Magnetic surface effects and low-temperature magnetoresistance in manganese perovskites

The low-field magnetoresistance and the magnetization of ceramic oxides have been studied as a function of the grain size. It is shown that these ceramics become magnetically harder when reducing the particle size, exhibiting large magnetic anisotropy that also increases when reducing the grain size. In concomitance with this enhancement of the magnetic hardness a gradual increase of the low-field magnetoresistance is also detected. We suggest that both phenomena are closely related and associated with the existence of some degree of spin disorder at the grain boundaries. Implications of these findings for improvements of the field response sensitivity of these materials are discussed.