Pratap Raychaudhuri

Enhanced room-temperature magnetoresistance in La0.7Sr0.3MnO3-glass composites

In this letter, we report the improved magnetotransport properties of La0.7Sr0.3MnO3-borosilicate glass composite with different weight percents of glass. All the composites showed ferromagnetic nature at room temperature. The microstructure of these composites was seen using a scanning electron microscope. The microstructure was reconfirmed using spot energy dispersive x-ray analysis. We observe an enhancement of the low-field magnetoresistance (<200 Oe) at room temperature for the optimal composition of 25 wt % of glass. It is argued that glass layer separating the grain boundaries may be acting as barrier for spin-polarized tunneling, thereby enhancing the low-field magnetoresistance.

Observation of Minority Spin Character of the New Electron Doped Manganite La0.7Ce0.3MnO3 from Tunneling Magnetoresistance

We report the magnetotransport characteristics of a trilayer ferromagnetic tunnel junction built of an electron doped manganite (La0.7Ce0.3MnO3) and a hole doped manganite (La0.7Ca0.3MnO3). At low temperatures the junction exhibits a large positive tunneling magnetoresistance (TMR), irrespective of the bias voltage. At intermediate temperatures below T(C) the sign of the TMR is dependent on the bias voltage across the junction. The magnetoresistive characteristics of the junction strongly suggest that La0.7Ce0.3MnO3 is a minority spin carrier ferromagnet with a high degree of spin polarization, i.e., a transport half-metal.

p–n diode with hole- and electron-doped lanthanum manganites

The hole-doped (p-) manganite La0.7Ca0.3MnO3 and the electron-doped (n-) manganite La0.7Ce0.3MnO3 undergo an insulator-to-metal transition at around 250 K, above which both behave as a polaronic semiconductor. We have fabricated an epitaxial trilayer (La0.7Ca0.3MnO3/SrTiO3/La0.7Ce0.3MnO3), where SrTiO3 is an insulator. At room temperature, i.e., in the semiconducting regime, it exhibits asymmetric current–voltage (I–V) characteristics akin to a p–n diode. The observed asymmetry in the I–V characteristics disappears at low temperatures where both the manganite layers are metallic. These results indicate that using the polaronic semiconducting regime of doped manganites, a p–n diode can be constructed.

Growth of epitaxial and polycrystalline thin films of the electron doped system La1−xCexMnO3 through pulsed laser deposition

The polycrystalline La1−xCexMnO3 manganites do not exist in single phase in bulk under the preparation conditions so far employed, but their polycrystalline and epitaxial films deposited by the pulsed laser deposition (PLD) technique form readily in single phase. The cerium oxide (CeO2) remains partially unreacted when the bulk sample is prepared through the solid state reaction route. The resistivity of the bulk La0.7Ce0.3MnO3 sample shows a broad metal insulator transition (MIT) clearly resolved into two peaks, suggesting the presence of a second (impurity) phase, which is identified as unreacted CeO2 by the intensity analysis of the x-ray diffraction (XRD) data. However, when prepared as thin films by PLD, La0.7Ce0.3MnO3 forms in single phase, as corroborated by the uniqueness and sharpness of the MIT peak and also by the XRD patterns of the polycrystalline films. We also performed a detailed study of the epitaxial films by a high-resolution XRD system with a four-circle goniometer and did not find any...

Direct observation of electron doping in La0.7Ce0.3MnO3 using x-ray absorption spectroscopy

We report an x-ray absorption spectroscopic (XAS) study on thin films of La0.7Ce0.3MnO3. The measurements clearly show that the cerium is in the Ce(IV) valence state, and that the manganese is present in a mixture of Mn2+ and Mn3+ valence states. These data demonstrate that La0.7Ce0.3MnO3 is an electron-doped colossal magnetoresistive manganite, a finding that may open up opportunities for device applications as well as for further research on the colossal magnetoresistance phenomenon in these materials.

