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. 
P. Raychaudhuri, T. K. Nath, A. K. Nigam, R. Pinto