Arnab Roy

Size dependent magnetic phase of nanocrystalline Co0.2Zn0.8Fe2O4

Co0.2Zn0.8Fe2O4 nanoparticles have been synthesized by the coprecipitation method. Nanoparticles with average particle size of 5–∼65 nm were obtained by sintering the samples from 393 to 1173 K. Magnetic measurements show that samples with particle size ⩽13 nm are superparamagnetic above the blocking temperature TB. Samples with average particle size larger than 13 nm have a wide particle size distribution and show the presence of two magnetic phases: superparamagnetic and cluster glass. Cation distribution of the samples show that in the samples with particle size <13 nm Fe ions are distributed between the tetrahedral and octahedral sites of the spinel lattice but in the larger particle size samples the tetrahedral sites have very few Fe ions that form Fe–Fe clusters and hence the high temperature sintered samples show a mixed phase, consisting of superparamagnetic and cluster glass.

Structural and electrical properties of metal–ferroelectric–insulator–semiconductor structure of Al/SrBi2Ta2O9/HfO2/Si using HfO2 as buffer layer

Ferroelectric SrBi2Ta2O9 (SBT) thin films have been deposited by the radio-frequency magnetron sputtering technique on bare p-Si as well as on HfO2 insulating buffer p-Si. XRD patterns revealed the formation of a well-crystallized SBT perovskite thin film on the HfO2 buffer layer. The electrical properties of the metal–ferroelectric–insulator–semiconductor (MFIS) structure were characterized by varying thicknesses of the HfO2 layer. The MFIS structure exhibits a maximum clockwise C–V memory window of 1.60 V when the thickness of the HfO2 layer was 12 nm with a lower leakage current density of 6.20 × 10−7 A cm−2 at a positive applied voltage of 7 V. However, the memory window reaches a maximum value of 0.7 V at a bias voltage of ±5 and then decreases due to charge injection in the case of the insulating buffer layer thickness of 3 nm. The density of oxide trapped charges at/near the buffer layer–ferroelectric interface is studied by the voltage stress method. Capacitance–voltage (C–V) and leakage current density (J–V) characteristics of the Al/SBT/HfO2/Si(1 0 0) capacitor indicate that the introduction of the HfO2 buffer layer prevents interfacial diffusion between the SBT thin film and the Si substrate effectively and improves the interface quality. Furthermore, the Al/SBT/HfO2/Si structures exhibit excellent retention characteristics, the high and low capacitance values clearly distinguishable for over 1 h and 30 min. This shows that the proposed Al/SrBi2Ta2O9/HfO2/Si structure is ideally suitable for high performance ferroelectric memories.

Temperature dependent leakage current behavior of pulsed laser ablated SrBi2Ta2O9 thin films

Polycrystalline SrBi2Ta2O9 (SBT) thin films were grown on Pt/Ti/SiO2/Si substrates by pulsed laser ablation technique. Phase analyses of the deposited films were studied by grazing incidence x-ray diffraction. Microstructural and interfaces of the SBT film were investigated using a field emission scanning electron microscope. The dc leakage current behavior was studied at different temperatures, and the current transport mechanism was investigated. The calculated activation energies from the Arrhenius plot were attributed to the shallow traps existing near the conduction band of the SBT thin films. The current-voltage plot could be clearly separated into three regions, i.e., Ohm’s law, trap-filled limited, and Child’s law. At a low electric field, the current density–voltage characteristics showed the Ohmic behavior. Lampert’s theory of space charge limited conduction was found to be suitable to explain the current conduction through SBT films. The trap-filled limited voltage increases with increasing tem...

Interfacial and electrical properties of SrBi2Ta2O9/ZrO2/Si heterostructures for ferroelectric memory devices

We have investigated the interfacial and frequency dependent electrical properties of metal-ferroelectric-insulator-semiconductor capacitors with SrBi2Ta2O9 (SBT) ferroelectric films grown on ZrO2 buffer layer coated Si. Heterostructure SBT and ZrO2 thin films were deposited using rf magnetron sputtering. Interfacial and surface roughness parameters of heterostructures were extracted from the simulation of specular x-ray reflectivity data. The structure exhibited clockwise capacitance-voltage hysteresis with a maximum memory window of 2.0 V at a bias voltage of ±7 V. Frequency dependent (5 kHz–1 MHz) measurements at room temperature indicated that the clockwise hysteresis originates from the ferroelectric domain reversal. A minimum leakage current density of 4×10−8 A/cm2 of fabricated capacitors at an applied voltage of ±5 V revealed that the ZrO2 buffer layer prevents the interfacial diffusion between SBT thin film and the substrate, resulting in an improvement of interface quality. The charge retention ...

