Generation of cylindrically symmetric magnetic fields with permanent magnets and µ-metal

A method that allows the calculation of magnetic fields produced by cylindrically symmetric configurations of permanent magnets and high permeability materials is presented. The method is based on a non-iterative finite-element algorithm and can be utilized on small-scale computing facilities. As an example, the design of a magnetic trap for neutral atoms is discussed. Comparisons of calculations with analytical and experimental data are also reported.     

Analytic construction of magnetic multipoles from cylindric permanent magnets

We discuss an analytic method for the design of three-dimensional magnetic multipoles from permanent magnet materials. The concept is explicited with an idealized, continuously varying magnetization. The effect of segmentation for realistic implementations is discussed. As an example we present an open, experimentally accessible cylindric structure for a dipole and a quadrupole field with high purity. The fields are useful over several cm³.     

A study on the order–disorder phase transformations of rapidly solidified Sm–Co-based permanent magnets

The structural transformation from the metastable, disordered TbCu₇-type SmCo₇ structure to the equilibrium, ordered Th₂Zn₁₇-type Sm₂Co₁₇ structure was revealed by X-ray diffraction analysis using Rietveld refinement. The magnetic properties depended strongly on the stage of the transformation, as the coercivity depended on the annealing temperature. The as-solidified alloys in the TbCu₇-type structure exhibited coercivity as high as 7.85 kOe, which increased to greater than 9 kOe after heat treatment. The magnetization processes were also strongly influenced by the structural state. Initially it was dominated by nucleation processes, which gave way to domain wall pinning-controlled magnetization processes with the development of the Sm₂Co₁₇ structure. Transmission electron microscopy revealed the development of antiphase domains during heat treatment, which apparently served as the domain-wall pinning sites during magnetization reversal.     

Optimisation of bit patterned media for 1 Tb/in

Micromagnetic simulations were used to investigate the influence of patterned media geometry on the signal to noise ratio (SNR), adjacent track erasure and write margin for a target recording density of 1 Tb/in². For an ideal patterned medium the readback noise was a maximum when the read head was directly over the dots and a minimum at the transitions. The SNR improved for smaller dots due to the larger dot separation. However, the ideal media with the highest SNR were also the most susceptible to dispersions of dot size and position. Low temperature simulations suggest that large write margins are available; however, at room temperature the write margin can be much reduced. Increasing the rise time of the write head had a deleterious effect on the write margin and the write margin was zero for rise times of more than 0.45 ns. Nevertheless, error-free writing at 1 Tb/in² could be achieved using appropriate head geometries and material parameters.     

Optimization of pick-up coils for weakly damped SQUID gradiometers

The performance of a superconducting quantum interference device(SQUID) gradiometer is always determined by its pick-up coil geometry, such as baseline and radius. In this paper, based on the expressions for the coupled flux threading a magnetometer obtained by Wikswo, we studied how the gradiometer performance parameters, including the current dipole sensitivity, spatial resolution and signal-to-noise ratio(SNR), are affected by its pick-up coil via Mat Lab simulation.Depending on the simulation results, the optimal pick-up coil design region for a certain gradiometer can be obtained.To verify the simulation results, we designed and fabricated several first-order gradiometers based on the weakly damped SQUID with different pick-up coils by applying superconducting connection. The experimental measurements were conducted on a simple current dipole in a magnetically shielding room. The measurement results are well in coincidence with the simulation ones, indicating that the simulation model is useful in specific pick-up coil design.     

High-field NMR using resistive and hybrid magnets

Resistive and resistive-superconducting hybrid magnets can generate dc magnetic fields much higher than conventional superconducting NMR magnets but the field spatial homogeneity and temporal stability are usually not sufficient for high-resolution NMR experiments. Hardware and technique development addressing these issues are presented for high-resolution NMR at magnetic fields up to 40T. Passive ferromagnetic shimming and magic-angle spinning are used effectively to reduce the broadening from inhomogeneous magnetic field. A phase correction technique based on simultaneous heteronuclear detection is developed to compensate magnetic field fluctuations to achieve high spectral resolution.     

Quality of the simulation of torus conditions with different three coil arrangements

The displacement and stress distributions in toroidally arranged circular superconducting coils producing the toroidal field for fusion experiments are computed by a finite element method. The results are compared with the displacement and stress distributions in one superconducting coil which forms part of different arrangements simulating torus conditions. Four arrangements with three coils are discussed; three of them consist of one superconducting coil with an inner coil winding diameter of 0.94 m and a maximum stored magnetic energy of 2.1 MJ, and two copper coils which were designed for experimental tests. The fourth 3-coil arrangement consists of three identical superconducting coils forming a sector of the torus.     

