A.K. Pradhan

Interesting magnetic properties of UCu2Si2

Abstract We report the results of magnetization and magnetic relaxation studies on the compound UCu 2 Si 2 with ThCr 2 Si 2 structure, highlighting the metastable magnetic character of this compound. Our study indicates the important role of a temperature dependent anisotropy in this system.

Improved thermogravimetric analysis and crystal growth of BiSrCaCuO

Superconducting crystals of Bi Sr Ca Cu O with and without Pb or Y dopant have been grown in platinum or alumina crucibles from KCl or self-flux. The improved thermogravimetric analysis (ITGA) enables the crystallization temperature to be measured and then the crystals to be grown by introducing a Pt wire as a nucleation centre or by top-seeding. At the end of crystal growth the crystals could be separated from the flux. Differential thermal analysis (DTA) is useful for getting some information to design the temperature program. The crystals of Bi Sr Ca Cu O, Bi(Pb) Sr Ca Cu O and Bi Sr Ca(Y) Cu O were identified as mainly 2212 phase with the two Cu O layer structure by X-ray diffraction and by electron probe microanalysis (EPMA). The as-grown crystals are superconducting with transition temperatures between 80 and 90 K.

The phase diagram and crystal growth of Y1xPrxBa2Cu3O7-y

Y 1 x Pr x )Ba 2 Cu 3 O 7- y is a unique superconductive material for the investigation of the interrelation between crystal structure, chemical composition and physical properties. In order to grow large crystals of high quality, an extensive study has been made of the Y 2O 3 -Pr 6 O I1 -BaO-CuO growth system. The eutectic composition of the BaO-CuO flux is confirmed by the improved thermogravimetric method (ITGA) as occurring at the molar composition of 7:18. The pseudo-binary phase diagram of (Y, Pr)BCO-(7BaO+18CuO) has been determined by means of differential thermal analysis (DTA), by X-ray diffraction and by study of crystal growth. Using the phase diagram, suitable compositions and temperature programs have been established for the growth of large crystals and for the separation of the crystals from the flux. Single crystals with a wide range of Pr doping have been obtained from a eutectic BaO-CuO flux. The dependence of the Pr concentration in the crystals on the composition of the initial melt has been determined by electron probe microanalysis and X-ray microanalysis. The superconducting transition temperature is found to decrease monotonically with increasing Pr concentration.

Peak effect and magnetization minima in single crystals

We report the effects of annealing at 400°C in oxygen and also in argon on the double-peak magnetization anomaly in as-grown BSCCO crystal. The diffusion or injection of oxygen does not change the peak structure. Argon annealing produces an increase in the pinning and the irreversibility field. The double-peak anomaly in BSCCO crystal that disappears after heavy-ion irradiation cannot be reconstructed on oxygen annealing. The effects of annealing on the irradiated crystal have been studied in detail. The origin of the magnetization minimum observed in the field-cooled magnetization is not clear and cannot be explained using the existing models.

Flux pinning by columnar defects in Bi2Sr2CaCu2O8+Y single crystals

Abstract We report the magnetization studies of as-grown and 200 MeV Ag-ion irradiated BSCCO single crystals. The maximum shift of the irreversibility line was observed for fluence level of 2.5 × 10 11 ions/cm 2 with a corresponding B Φ = 5 T. Ag irradiation induces damage in the form of amorphous columnar tracks which penetrate throughout the thickness of the crystal. These defects are not only responsible to shift the irreversibility line but also make the dislocation networks present in the CuO 2 plane ineffective and hence the disappearance of the double-peak anomaly generally observed in the as-grown crystal. The angle dependence of the irradiated BSCCO crystal predicts the presence of unidirectional pinning and interlayer Josephson coupling. The 2D pancake vortices are not completely decoupled even up to T = 50 K. The magnetic relaxation in BSCCO crystals predicts the enhanced pinning upon irradiation and the line behaviour of flux-lattice which can exist up to high temperature. The pinning energies calculated using predicted Anderson-Kim model shows enhanced pinning energy upon irradiation. The columnar defects by heavy-ion irradiation have been proved to be an effective tool for enhancement of flux-pinning. Even after heavy-ion irradiation 2D nature of BSCCO crystal is preserved.

Magnetic anomaly in Y1−xPrxBa2Cu3O7−y

Abstract Measurements of isothermal magnetization curves have been performed on the Y 1− x Pr x Ba 2 Cu 3 O 7− y system for 0≤ x ≤0.37 at various temperatures. The measurements reveal that the anomalous double peak structure emerges with increasing Pr concentrations ( x ) and becomes maximum for x =0.2 and then decreases. The magnetic relaxation experiments done at 20 K indicate that pinning energy behaves in a similar flux pinning and can be interpreted in the framework of the suppression of superconducting order parameters in the vicinity of Pr ions due to magnetic pair-breaking.

Observation of negative magnetoresistance in single crystal Y1−xPrxBa2Cu3O7−y

Abstract The negative magnetoresistance was observed in flux-grown single crystal Y 0.70 Pr 0.30 Ba 2 Cu 3 O 7−y with magnetic field applied either perpendicular or parallel to the ab plane up to 15T. The magnetoresistance was saturated at 8T after which it started increasing. The observed negative magnetoresistance may be due to the 2-D weak localization of charge carriers.

Magnetoresistance and upper critical fields in single crystal Y1−xPrxBa2Cu3O7−y

Abstract The magnetoresistance and upper critical fields of superconducting single crystal Y 1− x Pr x Ba 2 Cu 3 O 7− y alloy system with varying Pr-content have been measured. The crystals showed a remarkable narrowing of superconducting width and positive curvature in upper critical fields. The results were interpreted in terms of the negative exchange interaction between the Pr 4ƒ electrons and superconducting charge carriers.

Effect of sample shape on the low-field magnetisation peak in a C15 Laves phase superconductor

A low-field peak is observed in the magnetisation hysteresis curve of various hard type-II superconductors. We study the effect of sample shape on the position of this peak in a polycrystalline sample of a C15 Laves phase superconductor. The peak shifts towards zero field as the demagnetisation factor increases. This is discussed in terms of recent calculations, and also with reference to similar results in YBa 2 Cu 3 O 7 .

Effects of heavy-ion irradiation on the magnetic anomaly in Bi2Sr2CaCu2O8+y crystal near the transition temperature

Abstract We report the observation of an anomalous magnetization behavior in single crystal of Bi 2 Sr 2 CaCu 2 O 8+ y near the transition temperature after 200 MeV Ag ion irradiation with a fluence of 2.5 × 10 11 ions/cm 2 . We argue that the magnetization anomaly (the virgin curve lying above the return leg and the presence of a characteristic crossover temperature, T ∗ ) is the manifestation of fluctuation phenomena as were observed in as-grown crystal. The columnar defects produced by the heavy ions could not reduce the anisotropy and enhance the interlayer coupling to such an extent that the fluctuation effects near the transition temperature region were suppressed.