LARGE LOW-FIELD MAGNETORESISTANCE OVER A WIDE TEMPERATURE-RANGE INDUCED BY WEAK-LINK GRAIN-BOUNDARIES IN LA0.7CA0.3MNO3

Polycrystalline bulk porous samples with a large number of weak-link g rain boundaries and high density polycrystalline bulk samples with str ong-link boundaries were synthesized by conventional solid-state react ion and a partial melting technique. The weak-link samples showed larg e magnetoresistance (MR,Delta rho/rho(H)= [rho(0) - rho(H)]/rho(0)), 2 0-30%, at a low magnetic field of 300 mT and over a wide temperature r ange from the magnetic transition at 235 to 77 K. The partially melted samples exhibited the same magnetoresistance behavior as that of sing le crystals with a maximum peak MR of 15% at a narrow temperature rang e around the ferromagnetic transition. It is suggested that grain boun daries are necessary but not sufficient to account for the MR at low f ield over a large temperature range. Weak-link grain boundaries rather than strong-link boundaries are responsible for the MR at low field o ver a large temperature range. (C) 1998 American Institute of Physics.