• C M Srivastava

      Articles written in Bulletin of Materials Science

    • Permeability spectrum of garnets

      Om Prakash R Aiyer C M Srivastava

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      A detailed study of permeability spectra of polycrystalline pure and Aland Gd-Substituted garnets has been carried out for different values of grain diameter and magnetisation. The spectra are of three types—resonance, relaxed and mixed. It has been shown that the resonance spectra occur when the closure demains are absent and the domain wall is anchored at the grain boundary. The relaxed spectrum is found for domains having closure flux while the mixed spectrum is an indication of simulataneous presence of grains with and without closure domains.

    • Research in magnetism at Indian institute of technology, Bombay

      C M Srivastava

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      The experimental solid state physics group at the Indian Institute of Technology, Bombay, has been engaged in research on ferromagnetic resonance and relaxation, magnetic ordering and exchange phenomena in ferrimagnetic systems. A brief report on the facilities developed and the work done by the magnetism group is presented.

    • Exchange in insulators

      C M Srivastava

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      The exchange constants obtained using the three sublattice model for ferrites and garnets have been analysed using Anderson’s theory of superexchange. The transfer integrals for the 180° and 90° interactions obtained from the experimental values of exchange constants give approximate values of the covalency parameters which are consistent with the chemical theory of covalency.

    • The influence of Cu2+ substitution on the magnetic properties of Fe-Zn and Ni-Zn ferrites

      C M Srivastava M J Patni G Srinivasan T T Srinivasan

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      The influence of a Jahn-Teller ion on magnetization,g-values and linewidth in Znx Fe3−xO4 and Ni1−x Znx Fe2O4 spinel ferrites has been investigated by introducing small quantities of Cu2+ ions. The magnetization of Cu substitution in both the systems but the curie temperature is affected only in FeZn ferrites. The lattice constant in NiZn is significantly reduced while in FeZn there is only a slight increase. The change ingeff on copper substitution,Δgeff, is very different in the two systems. In NiZn,Δgeff has a uniform value, ∼ 0·2, for all values ofx while in FeZn it has a peak atx=0·3 and vanishes atx=0 andx⩾0·5. The ferromagnetic linewidthΔH does not change significantly on copper substitution in both the systems. An explanation based on the perturbation produced due to the local lattice distortion by the Jahn-Teller mechanism explains the observed results satisfactorily.

    • The verwey transition

      C M Srivastava

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      The origin of Verwey transition in magnetite is investigated. It is shown that the transport properties in the high temperature phase of magnetite can be accounted by a phonon-induced correlated electron transfer mechanism. Assuming that the condensation of this phonon mode leads to Verwey transition, the isotope and pressure effects have been explained. The electron energy band diagram consistent with the electrical transport behaviour in the low temperature phase has been constructed.

    • Influence of the presence of Fe2+ ion in nickel-zinc ferrite

      C M Srivastava S N Shringi M J Patni S M Joglekar

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      The presence of small amounts of Fe2+ ion in nickel zinc ferrite significantly influences some of its magnetic properties. The lattice parameter increases slightly and the increase Δa is independent ofx. The variation of magnetization with zinc concentration is explained on the basis of the Yafet-Kittel model. Increase in the Néel temperature on Fe2+ substitution in ZnxNi1-xFe2O4 is remarkable. This has been explained on the basis of a four sublattice model. Our analysis shows thatJAB (d5-d5) interaction is most affected. The influence of Fe2+ ions on the relaxation processes in the Mössbauer line shapes of Ni-Zn ferrite is also investigated and is compared with Cu2+ doped Ni-Zn ferrite.

