How do we find out the irreversible field from M-H loop, how is it different from Hc2(0) obtained from rho vs T plot?
The irreversibility field line, which is only present in type II superconductors, does not correspond to the transition from normal to superconducting state and is not a phase transition line.
It corresponds to a transformation of the vortex lattice. At a given temperature for fields below Hirr the vortex are pinned and above moves freely (“the vortex lattice melts”). Consequently there is not hysteresis loop in the magnetization and it appears electrical resistivity both types of measurements have been used to locate this line, but it depend on the involved times rates.
In field cooling (FC) and zero field cooling experiments (ZFC) a point of the irreversible line would be located where both magnetization versus temperature curves merge.
The irreversibility line separates a vortex liquid region from a region where the vortex are pinned. The Irreversibility lines can be obtained both from magnetization and resistivity measurements as a function of temperature for different applied magnetic fields in FC and ZFC regime.
The experimental data (Tirr(H)) vs. T usually fit well to the de Almeida-Thoules power law characteristic of grain coupled systems.
Some models asume the irreversibility line as a phase transition but it is not considered as a phase transition from normal state to superconducting state
Irreversibility field is a field at which the hysterisis curve do not follow its reversible trend. This is because at a certain temperature, beyond a certain field, the pinned vortex starts moving and MH loop no longer behaves in a reversible way. MH loops, MT ZFC FC measurements can be used to determine the Hirr. Hirr and Hc2 can be determined also by the Temperature Vs Resistivity plot using 10% and 90% of transition point temperature criteria. Hirr as previous intellectuals have said gives the field and temperature at which the superconducting vortex starts melting or moving.
The irreversibility line is resulted from the short coherence length, large anisotropy and high working temperature of high-Tc oxides. Taking the values of the temperature-dependent resistivity under magnetic fields at 10 % and 90 % of the normal state resistivity (ρ0) , we can found μ0Hirr(T ) and μ0Hc2(T ). The irreversibility field is determined (i) from M-vs.-H curves using the onset of irreversibility and (ii) from the peak value of the imaginary part of the ac susceptibility measured with an ac field.
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