Giant magnetoresistance in bulk samples of La<SUB>1−x</SUB>A<SUB>x</SUB>MnO<SUB>3</SUB> (A=Sr or Ca)

Magnetoresistance measurements have been carried out on bulk samples of several members of the La<SUB>1&#8722;x</SUB>A<SUB>x</SUB>MnO<SUB>3</SUB> family (A=Sr and Ca) with varying x or Mn<SUP>4+</SUP> content. The magnitude of magnetoresistance (MR) is highest at the insulator-metal (I-M) transition when the transition temperature is relatively low (&#8804;250 K). The relative MR (%MR) is essentially the same (40-50%) at 4.2 K for all the compositions showing the I-M transition, namely ~15% at T&#8805;300 K. Insulating compositions (small or large x) show smaller %MR than the compositions that undergo the I-M transition