Field and Temperature Dependent Suspension Studies of High Temperature Superconductors
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FIELD AND TEMPERATURE DEPENDENT SUSPENSION STUDIES OF HIGH TEMPERATURE SUPERCONDUCTORS ,
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Y. HUANG AND S.A. SOLIN *Michigan State University, Center for Fundamental Materials Research and Dept. of Physics, East Lansing MI 48824-1116, Current Address: Argonne National Laboratory, Argonne IL **NEC Research Institute, 4 Independence Way, Princeton, NJ 08540 and Michigan State University, Center for Fundamental Materials Research and Dept. of Physics, East Lansing MI 48824-1116
ABSTRACT We have employed a novel variable-temperature, variable-field/field-gradient cryogenic levitometer, previously used in levitation experiments, to study the suspension properties of the thallium-based high temperature superconductors. Our results are compared and contrasted with the levitation studies and the singular features of the suspension properties are noted. INTRODUCTION One of the most striking demonstrations of the realization of high temperature superconductivity in oxides is the levitation of a small magnet above a superconducting disc held at liquid nitrogen temperature or vice versa (disc levitated above a magnet). Several authors1-3 have explored the qualitative features of the physics of the levitation process at fixed field and fixed temperature but only recently have the temperature and field/fieldgradient dependence of the levitation of high temperature superconductors been reported by Huang and Solin.4,5 In those studies we measured the levitation height as a function of temperature for a series of fixed fields and field gradient and found that the zero-field-cooled data for the 123, Bi-and Tlbased high temperature superconductors exhibited a low temperature slope discontinuity (at -20K, 15K and 25K respectively) and a precipitous cessation of levitation at TL the maximum temperature at which levitation could be sustained for the particular field profile in our apparatus. These results for the 123 and Bi materials were quantitatively explained using a model in which the magnetization is treated as field independent for applied fields > H11'" cl 11 but < z0. z0
Thus since M is negative in the vortex state for the increasing field associated with lowering the sample into levitation from above z 0 , the magnetic force is along the +z direction and levitation is possible. In contrast, because of the hysteresis associated with flux pinning in the specimen, M can be positive under the conditions in which the sample is lowered in to a suspended state in the region below z 0 . The product of the positive gradient and positive magnetization also yields a magnetic force along the +z direction which provides for the suspension of the sample. In our previous studies of the levitation of high temperature superconductors6,7 we showed that the approximate field independence of the magnetization for fields in the range 1 - 10 kG yielded the relation
LS=T2c _ [(8,pgT2)/Hcl(0)][MHH(z)/MzL cc oy
L
(3)
where (0) is an effective critical field associated with the random orienci tation of individual micrograins in a sinter or pre
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