Third harmonic ac susceptibility measurements in MgB2 bulk: frequency behavior of IL and 3D glass pinning analysis

The third harmonics of the ac susceptibility χ 3 of MgB 2 bulk samples have been measured in the frequency range 61–1070 Hz as a function of the temperature for different applied dc magnetic fields. The irreversibility line ( T irr versus H dc ) deduced from the temperature onsets of χ 3 for each field shows a frequency dependence: The onset temperature increases as the frequency increases. This frequency dependence has been analyzed using the 3D vortex-glass model. Moreover the real versus imaginary components of the χ 3 for each dc magnetic field have been interpreted with a 3D glass creep using a numerical analysis based on a non-linear diffusion equation of the magnetic field.