Department of Physics

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At room temperature, Mössbauer spectra of Sn4+/Nb 5+substituted Mn-Zn ferrites have been taken to understand the site occupancy of substituted ions. The results indicate that both the substituted ions occupy tetrahedral (A) site only at lower concentration of substitution while at higher concentration they occupy both tetrahedral and octahedral sites. At higher concentration of Nb substitution a doublet has been observed besides sextets. The possibility of canting existence on octahedral sites has been discussed. The observed variations of the hyperfine fields, isomer shift, line widths and B/A peak area ratio with the ferrite composition are interpreted.

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Bulletin of Materials Science

Mössbauer studies of Sn4+/Nb5+ substituted Mn-Zn ferrites

Demagnetisation field arising from the free poles at the ribbon ends makes the measurement of hysteresis loop inaccurate for melt-spun high permeability ribbon shaped sample. No standard equation is also available for calculation of demagnetisation factor for such samples. In the present paper, attempts have been made to use empirical relations for demagnetisation correction of ribbon shaped FeNbCuSiB based material having thickness of the order of 35μm. To the first approximation the material was considered as a cylinder with the equivalent cross-sectional area of the ribbon. A pre-factor, φ, was multiplied with the demagnetisation factor for equivalent cylindrical specimen to get the corrected demagnetisation factor of the measured sample. φ = 0. 65 was found to be the optimum pre-factor for the demagnetisation corrected hysteresis loop. © 2002 Elsevier Science B.V. All rights reserved.

Journal of Magnetism and Magnetic Materials

Demagnetisation effect and its correction on the measurement of magnetic hysteresis loop of melt-spun ribbons

Differential scanning calorimeter (DSC) has been used to study the crystallization kinetics of Al87.5Ni7Mm5Fe0.5 alloy. Both isochronal and isothermal DSC plots showed that the Al87.5Ni7Mm5Fe0.5 alloy undergoes three stages crystallization process. The first peak corresponds to the precipitation of fcc-Al from the amorphous matrix at around 160°C. Isothermal DSC traces showed that there is a short incubation stage for the formation of nanoscale Al particles. The value of Avrami exponent suggests that the crystallization of Al crystals starts with a nucleation process for a short time and finally controlled by a growth mechanism. The second peak corresponds to the crystallization of Al11(La,Ce)3 from the matrix at around 320°C with a nucleation rate, which decreases with time, and a constant growth rate. The activation energy calculated for the crystallization of Al11(La,Ce)3 phase is 288 ± 5.9 kJ/mol, 324.6 ± 18 kJ/mol, 292 ± 6 kJ/mol by Kissinger, Johnson-Mehl-Avrami and Arrhenius type equations. The difference in the values of kinetic energy is mainly due to different annealing conditions. The third peak at around 340°C corresponds to the precipitation of intermetallic compounds from the remaining matrix.

Materials Transactions

Crystallization kinetics in an amorphous Al-Ni-Mm-Fe alloy

We report on the synthesis and characterization of silicon nanoparticles by ablating silicon wafer in an ambient atmosphere of helium at 1 Torr. The mean cluster size ranging from 1.8 nm to 4.4 nm deposited on silicon substrate at room temperature is observed to depend on the laser fluence. The size of the nanoparticles decreases with laser fluence. Photoluminescence of the deposited films using Nd:YAG laser and Ar+ ion laser at 355 nm and 457.9 nm respectively shows emission peaks at 1.7, 2.2, and 2.7 eV. The luminescence peak at 2.2 eV and 2.7 eV are attributed to oxygen related impurities and the peak at 1.7 eV is attributed to quantum confinement.

Modern Physics Letters B

Photoluminescence from silicon nano-particles synthesized by pulsed laser ablation

The effect of stresses is known to modify the magnetic properties in magnetostrictive amorphous wires. A comparative study of the giant magnetoimpedance (GMI) for amorphous wires with compositions Fe77.5Si7.5B15, Co72.5Si12.5B15 and Co68.15Fe4.35Si12.55B15 and positive (30 × 10-6), negative (-2 × 10-6) and vanishing (0.1 × 10-6) magnetostriction have been studied. The influence of torsional stress on hysteresis loops and GMI is also reported. Modifications of GMI response are ascribed to the magnetoelastic anisotropy induced by torsion. © 2002 Elsevier Science B.V. All rights reserved.

