• N Y Vasanthacharya

      Articles written in Bulletin of Materials Science

    • Near-neighbour impurity effect on the spin-state transitions in LaCoO3 at low temperature (12<T<300 K)

      N Y Vasanthacharya P Ganguly

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      The low-temperature magnetic susceptibility behaviour of LaCoO3 prepared under different conditions as well as substituted samples such as LaCo0.95M0.05O3 (M=Al, Ga, Cr, Fe, Mn, Ni) and La0.98Sr0.02CoO3 have been investigated in the temperature range 12–300 K. Earlier interpretations of the magnetic susceptibility have been reexamined. In the case of LaCoO3 samples containing Al, Ga, Cr and Fe impurities spin-state transitions involving a temperature independent activation energy (∼0.01 eV) are observed atT<200 K. Analysis of the data indicates that either the excited state has an intermediate-spin (t2g5eg1) configuration or only half the Co ions are involved in the activated transition to the high-spin (t2g4eg2) configuration. Al3+, and Cr3+ increases the activation energy considerably. Substitution of L ≠ 0 ions such as Mn3+ and Ni3+ or Co4+ (low-spin) seems to introduce ferromagnetic interactions and stabilizes the paramagnetic state. LaCoO3, when Co is substituted by Mn (5%) or La is substituted by Sr (2%) show giant magnetic moments. When Co is substituted by Ni (5%) a ferromagnetic ground state is observed.

    • Chemical stability of the high-Tcsuperconductor YBa2Cu3O7−δ+

      A K Ganguli N Y Vasanthacharya

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      YBa2Cu3 O7−δ samples stored in different environments have been studied over a period of 30 days employing TGA, electrical resistivity and X-ray diffraction measurements. Oxygen content andTcdo not change significantly in samples stored in the normal laboratory environment, in a sealed tube or in a vacuum desiccator. Superconductivity is lost only in samples stored in a very humid atmosphere around 320 K.

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