Abstract
The third harmonic of the ac susceptibility of MgB2 high pressure (HP) bulk samples have been measured at the frequency of 1070 Hz as a function of the temperature for different dc magnetic fields. The irreversibility line (IL) has been extracted from the temperature onsets. The IL has a (1−t)n dependence, with an “effective power law” exponent n=1.27. This exponent takes into account the interaction between the thermal fluctuations and flux pinning feature. Another IL data of HP samples, drawn out of VSM measurements at few Hertz has similar behaviour. This shows that the superconducting response is independent from frequency in the ranges of the temperature and the magnetic field studied. This analysis supports the fact that when the magnetic field is on and the critical state is formed the frequency creep decay is outside the frequency range analyzed.
Published Version
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