Magnetic transition of ferromagnetic material at high pressure using a novel system

A system for the investigation of the magnetic properties of materials under high pressure is fabricated based on diamond anvil cell （DAC） technology. The system is designed with an improved coil arranged around the diamond of a non-magnetic DAC. Using this system, the magnetic transition of ferromagnetic （Fe） sample under increasing pressure can be observed. We successfully obtain the evolution of magnetic properties as a function of applied pressure reaching 26.9 GPa in the Fe sample. A magnetic transition is observed at approximately 13 GPa, which is consistent with the theoretical prediction.     

Effect of deformation of diamond anvil and sample in diamond anvil cell on the thermal conductivity measurement

Studies show that the sample thickness is an important parameter in investigating the thermal transport properties of materials under high-temperature and high-pressure (HTHP) in the diamond anvil cell (DAC) device. However, it is an enormous challenge to measure the sample thickness accurately in the DAC under severe working conditions. In conventional methods, the influence of diamond anvil deformation on the measuring accuracy is ignored. For a high-temperature anvil, the mechanical state of the diamond anvil becomes complex and is different from that under the static condition. At high temperature, the deformation of anvil and sample would be aggravated. In the present study, the finite volume method is applied to simulate the heat transfer mechanism of stable heating DAC through coupling three radiative-conductive heat transfer mechanisms in a high-pressure environment. When the temperature field of the main components is known in DAC, the thermal stress field can be analyzed numerically by the finite element method. The obtained results show that the deformation of anvil will lead to the obvious radial gradient distribution of the sample thickness. If the top and bottom surfaces of the sample are approximated to be flat, it will be fatal to the study of the heat transport properties of the material. Therefore, we study the temperature distribution and thermal conductivity of the sample in the DAC by thermal-solid coupling method under high pressure and stable heating condition.     

Nonmetallic gasket and miniature plastic turnbuckle diamond anvil cell for pulsed magnetic field studies at cryogenic temperatures

A plastic turnbuckle diamond anvil cell (DAC) and a nonmetallic gasket have been developed for pulsed magnetic field studies to address issues of eddy current heating and Lorentz forces in metal cells. The plastic cell evolved from our Ø 6.3 mm metal turnbuckle DAC that was designed in 1993 to rotate in the 9 mm sample space of Quantum Design's MPMS. Attempts to use this metal DAC in pulsed magnetic fields caused the sample temperature to rise to T>70 K, necessitating the construction of a nonconductive cell and gasket. Pressures of 3 GPa have been produced in the plastic cell with 0.8 mm culets in an optical study conducted at T=4 K. Variations of the cell are now being used for fermiology studies of metallic systems in pulsed magnetic fields that have required the development of a rotator and a special He-3 cryostat which are also discussed.     

金刚石压砧内单轴应力场对物质状态方程测量的影响

金刚石压砧的几何结构使得在高压下封垫内的样品通常处于单轴应力场中：压砧轴向加载应力最大,径向应力最小.由于金刚石压砧内非静水压单轴应力场的影响,用传统的高压原位X射线衍射方法测得的物质压缩曲线一般位于理想静水压压缩曲线之上.利用金刚石压砧径向X射线衍射技术以及晶格应变理论,结合最近的钨、金刚石和硼六氧样品的高压原位同步辐射径向X射线衍射实验结果,从宏观差应力、样品强度、标压物质和待测物质强度的关系三个方面分析讨论了金刚石压砧内单轴应力场对物质状态方程测量的影响及解决方案. 关键词： 金刚石压砧 单轴应力场 高压原位X射线衍射 状态方程     

Investigating the thermal conductivity of materials by analyzing the temperature distribution in diamond anvils cell under high pressure

Investigating the thermal transport properties of materials is of great importance in the field of earth science and for the development of materials under extremely high temperatures and pressures. However, it is an enormous challenge to characterize the thermal and physical properties of materials using the diamond anvil cell (DAC) platform. In the present study, a steady-state method is used with a DAC and a combination of thermocouple temperature measurement and numerical analysis is performed to calculate the thermal conductivity of the material. To this end, temperature distributions in the DAC under high pressure are analyzed. We propose a three-dimensional radiative-conductive coupled heat transfer model to simulate the temperature field in the main components of the DAC and calculate in situ thermal conductivity under high-temperature and high-pressure conditions. The proposed model is based on the finite volume method. The obtained results show that heat radiation has a great impact on the temperature field of the DAC, so that ignoring the radiation effect leads to large errors in calculating the heat transport properties of materials. Furthermore, the feasibility of studying the thermal conductivity of different materials is discussed through a numerical model combined with locally measured temperature in the DAC. This article is expected to become a reference for accurate measurement of in situ thermal conductivity in DACs at high-temperature and high-pressure conditions.     