Engineering issue study of triple harmonic method for in situ flying height analysis

In situ flying height testing technology is becoming more and more important in slider–disk interaction analysis and manufacturing quality control of disk drives and head-related components. Triple harmonic method is a quite promising choice for in situ flying height analysis, compared with other in situ methods reported up to now. This paper reports results of investigations on engineering issues of applying triple harmonic method for in situ flying height analysis. The paper reports results of analysis on the effects of various testing conditions on flying height testing repeatability and accuracy. Results suggest that working at reasonable high channel density and working on the ratio between third and first harmonics will be an advantage in terms of both flying height testing sensitivity and testing repeatability. Comparing with media thickness effect, the gap-length variation among different heads will be important if it is to study flying height difference among different heads and the testing is at high channel density. Also, it is suggested to work at AC erased track, in order to reduce the non-linearity caused by hard transition.     

Radio frequency characteristics of Fe-filled carbon nanotube film

The conduction noise suppression in radio frequency region using film type of the Fe-filled carbon nanotubes and its epoxy composite was evaluated on a coplanar waveguide. Fe in carbon nanotubes have shown α-Fe crystalline structure and had a coercivity of 650 Oe. The magnitudes of the signal attenuation of Fe-filled carbon nanotubes on coplanar waveguide were shown in the range of about 10–18 dB/cm at 20 GHz (the stop-band frequency region). The power losses of these films exhibited 65–85% at 20 GHz in the stop-band frequency.     

超导磁体气冷电流引线优化设计

提出了一种较为准确的计算超导磁体气冷电流引线最优长横比与电流引线末端流向低温容器热量的方法。根据电流引线的热平衡方程,利用程序设计,采用反复迭代、逐步逼近的方法,并以无氧铜材料为例,计算了10kA电流引线的长横比与流入低温容器的最小漏热。     

Read channel simulation based on microtrack and micromagnetic modeling for 1 Tb/in magnetic recording

Channel design for ultra-high-density perpendicular magnetic recording requires fast and precise modeling to generate readback signals corrupted by media noise. In this paper, we present a geometric-dependent approach to model random readback pulses, where a three-dimensional head and media combination for 1 Tb/in² density design is considered in the micromagnetic simulation. A systematic approach is developed to extract media noise statistics from micromagnetic modeling and generate readback pulses based on a fast microtrack model incorporating nonlinear effects. The feasibility of the proposed model is demonstrated through bit-error-rate (BER) simulation of a turbo equalization scheme over a low-density parity-check (LDPC) coded, general-partial-response (GPR) equalized perpendicular recording channel.     

Complex impedance spectra of chip inductor using Li–Zn–Cu–Mn ferrite

A multi-layer chip inductor (MCI) was fabricated using polycrystalline Li–Zn–Cu–Mn ferrite and the green-sheet technique, and its complex impedance spectrum was evaluated with the help of numerical calculations. The complex impedance spectra of the MCI component using Ni–Zn–Cu ferrite, which have been widely used for this application, were very sensitive to the residual stress and deviated much from the calculated values; however, it was found that the complex impedance spectrum of the MCI component using Li–Zn–Cu–Mn ferrite is quite well reproduced by calculation, where the complex permittivity and permeability of the polycrystalline ferrite as well as the MCI dimensions, were used. It implied that the magneto-striction effect was negligible in case of MCI using Li–Zn–Cu–Mn ferrite, and that the difference was related to magneto-strictive coefficient of the polycrystalline ferrite. Consequently, utilization of Li–Zn–Cu–Mn ferrite enabled us to easily design the complex impedance of MCI component.     

Simulations of continuous/cluster-pinned recording media

Continuous/cluster-pinned recording media, consisting of cuboid clusters of various sizes exchange-coupled to a continuous hard layer, were modelled to investigate their suitability for high density data storage. The pinning field due to the clusters was determined by modelling domain-wall motion in the continuous layer. Larger clusters, a thinner continuous layer and increased saturation magnetisation of the clusters all increased the pinning field. Simulations of recording demonstrated the feasibility of using domain-wall pinning to control the written bit size in continuous media.     

