Flux dynamics of MgB[sub 2] superconductor by AC susceptibility measurements.

Flux dynamics of an MgB[sub 2] bulk sample have been studied by systematic measurements of the fundamental and third harmonic ac susceptibility. The irreversibility line T[sub irr](B) has been obtained by fitting the onset temperature T[sub on] (f,B) of the third harmonics at different frequencies to T[sub on](f,B) = C(2πf)[sup 1/[(z-1)ν]] + T[sub irr](B). The flux creep activation energy U(j,B,T) has been derived as U[sub 0][1-(T/T[sub irr])²]²(B/B[sub 0])[sup -n](j/j[sub 0])[sup -μ], with n=1.33 and μ=0.2. The steep decline of the activation energy as a function of the magnetic field U(B) ∝ B[sup -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[sup -4]∼10[sup -1]s, as the frequency is in the range of 10Hz-10kHz, which extends the time window l∼10[sup 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 MgB[sub 2] sample will need to be improved for practical applications. [ABSTRACT FROM AUTHOR]