COMPETITION BETWEEN METAL-SEMICONDUCTOR AND CHARGE ORDERING TRANSITIONS IN THE CMR CERAMIC PR1 2-XYXSR1/2MNO3/

The systematic study of the solid solution Pr1/2-xYxSr1/2MnO3 shows th e coexistence of two types of CMR effect on the same sample associated with a transition from an antiferromagnetic semiconducting to a ferro magnetic metallic state (at T-co) and then to a paramagnetic semicondu cting state (at T-c) as T increases. The competition between these two types of CMR effect is shown for the first time. For low x values, T- co (for charge ordering) increases and T-c decreases as the mean size of the interpolated cation decreases. Beyond a critical x value the tw o antagonist effects interpenetrate each other so that they tend to di sappear. The rho(T) curves, registered in the Earth's magnetic field ( B = 0) and in B = 5 T show a negative CMR effect similar to that obser ved for Pr1/2Sr1/2MnO3 i.e. a R(0)/R(B=5T) congruent to 10(2) at 120 K for x = 0.04. It is remarkable that the application of a magnetic fie ld has the same effect as increasing the size of the interpolated cati on: it decreases T-co and increases T-c. The electron microscopy study of these samples shows that they exhibit a 'pure' body centered cell (Imma) derived from the perovskite (a approximate to c approximate to a(p) root 2 and b approximate to 2 a).