Deuterium nuclear magnetic resonance spin-lattice relaxation of analytically relevant solvent systems

We previously reported that the nuclear magnetic resonance (NMR) 14N spin-lattice relaxation times (T1) of CH3CN in CH3CN-H2O mixtures directly correlate with the solution viscosity when scaled for temperature (eta/T) in this common chromatographic mobile phase system.' Here, we demonstrate that the 2HT1 relaxation times also correlate with viscosity, contrary to a previous report. (2) This establishes 2HT1 relaxation times as a useful means of measuring changes in solution viscosity in CH3CN-H2O mixtures. We show thermal convection to result in grossly decreased, apparent T1's, by as much as approximately 40%, in nonspinning samples. This effect can be eliminated by moderate sample rotation or confinement of the sample to within the rf-irradiated region. The problem of thermal convection is revealed in systems having long Ti's and has implications in T1 experiments employing nonspinning samples at elevated temperatures, including inherently nonspinning systems, such as those used in high-pressure studies.     

磁场对PET表面碳氟等离子体改性的影响：II.磁场位形与PET？…

研究了不同磁场条件下碳氟等离子体对涤纶（ＰＥＴ）表面改性的影响,通过变角Ｘ光电子能谱（ＸＰＳ）方法研究改性后ＰＥＴ表面结构和性质的变化,并探讨了磁场对碳氟等离子体改性的影响,实验结果表明,在磁场存在的条件下,碳氟等离子体处理ＰＥＴ的表面结构和性质的变化,依赖于磁场的强度而不是磁场的分布,磁场强度的增强,导致了改性后ＰＥＴ表面Ｆ／Ｃ比的提高及等离子体事物或蚀程度的加强。     

直流磁场对Mg-Y-Cu-Zr合金结晶取向和性能的影响

研究了0~1.4T的直流磁场对陶瓷型铸造Mg-Y-Cu-Zr合金结晶取向和力学性能的影响。结果表明直流磁场对合金的初生相没有明显影响,但磁场强度大于1.12T时,合金第二相的分布变得不连续。当磁场强度小于0.84T时,α-Mg在(002)面的衍射峰逐渐增强,但变化不明显;当磁场强度大于0.84T时,α-Mg基体在(002)、(101)面的衍射峰强度明显减弱,而在(100)晶面衍射峰强度有所增强。磁场处理后,合金的综合力学性能得到了改善;当磁场强度为1.4T时,其抗拉强度和伸长率最高,分别为237 MPa和8.5%,较无磁场处理时分别提高了38.6%和33.4%。     

Effect of magnetic field on metallurgical structure of magnetostrictive alloys solidified unidirectionally in microgravity

TbFe₂ alloy solidification experiments were conducted in a static magnetic field in microgravity using a 10 m drop tower. When TbFe₂ melt was solidified in a magnetic field from 0 to 0.12T in microgravity, a [111] crystallographic alignment dominated with an increased magnetic field, but the planar macrostructure was random. The magnetostrictive constant of TbFe₂ solidified in magnetic field of 0.12T in microgravity was 2000 ppm at the external 1. 6T magnetic field. When TbFe₂ melt was solidified unidirectionally in a 0. 1 T magnetic field in microgravity, a [111] crystallographic alignment dominated, and the planar structure grew and oriented along the solidification direction. The magnetostrictive constant of TbFe₂ solidified unidirectionally in a 0. 1 T magnetic field in microgravity was 4500 ppm at the external 1. 6T static magnetic field. For all solidification in normal gravity, the maximum magnetostrictive constant remained at 2000 ppm at the external 1. 6T static magnetic field. TbFe₂ crystals grew predominantly along the same direction as the magnetic field, and the planar structure oriented along the solidification direction in microgravity.     

Superconductivity in an Organic Conductor Stabilized by a High Magnetic Field

The application of a sufficiently strong magnetic field to a superconductor will in general destroy the superconducting state. However, for the quasi‐two‐dimensional organic conductor λ‐(BETS)₂FeCl₄, where BETS is bis(ethylenedithio)tetraselenafulvalene, we find that superconductivity is induced under magnetic fields in the range between 18 T and 42 T when the magnetic fields are applied exactly parallel to the conducting layers of the crystals, although it has an insulating ground state below 10 T. This is the first material whose superconducting phase is stabilized only under a magnetic field. The resulting phase diagram indicates that the internal magnetic field due to the negative exchange interaction between the localized Fe moments and the conduction electron spins is crucial.     

