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(X)MNO(3) (LN=LA,ND,GD,Y, A=CA,SR,BA,PB)

Based on an extensive study of a large variety of rare earth manganate s (rare earth = La, Nd, Gd, or Y) with Ca, Sr, Ba, and Pb as the subst ituents, 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 maximu m at 1.23 r +/- 0.01 Angstrom. The essentially linear region ([r(A)] = 1.18-1.24 Angstrom) demarcates the ferromagnetic metal and paramagnet ic insulator regimes. The peak resistivity at the I-M transition, as w ell as the magnitude of the magnetoresistance (MR), decreases with inc reasing [r(A)] Only below a [r(A)] of 1.23 +/- 0.01 Angstrom is the MR greater than 50%. (C) 1995 Academic Press, Inc.