RESTORING THE AFMI-FMM TRANSITION BY CALCIUM DOPING IN THE CMR MANGANITES PR0.52SR0.48-XCAXMNO3

A small deviation from the ratio Mn3+:Mn4+ = 1 in the manganite Pr0.5ySr0.5-yMnO3 leads to a disappearance of the AFMI-FMM transition, the AFMI state being replaced by a FMM state. The present study of the ser ies Pr0.52Sr0.48-xCaxMnO3 shows for the first time that the AFMI-FMM t ransition can be restored for a Mn3+:Mn4+ ratio different from 1 by ca lcium doping. This result is interpreted by the fact that the decrease of the average size of the A-site cation. [r(A)], and of the mismatch , sigma(2), favors the AFMI state at the expense of the FMM state. The deviation from the perfect charge ordering of the 1:1 compound is the n compensated by these two effects. The application of a magnetic fiel d of 7 T shows that all the compositions exhibit a CMR effect. The max imum R-0/R-7 T ratio is equal to 3500 at 127 K for the Pr0.52Sr0.26Ca2 2MnO3 compound.