Rigid vortices in MgB2

Magnetic relaxation of high-pressure synthesized MgB2 bulks with different thickness is investigated. It is found that the superconducting diamagnetic moment depends on time in a logarithmic way: the flux-creep activation energy decreases linearly with the current density (as expected by Kim-Anderson model) and the activation energy increases linearly with the thickness of sample when it is thinner than about 1 mm. These features suggest that the vortices in the MgB2 are rather rigid, and the pinning and creep can be well described by Kim-Anderson model. (C) 2003 American Institute of Physics.