Magnetic anisotropy and spin diffusion through spin disordered interfaces in magnetoresistive manganites

The magnetoresistance and the magnetization 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 large magnetic anisotropy that 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, signaling the relevance of the surface and interfaces contribution to the magnetic and transport properties. 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. (C) 1998 American Institute of Physics. [S0021-8979(98)29211-5].