The third harmonic components of the alternating-current susceptibility of MgB2 bulk samples were measured as a function of applied magnetic fields, together with standard magnetization cycles. The irreversibility line (IL) of the magnetic field was extracted from the onset of the third harmonic components. Using a (1-t)α glass/liquid best fit where α = 1.27 IL shows a coherent length ξ divergence with exponent ν = 0.63, which indicates a 3D behavior. Moreover, using the numerical solution of the non-linear magnetic diffusion equation, considering the creep model in a 3D vortex glass, a good description of the vortex dynamics was obtained. The behavior of the magnetization amplitude (~Hz) and the alternating-current susceptibility signals (kHz), at different applied magnetic fields, 3.5T < HdC < 4.5T, and at reduced temperatures of 0.86 to 0.93 (T = 22K), shows that the superconducting dynamic response of vortices in the MgB2 samples was not evidently dependent on the frequency.

Irreversibility Line and Magnetic Field Dependence of the Critical Current in Superconducting MgB2 Bulk Samples. D.Di Gioacchino, U.Gambardella, P.Tripodi, G.Grimaldi: Superconductor Science and Technology, 2003, 16, 534-7