SCOPE AND LIMITATIONS OF DC AND AC MAGNETIZATION TECHNIQUES TO LOCATE IRREVERSIBILITY LINE IN CONVENTIONAL SUPERCONDUCTORS

In our continuing efforts to look for the behavioural similarities between the specimens of high-temperature and conventional superconductors, we have searched for the irreversibility (T(r), H(r)) line via DC and AC magnetization techniques in a few specimens of lead (type-I) and niobium (type-II). A comparison has been made of T(r) and H(r) values determined by four different methods, viz., (i) the merger of zero-field-cooled and field-cooled magnetization curves, (ii) the vanishing of hysteresis in isothermal DC magnetization, (iii) the appearance of a differential paramagnetic effect (DPE) in the in-phase AC susceptibility chi'-H(T) measured in DC field, and (iv) the peak in the out-of-phase AC susceptibility chi"-H(T). The first method gives a lower limit of T(r)(H) values whereas the second method appears to give H(r)(T) values smaller than those evident from isothermal chi'-H data. The identification of peak temperature in chi"-H(T) data with T(r)(H) value is appropriate only if the contribution from the normal state electrodynamics can be isolated and the peak in chi"-H(T) is narrow. The observation of DPE in chi'-H(T) data is a qualitative feature whose presence is adequate to imply reversibility in magnetization response. However, its efficacy to locate very precisely the T(r) and H(r) values remains to be established.