Giant Magnetoresistance in Bulk Samples of La1-xAxMnO3 (A = Sr or Ca)

Magnetoresistance measurements have been carried out on bulk samples of several members of the La1-x A x MnO3 family (A = Sr and Ca) with varying x or Mn4+ content. The magnitude of magnetoresistance (MR) is highest at the insulator-metal (I-M) transition when the transition temperature is relatively low (≤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 ≥ 300 K. Insulating compositions (small or large x) show smaller %MR than the compositions that undergo the I-M transition.