Unusual noise in the magnetization relaxation in MgB₂ superconductors

Magnetization relaxation studies have been made on MgB₂ powders as a function of magnetic field and temperature. An H-T phase diagram is constructed representing different regimes of flux dynamics. It is found that at high temperature and field the magnetization relaxation, m(t) is logarithmic and follows the Anderson-Kim behavior. At low T (T<10K) and moderate fields (4kOe<H<8kOe), it becomes nonlogarithmic. The most interesting region of flux creep is, however, at low T and low H, where it is observed that the magnetization at fixed field and temperature becomes noisy. The noise levels are much larger than our superconducting quantum interference device resolution.