The results of x-ray diffraction, electrical resistance, DC and AC magnetization studies on two specimens of compositions Y_2.1Ba_3.9Cu₆O_14−δ and Y_1.8Ba_4.2Cu₆O_14−δ are presented. Our studies confirm that La₃Ba₃Cu₆O₁₄ type orthorhombic phase is responsible for superconductivity in the 90 K range. The heat treatment that yields high quality samples with sharp transition in electrical resistance as well as in AC magnetic susceptibility measurements is described. Magnetic response just belowT_c is found to be sensitive to the measuring field.

The existence of a remanent magnetization (M_rem) on switching off the field of a field cooled (FC) sample of a highT_c superconductor is often reported. It has recently been argued thatM_rem should equal the difference in FC and zero field cooled (ZFC) magnetizations (M_FC —M_ZFC) in hard superconductors and this has been demonstrated to hold in single crystals of YBCO at 4.2K over a limited range ofH values. We report the detailed magnetization measurements under various thermomagnetic histories (of whichM_rem is one special case) on two specimens of Nb, which show different extents of flux trapping. We find that there are in general three regions inH, T space, corresponding toM_rem+M_ZFC−M_FC=0,M_rem<(M_FC−M_ZFC) andM_rem>(M_FC−M_ZFC). At anyT, the equality holds forH<H_c1(T), and forH→H_c2 (M_FC−M_ZFC) asymptotically vanishes and thereM_rem>(M_FC−M_ZFC). We show that there exists an intermediate region in all hard superconductors, whereM_rem<(M_FC−M_ZFC). The range over which this situation persists, however, depends on the degree of irreversibility in a sample. We can explain qualitatively all the history dependent magnetization data in terms of the critical state model. We point out an inconsistency in an earlier analysis to determineH_c1(T) from such data in YBCO. We also propose a new criterion for putting limits onH_c1(T) in hard superconductors.

A comparison has been made of irreversibility temperature determined by four different methods in few specimens of lead (type-I) and niobium (type-II). The merger ofM_ZFC(T) andM_FC(T) curves giveT_r(H) values lower than those evident from vanishing the hysteresis in isothermal DC magnetization. The identification of peak temperature inx″_H(T) data withT_r(H) is appropriate only if the contribution from changes in the normal state electrodynamics can be isolated and the peak is narrow. The appearance of differential paramagnetic effect inx′_H(T) data is adequate to imply reversibility, however, its efficacy to precisely locate irreversibility line remains to be established.

The results of experimental studies on hysteresis in magnetization, thermomagnetic history effects, anomalous variations in magnetic hysteresis curves and the decay rates of magnetization obtained under different thermomagnetic histories in specimens of conventional and high temperature superconductors are presented. The Bean’s critical state model is considered adequate to explain magnetic behaviour in conventional hard superconductors. The similarity in the general features of the results of different experiments on specimens of the two families of superconductors underscores the efficacy of the said model to understand some aspects of the macroscopic magnetic response of high temperature superconductors as well. For instance, the isothermal magnetization hysteresis loop which comprises of magnetization curves along forward (−H_max to +H_max) and reverse (+H_max to −H_max) paths define an envelop within which all isothermal magnetization data along different thermomagnetic histories lie. There exist inequality relationship between various field values identified asH_peak,H_I,H_II etc. in isothermal magnetization hysteresis as well as magnetic relaxation data. The entire field span of an isothermal magnetization hysteresis data set can be considered to comprise of three parts corresponding to (M_rem(H)−M_FC(H)+M_ZFC(H)) being equal to, less than or greater than zero, whereM_rem(H) are the remanent magnetization values obtained on reducing field to zero after having the specimen in different applied field (H) values. There are, however some situations amongst thermomagnetic history effects in specimens which show incomplete flux trapping on field cooling, where the critical state model has been found inadequate.