arXiv subjects

P. Raychaudhuri

Publications and source records attributed to P. Raychaudhuri.

23 records · Page 2Linked to original sources

arXiv1998

The effect of the total density of states at Fermi level on spin polarised tunnelling in granular La0.55Ho0.15Sr0.3MnO3

We study the spin polarised tunnelling mechanism through magnetisation and magnetoresistance in the granular polycrystalline colossal magnetoresistive manganite, La0.55Ho0.15Sr0.3MnO3. This system has a ferromagnetic transition temperature (Tc) of 255 K associated with a metal-insulator transition around the same temperature. We have investigated dependence of the magnetoresistance due to spin polarised tunnelling on temperature and reduced magnetisation of the sample. We discuss the significance of our results within the realm of a model recently proposed by us to explain the spin polarised tunnelling in granular La0.7Sr0.3MnO3, in the light of the recent finding by A. Biswas et al. (cond-mat/9806084) regarding the evolution of the total density of states at Fermi level as a function of temperature in colossal magnetoresistive materials.

cond-mat.str-el↗

arXiv1998

Spin polarized tunneling in the half-metallic ferromagnet La0.7Sr0.3MnO3: experiment and theory

The magnetoresistance (MR) in polycrystalline colossal magnetoresistive compounds follows a behavior different from single crystals below the ferromagnetic transition temperature. This difference is usually attributed to spin polarized tunneling at the grain boundaries of the polycrystalline sample. Here we derive a theoretical expression for the contribution of spin polarized tunneling to the magnetoresistance in granular ferromagnetic systems under the mean field approximation. We apply this model to our experimental data on the half metallic ferromagnet La0.7Sr0.3MnO3, and find that the theoretical predictions agree quite well with the observed dependence of the spin polarized MR on the spontaneous magnetization.

cond-mat.str-el↗

arXiv1998

Metal-Insulator Transition and Ferromagnetism in the Electron Doped Layered Manganites La2.3-xYxCa0.7Mn2O7 (x=0,0.3,0.5)

Bulk samples of La2.3-xYxCa0.7Mn2O7, x=0,0.3,0.5, with layered perovskite structure have been synthesized and investigated with respect to their electrical, electronic and magnetic properties. It is found that La1.8Y0.5Ca0.7Mn2O7 has tetragonal structure and is a metallic ferromagnet with a magnetic transition temperature of 170 K. The compound shows metallic behavior below 140 K and has a large magnetoresistance (MR) Delta-rho/rho(0)~94% at 100 K at 34 kOe. For x=0 and 0.3 the structure is monoclinic with a suppression of metallicity. For x=0 the material is an ferromagnetic insulator. We observed a large increase in the coefficient of the linear term in specific heat with decreasing x. As far as we are aware, this is the first report of an electron doped manganite showing metal-insulator transition and ferromagnetism.

cond-mat.mtrl-sci↗

arXiv1998

A phenomenological model for magnetoresistance in granular polycrystalline colossal magnetoresistive materials: the role of spin polarised tunnelling at the grain boundaries

It has been observed that in bulk and polycrystalline thin films of collossal magnetoresistive (CMR) materials the magnetoresistance follows a different behaviour 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 polarised tunnelling 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 polarised tunnelling 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 double exchange model for all values of field.

cond-mat.mtrl-sci↗

arXiv1998

Magnetic field dependent variable range hopping behaviour in resistivity at low temperatures in polycrystalline colossal magnetoresistive manganites: evidence of spin polarised tunnelling

It has been observed that the low temperature magnetoresistance behaviour in polycrystalline colossal magnetoresistive (CMR) manganites differ significantly from the single crystals. The polycrystalline samples show large magnetoresistance at temperatures much below the ferromagnetic transition temperature where the magnetoresistance of single crystals is very small. This has conventionally been attributed to spin polarised tunnelling at the grain boundaries in polycrystalline samples. In this paper we show the existence of a variable range hopping behaviour in resistivity at low temperatures in polycrystalline CMR samples. This behaviour gets gradually suppressed under the application of magnetic field. We discuss the significance of these results with respect to spin polarised tunnelling.

cond-mat.str-el↗