Magnetic and transport properties of La0.7 Ca0.3 Mn1-x Mx′ O3 (M′ =Al,Ti) are studied. The dilution of the Mn network results in a weakening of the ferromagnetism, a deterioration of the metallic conductivity, and a strong enhancement of the magnetoresistance. Although Tc linearly decreases with x in the low substitution ranges for both M′ series, the scaling behavior Tc (np) previously observed for La0.7 Sr0.3 Mn1-x Mx′ O3 is no longer obtained. Extrapolations of the Tc (np) linear curves to Tc =0K give np values much smaller than one. These results suggest that, according to a molecular-field approximation, antiferromagnetic superexchange between Mn ions is significant in La0.7 Ca0.3 MnO3, in contrast to what was observed in La0.7 Sr0.3 MnO3. Additionally, the structural data of the Al-substituted samples suggest that variations of the eg -electron bandwidth W cannot explain the decrease in Tc in magnetically diluted manganites. © 2008 The American Physical Society.

Nam D.N.H., Dai N.V., Thanh T.D., Tuong L.T.C., Hong L.V., Phuc N.X., Hong H.S., Khien N.V.

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