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A simple gradiometer, capable of sensing submicrotesla changes in magnetic field with 10μs resolution, is constructed from a pair of lowcost, wideband Halleffect sensors output to a voltage preamplifier and digital storage oscilloscope. The linear operational range of the sensors ±7.5 mT limits typical applications, such as gradient coil development and testing, to environments where background fields are very low. The gradiometer is ideal for characterizing gradient field rise times and waveforms in this limit and applications pertinent to verylowfield nuclear magnetic resonance and magnetic resonance imaging are presented. © 2010 Wiley Periodicals, Inc. Concepts Magn Reson Part B Magn Reson Engineering 37B: 1–6, 2010

We consider eddy current generation in weakly conducting spheres as a means to characterize RF power dissipation over the frequency range 0.1–1.2 MHz, which is relevant to MRI in very low magnetic fields. We provide a complete, yet simple, theoretical analysis of the homogeneous sphere in a uniform RF field and summarize expressions for the current density, power dissipation, and magnetic field for the limiting case of the nonmagnetic conducting sphere. Experiments are performed on spherical saline phantoms using a whole‐body B1‐coil as the RF field source. Changes in the quality factor and magnetic field profile of the coil are measured and used to extract a value for conductivity of the saline solution. This conductivity is known independently from the NaCl concentration in the phantom. Excellent agreement between the two values confirms the theoretical analysis. © 2007 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 31B: 191–202, 2007

We have used the technique of muon spin relaxation to study the competition between antiferromagnetism (AFM) and superconductivity (SC) in oxygen deficient YBa2Cu3Oxfor 6.0 < x < 6.5. Average Néel temperatures range from 450K for x= 6.0 down to 2.5K for x = 6.40. Samples with x near 6.4 were both SC and AFM. In the x = 6.1 sample, where there is a narrow distribution of TN's, we have observed the critical slowing down of the Cu spins at the average TN.