• L Madhav Rao

      Articles written in Pramana – Journal of Physics

    • Non-spherical magnetic moment in MnAlGe

      S K Paranjpe S R Tendulkar L Madhav Rao N S Satya Murthy

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      The magnetic structure factors of MnAlGe (space groupP4/nmm) measured with polarised neutrons have been expressed in terms of the magnetic moment of the Mn atom (site symmetry tetrahedral with tetragonal distortion), the Bessel transforms 〈jn〉 of the Mn radial functions and the fractional occupancies of the moment density in the various crystal field orbitals. The measured structure factors were least-squares fitted with the theoretical expression involving 〈jn〉 appropriate to the Mn0, Mn+ and Mn2+ atoms. The best fit was got using Mn0 transforms, yielding 1·45µB as the Mn magnetic moment. The fractional occupancies of the moment density in the crystal field orbitalsA1g,B1gEg andB2g were obtained. This analysis shows the magnetic moment to be highly non-spherical with a large fractional occupancy (38%) in theA1g orbital directed along the tetragonal axis while the fractional occupancies ofB1g andB2g are found to be 31% and 30% respectively. The fractional occupancy of the moment in theEg orbital directed towards the Ge and Al atoms is very low (1%). The spatially averaged moment density of Mn in MnAlGe is more diffuse than that of Mn I and Mn II in isostructural Mn2Sb.

    • Magnetic moment distributions and form factors in ferromagnetic nickel-ruthenium alloys

      R Chakravarthy L Madhav Rao N S Satya Murthy

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      Using polarised neutrons, the full three-dimensional magnetic structure amplitudes in the Ni1−c Ruc single crystals forc = 0·027, 0·033 and 0·046 were measured. Moment density maps in various portions of the Wigner-Seitz cell were obtained. It is seen from these maps that unlike Ni-based alloys with 3d impurities, the introduction of Ru to the Ni matrix produces extensive perturbations in the diffuse moment density, giving rise to a netpositive diffuse moment which tends to increase with Ru concentration. The asphericity of the host moment at first increases and then decreases with increasing Ru content. Another significant outcome of the present study is the evidence for the reversal of the sign of the Ru moment, from negative to positive, obtained by comparing the shape of the spherical site form factors of the three-alloy concentrations with the Ni spherical form factor itself. The sign reversal of the impurity moment is confirmed by the form factor analyses. Strong local environmental effects seem to play a major role in this alloy system.

    • Thermal neutron polarisation

      N S Satya Murthy L Madhav Rao

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      The basic principle for the production of polarised thermal neutrons is discussed and the choice of various crystal monochromators surveyed. Brief mention of broad-spectrum polarisers is made. The application of polarised neutrons to the study of magnetisation density distributions in magnetic crystals, the dynamic concept of polarisation, principle and use of polarisation analysis, the neutron spin-echo technique are discussed.

    • Reanalysis of polarised neutron data on NiRu using projection operator formalism

      R Chakravarthy L Madhav Rao

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      Projection operator formalism has been applied to analyse magnetic structure amplitudes in Ni1−xRux (x=0·027, 0·033 and 0·046) alloys. This method is suitable for metals and alloys as it does not assume any free ion form factor. A new prescription has been discussed for the local site moment and diffuse moment in this frame work. The results of the present analysis are compared with those obtained using form factor analysis.

    • A neutron polarization analysis study of Ce2Fe17 in its paramagnetic phase

      Sk Mohammad Yusuf L Madhav Rao K B Bhat

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      Spin-flip (paramagnetic) scattering and neutron depolarization studies were performed on Ce2Fe17 in its paramagnetic phase on the Dhruva neutron polarization analysis spectrometer. The absence of normalQ dependence of the scattered spin flip intensity shows that Ce2Fe17 is not a normal paramagnetic and there exist superparamagnetic clusters of sufficiently large dimensions (∼100Å). The observed neutron depolarization gives an indication of the dynamics of these Ce2Fe17 superparamagnetic clusters.

    • Magnetic studies in mesoscopic length scale using polarized neutron spectrometer at Dhruva reactor, Trombay

      Sk Mohammad Yusuf L Madhav Rao

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      The details of construction and principle of polarized neutron spectrometer at Dhruva reactor, Trombay, for neutron depolarization studies have been described. The feasibility in carrying out neutron depolarization studies in order to know the nature of magnetic ordering in various types of magnetic systems on mesoscopic length scale has been shown.

    • Local canted spin behaviour in Co1.4−xZnxGe0.4Fe1.2O4 spinels: A macroscopic, mesoscopic and microscopic study

      MV Subbarao SM Yusuf RG Kulkarni L Madhav Rao

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      DC magnetization, neutron depolarization and neutron diffraction (with both polarized and unpolarized neutrons) measurements have been reported for the Co1.1−xZnxGe0.1Fe1.2O1 spinels with x=0.5, 0.6 and 0.7. Neutron depolarization and neutron diffraction measurements confirm the presence of a long range ferrimagnetic ordering of the local canted spins in these ferrite samples. The observed features of low field magnetization have been explained under the framework of thermally activated domain wall movement of ferrimagnetic arrangement of local canted spins. An important role of magnetic anisotropy (due to the presence of Co2+ ions) in establishing the magnetic ordering and domain kinetics in these ferrites has been observed.

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