Dependence of error patterns and BER performance on write pre-compensation in GPRML channels

Nonlinear transition shift (NLTS) influences the performance of PRML channels in perpendicular magnetic recording. For NLTS, it is thought that write pre-compensation is useful to write the transition at the intended position. In this paper, we study the influence of NLTS on the BER performance of GPRML channels and show the appropriate amount of pre-compensation for GPRML channels both with and without a post-processor (PP) to achieve better BER performance. The results show that the widening the shortest bit length by NLTS brings the BER improvement to GPRML channel and the write pre-compensation is useful for a GPRML channel with PP.     

On the contactless suspension of objects by electric and magnetic fields

In connection with the contactless suspension of objects by electric and magnetic fields, an electrodynamical version of Kapitza's theorem on the stabilizing influence of a rapidly oscillating field is deduced.     

Performance of GPRML-AR system in perpendicular magnetic recording channel with thermal decay

The bit-error rate (BER) performance of the generalized partial response maximum likelihood with autoregressive (GPRML-AR) channel model system in perpendicular magnetic recording (PMR) channel with thermal decay is obtained. The 128/130(0,16/8) run-length-limited (RLL) code is used as a recording code. The GPR channel consists of the PR1 channel followed by the reduction circuit of predicted noise. The BER performance is evaluated by computer simulation using a thermal decay model. The model has been obtained by using an approximate equation that represents amplitude degradation of the reproducing waveform with elapsed time based on the experimental data for CoPtCr-SiO₂ PMR media. The Viterbi detector with an AR channel model is employed. Furthermore, long-term degradation of the required SNR to achieve a BER of 10⁻⁴ with elapsed time is obtained and the performance is compared with that of PR1ML system. The results show that the poorer the thermal stability of the medium becomes, the larger the SNR gain of the GPR1ML-AR system over the PR1ML system becomes. The SNR gain also increases with elapsed time.     

Analytical characterization of detection SNR for PR target design in Viterbi-based receivers

In this paper, we address the problem of designing the optimum partial-response (PR) target that results in the best bit-error-rate (BER) performance in Viterbi-like detectors when applied for signal detection in digital magnetic recording systems. We present a detailed analysis of the effective detection signal-to-noise ratio (SNR_eff), which is closely related to the BER performance of a Viterbi detector. We show that SNR_eff is a concave function with a unique global maximum corresponding to the magnitude frequency response of the optimum targets. Thus, any simple search approach is guaranteed to reach the optimum target. We consider the cases of single dominant error event and multiple dominant error events. Numerical and simulation results are presented to corroborate our analytical results for perpendicular magnetic recording channels.     

Nuclear magnetic resonance measurement station in SECUF using hybrid superconducting magnets

Nuclear magnetic resonance(NMR) is one of the most powerful tools to explore new quantum states of condensed matter induced by high magnetic fields at a microscopic level. High magnetic field enhances the intensity of the NMR signal, and more importantly, can induce novel phenomena. In this article, examples are given on the field-induced charge density wave(CDW) in high-Tc superconductors and on the studies of quantum spin liquids. We provide a brief introduction to the high magnetic field NMR platform, the station 4 of the Synergetic Extreme Condition User Facility(SECUF), being built at Huairou, Beijing.     

Micromagnetic modeling for heat-assisted magnetic recording

Heat-assisted magnetic recording (HAMR) is one of the candidate systems beyond the perpendicular recording technology. Here, a micromagnetic model and a heat transfer model are introduced to study the heating and cooling processes in the HAMR media; then, by integration of the SPT head and the laser heating source, the recording performance is simulated and investigated on a single track at an area density of 1 Tb/in². In the HAMR system, the temperature in the medium under the laser wave guide is increased by heating, and decreased by air bearing and heat conduction when the write process really occurred. The target of this study is to find the proper design of the head-laser assembly for optimum recording. It is found that the proper distance between the laser wave guide and the head's main pole rear/front edge is only 41.4/1.4 nm for optimum recording performance.     

用模拟退火法进行纯永磁波荡器磁块组合优化

纯永磁波荡器由多个磁块组成,磁块的剩磁离散性会引起波荡器磁场误差,从而影响储存环工作状态和自发辐射谱质量。在波荡器磁块安装之前,使用模拟退火法对磁块进行组合排序优化,可以使峰值场强误差降低到10-4量级以下,磁场一次积分降低到10-6 T·m量级,二次积分降低到10-6 T·m2量级,优化结果不依赖于初始状态的选择。给出优化的详细过程,提出了根据磁块剩磁快速计算波荡器峰值场强误差和积分场的方法。