A technique for studying magnetic field induced structural changes at the atomic resolution is described. The instrument involves the coupling of a high resolution laboratory x-ray powder diffractometer, a helium flow cryostat, and a split-coil superconducting magnet allowing for in situ structural studies in a magnetic field between 0 to 35 kOe, and a temperature between 2.2 to 315 K. The data collected from a copper sample, which is used as a standard, at temperatures down to 4.3 K and in fields up to 10 kOe are presented. The ability to image massive magnetic field induced structural transformations is demonstrated utilizing powder diffraction data of Gd5Ge4 collected under both isothermal and isofield conditions at various temperatures below 15 K and magnetic fields up to 35 kOe. These results show the utility of our approach to obtain high precision structural information in the presence of a strong magnetic field.
We have measured the specific heat of Allen-Bradley glassy carbon (heattreated at 2500~ in the temperature range 1.2 < T <_ 20 K. The material was further characterized by x-ray, de electrical conductivity, magnetoresistance, and Hall effect measurements. The specific heat results are compared to those for natural graphite and other disordered carbons. Below T = 2.2 K the glassy carbon data can be represented by the equation C v = (0.054 mJ/mole-KZ)T + (0.101 mJ/mole-K4)T 3. The lattice specific heat was found to be larger than that of natural graphite at all temperatures between 1 and 20 K. We attribute this enhancement primarily to a much reduced value ofC3a, the elastic constant characterizing compressive displacements between graphite-like planes.
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