Flux dynamics of MgB2 superconductor by AC susceptibility measurements

Flux dynamics of an MgB2 bulk sample have been studied by systematic measurements of the fundamental and third harmonic ac susceptibility. The irreversibility line T-irr(B) has been obtained by fitting the onset temperature T-on(f,B) of the third harmonics at different frequencies to T-on(f,B) = C(2pif)(1/[(z-1)v]) + T-irr(B). The flux creep activation energy U(j,B,T) has been derived as U-0[1-(T/T-irr)(2)](2) (B/B-0)(-n) (j/j(0))(-mu), with n = 1.33 and mu = 0.2. The steep decline of the activation energy as a function of the magnetic field U(B) proportional to B-1.33 leads to the steep drop in the critical current density with magnetic field. The relaxation of the current density j(t) of the sample has been detected in a time window of 10(-4)similar to10(-1)s, as the frequency is in the range of 10Hz-10kHz, which extends the time window 1similar to10(4)s of the conventional dc magnetic relaxation to a much smaller one. The obtained activation energy and the irreversibility line indicate that the pinning properties of the MgB2 sample will need to be improved for practical applications.