TRANSPORT-PROPERTIES OF CE-DOPED RMNO3 (R = LA, PR, AND ND) MANGANITES

in Ce-doped RMnO3 the rare: earth (R) is partially replaced with ceriu m ions instead of divalent alkaline-earth metals. These compounds are very sensitive to annealing. The resistivity and thermoelectric power (TEP, S) of R0.7Ce0.3MnO3 (R = La, Pr, and Nd) have been studied as a function of annealing conditions. They show a ferromagnetic to paramag netic transition, and a resistive peak near T-c, marking a metal-insul ator (MI) transition. A pronounced giant magnetoresistance effect is a lso observed in these compounds, and T-c can be changed by annealing i n various atmospheres. The rho(T) curve of oxygen overdoped samples sh ows double peaks slang with a hysteresis loop (in the heating and cool ing cycles), which bears the signature of the first-order nature of th e transition. The thermopower of as-prepared samples show a peak near the MI transition, and changes sign from positive to negative at tempe ratures below the ferromagnetic transition. The magnitude of S at low temperatures is enhanced to an abnormally large value. With increasing of oxygen content. several characteristic features develop in the tem perature dependence of the thermopower which is consistent with the re sistive behavior. At high temperatures, the resistivity and thermopowe r follow the predictions of the Emin-Holstein theory of adiabatic pola ron hopping.