Exchange bias in ferrimagnetic-antiferromagnetic nanocomposite produced by mechanical attrition

Nanoparticles comprising of magnetite (Fe₃O₄) core-hematite (α-Fe₂O₃) shell with mean diameter around 9 nm were synthesized by mechanical grinding. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization and hysteresis loop measurements were carried out over the temperature range 5-300 K. Exchange bias varying from 140 to 10 Oe was observed. This is explained as arising due to ferrimagnetic-antiferromagnetic coupling at the nanointerfaces between the two phases. Analysis of data shows a pronounced increase in the effective anisotropy constant as the milling time is increased.     

Effect of nickel doping concentration on structural and magnetic properties of ultrafine diluted magnetic semiconductor ZnO nanoparticles

The ZnO:Ni²⁺ nanoparticles of mean size 2-12 nm were synthesized at room temperature by the simple co-precipitation method. The crystallite structure, morphology and size were determined by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The wurtzite structure of ZnO gradually degrades with the increasing Ni doping concentration and an additional NiO-associated diffraction peak was observed above 15% of Ni²⁺ doping. The change in magnetic behavior of the nanoparticles of ZnO with varying Ni²⁺ doping concentration was investigated using a vibrating sample magnetometer (VSM). Initially, these nanoparticles showed strong ferromagnetic behavior, however, at higher doping percentage of Ni²⁺, the ferromagnetic behavior was suppressed and paramagnetic nature was observed. The enhanced antiferromagnetic interaction between neighboring Ni-Ni ions suppressed the ferromagnetism at higher doping concentrations of Ni²⁺.     

Luminescence studies and formation mechanism of symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix

Surface effects significantly influence the functionality of semiconductor nanocrystals. In the current work we present synthesis of ZnO quantum dots (QD) vis-a-vis symmetrically dispersed ZnO quantum dots embedded in SiO₂ matrix and discussed their optical properties to understand the role of the surface effects. These nanomaterials were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), fourier transform infrared (FTIR), absorption (UV-visible) and photoluminescence (PL) spectroscopy. TEM studies confirm the formation of ZnO nanophosphors inside the SiO₂ matrix in highly symmetrical manner. These symmetrically dispersed ZnO@SiO₂ nanophosphors exhibited enhanced stable emission over uncoated sample and would permit the conjugation of the nanocrystals to biological entities after functionalization. Furthermore, the mechanism behind the formation of symmetrically dispersed ZnO quantum dots embedded in SiO₂ matrix was discussed in detail.     

Graphite C-axis thermal conductivity

This paper models the c$c$-axis thermal conductivity of thin graphite layers taking into account phonon confinement. A Debye model is used to calculate graphite c$c$-axis thermal conductivity, which is found to be 4 orders of magnitude smaller than in the graphite basal plane. This reduced thermal conductivity is promising for devices with improved thermoelectric figure of merit, ZT$ZT$, and thermal conduction along graphite c$c$-axis. Results of graphite thermal conductivity in the basal plane are also presented and discussed. These calculations have been done for ideal graphite structures that are a few monolayers thick, free of defects, and free of boundary scattering processes. To achieve the low calculated values of thermal conductivity, it will be necessary to fabricate high-quality graphite structures; this will pose significant fabrication challenges.     

The zero-norm subspace of bounded cohomology

Let be a closed, orientable surface of genus > 1. In this paper, non-trivial elements of the third bounded cohomology with are given constructively by using both a hyperbolic metric and a singular euclidean metric on . Furthermore, it is shown that the dimension of the subspace of consisting of zero-norm elements is the cardinality of the continuum. Received: November 26, 1996     

Microwave spectral study of 2-fluorophenol: cis conformer

The microwave spectra of 2-fluorophenol and its deuterated species have been observed and analyzed in the frequency ranges 12.5–18.0 GHz (KU band) and 21.5–26.0 GHz (K band) in the ground vibrational state at room temperature. For the normal species, the radio frequency-microwave double resonance spectrum has been recorded in the frequency range 30.0–38.0 GHz. Three rotational and five quartic centrifugal distortion constants for the normal species, $\text{A}\text{?}\text{= 3337.86 ± 0.02}$, $\text{B}\text{?}\text{= 2231.92 ± 0.01}$, $\text{C}\text{?}\text{= 1337.52 ± 0.01}$, d_J = (3.5 ± 2.9) × 10^?4, d_JK = (?4.9 ± 1.5) × 10^?3, d_K = (?3.2 ± 1.0) × 10^?3, d_WJ = (?2.0 ± 1.0) × 10^?7, d_WK = (2.6 ± 0.8) × 10^?6 (in MHz), and three rotational constants for the deuterated species, $\text{A}\text{?}\text{= 3324.70 ± 0.03}$, $\text{B}\text{?}\text{= 2177.95 ± 0.03}$, $\text{C}\text{?}\text{= 1315.96 ± 0.03}$ (in MHz), have been obtained. Consideration of the r_s coordinate of the hydroxyl hydrogen atom leads to the assignment of the spectra to the cis conformer of the molecule. An r₀ structure for the cis conformer has been proposed. The nonbonded OH ? F distance is lower by about 0.3 Å than the sum of the van der Waals radii.     

Measurement of isotope shift and hyperfine splitting of190, 191, 193, 197Pb isotopes by collinear laser spectroscopy

We report here on the measurement of isotope shift and hyperfine splitting of^{190, 191, 193, 197}Pb for the 723 nm atomic optical transition. Detailed analysis of the optical data has been done by combining them with the available muonic and electronicx-ray isotope shift data. The magnetic dipole moments and the electric quadrupole moments of the odd isotopes have been extracted from the hyperfine coupling constants of the atomic states involved in the optical transition used.     

Monolithic integration of photodetector with vertical cavity surface emitting laser

For the first time, monolithic integration of a photodetector with a vertical-cavity surface emitting laser (VCSEL) is demonstrated. The authors have designed and fabricated VCSELs with monolithically integrated pin photodetectors which dynamically monitor (and can thus control) the output power of the lasers. The VCSELs lase at room temperature and emit a highly coherent, low-divergence, circular beam directly from the top surface. The integrated photodetector shows a linear response to the laser emission with an effective responsivity of 0.25 A/W.<>     

Quantization on the Grassmann Manifold

The problem of quantization in an Euclidean space with unitary constraints can be formulated as an unconstrained problem on a Grassmann manifold. Such constraints arise in areas such as wireless communication with multiple antennas at the transmitter and at the receiver. Due to the constraints, the distortion rate analysis developed for Euclidean spaces cannot be applied directly. This paper extends Gersho's asymptotic (large rate, small distortion) distortion bounds to the case when the source is distributed on the complex Grassmann manifold. The special structure of the Grassmann manifold and the distortion measures defined on it differentiate this problem from the traditional vector quantization in Euclidean spaces.     

Orthogonality constrained general variational calculation of excited state wavefunctions and energies: A new strategy

A viable strategy is developed for the general variational calculation of excited state wavefunctions which are constrained to remainorthogonal to all the lower-lying states of the samesymmetry. A key element of the strategy is to employ the penalised functional procedure for enforcing the relevant orthogonality constraints and the method of steepest descent to locate the constrained minima with respect to all variables, linear as well as nonlinear. The workability of the algorithm is tested by applying the technique for the optimization of nonlinear parameters in trial functions for the 2s state of H atom and singlet 1s2s states of helium atom and some isoelectronic ions.