Effect of the Internal Pressure Due to the A-Site Cations on the Giant Magnetoresistance and Related Properties of Doped Rare Earth Manganates, Ln1-xAxMnO3 (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 LaMnO3 samples with varying Mn4+ content, it is shown that the ferromagnetic transition temperature, T c, as well as the insulator-metal (I-M) transition temperature, T p , increases with the weighted average radius of the A-cations, 〈r A〉, showing a maximum at 1.23 ± 0.01 Å. The essentially linear region (〈r A〉 = 1.18-1.24 Å) 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 〈r A〉. Only below a 〈r A〉 of 1.23 ± 0.01 Å is the MR greater than 50%.