Effect of Substitution of $Cr^{3+}$ in Place of $Mn^{3+}$ in Rare-Earth Manganates on the Magnetism and Magnetoresistance: Role of Superexchange Interaction and Lattice Distortion in $LnMn_{1-x}Cr_xO_3$

Magnetic and electrical properties of oxides of the formula $LnMn_{1-x}Cr_xO_3$ (Ln= rare earth) have been investigated to understand the importance of the $Mn^{3+}-O-Cr^{3+}$ superexchange interactions, in contrast to the $Ln_{1-x}A_xMnO_3$ (A= divalent cation) system, where double exchange interactions predominate. All the $LnMn_{1-x}Cr_xO_3$ compositions are ferromagnetic insulators and exhibit small values of negative magnetoresistance (MR) at ordinary temperatures, the highest value of MR at 6 T being \sim 20% at 120 K for some of the compositions. The results underscore the important role of double exchange, as well as lattice distortion effects, in giving rise to high values of MR in $Ln_{1-x}A_xMnO_3$