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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).