Temperature and magnetic field dependence of the n-values of MgB2 superconductors

4 pages, 5 figures, 1 table.-- Trabajo presentado al: "Applied Superconductivity Conference" celebrado en Seattle(USA) el 27/08/2006.-- et al.We analyse the temperature and magnetic field dependence of the power-law n-values characterizing the electric field-current density (E-J) curves of MgB2 superconducting samples. This is done by means of magnetic relaxation measurements, using the normalized relaxation rate of irreversible magnetization Mirr, S = -dlnMirr/dlnt, which directly relates to the E-J curve E infin Jn with the n-exponent approximately equal to 1 + 1/S. The study has been done at temperatures and fields ranging from (T = 5 K, B = 1 T) and up to (T, B) near the irreversibility line. Different samples such as undoped and SiC-doped bulk samples and wires have been analysed. We have observed a strong decrease in the n-values when increasing temperature and magnetic field with some differences among the analysed samples. We found correlation between critical current density (Jc) and n-values. The Jc(T, B) and n(T, B) dependences show striking similarities and the high n-values (n > 30) are always related to the high critical current densities (Jc > 108 A/m2).This work was supported by the EU FP6 project NMP3-CT2004-505724 and by the Spanish CICYT Project: MAT2005-06279-C03-01.Peer reviewe