• G Ravikumar

      Articles written in Pramana – Journal of Physics

    • Thermomagnetic history effects in niobium and its implication for Hc1 in highTc superconductors

      A K Grover P L Paulose P Chaddah G Ravikumar

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      The existence of a remanent magnetization (Mrem) on switching off the field of a field cooled (FC) sample of a highTc superconductor is often reported. It has recently been argued thatMrem should equal the difference in FC and zero field cooled (ZFC) magnetizations (MFCMZFC) 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 whichMrem 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 toMrem+MZFCMFC=0,Mrem<(MFCMZFC) andMrem>(MFCMZFC). At anyT, the equality holds forH<Hc1(T), and forHHc2 (MFCMZFC) asymptotically vanishes and thereMrem>(MFCMZFC). We show that there exists an intermediate region in all hard superconductors, whereMrem<(MFCMZFC). 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 determineHc1(T) from such data in YBCO. We also propose a new criterion for putting limits onHc1(T) in hard superconductors.

    • Field and temperature dependence of intergranular hysteresis in YBa2Cu3O7

      Shailendra Kumar G Ravikumar P K Mishra P Chaddah B A Dasannacharya Ram Prasad N C Soni

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      We have studied the variation of low field magnetization hysteresis in YBa2Cu3O7 as a function of the maximum magnetic field applied during a hysteresis cycle (1 G<Hmax<7.3 G) and also as a function of temperature (77 K<T<95 K). The remnant magnetization is studied as a function ofHmax andT and the measured dependences are explained using the extended critical state model. The potential of this technique as a contactless method of probing the temperature dependence ofJc is discussed.

    • Commensurability oscillations in NdBa2Cu3Oy single crystals

      H Küpfer G Ravikumar Th Wolf AA Zhukov H Wühl

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      Commensurability between inter-vortex distance and crystal lattice constant is investigated by angular dependent magnetization in very pure twinned and twin-free NdBa2 Cu3 Oy single crystals. With increasing temperature the incommensurate states split up and become finally commensurate with half the vortex distance. These new commensurate states are related to a substructure of the intrinsic pinning potential within the unit cell and discussed with respect to temperature, field, anisotropy, and twin structure.

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