放电等离子烧结Gd/Gd5Si2Ge2复合材料磁热效应研究

以高纯钆和Gd5Si2Ge2合金为原料,采用放电等离子烧结技术制备了两组元Gdx(Gd5Si2Ge2)1-x(x=0,0.33,0.5,0.7,1)层状复合磁制冷材料.通过自制的磁热效应测量仪器直接测量了复合材料在外加磁场1.5 T下的磁热效应(ΔTad).随着复合比例的变化,材料的最大绝热温变(ΔTad)从x=0.3时的1.6 K增加到x=0.7时的2.0 K,而最大绝热温变峰的位置从286K变到了293 K.同时,与单组元的Gd5Si2Ge2合金相比,随着钆的含量增加时,复合材料的最大绝热温变峰变宽.当x=0.7时,层状复合磁制冷材料在外加磁场1.5 T下的最大绝热温变(ΔT)在260-310K范围里从1.1 K变到2.0 K,这种材料非常适合作为室温磁制冷材料.     

Ordered, self-organized cobalt nanodots in Co-diamond-like carbon thin films

A formation process for ordered, self-organized cobalt (Co) nanodots in diamond-like carbon (DLC) thin films deposited by magnetron sputtering in a plasma-assisted Ar/CH4 discharge is presented. episilon-Co dots -5 nm in diameter, separated by 1-2 nm DLC boundaries and arranged in hexagonal arrays were produced on Si substrates. The formation mechanism relies on a self-organization process which is based on surface energy minimization and local magnetic field interaction. The proposed plasma-assisted process presents a controlled and cost-effective bottom-up nanofabrication approach for the production of well-ordered magnetic nanodots based on self-organization.     

Magnetophoretic velocimetry of manganese(II) in a single microdroplet in a flow system under a high gradient magnetic field generated with a superconducting magnet

An experimental system for magnetophoretic velocimetry, which could determine the volume magnetic susceptibility of a single particle dispersed in a liquid phase from a magnetophoretic velocity, has been developed. A micrometer-sized high-gradient magnetic field could be generated in a capillary by a pair of iron pole pieces in a superconducting magnet (10 T). The magnetophoretic behavior of a single particle in a capillary flow system was investigated under the inhomogeneous magnetic field. From the magnetophoretic velocity of a polystyrene latex particle dispersed in a MnCl2 aqueous solution, the product of the magnetic flux density and the gradient, B(dB/dx), was determined as a function of the position along the capillary. The maximum value of B(dB/dx) was 4.7 x 10(4) T2 m(-1), which was approximately 100 times higher than that obtained by two Nd-Fe-B permanent magnets (0.4 T). Organic droplets extracting manganese(II) with 2-thenoyltrifluoroacetone and tri-n-octylphosphine oxide from MnCl2 solution were used as test samples. The difference of the volume magnetic susceptibility between the droplet and the medium could be determined from the magnetophoretic velocity. This method allowed us to continuously measure a volume magnetic susceptibility of 10-6 level for a picoliter droplet and to determine manganese(II) in the single droplet at the attomole level.     

Estimation on Magnetic Refrigeration Material （Gd1-xREx）5Si4 （RE=Dy, Ho）

A systematic (Gd1-xREx)5Si4 (RE=Dy, Ho) alloys are investigated to estimate their magnetocaloric effect.The Curie points of (Gd1-xREx)Si4 alloys can tunable from 266 K to 336 K when RE=Dy, Ho; x=0～0.35 and 0～0.15, respectively, and decrease nearly linearly with increasing x. These alloys keep orthorhombic structures Ge5Sm4 and exhibit second order transition when they experience in a change magnetic field at about Curie when magnetic field changes 0～2 T. The adiabatic temperatures changes (△Tad) of these alloys at Curie points are larger than 1 K in a field change 0～1.4 T, the curve of △Tad is wide as that of Gd. The relative cooling power is about 0.8～0.9 J/cm3 when field changes 0～2 T, 55% of that of Gd. Comparing with Gd5(Si1-xGex)4, these alloys do not contain expensive element Ge, so that their cost are lower than the former. Because they could work at temperature region 260～340 K due to their Curie points can be tuned, which is an advantage comparing with Gd, these alloys are potential magnetic refrigerants working in a magnetic refrigerator with a low magnetic field at room temperatures.     

Thermometers in Low Magnetic Fields

In this article the effect of low amplitude DC magnetic fields on different types of thermometers is discussed. By means of a precision water-cooled electromagnet, the effect of a magnetic field on platinum resistance thermometers, thermistors, and type T, J, and K thermocouples was investigated, while thermometers were thermally stabilized in thermostatic baths. Four different baths were used for temperatures from 77 K (?196 °C) to 353 K (80 °C): liquid nitrogen bath (nitrogen boiling point at atmospheric pressure), ice-point bath, room-temperature air bath, and hot-water bath. The generated DC magnetic field of high relative precision (2 × 10^?4 at 1 T, 4 × 10^?5 short-term stability) and high relative uniformity (2 × 10^?5 over 1 cm², 10 mm gap) had a magnetic flux density of 1 T in the center of the gap between the magnet pole caps. The results indicate a magnetic effect of up to 100 mK due to a 1 T magnetic field for the types of thermocouples composed of ferromagnetic materials (Fe, Cr, Ni). For platinum resistance thermometers, thermistors, and non-magnetic type T thermocouples, the detected magnetic effect was weaker, i.e., under 10 mK.     

