Flux trapping and its effect on AC susceptibility of granular superconductors

Experimental data on the real (x′) and imaginary (x″) parts of AC magnetic susceptibility as a function of DC bias field (H) shows the effect of trapped flux in granular highT _c superconductors. The aim was to substantiate our recent theoretical findings on the basis of a two-component critical state model suitable for granular highT _c superconductors. Stress has been given to understanding the origin of hysteresis inx′(H) andx″(H). It was seen in the experimental data that above a certain value of DC field range irreversibility appears inx′(H) andx″(H) creating hysteresis like loops. Comparison of data with calculated loops shows good agreement.x′(H) andx″(H) curves show considerable asymmetry in presence of trapped flux.     

On the variation of a.c. susceptibility with temperature for some Cu-Li ferrites

The a.c. susceptibility vs. temperature of the system (Li_0·5Fe_0·5)_1−x Cu _x Fe₂O₄, for 0⩽x⩽1·0 has been studied. While pure CuFe₂O₄ (x=1) shows a dramatic peak in susceptibility just before its Curie temperature (T _c), the same is supressed with the addition of a small amount of Li to the system. The variation of lattice parameter with Li content has also been investigated. Samples up tox=0·4 show tetragonal structure while those withx>0·4 are cubic. This change of structure can be explained on the basis of reduction ofdsp ² bonding of Cu ions on B-sites of the spinel lattice.     

Al-Si合金熔体中磁性杂质颗粒的电磁分离

分析了处于交变磁场中的磁性杂质颗粒的受力情况，比较了由交变磁场和稳恒磁场产生的磁场力。结果表明这两种力的本质相同，可以统一起来，而且磁性杂质颗粒在其驱动下的运动规律也相同。Al-Si合金中的富铁相杂质是弱磁性，电磁力和磁场力都可以促使其运动，电磁分离试验结果表明，当磁感应强度大于145mT时，电磁力起主要作用，杂质颗粒向分离器内侧运动；当磁感应强度小于120mT时，磁场力起主要作用，杂质颗粒向分离器侧面运动，据此可以指导分离器的结构设计。     

Optimized EPI for fMRI studies of the orbitofrontal cortex: compensation of susceptibility-induced gradients in the readout direction

Object Most functional magnetic resonance imaging (fMRI) studies record the blood oxygen leveldependent (BOLD) signal using gradient-echo echo-planar imaging (GE EPI). EPI can suffer from substantial BOLD sensitivity loss caused by magnetic field inhomogeneities. Here, BOLD sensitivity losses due to susceptibility- induced gradients in the readout (RO) direction are characterized and a compensation approach is developed. Materials and Methods Based on a theory describing the dropout mechanism, an EPI sequence was optimized for maximal BOLD sensitivity in the orbitofrontal cortex (OFC) using a specific combination of an increased spatial resolution in the RO direction and a reduced echo time. Using measured BOLD sensitivity maps and a breath hold experiment, the model and compensation approach were tested. Results Using typical fMRI EPI parameters, susceptibility-induced gradients in the RO direction caused dropouts in the OFC and the inferior temporal lobe. Optimizing the echo time and spatial resolution effectively reduced the dropout as predicted by the theory. Conclusion The model-based compensation approach effectively reduces BOLD sensitivity losses due to susceptibility-induced gradients in the RO direction. It retains the high temporal resolution of single-shot EPI and can be readily combined with methods for the compensation of susceptibility-induced field gradients in the phase-encoding and through-plane direction.