    • Mössbauer spectroscopy of titanium-substituted iron garnets

      C M Srivastava S N Shringi B Uma Maheshwar Rao D Bahadur Om Parakash

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      The Mössbauer spectroscopy of polycrystalline Y3−xCaxFe5−xTixO12 (x=0·2, 0·5, 0·7) garnets has been studied at room temperature. The hyperfine fields both at the octahedral and tetrahedral sites decrease with increasing titanium content. The hyperfine fields both ata andd-sites are linearly proportional to magnetization. The quadrupole splitting fora-site shows no dependence on Ti content while the quadrupole splitting ford-site increases slightly withx. The results are explained on the basis of lattice distortion due to Ti substitution ona-site.

    • Magnetic structure of zinc ferrite

      C M Srivastava S N Shringi M Vijaybabu

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      A detailed analysis of the available experimental data has been made for the ordered phase of ZnFe2O4. The abnormally lowTN and the observed spin arrangement have been explained on the basis of the hybridization of orbitals on the oxygen ions which gives rise to an anisotropic exchange interaction. The variation of hyperfine field and susceptibility with temperature along with the zero point reduction of the magnetic moment indicate that the ZnFe2O4 is a two-dimensional antiferromagnet.

    • Dependence of microstructure on process variables in manganese zinc ferrites

      N Venkataramani R Aiyar P S Sekhar C M Srivastava

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      The dependence of microstructure on sintering conditions during hot pressing have been investigated and the densification mechanisms have been used to explain the observed results. The effect of addition of excess ZnO content over stoichiometry on permeability has been studied in the normal sintered Mn-Zn ferrites.

    • On the mechanism of pairing in thebcs theory

      C M Srivastava

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      An explanation has been offered for the mechanism of cooper pairing in thebcs theory of superconductivity on the basis of the relativistic Darwin interaction. The theory leads to an expression for critical temperature which depends on a few atomic parameters. Calculated values ofTc for elements and alloys are in good agreement with experiment.

    • Spin wave relaxation processes in polycrystalline yttrium iron garnets

      S N Bhatia P B Joshi C M Srivastava

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      The wavevector and temperature dependence of the spinwave linewidth in yttrium iron garnets has been studied for polycrystalline samples of average grain diameter 12·8µm and has been compared with the calculated relaxation times of basic three-magnon confluence and four-magnon scattering processes.

    • Infrared absorption in spinel ferrites

      T T Srinivasan C M Srivastava N Venkataramani M J Patni

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      Their spectra of a number of ferrites, MFe2O4, M(Mn, Fe, Co, Ni, Cu and Zn) have been studied at room temperature in the range 200–1000 cm−1. In all the cases the spectrum consists of four bands, two of which have strong absorption while the other two have weak absorption and often appear as shoulders on the main band. However up to 20% of the inverse ferrite, the shoulder in the low energy side appears as a band. In this paper we have analysed the origin of the above bands.

    • Influence of spin orbit coupling and lanthanide contraction on the exchange interaction in rare earth garnets

      C M Srivastava C Srinivasan R Aiyar

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      The exchange constants observed in the rare earth iron garnets, obtained from fitting magnetization and paramagnetic susceptibility as a function of temperature have been analysed in terms of Anderson’s theory of superexchange. The transfer integrals and covalency parameters have been obtained for various rare earth ions overlapping with oxygen.

    • Magnetic relaxation in rare earth garnets

      C M Srivastava B Uma Maheshwar Rao N S Hanumantha Rao

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      An attempt has been made to review the ferromagnetic resonance and relaxation data on rare earth iron garnets and in the light of it, to analyze the data on Sm and Eu thin films and Gd and Dy bulk polycrystalline samples obtained in our laboratory. A phenomenological approach adopted to explain the temperature and composition dependence of the line width andgeff appears to explain the complex spectra in most cases.

    • Investigation of high temperature superconductivity through microwave absorption method

      C M Srivastava N Venkatramani A Datta N S H Rao

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      The field dependence and near zero magnetic field microwave absorption as a function of rf power in YBa2Cu3O7−δ and Bi-Ca-Sr-Cu-O has been studied using a varian ESR spectrometer. A model of microwave absorption built on diamagnetic tensor susceptibility has been proposed which explains the observed results satisfactorily.

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