Giant Magnetoresistance in Bulk Samples of $La_{1-x}A_xMnO_3$ (A = Sr or Ca)

Magnetoresistance measurements have been carried out on bulk samples of several members of the $La_{1-x}A_xMnO_3$ family (A = Sr and Ca) with varying x or $Mn^{4+}$ content. The magnitude of magnetoresistance (MR) is highest at the insulator-metal (I-M) transition when the transition temperature is relatively low $(\leq 250 K)$, The relative MR (%MR) is essentially the same (40-50%) at 4.2 K for all the compositions showing the I-M transition, namely $\sim 15\%$ at $T \geq 300 K$. Insulating compositions (small or large x) show smaller %MR than the compositions that undergo the I-M transition