Effect of the internal pressure due to the A-site cations on the giant magnetoresistance and related properties of doped rare earth manganates, Ln<SUB>1&#8722;x</SUB>A<SUB>x</SUB>MnO<SUB>3</SUB> (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, Tc, as well as the insulator-metal (I-M) transition temperature, Tp, increases with the weighted average radius of the A-cations, &lt;rA&gt;, showing a maximum at 1.23&#177;0.01 &#197;. The essentially linear region (&lt;rA&gt;=1.18-1.24 &#197;) 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 &lt;rA&gt;. Only below a &lt;rA&gt; of 1.23&#177;0.01 &#197; is the MR greater than 50%