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

摘要

It has been observed that the low temperature magnetoresistance behaviour inpolycrystalline colossal magnetoresistive (CMR) manganites differ significantlyfrom the single crystals. The polycrystalline samples show largemagnetoresistance at temperatures much below the ferromagnetic transitiontemperature where the magnetoresistance of single crystals is very small. Thishas conventionally been attributed to spin polarised tunnelling at the grainboundaries in polycrystalline samples. In this paper we show the existence of avariable range hopping behaviour in resistivity at low temperatures inpolycrystalline CMR samples. This behaviour gets gradually suppressed under theapplication of magnetic field. We discuss the significance of these resultswith respect to spin polarised tunnelling.