• D Bahadur

      Articles written in Bulletin of Materials Science

    • Characterisation of bubble materials

      M J Patni Om Parkash D Bahadur

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      The rapid development of magnetic bubble technology has required growth to exacting specifications of a large number of magnetic films of garnets and some amorphous binary and ternary alloys of rare-earth and transition metals. The characterisation of these films is an essential part in the search for newer materials which hold out promise for better device performance and cost viability. Many methods of films characterisation have been reported from time to time. By and large, these methods can be divided into two groups: one, bulk measurements made on the film and two, measurements made on the domains. We have attempted to collate and briefly introduce various techniques to characterise magnetic bubble materials in this review.

    • Electron transport in hot pressed Y3−xGdxFe5O12

      D Bahadur OM Parkash Devendra Kumar

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      Electron transport properties of a few hot-pressed garnets of the series Y3−xGdxFe5O12 (wherex=0, 1 and 2·4) have been measured. For comparison, a normal sintered YIG has been studied to see the effect of porosity and microstructure. The electron transport properties have been discussed on the basis of the model suggested by Austin and Mott keeping in view the distortion caused by the substitution.

    • Spin-state equilibria in YbCoO3

      D Bahadur

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      The relative properties of high spin and low spin Co8+ in HoCoO3, ErCoO3 and LuCoO3 are about 1 : 1 above a characteristic temperature. YbCoO3 seems to show a similar behaviour with no evidence of electron transfer between the high spin and low spin states.

    • 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.

    • Electron transport in non-crystalline garnet films

      D Bahadur D Roy B Singh D Saran

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      Thin films of yttrium iron garnet (yig) and Gd-substitutedyig of different thickness have been prepared by flash evaporation. The surfacedc andac electrical resistivity and thermopower in these films have been studied. The results are explained on the basis of Mott and Davis model in which narrow tails of localised states exist at the extreme valence and conduction bands and a band of localised levels near the middle of the gap. For the temperature range studied, the main conduction mechanism is on account of excitation of carriers into localised states at the band edges and hopping at energies close to the band tails.

    • Characterization of RF-sputtered garnet films

      Rambilas D Bahadur

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      Noncrystalline garnet films of nominal composition Y3Fe5O12 and Y2GdFe5O12 were synthesized by RF sputtering. The AC and DC resistivity data have been discussed in line with the model of Mott and Davis where conduction occurs through excitation of carriers into localized states at the band edges and hopping at energies close to the band tails.

    • Current trends in applications of magnetic ceramic materials

      D Bahadur

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      Magnetic oxides, a major constituent of magnetic ceramic materials, are most extensively used in a variety of applications as soft, moderate and hard ferrites. I review here its applications with special emphasis on some recent developments in magnetic recording materials.

    • Resistivity studies of melt spun and annealed Al-Mn-Fe-Si multiply twinned samples

      Kulvir Singh D Bahadur GVS Sastry

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      The quaternary Al75Mn10Fe5Si10 alloy was synthesized by induction melting, melt spinning and subsequent annealing. The melt spun sample shows the multiply twinned nodular-type microstructure. These plates surround an icosahedral seed of the nodule. Annealing induces relaxation/transformation of the seeds due to diffusion process. We studied the electrical properties of all the three states (as-cast, quenched and annealed) of Al75Mn10Fe5Si10 alloy. The results are discussed in the framework of Ziman-Faber theory.

    • Studies on synthesis of calcium ferrite-based bio glass ceramics

      B R Jagadish N Prabhu D Bahadur

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      The possibility of synthesizing a Ca-ferrite based biocompatible glass ceramic has been explored in the following two glass compositions: (i) 28Na2O-8CaO-3P2O5-llFe2O3-50SiO2, and (ii) 25Na2O-8CaO-3P2O5-20Fe2O3-41SiO2-3B2O3 (in weight ratio). The effect of simulated body fluid on the different glasses and glass ceramics was also investigated. While there is no direct evidence for apatite formation, the weight losses recorded and formation of a Si-rich layer at the surface appears to be an indication of onset of apatite formation. The rate of apatite formation is presumably retarded due to the presence of Al3+ (picked up from AL2O3 crucible). Ferromagnetic resonance experiments at 9.03 GHz demonstrate that these glass ceramics can possibly be used for microwave hyperthermia.

    • Importance of structural tuning in manganites

      D Bahadur R A Dunlap

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      Properties of substituted La-manganites are known to be very sensitive to La site substitution. The critical parameters appear to be (a) the charge and concentration of the substituted ions, which determine the Mn3+/Mn4+ ratio, (b) the average ionic radius at theA site and (c) the tolerance factor,t. The latter two parameters determine the distortion and structural tuning of the perovskite structure. Here we report the influence of minor variations in the above parameters on the magnetotransport of compounds around the basic composition La0·6M0·07Ca0·33MnO3 (M=Er, Yb and Bi). The influence of the method of synthesis on different properties is also discussed. The ionic radii of Er3+, Yb3+ and Bi3+ ions are 1·00, 0·985 and 1·11 Å, respectively, compared to 1·32 Å for La3+ and this provides a systematic variation for the investigation of the effects ofA site substitution. The zero field resistivity data are essentially similar above the maxima which corresponds, at least approximately, to the Curie temperature as determined by magnetization measurements. However, there are variations below the transition temperature, where there is magnetic ordering, and large variations are often seen in the magnetic data between samples which are very similar in composition. Substantial similarities between field dependent magnetization and resistivity measurements appear to be related to domain wall displacement and rotation. These results will be discussed in the framework of structural tuning which has a strong influence on exchange parameters such as the Mn-O-Mn bond length and angle.

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