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It has been observed that in bulk and polycrystalline thin films of collossalmagnetoresistive (CMR) materials the magnetoresistance follows a differentbehaviour compared to single crystals or single crystalline films below theferromagnetic transition temperature Tc. In this paper we develop aphenomenological model to explain the magnetic field dependence of resistancein granular CMR materials taking into account the spin polarised tunnelling atthe 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 haveseparated out, in La0.55Ho0.15Sr0.3MnO3, the intrinsic contribution from theintergranular contribution to the magnetoresistance coming from spin polarisedtunnelling at the grain boundaries. It is observed that the temperaturedependence of the intrinsic contribution to the magnetoresistance inLa0.55Ho0.15Sr0.3MnO3 follows the prediction of double exchange model for allvalues of field.Comment: 14 pages + 5 figures, postscript (to appear in Journal of Applied  Physics

journal of applied physics

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