• C K Majumdar

      Articles written in Bulletin of Materials Science

    • Antiferromagnetism in the heisenberg hamiltonian

      C K Majumdar

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      The shortcomings of the classical theories of antiferromagnetism are reviewed. On the basis of known exact results of several antiferromagnetic models, it is argued that the spin correlation functions may have oscillating behaviour anticipated from the classical Neel states even though the exact ground state is quite different from Neel state. It is also found that as the spin becomes large, the classical results become more and more applicable as seen in Fisher’s model and verified experimentally. A few remarks on the broken symmetry are made. The existence of an anisotropic antiferromagnetic model with Neel states as ground state is pointed out.

    • Mössbauer, xrd and positron annihilation studies on natural magnetite and hematite ore from Ari Dongri, central India

      D Das M B Chakraborti K Choudhury P M G Nambissan B R S Babu P Sen Sangeeta C K Majumdar

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      Natural magnetite and hematite samples taken from iron ore deposits associated with Precambrian banded iron-formation (BIF) at Ari Dongri (20°23′N:81°3°E), Bastar district in Central India have been studied by Mossbauer, XRD and positron annihilation techniques. Three magnetite samples show a genetic association with α-Fe2O3 with a wide range of variations in Fe3O4:α-Fe2O3 ratio. The fourth sample, a typical specular hematite, shows α-Fe2O3 content of the order of 90%, the rest being magnetite. The magnetite present in the samples was found to be stoichiometric. None of the samples contains maghemite (γ-Fe2O3). Some geological implications of the observed variation in the oxidation states of the samples are considered.

    • 57Fe Mössbauer studies on natural chromites

      D Das M Sudarshan S N Chintalapudi K L Chakravorty C K Majumdar

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      Five chromite samples procured from two different belts of India have been studied by57Fe Mössbauer spectroscopy. It is shown that four symmetric doublets are sufficient to fit the chromite spectrum; the Mössbauer parameters and the Fe3+/Fe2+ ratio are computed. For fully oxidized chromite with only Fe3+, two doublets are needed and the parameters are computed.

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