Probing magnetic anisotropy and spin polarization in spintronic materials

Magnetic anisotropy and spin polarization are fundamental parameters in ferromagnetic materials that have use in spintronic device applications. As the need for screening properties of new magnetic materials rises, it is important to have measurement probes for quantities such as anisotropy and spin polarization. We have developed two unconventional yet powerful techniques to study these parameters. A resonant RF transverse susceptibility method is used to map the characteristic anisotropy and switching fields over a wide range in temperature and magnetic fields. For studies of spin polarization, the phenomenon of Andreev reflection across ferromagnet-superconductor junctions is used to extract values of the transport spin polarization. The effectiveness of these approaches is demonstrated in candidate spintronic materials such as half-metallic CrO/sub 2/ thin films and arrays of monodisperse, single-domain Fe nanoparticles.     

Improved i-vector extraction technique for speaker verification with short utterances

A major challenge in ASV is to improve performance with short speech segments for end-user convenience in real-world applications. In this paper, we present a detailed analysis of ASV systems to observe the duration variability effects on state-of-the-art i-vector and classical Gaussian mixture model-universal background model (GMM-UBM) based ASV systems. We observe an increase in uncertainty of model parameter estimation for i-vector based ASV with speech of shorter duration. In order to compensate the effect of duration variability in short utterances, we have proposed adaptation technique for Baum-Welch statistics estimation used to i-vector extraction. Information from pre-estimated background model parameters are used for adaptation method. The ASV performance with the proposed approach is considerably superior to the conventional i-vector based system. Furthermore, the fusion of proposed i-vector based system and GMM-UBM further improves the ASV performance, especially for short speech segments. Experiments conducted on two speech corpora, NIST SRE 2008 and 2010, have shown relative improvement in equal error rate (EER) in the range of 12–20%.     

Synthesis of linear array using Taylor distribution and Particle Swarm Optimisation

In this article, non-uniformly excited linear arrays are optimised using Taylor distribution and classical particle swarm optimisation (CPSO) algorithm for obtaining desired equal side lobe level (SLL). Elements of the array are considered to be isotropic in nature with uniform interelement spacing. Excitation amplitudes of each element are taken as optimisation parameters. Taylor distribution defines the range of excitation amplitude in which CPSO algorithm searches for the optimum value of excitation amplitude, with the objective of obtaining desired equal SLL. The proposed method eliminates the initial randomness of defining search space for CPSO algorithm. Comparison with other methods has been made whenever possible. The results reveal that the proposed method can be used to obtain the desired SLL.     

Sensorless field-oriented control for double-inverter-fed wound-rotor induction motor drive

A novel control technique for sensorless vector control operation of a double-inverter-fed wound-rotor induction motor is presented. Two current controllers control the stator-side currents based on a vector control algorithm. Another V/f-type flux and frequency controller controls the rotor-side frequency directly. A novel frequency command profile for the rotor-side controller is suggested to make this sensorless drive operation reliable and reduce dependence on motor parameters at any rotor speed. A complete inverter power flow analysis is presented to show that the drive can deliver full torque from 0- to 2-p.u. speed for either direction of rotation. Thus, double the rated power can be extracted from the induction motor without overloading it. The proposed algorithm allows the drive to start on-the-fly without any rotor transducer. Results from a prototype 50-hp drive are presented.     

Transesterification of Chromenes Employing Immobilized Lipase in Ionic Liquids

This is the first report for the transesterification of chromene catalyzed by lipase with vinyl acetate in Ionic liquids (ILs) at room temperature. The influence of the different ILs and additive in an organic media at this reaction is discussed. It was observed that the reaction catalyzed by enzyme in ILs took place more rapidly and selectively than in organic media. Moreover, both enzyme and IL could be recycled for several runs without sizeable diminution in activity.     

A FORTRAN program to compute vertical magnetic field of a horizontal rectangular loop on n-layered earth for em depth sounding

A FORTRAN program is described to compute the vertical magnetic field anywhere inside or outside a rectangular loop which carries a sinusoidally varying current and is horizontally placed on the surface of a n-layered earth. The program utilizes the concept of reciprocity and the known solution for a horizontal magnetic dipole source placed over a n-layered half-space. The computing algorithm is executable on a minicomputer like PDP 11/40 and as such is useful to many geoscientists who do not have access to mainframe computer, for computing model data to fit and interpret field observations from electromagnetic depth sounding experiments.     

Superparamagnetic Polymer Nanocomposites with Uniform Fe3O4 Nanoparticle Dispersions

Magnetic nanoparticles embedded in polymer matrices are good examples of functional nanostructures with excellent potential for applications such as electromagnetic interference shielding, magneto‐optical storage, biomedical sensing, flexible electronics, etc. Control over the dispersion of the nanoparticle phase embedded in a polymer matrix is critical and often challenging. To achieve excellent dispersion, competition between polymer–polymer and polymer–particle interactions have to be balanced to avoid clustering of particles in polymer nanocomposites. We report the first deposition of magnetic nanocomposite poly(methyl methacrylate)/polypyrrole bilayers from solution using spin‐coating. Fe₃O₄ nanoparticles have been synthesized using a chemical co‐precipitation route. Using a combination of dissolving the polymer and mixing fatty acid surfactant coated Fe₃O₄ nanoparticles, we have demonstrated the formation of nanocomposites with uniform nanoparticle dispersion. Cross‐sectional scanning electron microscopy, transmission electron microscopy, and magnetic measurements confirm the excellent dispersion and superparamagnetic response. Low‐frequency impedance measurements on these bilayers are also presented and analyzed.     

Nanocomposite magneto-rheological fluids with uniformly dispersed Fe nanoparticles

A systematic study of the magnetic and rheological properties of magneto-rheological (MR) fluids containing micron-size and nano-size iron particles is presented. The MR fluids were prepared with hydraulic oil as the carrier liquid and lecithin as an effective surfactant medium that promotes uniform particle dispersion. Magnetic measurements on micron-, hybrid-(nano + micron), and nano-MR fluids clearly indicate that the partial replacement of the micro-size particles by nanoparticles results in a better suspension and robust chain formation under applied external magnetic fields. For nano-MR fluids, the measured yield stress was found to be lower than micron-MR fluids. However, better flow properties and sharper magnetic switching make nanoparticle-based MR fluids appealing for microfluidics device applications where higher yield stress is not required.