Significant enhancement of irreversibility field in clean-limit bulk MgB2

Low resistivity MgB2 bulk samples annealed in Mg vapor show an increase in irreversibility field mu(0)H(*)(T) by a factor of similar to2 in both transport and magnetic measurements. The best sample displayed a magnetic irreversibility field mu(0)H(M)(*)>14 T at 4.2 K and similar to6 T at 20 K. These changes were accompanied by an increase of the 40 K resistivity from 1 to 18 muOmega cm and a lowering of the resistivity ratio from 15 to 3, while the critical temperature T-c decreased by only 1-2 K. These results show that systematic processing changes can make MgB2 attractive for magnet applications.