial thin films and sintered polycrystalline ceramics of RuSr2GdCu2O8 show evidence for the occurrence of a spin-flop transition, consistent with earlier neutron diffraction data. Realignment of spins in a magnetic field enhances Tc by up to 4K, presumably due to reduced magnetic pair-breaking. Specific heat and powder AC susceptibility measurements give strong evidence for bulk superconductivity. High temperature magnetization measurements indicate in-plane ferromagnetic correlations which remain to be reconciled with low-temperature neutron diffraction evidence for conventional G-type antiferromagnetism.