3D modeling of electromagnetic levitation coils

Electromagnetic levitation (EML) enables the handling of samples in a containerless manner, facilitating more precise measurements or manipulation of materials. Currently, EML coil design is based on two-dimensional (2D) mathematical models combined with empirical data and experience. We propose that employing a truly 3D model enables a more accurate and rational coil design. This approach shows significant differences in magnetic field, and the resulting lift and heating properties, when comparing a 2D and a 3D coil. A 3D coil, resembling the geometry of a 2D coil, requires more current to generate the same lift. Reversing the situation, a 3D coil producing the exact same magnetic field as a 2D coil has a vastly different design. Furthermore, we show that the assumption that the magnetic field is homogeneous in the axial plane within the sample volume is invalid, even for small samples, in the upper and lower regions of the coil. Using a 3D model, we design a coil capable of levitating and melting an iridium sphere with a diameter of 6 mm.     

Ultrahigh-pressure experiment with a motor-driven diamond anvil cell

A Pt sample was compressed to ultrahigh pressures in a diamond anvil cell (DAC) using a motorized gearbox to change pressure remotely from outside the synchrotron x-ray hutch. In?situ angle-dispersive x-ray diffraction (XRD) was used to determine pressure from known equations of state (EOS). The sample position was unperturbed during motor-driven pressure changes. By eliminating the need to realign the sample to the x-ray position after each pressure increment, 142 XRD patterns could be collected continuously over the course of three hours, and the maximum pressure of 230?GPa was reached before diamond failure ended the experiment. We demonstrate the advantages of this motor-driven assembly for smooth and efficient pressure change, and the possibility for fine pressure and temporal resolution.     

Neutron diffraction experiments in diamond and sapphire anvil cells

Diamond anvil cells (DAC) provide the highest static pressures ≥1?Mbar. Because of the low intensity of neutron sources, for a long time it was thought impossible to use DAC or other anvil cells in neutron experiments. We describe pressure cells with diamond and sapphire anvils and neutron instrumentation allowing neutron diffraction experiments to be carried out under pressures as high as 50?GPa, temperatures down to 0.1?K, and applied magnetic fields up to 7.5?T.     

Resistively heated,high pressure,membrane and screw driven diamond anvil cell

ABSTRACT

High temperature is of paramount importance in high pressure science. One of the leading tools in this respect is the resistively heated diamond anvil cell (DAC), where the heat is provided by small heaters, positioned close to the diamond/gasket/sample region (internally heated DAC, IHDAC) or by wrapping the DAC body into bigger heaters (externally heated DAC, EHDAC). Although IHDACs can reach sample temperatures higher than 1000?K, they are difficult to handle and the heater/diamond/gasket/sample region may be affected by strong thermal gradients potentially hindering accurate temperature measurements. Here we present a novel EHDAC, which overcomes these issues by uniquely joining: (i) high mechanical precision for multi-Mbar, (ii) high temperature alloys for operating to 1000?K, (iii) membrane or screw driven, easily switchable between each other, (iv) operation into a vacuum chamber, (v) uniform temperature, (vi) facile handling, and (vii) possibility to add internal heaters for achieving even higher temperatures.     

FDTD Analysis of a Radiofrequency Knee Coil for Low-Field MRI: Sample-Induced Resistance and Decoupling Evaluation

Magnetic resonance imaging (MRI) using high static field (>3T) generates high-quality images, thanks to high homogeneity in transmission as well as high signal-to-noise ratio (SNR) in reception. On the other hand, biological effects are proportional to the magnetic field strength and moreover the diagnostic accuracy is not always linked to high-quality imaging. For these reasons, the interest in low-field imaging becomes greater, also because of cheaper setting, greater patients comfort and more safety profile. In simple cases, as for surface coil, the coil performance is evaluated using classical electromagnetic theory, but for more complex geometry and in presence of a sample, is more difficult to evaluate the solution and often is necessary to follow a trial-and-error approach. Numerical methods represent a solution to this problem. In this work, we performed numerical simulation on a two-channel knee coil for low-field (0.5 T) MRI scanner. We are interested in seeing the effect of a sample placed inside the coil on the sample-induced resistance and decoupling between channels. In particular, we observe how the position of the sample inside the channel influences the resistance value and for performing this we compared an innovative method based on the exponential fitting on voltage oscillation damping with a validated method (estimation using quality factor). Finally, for the complete coil, the scatter parameters were calculated in loaded and unloaded conditions.     

