Magnetization decay in neutron irradiated MgB2 bulk samples

In this paper the magnetization decay behavior of neutron irradiated MgB2 bulk samples and an estimate of the strength of the introduced pinning centers are presented. The magnetization decay measurements appear much more complex than usual, in fact, they may be logarithmic or not, depending on the values of temperature and magnetic field and, as the region of low temperatures and fields is approached, they are affected by a 'noise' that is out of experimental uncertainties. This 'noisy' behavior is attributed to the small flux jumps, related to the dendritic flux penetration, widely observed in MgB2 samples. We identify a phase diagram that divides the H-T plane in two regions: one, at relatively low temperatures and fields, where the decay observed is noisy, the other, outside it, where the decay is 'clean.' This noisy region extends up to T/T-c congruent to 0.5 and up to a field of some teslas, i.e., in a range of instability much more extended than that found in previous measurements. At high magnetic field the pinning energy turns out to be higher in irradiated samples than in the pristine one, in agreement with the observed increase in the critical current density. However, at high enough fields and temperatures, pinning is no longer effective and, by varying the measurement frequency, differences in the magnetic valuation of critical current density are observed. This fact may be of importance because it occurs at magnetic field and temperature conditions where technological applications of MgB2 are expected. (c) 2008 American Institute of Physics.