V. Prasad

Colossal magnetoresistance in epitaxial La(1−x−y)NayMnO3 thin film

We have synthesized La0.83Na0.11MnO2.93 by heating La2O3 and MnCO3 in NaCl melt at 900 °C. The exact composition was arrived by analyzing each ion by an independent chemical method. The compound crystallized in a rhombohedral structure and showed an insulator-to-metal transition at 290 K. Epitaxial thin films were fabricated on LaAlO3 (100) using a pulsed laser deposition technique. The film also showed an insulator-to-metal transition at 290 K. Magnetoresistance [ΔR/R0=(RH−R0)/R0] was −71% near the insulator-to-metal transition temperature of 290 K at 6 T magnetic field.

High emission currents and low threshold fields in multi-wall carbon nanotube-polymer composites in the vertical configuration

In this work, we present field emission characteristics of multi-wall carbon nanotube (MWCNT)-polystyrene composites at various weight fractions along the cross-section of sample. Scanning electron microscope images in cross-sectional view reveal that MWCNTs are homogeneously distributed across the thickness and the density of protruding tubes can be scaled with weight fraction of the composite film. Field emission from composites has been observed to vary considerably with density of MWCNTs in the polymer matrix. High current density of 100 mA/cm2 was achieved at a field of 2.2 V/μm for 0.15 weight fraction. The field emission is observed to follow the Fowler–Nordheim tunneling mechanism, however, electrostatic screening is observed to play a role in limiting the current density at higher weight fractions.

Electrical resistivity and enhanced magnetoresistance in (La0.6Pb0.4MnO3/La0.85MnO3−δ)n superlattices

Resistivity and magnetoresistance studies have been carried out on (La0.6Pb0.4MnO3/La0.85MnO3−δ)n (n=30 periods) ferromagnetic/paramagnetic oxide multilayers fabricated by pulsed laser deposition. An epitaxial film of La0.6Pb0.4MnO3 is ferromagnetic and metallic below 320 K, whereas that of La0.85MnO3−δ is a paramagnetic (and antiferromagnetic at low temperatures) insulator. The multilayers showed an insulator-to-metal transition at 285±5 K. The magnetoresistances of these multilayers were about 60%–75% at the transition temperature in 6 T magnetic field. The magnetoresistance is also shown to be dependent on the thickness of the ferromagnetic oxide layer. A maximum magnetoresistance of 75% was observed for a [La0.6Pb0.4MnO3(50 A)/La0.85MnO3−δ(20 A)]30 superlattice as against 25% in a single La0.6Pb0.4MnO3 film at 285 K.

Transport properties and colossal magnetoresistance in epitaxial La0.67Cd0.33MnO3 thin film

An optimal composition of La0.67Cd0.33MnO3 was synthesized by ceramic route. The compound crystallized in a rhombohedral structure with lattice parameters a = 5.473(4) A and α = 60°37′. Resistivity measurement showed an insulator-to-metal transition coupled with a ferromagnetic transition of around 255 K. Epitaxial thin films were fabricated on the LaAlO3 (100) substrate by a pulsed laser deposition technique. The psuedocubic lattice parameter a of the film is 3.873(4) A. The insulator-to-metal transition of the film was observed at 250 K which is comparable with the bulk value. The film was ferromagnetic below this temperature. Magnetoresistance defined as ΔR/R0 = (RH−R0)/R0 was over −86% near the insulator-to-metal transition temperature of 240 K at 6 T magnetic field and over-30% at relatively low fields of 1 T. No magnetoresistance was observed at low temperatures in the film unlike in the polycrystalline sample, where about a 40% decrease in resistance was observed on applying 6 T magnetic field due to the spin dependent scattering at the grain boundaries.

Probing the inter-tube transport in aligned and random multiwall carbon nanotubes

The temperature and magnetic field dependence of conductivity has been used to probe the inter-tube transport in multiwall carbon nanotubes (MWNTs). The scanning electron microscopy images show highly aligned and random distribution of MWNTs. The conductivity in aligned carbon nanotube (ACNT) and random carbon nanotube (RCNT) samples at low temperature follows T-1/2 (at T 8 K) dependence in accordance with the weak localization and electron-electron (e-e) interaction model. The values of diffusion coefficient in ACNT and RCNT are 0.25 x 10(-2) and 0.71 x 10(-2) cm(2) s(-1), respectively, indicating that larger number of inter-tube junctions in later enhances the bulk transport. The positive magnetoconductance (MC) data in both samples show that the weak localization contribution is dominant. However, the saturation of MC at higher fields and lower temperatures indicate that e-e interaction is quite significant in RCNT. The T-3/4 and T-1/2 dependence of inelastic scattering length (l(in)) in ACNT and RCNT samples show that the inelastic e-e scattering is more important in aligned tubes

Enhancement of magnetoresistance in manganite multilayers

Manganite multilayers have been fabricated using

High coercivity below percolation threshold in polymer nanocomposite

La_{0.67}

Low temperature electrical transport properties of carbon matrix containing iron nanoparticles

Ca_{0.33}