Scope and limitations of DC and AC magnetization techniques to locate irreversibility line in conventional superconductors

Abstract

In our continuing efforts to look for the behavioural similarities between the specimen 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 χ'_H(T) measured in DC field, and (iv) the peak in the out-of-phase AC susceptibility χ'_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 χ'_H data. The identification of peak temperature in χ'_H(T) data with T_r(H) value is appropriate only if the contribution from the normal state electrodynamics can be isolated and the peaks in χ'_H(T) is narrow. The observation of DPE in χ'_H(T) data is 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.