Distribution of Pinning Current and Irreversibility Properties in Underdoped

                                                   Bi-2212 Single Crystal



             S. Yamaura,1  T. Matsushita,1;2 Y. Nakayama,3  J. Shimoyama,3  and K. Kishio3
                  1Kyushu Institute of Technology, 680-4 Kawazu, Iizuka 820-8502, Japan

                2Kyushu University, 6-10-1 Hakozaki, Higashi-ku Fukuoka 812-8581, Japan

                3The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan



    Contributions to the irreversibility from the bulk and the surface are investigated in detail for

an  underdoped  Bi-2212  single  crystal  using  Campbell's  technique  under  a  magnetic  field  along

the  c-axis  with  superposition  of  small  AC  field  in  the  a-b  plane.   The  contribution  from  the

surface was not found in wide regions of magnetic field and temperature.  It is concluded that the

irreversibility comes from the bulk pinning.  The irreversibility field and the magnetic relaxation

rate are compared with the result of flux creep theory using the bulk pinning property.