Colossal enhancement of magnetoresistance in La0.67_{0.67}Sr0.33_{0.33}MnO3_{3} / Pr0.67_{0.67}Ca0.33_{0.33}MnO3_{3} multilayers: reproducing the phase-separation scenario

Colossal enhancement of magnetoresistance has been achieved over a broad temperature range which extends upto the room temperature, in ferromagnetic metal-charge ordered insulator manganite multi-layers. The artificially created phase coexistence in the multilayers reproduce the characteristic signatures of metastability in the magnetotransport properties commonly observed in electronically phase-separated manganites

Inversion of magnetoresistance in magnetic tunnel junctions : effect of pinhole nanocontacts

Inverse magnetoresistance has been observed in magnetic tunnel junctions with pinhole nanocontacts over a broad temperature range. The tunnel magnetoresistance undergoes a change of sign at higher bias and temperature. This phenomenon is attributed to the competition between the spin conserved ballistic transport through the pinhole contact where the transmission probability is close to unity and spin polarized tunneling across the insulating spacer with weak transmittivity.Comment: Replaced with revised version and new figure, 6 figures, RevTex

Giant enhancement of room temperature magnetoresistance in La_{0.67}Sr_{0.33}MnO_{3}/Nd_{0.67}Sr_{0.33}MnO_{3} multilayers

The metal-insulator transition temperature in CMR manganites has been altered and brought close to the room temperature by preparing La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO)/ Nd$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (NSMO) multilayers with ultra thin individual layers of LSMO and NSMO. The LSMO/NSMO multilayers with ultra thin individual layers of thickness of about $10\AA$ exhibits 150% magnetoresistance at 270 K whereas LSMO/NSMO multilayers with moderate individual layer thickness of about $40\AA$ each exhibits a mere 15% magnetoresistance at the same temperature. We have shown that the reduction in thickness of the individual layers leads to increased spin fluctuation which results in the enhancement of magnetoresistance.Comment: Replaced with revised version and new figure, 9 pages, 4 figure

Aging during Phase Separation in Long-Range Ising Model

The kinetics of domain growth and aging in conserved order parameter systems, in the presence of short-range interaction, is widely studied. Due to technical difficulties and lack of resources, regarding computation, the dynamics is still not well established in the cases where long-range interactions are involved. Here we present related results from the Monte Carlo simulations of the two-dimensional long-range Ising model (LRIM). Random initial configurations, for $50:50$ compositions of up and down spins, mimicking high temperature equilibrium states, have been quenched to temperatures inside the coexistence curve. Our analysis of the simulation data, for such a protocol, shows interesting dependence of the aging exponent, $\lambda$, on $\sigma$, the parameter, within the Hamiltonian, that controls the range of interaction. To complement these results, we also discuss simulation outcomes for the growth exponent. The obtained values of $\lambda$ are compared with a well-known result for the lower bounds. For this purpose we have extracted interesting properties of the evolving structure.Comment: 9 pages, 5 figure

Evaluation and design of surfactant formulations for wettability alteration

Only about 35% of oil is recovered from carbonate reservoirs through primary and secondary flooding because of oil wet surfaces and unfavorable capillary pressures. Surfactants, with their dual hydrophobic and hydrophilic nature have been known to improve oil recovery significantly by lowering oil-water interfacial tension and by altering wettability of surfaces. However, the process of selecting an efficient surfactant for wettability alteration is dependent on several factors, including mineral type, porosity, temperature, salinity, nature of adsorbed oil, molecular structure and surfactant adsorption. Core-flood experiments usually used for evaluating surfactants tend to be time-consuming and provide very little information on the actual mechanism of surfactant action. A fast evaluation scheme is hence required to measure surfactant performances corresponding to the above mentioned parameters. The current work focusses on macro and molecular scale analysis of surfactants to understand relevant structure-property relationships and mechanism of wettability alteration. Surfactants are first evaluated and screened through a series of phase behavior, contact angle and oil-film experiments. The experimental observations have been used to correlate parameters like molecular structure, temperature and brine salinity to macroscopic properties like wettability alteration, adsorption and capillary driving force. Oil-film experiments have been used to understand the surfactant-aided wettability alteration. The role of surfactant adsorption in wettability alteration is investigated by static adsorption experiments. Adsorption isotherms are measured for different surfactant hydrophilicities at different temperatures and surfactant cloud point is used to develop a thermodynamic model explaining the universal surfactant behavior. Along with experiments, molecular dynamics simulations are also performed to understand the mechanism of aggregative adsorption of the nonionic surfactants. To address the issue of high temperature, high salinity applications, mixed surfactant formulations of nonionic surfactants and anionic hydrotropes are developed. Detailed investigations are performed to understand the role of hydrotrope structure, concentration and temperature on the mechanism of aqueous stabilization and adsorption and their effect on wettability alteration. Overall, the current work first establishes a macro and molecular-scale understanding of the phenomenon of surfactant-assisted wettability alteration and associated structure-property relationships. While shorter surfactant hydrophilic units and high temperatures are found to exhibit better wettability alteration, in fact it is proximity to surfactant cloud point which is the determining thermodynamic descriptor. Improved wettability alteration is correlated with surfactant adsorption which occurs in an aggregative manner. It also means there is a tradeoff between surfactant adsorption and wettability alteration. Using this knowledge, surfactant formulations are developed to observe and predict enhanced oil recoveries from representative porous media.Chemical Engineerin

Anisotropic generalization of Buchdahl bound for specific stellar models

Anisotropy is one factor that appears to be significantly important in the studies of relativistic compact stars. In this paper, we make a generalization of the Buchdahl limit by incorporating an anisotropic effect for a selected class of exact solutions describing anisotropic stellar objects. In the isotropic case of a homogeneous distribution, we regain the Buchdahl limit $2M/R \leq 8/9$. Our investigation shows a direct link between the maximum allowed compactness and pressure anisotropy vi-a-vis geometry of the associated $3$-space.Comment: To appear in Euro. Phys. J