A phenomenological model for magnetoresistance in granular polycrystalline colossal magnetoresistive materials: The role of spin polarized tunneling at the grain boundaries

It has been observed that in bulk and polycrystalline thin films of colossal magnetoresistive (CMR) materials the magnetoresistance follows a different behavior compared to single crystals or single crystalline films below the ferromagnetic transition temperature Tc. In this paper we develop a phenomenological model to explain the magnetic field dependence of resistance in granular CMR materials taking into account the spin polarized tunneling at the grain boundaries. The model has been fitted to two systems, namely, La0.55Ho0.15Sr0.3MnO3 and La1.8Y0.5Ca0.7Mn2O7. From the fitted result we have separated out, in La0.55Ho0.15Sr0.3MnO3, the intrinsic contribution from the intergranular contribution to the magnetoresistance coming from spin polarized tunneling at the grain boundaries. It is observed that the temperature dependence of the intrinsic contribution to the magnetoresistance in La0.55Ho0.15Sr0.3MnO3 follows the prediction of the double exchange model for all values of field.

Mechanism of the Size Dependence of the Superconducting Transition of Nanostructured Nb

In nanocrystalline Nb films, the superconducting Tc decreases with a reduction in the average particle size below 20nm. We correlate the decrease in Tc with a reduction in the superconducting energy gap measured by point contact spectroscopy. Consistent with the Anderson criterion, no superconducting transition was observed for sizes below 8 nm. We show that the size-dependence of the superconducting properties in this intermediate coupling Type II superconductor is governed by changes in the electronic density of states rather than by phonon softening.

Transport and magnetic properties of laser ablated La0.7Ce0.3MnO3 films on LaAlO3

La0.7Ce0.3MnO3 is a relatively new addition to the family of colossal magnetoresistive manganites, in which the cerium ion is believed to be in the Ce4+ state. In this article, we report the magnetotransport properties of laser ablated La0.7Ce0.3MnO3 films on LaAlO3, and the effect of varying the ambient oxygen pressure during growth and the film thickness. We observe that the transport and magnetic properties of the film depend on the oxygen pressure, surface morphology, film thickness, and epitaxial strain. The films were characterized by x-ray diffraction using a four-circle goniometer. We observe an increase in the metal-insulator transition temperature with decreasing oxygen pressure. This is in direct contrast to the oxygen pressure dependence of La0.7Ca0.3MnO3 films and suggests the electron doped nature of the La0.7Ce0.3MnO3 system. With decreasing film thickness we observe an increase in the metal-insulator transition temperature. This is associated with a compression of the unit cell in the a-b plane due to epitaxial strain. On codoping with 50% Ca at the Ce site, the system (La0.7Ca0.15Ce0.15MnO3) is driven into an insulating state suggesting that the electrons generated by Ce4+ are compensated by the holes generated by Ca2+, thus making the average valence at the rare-earth site 3+ as in the parent material LaMnO3.La0.7Ce0.3MnO3 is a relatively new addition to the family of colossal magnetoresistive manganites, in which the cerium ion is believed to be in the Ce4+ state. In this article, we report the magnetotransport properties of laser ablated La0.7Ce0.3MnO3 films on LaAlO3, and the effect of varying the ambient oxygen pressure during growth and the film thickness. We observe that the transport and magnetic properties of the film depend on the oxygen pressure, surface morphology, film thickness, and epitaxial strain. The films were characterized by x-ray diffraction using a four-circle goniometer. We observe an increase in the metal-insulator transition temperature with decreasing oxygen pressure. This is in direct contrast to the oxygen pressure dependence of La0.7Ca0.3MnO3 films and suggests the electron doped nature of the La0.7Ce0.3MnO3 system. With decreasing film thickness we observe an increase in the metal-insulator transition temperature. This is associated with a compression of the unit cell in the a-b ...

Superconducting properties and Hall effect of epitaxial NbN thin films

We have measured the magnetotransport properties and Hall effect of a series of epitaxial NbN films grown on (100) oriented single crystalline MgO substrate under different conditions using reactive magnetron sputtering. Hall effect measurements reveal that the carrier density in NbN thin films is sensitive to the growth condition. The carrier density increases by a factor of 3 between the film with highest normal state resistivity

Role of critical current on the point-contact Andreev reflection spectra between a normal metal and a superconductor

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