Propagation Characteristics of Wave Packet in Non-Uniform Magnetic Fields

Experimental observations on a wave packet in a positive column of helium discharge with magnetic fields are reported. The wave packet is a kind of ionization wave and is created by applying voltage pulses to a mesh grid. When an axial non-uniform magnetic field is applied to a positive column, the plasma parameters change inhomogeneously near the magnetic coil. So various characteristics (amplitude, frequency, wavelength and so on) of the wave packet are changed at the both sides of the coil. The wavelength of the wave making up the wave packet varies continuously with a magnetic field. On the contrary, its amplitude and frequency vary remarkable near the magnetic coil, as a strong magnetic field is applied.     

Microwave field distribution in a magic angle spinning dynamic nuclear polarization NMR probe

We present a calculation of the microwave field distribution in a magic angle spinning (MAS) probe utilized in dynamic nuclear polarization (DNP) experiments. The microwave magnetic field (B(1S)) profile was obtained from simulations performed with the High Frequency Structure Simulator (HFSS) software suite, using a model that includes the launching antenna, the outer Kel-F stator housing coated with Ag, the RF coil, and the 4mm diameter sapphire rotor containing the sample. The predicted average B(1S) field is 13μT/W(1/2), where S denotes the electron spin. For a routinely achievable input power of 5W the corresponding value is γ(S)B(1S)=0.84MHz. The calculations provide insights into the coupling of the microwave power to the sample, including reflections from the RF coil and diffraction of the power transmitted through the coil. The variation of enhancement with rotor wall thickness was also successfully simulated. A second, simplified calculation was performed using a single pass model based on Gaussian beam propagation and Fresnel diffraction. This model provided additional physical insight and was in good agreement with the full HFSS simulation. These calculations indicate approaches to increasing the coupling of the microwave power to the sample, including the use of a converging lens and fine adjustment of the spacing of the windings of the RF coil. The present results should prove useful in optimizing the coupling of microwave power to the sample in future DNP experiments. Finally, the results of the simulation were used to predict the cross effect DNP enhancement (?) vs. ω(1S)/(2π) for a sample of (13)C-urea dissolved in a 60:40 glycerol/water mixture containing the polarizing agent TOTAPOL; very good agreement was obtained between theory and experiment.     

基于金刚石色心自旋磁共振效应的微位移测量方法

基于压电陶瓷精密微位移系统的扫描探测技术是目前精密测量仪器进行微纳区域/结构性能测试的核心系统,但压电陶瓷材料存在迟滞、非线性问题,限制了对微位移分辨能力的提升.本文以金刚石氮空位色心为敏感单元,利用电子自旋效应对磁场强度的高分辨敏感机理,结合永磁体周围不同位置对应的磁场强度变化关系,提出了一种基于金刚石氮空位色心电子自旋敏感机理的微位移检测方法.通过建立电子自旋效应与微位移的关联模型,搭建了相应的微位移测量系统.经实验验证,该系统对微位移测试的灵敏度为16.67 V/mm,检测分辨率达到60 nm,实现了对微位移的高分辨率测量.并通过理论分析,该系统的微位移测量分辨率可进一步提升至亚纳米级水平,为新型微位移测量技术提供了发展方向和研究思路.     

Unilateral MRI using a rastered projection

Unilateral magnetic resonance techniques, where magnet and radio frequency (RF) coil are placed on one side of the sample, can provide valuable information about a sample which otherwise cannot be accommodated in conventional high spectral resolution magnetic resonance systems. A unilateral magnetic resonance imaging approach utilizing the stray field from a disc magnet and a butterfly geometry RF coil is described. The coil excites spins in a volume centered around an arc through the sample. Translating the RF coil relative to the magnet and recording the signal at each translational location creates a projection of the signal in a tomographic slice through the sample. Rotating the RF coil relative to the sample and repeating the translation creates projections through the sample at different angles. Backprojecting this information yields an image. A proof of concept device operating on this principle at 12.4 MHz was constructed and characterized. Projections through three phantoms are presented with a 1.2-4 cm field of view, thickness of 102 microm, and at a distance of 3mm from the RF coil and 14 mm from the magnet. The edge spread function (ESF) was measured resulting in a 4mm full width at half maximum (FWHM) line spread function (LSF) estimation using a Gaussian model. An example of one reconstructed image is presented.     

