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 i s found that these ceramics become magnetically harder when reducing the pa rticle size exhibiting a large magnetic anisotropy that increases when redu cing the grain size. In concomitance with this enhancement of the magnetic hardness, a gradual increase of the low-field magnetoresistance is also det ected, signaling the relevance of the surface and interfaces contribution t o the magnetic and transport properties. We suggest that both phenomena cou ld be closely related and associated to the existence of some degree of spi n disorder at the grain boundaries. We propose that these features are fing erprints of quenched spin disorder at interfaces that can be tuned through bandwidth modulation of the magnetic frustration. (C) 1998 American Institu te of Physics. [S0021-8979(98)29211-5].