A two-axis magnetometer using a single magnetic tunnel junction

Based on a qualitative study of the Stoner-Wohlfarth model, we point out that driving a magnetic tunnel junction (MTJ) with an alternative two-dimensional magnetic field allows to measure simultaneously two components of an external magnetic field. Only one single MTJ without a pinning layer is needed to measure both components of a magnetic field parallel to the junction plane. The response of the magnetometer does not depend on the resistance of the junction or the amplitude of its variations. A prototype has been manufactured and encouraging experimental results are presented. Sensitivities higher than 500 V/T and a noise level of 2 /spl mu/T//spl radic/Hz are reported.     

磁场诱导自蔓延高温合成钡铁氧体

本文采用外加磁场诱导自蔓延高温合成钡铁氧体,试验用的电磁场强度最高可达1.3T,对无磁场和不同磁场强度下合成的铁氧体的形貌、相组成和磁性能分别进行了表征.研究结果表明:外加磁场对燃烧温度有影响,燃烧温度影响产物转换,燃烧温度较低时,产物为BaFe2O4与BaFe12O19相共存;本试验条件下,磁场强度为0.86T时,合成为M型的钡铁氧体(BaFe12O19),产物结晶完整,有六角片状的钡铁氧体,且性能达到了最佳,矫顽力达到1083(4π)-1·kA·m-1,比剩余磁化强度为16.16 A·m2/kg,比不加磁场条件下分别提高50%和提高32%,说明适当的磁场强度诱导自蔓延高温合成可以改善BaFe12O19的磁性能.     

纳米复合磁流变液的流变特性

在不同的磁场强度H、剪切速率γ和温度T下,测试了纳米复合磁流变(MR)液的磁致剪切应力△τ以及响应时间△tο结果表明,H、γ和T对纳米复合MR液的△t的影响较小,纳米复合MR液对撤消磁场的响应比施加磁场的响应略慢;随H增加,△τ增加;随户提高、△τ出现最大值;随T升高,△τ缓慢降低。它们之间的定量关系可用公式△τ=kHH+(Kγ1-kγ2γ)γ-kt△T来描述,其中:kH=28.32,kγ1=2.85,kγ2=0.0013,kt=3.8。用它能较全面地评价纳米复合MR液的性能,并可作为设计磁流变液智能器件的基础。     

Electronic transport properties of incipient graphitic domains formation in PAN derived carbon nanofibers

The carbon nanofibers used in this work were derived from a polyacrylonitrile (PAN)/N, N-dimethyl formamide (DMF) precursor solution using electrospinning and vacuum pyrolysis techniques. Their conductivity, /spl sigma/, was measured at temperatures between 1.9 and 300 K and transverse magnetic field between -9 and 9 T. Zero magnetic field conductivity /spl sigma/(0,T) was found to increase monotonically with the temperature with a convex /spl sigma/(0,T) versus T curve. Conductivity increases with the external transverse magnetic field, revealing a negative magnetoresistance at temperatures between 1.9 and 10 K, with a maximum magnetoresistance of -75 % at 1.9 K and 9 T. The magnetic field dependence of the conductivity and the temperature dependence of the zero-field conductivity are best described using the two-dimensional weak localization effect.     

The influence of a magnetic field on an AuMn resistor at low temperatures

Measurements of the magnetoresistance of an AuMn (0.9 at %) based low-temperature resistance thermometer were made at 4.2 K in a magnetic field up to 13.5 T and 2.8 K and 1.8 K in fields up to 4.2 T. The change in the scattering cross-section of the electrons by the manganese spins due to the magnetic field produces an important decrease (up to 50% at 10 T) of the resistivity. Experimental relationships for the temperature and magnetic field characteristics of the resistor are indicated     

