Effect of the Internal Pressure Due to the A-Site Cations on the Giant Magnetoresistance and Related Properties of Doped Rare Earth Manganates, $Ln_{1-x}A_xMnO_3$ (Ln = La, Nd, Gd, Y; A = Ca, Sr, Ba, Pb)

Based on an extensive study of a large variety of rare earth manganates (rare earth = La, Nd, Gd, or Y) with Ca, Sr, Ba, and Pb as the substituents, as well as of $LaMnO_3$ samples with varying $Mn^{4+}$ content, it is shown that the ferromagnetic transition temperature, $T_{c'}$ as well as the insulator-metal (I-M) transition temperature, $T_{r'}$ increases with the weighted average radius of the A-cations, $\langle r_\lambda\rangle$, showing a maximum at 1.23 ± 0.01 A. The essentially linear region $(\langle r_\lambda\rangle$ = 1.18-1.24 A) demarcates the ferromagnetic metal and paramagnetic insulator regimes. The peak resistivity at the I-M transition, as well as the magnitude of the magnetoresistance (MR), decreases with increasing $\langle r_\lambda\rangle$. Only below a $\langle r_\lambda\rangle$ of 1.23 ± 0.01 A is the MR greater than 50%