Development of a spin polarized low energy electron diffraction system

We have designed and constructed a spin polarized low energy electron diffraction system working in the reflected electron pulse counting mode. This system is capable of measuring asymmetries due to spin-orbit and exchange interactions. Photoemission from a strained GaAs/GaAsP super lattice is used as the source of spin polarized electrons. Spin-orbit asymmetry is evaluated for Ir(100) single crystal at various energies. Subsequently, exchange asymmetry has been evaluated on 40 monolayer Fe deposited on Ir(100). This instrument proves to be useful in understanding structure and magnetism at surfaces.

Avoiding acidic region streaking in two-dimensional gel electrophoresis: Case study with two bacterial whole cell protein extracts

Acidic region streaking (ARS) is one of the lacunae in two-dimensional gel electrophoresis (2DE) of bacterial proteome. This streaking is primarily caused by nucleic acid (NuA) contamination and poses major problem in the downstream processes like image analysis and protein identification. Although cleanup and nuclease digestion are practiced as remedial options, these strategies may incur loss in protein recovery and perform incomplete removal of NuA. As a result, ARS has remained a common observation across publications, including the recent ones. In this work, we demonstrate how ultrasound wave can be used to shear NuA in plain ice-cooled water, facilitating the elimination of ARS in the 2DE gels without the need for any additional sample cleanup tasks. In combination with a suitable buffer recipe, IEF program and frequent paper-wick changing approach, we are able to reproducibly demonstrate the production of clean 2DE gels with improved protein recovery and negligible or no ARS. We illustrate our procedure using whole cell protein extracts from two diverse organisms, Escherichia coli and Mycobacterium smegmatis. Our designed protocols are straightforward and expected to provide good 2DE gels without ARS, with comparable times and significantly lower cost.

Giant planar Hall effect in pulsed laser deposited permalloy films

Ni80Fe20 thin films with high orientation were grown on Si(1 0 0) using pulsed laser ablation. The anisotropic magnetoresistance (AMR) and the planar Hall measurements show a 2.5% resistance anisotropy and a 45% planar Hall voltage change for magnetic field sweep of 10 Oe. The planar Hall sensitivity dR/dH was found to be 900 Omega T-1 compared with a previously reported maximum of 340 Omega T-1 in the same system.Also these films are found to withstand repeated thermal cycling up to 110 degrees C and the Hall sensitivity remains constant within this temperature range. This combination of properties makes the system highly suitable for low magnetic field sensors, particularly in geomagnetic and biosensor applications. To elucidate this, we have demonstrated that these sensors are sensitive to Earths magnetic field. These results are compared with the sputter deposited films which have a very low AMR and planar Hall voltage change as compared with the films grown by PLD. The possible reasons for these contrasting characteristics are also discussed.

Simulation Based Verification using Temporally Attributed Boolean Logic

We propose a specification logic called temporally attributed Boolean (TAB) logic for assertion based verification which allows us to: (i) represent assertions succinctly, (ii) incorporate data-orientation and (iii) associate timing in design intentions. We present examples to show the motivation for this logic especially in the context of high level modeling of complex real time systems. We formally define TAB logic, formulate the problem of verification on a simulation trace and present efficient algorithms to check TAB assertions. We present results of application of TAB logic for instruction semantics and bus transaction verification of a bus integrated pipelined processor implementation

Role of ferric ions in the magnetic interactions of substituted iron molybdate

The magnetic interactions in titanium and zinc substituted Fe2MoO4 has been analyzed with respect to Fe2+ and Fe3+ ion concentrations on the two sites of the spinel lattice. The concentrations of Fe2+ and Fe3+ ions on the two sites of the spinel lattice were determined from the Mossbauer spectra of the samples recorded in their paramagnetic state. Substitution of molybdenum by titanium decreases the Fe3+ ions on both the sites of the ferrite. The decrease in Fe3+ ion concentration leads to a decrease in the A–O–B and B–B interactions and thus decreases the ferrimagnetic Curie temperature (TC). Substitution of Fe2+ on the tetrahedral site (A) of Fe2MoO4 by Zn2+ leads to an increase in Fe3+ concentration on the tetrahedral site and thus the A–A and A–O–B interactions become strong which explains the absence of compensation temperature. Also, the increase in Fe3+ content on the A site of the zinc substituted samples initially leads to a relatively small decrease in TC, compared to the titanium substituted sa...

Suppression of Barkhausen noise due to exchange biasing in Fe-FeMn bilalayers

Exchange biased Fe(FM)-FeMn(AFM) bilayers were grown by pulsed laser ablation in UHV and probed by SQUID magnetometer and planar Hall effect measurements. A suppression of barkhausen avalanches was observed during the switching of the bilayer when compared to that of pure Fe, which is indicative of a change in the reversal mechanism.