Use of paired, bonded NdFeB magnets in redox magnetohydrodynamics

Bonded neodymium-iron-boron (NdFeB) permanent magnets in a paired configuration were successfully used to control mass transport in redox-based, magnetohydrodynamics (MHD). Control of fluid flow based on magnetic fields has potential for use in portable lab-on-a-chip (LOAC) and analytical devices. Bonded magnets, composed of magnetic powder and organic binder materials, are less expensive and easier to fabricate and pattern than electromagnets and sintered permanent magnets, which have been previously used in MHD studies on electrochemical systems. The ability to pattern bonded magnets near and around the electrodes is expected to allow for better control over the magnetic field distribution and solution flow. Current was generated at an 800-microm-radius platinum disk electrode in a solution of 0.06 M nitrobenzene and 0.5 M tetra-n-butylammonium hexafluorophosphate in acetonitrile. Increases in limiting current in the presence of the magnetic field, which indicate enhancement in mass transport, for sintered (210+/-14%, N = 4, where B(r) = 1.23 T and magnetic field strength is 0.55 T) and bonded (94+/-8%, N = 4, where B(r) = 0.41 T and magnetic field strength is 0.20 T) magnets, were similar to those obtained using an electromagnet with the same magnetic flux densities. The magnetic field strength and not the magnet type is important in controlling fluid flow, which is encouraging for integration of bonded permanent magnets into LOAC devices.     

Development and analysis of a PCB vector 2-D magnetic field sensor system for electronic compasses

A high-sensitivity vector two-dimensional (2-D) magnetic sensor system for low magnetic field measurements has been realized and tested. The system, made in PCB technology, consists of a double-axis Fluxgate magnetic sensor and the readout electronic circuitry, based on second-harmonic detection. The amorphous magnetic materials Vitrovac 6025X (25 /spl mu/m thick) and Vitrovac 6025Z (20 /spl mu/m thick) were used as the ferromagnetic core of the sensor. By applying a sinusoidal excitation current having a 450-mA peak at 10 kHz with Vitrovac 6025Z, the measured magnetic sensitivity was about 1.25 mV//spl mu/T. This value seems to be adequate for the Earth's magnetic field detection (/spl plusmn/60 /spl mu/T). The full-scale linearity error was about 1.5%. By using the thicker Vitrovac 6025X and a sinusoidal excitation current having a 600-mA peak at 10 kHz, a maximum sensitivity of approximately 1.68 mV//spl mu/T with a linearity error of about 1.55% full scale in the range of /spl plusmn/60 /spl mu/T were measured. Due to the use of commercially available ferromagnetic materials, the vector 2-D magnetic sensor system presented is characterized by a very simple fabrication process, thus allowing low-cost devices to be designed.     

Simulation of the Magnetic Induction Vector of a Magnetic Core to be Used in FFC NMR Relaxometry

This paper is a contribution for the assessment and comparison of magnet properties based on magnetic field characteristics particularly concerning the magnetic induction uniformity in the air gaps. For this aim, a solver was developed and implemented to determine the magnetic field of a magnetic core to be used in Fast Field Cycling (FFC) Nuclear Magnetic Resonance (NMR) relaxometry. The electromagnetic field computation is based on a 2D finite-element method (FEM) using both the scalar and the vector potential formulation. Results for the magnetic field lines and the magnetic induction vector in the air gap are presented. The target magnetic induction is 0.2 T, which is a typical requirement of the FFC NMR technique, which can be achieved with a magnetic core based on permanent magnets or coils. In addition, this application requires high magnetic induction uniformity. To achieve this goal, a solution including superconducting pieces is analyzed. Results are compared with a different FEM program.     

Fabricating Co doped ZnO nanocrystallines by hydrothermal method with high pulsed magnetic field

Co-doped ZnO nanocrystallines were fabricated by hydrothermal method with high pulsed magnetic field. As a new preparation method, more refined grains and diluted magnetic semiconductors (DMSs) with better performance could be obtained by the hydrothermal synthesis process under pulsed magnetic field. The samples were tested by X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM), Vibrating Sample Magnetometer (VSM) and Raman scattering spectrum. The results show that the sample prepared under pulsed magnetic field has wurtzite structure with tiny crystal lattices changes. Doping Co ions into ZnO crystal lattice is also improved. Curie temperature of 2% Co doped ZnO nanocrystallines synthesized with 4T pulsed magnetic field is higher than that for without magnetic field process, and very close to the room temperature.     

Is the Two Dimensional Organic Conductor, τ-(EDO-S,S-DMEDT-TTF)2(AuCl2)1+y Clean or Dirty?

The transport properties of organic τ-type conductors seem to be very clean as well as very dirty depending on what we observe. To clarify this problem, we studied temperature and magnetic field dependence of resistivity, ρ(T, B), in τ-(EDO-S,S-DMEDT-TTF)₂(AuCl₂)_1+y. The properties that favor “probably dirty” are: i) stepwise ρ_a(T) increase below 20 K, which is suppressed by magnetic field, and ii) contrasted difference in ρ(T) as well as magnetization M(T) between field cooled (FC) and zero field cooled (ZFC). On the other hand, Shubnikov de Haas oscillations with very low Dingle temperature (T_D = 1.5 K) are typical of clean system. Based on these observations, we conclude this system changes from “dirty” to “clean” system by increase of magnetic field.