Paramagnetic Meissner effect in high Tc superconductors （I） --Flux trapping in small particles and typical behavior of paramagnetic moments

Theoretical and experimental results show that the state of minimum Ginsburg Landau free energy of small superconducting particles contains no flux line if the applied field is smaller than its critical field. For such micron sized particles, after being cooled in a small magnetic field (e.g. less than 100 μT), the moment remaining during subsequent zero field warming contains almost no contribution from trapped flux and is dominated entirely by the paramagnetic Meissner effect (PME) of the particles. A systematic study of such a moment has been carried out to reveal its characteristic behavior of temperature, magnetic field and cooling or warming rate dependence. Methods for removing and recovering the PME of small superconducting particles are also reported.