“磁通浓缩器”无浓缩磁通作用

本文通过解麦克斯韦方程给出了把“浓缩体”插入初级线圈前后的磁场分布。方程的解表明,如果在“浓缩体”插入前后,保持初级线圈中的电流不变,则二者在中空区产生的磁场不变;如果供应M者的能量不变(即C,V固定),则插入“浓缩体”前后场值之比B/B₀∝(R₀)^1/2,但这完全是由于磁场的再分布引起的。这时中空区的总磁通减小,磁通并不守恒,不存在使磁通浓缩的问题,因而用它产生高强脉冲磁场存在着原理性错误。本文指出了把“浓缩器”看作脉冲变压器的错误所在。理论计算还表明用端面为截头圆锥形装置能产生较高场是结构效应所致。 关键词：     

Investigation of superconductivity in a diamond anvil cell by mutual inductance bridge

Abstract

The method for the investigation of the superconductivity under high pressure in a DAC by mutual inductance bridge is proposed. Low frequency magnetic modulation system was used to distinguish signal due to superconducting transition from the temperature dependent background and to increase the sensibility. The sensibility was sufficient to detect the superconductivity of the sample with linear dimensions of 50 microns.     

Analysis of thermal energy harvesting using ferromagnetic materials

This Letter aims at giving a preliminary investigation of the thermal energy harvesting capabilities of a technique using the temperature-dependent permeability of ferromagnetic materials. The principles lie in the modification of the magnetic field caused by the variation of the permeability due to the temperature change, hence generating a voltage across a coil surrounding the circuit. The technique can be made truly passive by the use of magnets for applying bias magnetic field. Theoretical results, validated by experimental measurements, show a voltage output of 1.2 mV at optimal load of 2 Ω under 60 K temperature variation in 5 s (with a maximum slope of 25 K s⁻¹). Further improvements, such as the use of low resistivity coil and magnet with high remnant magnetic field, indicate that it is possible to convert up to 7.35 μJ cm⁻³ K⁻² cycle⁻¹.     

Surface magnetostatic oscillations in elliptical bubble domains

A theory of surface magnetostatic oscillations in magnetic bubble domains with an elliptical cross section is presented. The dependences of the eigenfrequencies of resonant modes on the applied magnetic field are analyzed for a barium hexaferrite sample with allowance made for the change in the domain size due to a variation in the bias magnetic field. The range of frequency tuning in response to a magnetic field ranging from the elliptical instability field to the collapse field is estimated. It is demonstrated that elliptical bubble domains can be used as microminiature resonators operating in the millimeter range.     

核磁共振高信噪比弱信号检测的理论与实验研究

基于小型射频线圈的核磁共振检测探头在波谱分析和成像研究中具有广泛的应用,如化学位移波谱分析、磁共振成像和勘探测井等技术领域。但是,由于外加静磁场作用下,自旋体系发生塞曼能级分裂后,高低能态之间的核自旋数量之差很小,普遍存在检测信噪比很低的问题,而且初级磁共振接收信号的质量受所用探头线圈电气参数的影响较大。研究结果表明,在特定的被测样品和接收线圈占空比以及静磁场等条件不变的情况下,检测信噪比与单位电流产生的射频磁场成正比,而与线圈高频电阻的平方根成反比。在永磁0.39Tesla主磁场条件下,研究了趋肤效应影响下小型螺线管线圈几何参数的优化设计方法。理论仿真和实际的测量结果表明,几何参数为线径0.5 mm、直径5.5 mm的10匝微螺线管线圈,在16.9 MHz谐振频率上,相对信噪比取得一个极大值点,对应的Q值约为199.8,与阻抗分析仪测得结果有较好的吻合,验证了该核磁共振检测线圈设计新方法是合理的。本文提出的基于线圈电磁特性的高信噪比检测探头设计方法,可推广到目前的质子密度成像、岩心弛豫谱分析等应用中。     

感生电场与动生电场的等效性探究

本文认为在感生电场的情况下,磁场的强弱变化可以引起磁场自身的横向运动,使得线圈中电子相对于磁场发生运动,从而等效为一个动生电场,受到洛伦兹力的作用.借助磁感线模拟磁场的运动方式,得到圆形回路中任意一点与磁场相对运动速度的表达式,进而推得该＂等效动生电场＂中的洛伦兹力.以螺线管为例,验证该方法可以解释感生电场所满足的规律.将感生电场与动生电场的产生原因统一为导体中电子与磁场的相对运动,相应电动势的非静电力统一为洛伦兹力.