Torsion dependence of giant magnetoimpedance in amorphous wires

A novel wet-chemical precipitation method is optimized for the synthesis of ZnS nanocrystals doped with Cu+ and halogen. The nanoparticles were stabilized by capping with polyvinyl pyrrolidone (PVP). XRD studies show the phase singularity of ZnS particles having zinc-blende (cubic) structure. TEM as well as XRD line broadening indicate that the average crystallite size of undoped samples is ∼2nm. The effects of change in stoichiometry and doping with Cu+ and halogen on the photoluminescence properties of ZnS nanophosphors have been investigated. Sulfur vacancy (VS) related emission with peak maximum at 434nm has been dominant in undoped ZnS nanoparticles. Unlike in the case of microcrystalline ZnS phosphor, incorporation of halogens in nanoparticles did not result VZn related self-activated emission. However, emission characteristics of nanophosphors have been changed with Cu+ activation due to energy transfer from vacancy centers to dopant centers. The use of halogen as co-activator helps to increase the solubility of Cu+ ions in ZnS lattice and also enhances the donor-acceptor type emission efficiency. With increase in Cu+ doping, Cu-Blue centers (CuZn -Cui+), which were dominant at low Cu + concentrations, has been transformed into Cu-Green (Cu Zn-) centers and the later is found to be situated near the surface regions of nanoparticles. From these studies we have shown that, by controlling the defect chemistry and suitable doping, photoluminescence emission tunability over a wide wavelength range, i.e., from 434 to 514nm, can be achieved in ZnS nanophosphors. © 2003 Elsevier B.V. All rights reserved.

Materials Chemistry and Physics

Synthesis and photoluminescent properties of ZnS nanocrystals doped with copper and halogen

The dynamics of an open quantum system exhibiting the quantum Brownian motion is analyzed when the coupling between the system and its environment is nonlinear, and the system and the reservoir are initially correlated. For couplings quadratic in the environment variables, the influence functional for the system is obtained perturbatively up to second order in the coupling constant, and then the propagator is explicitly evaluated when the particle is under the influence of a harmonic potential and an additional anharmonic potential, the so-called washboard potential. As an application of the propagator, the master equation and the Wigner equation are obtained for the quantum Brownian particle moving in a harmonic potential for the generalized correlated initial condition, and then for the specific case of the simplified “thermal” initial condition. The system is shown to obey the corresponding fluctuation-dissipation theorem. © 2003 The American Physical Society.

Physical Review E - Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics

General quantum Brownian motion with initially correlated and nonlinearly coupled environment

The metalloids Si and B in the melt-spun Fe73.5Nb3Cu1Si22.5-XBX (X = 5, 9, 10, 11.25, 19) alloys play a major role in their crystallization and magnetic behaviour. In the present work, the effect of nanocrystallization on the transport properties was studied using thermal variation of electrical resistivity. An X-ray diffractogram study showed that the formation of different phases was controlled by the metalloid content in the alloy. Nanocrystalline α-Fe(Si) and/or Fe3Si particles were formed in all the measured alloys except the alloy with 19 at% boron where α-Fe phase appeared. The lattice parameter as well as Si content within α-Fe(Si) was also found to depend on the metalloid content of the alloy. The Debye temperature was derived from thermal electrical resistivity study and was found to depend on the metalloid content of the alloys. However, Curie temperature of these alloys was not strongly dependent on the metalloid. The formation of α-Fe(Si) and/or Fe3Si nanoparticles was responsible for the superior soft magnetic properties of the 9at% boron alloy annealed around 800 K. The early appearance of boride phases, which have high magnetocrystalline anisotropy, was the cause of deterioration in soft magnetic properties in X = 5, 10, 11.25 and 19 alloys. © 2002 Elsevier Science B.V. All rights reserved.

Crystallization and soft magnetic properties of rapidly solidified Fe73.5Nb3Cu1Si22.5-XBX (X = 5, 9, 10, 11.25, 19) alloys

Amorphous alloys with compositions Fe40Co40Cu 0.5Al2Zr9Si4B4.5 and Fe62Co9.5Gd3.5Si10B15, were prepared by the melt spinning technique in the form of ribbons. Three stages of crystallization process were found in both the alloys and the crystallization kinetics was studied. Supper cooled region was found in Fe 62Co9.5Gd3.5Si10B15 alloy indicating the ability of formation of bulk amorphous alloy and 1 mm diameter rod was prepared by Cu-mould casting. Curie temperature and the saturation magnetisation of this alloy were found to be 596 K and 0.98 T respectively. Relatively, higher Curie temperature (736 K) and saturation megnetisation (1.18 T) were observed in the as-spun state of Fe 40Co40Cu0.5Al2Zr9Si 4B4.5 alloy. Present study also indicated that partial crystallization of Fe40Co40Cu0.5Al 2Zr9Si4B4.5 alloy would further enhance the Curie temperature and saturation magnetisation.

Crystallization and Magnetic Properties of Fe40Co 40Cu0.5Al2Zr9Si4B 4.5 and Fe62Co9.5Gd3.5Si 10B15 Amorphous Alloys

The effect of hydrogenation on the mechanical and magnetic response of amorphous Fe73.5Nb3Cu1Si13.5B 9 alloys was studied. Hydrogen was charged electrochemically by varying charging current density. Magnetic Barkhausen emissions and coercivity were measured in charged and degassed samples. It was found that both responses were reversible. The hydrogen induced stress was responsible for these reversible effects. The kinetics of hydrogenation and of degassing are reported. © 2003 Elsevier Science B.V. All rights reserved.