Magnetic surface anisotropy and low-temperature magnetoresistance in manganese perovskites

The magnetoresistance and the magnetisation of ceramic La(2/3)A(1/3)MnO(3) (A=Sr, Ca) oxides have been studied as a function of the grain size. It is found that these ceramics become magnetically harder when reducing the particle size exhibiting a huge magnetic anisotropy (K(1)) of about 10(7)erg/cm(3) that increases when reducing the grain size; thus signalling the relevance of the surface contribution to the magnetic anisotropy. In concomitance with this enhancement of K(1) a gradual increase of the low-field magnetoresistance is also detected. We suggest that both phenomena could be closely related and associated to the existence of some degree of spin disorder at the grain boundaries. We propose that these features are fingerprints of quenched spin disorder at interfaces that can be tuned through bandwidth modulation of the magnetic frustration.