Effects of dilution on magnetic and transport properties of La0.7Ca0.3Mn1-xM'xO3
Magnetic and transport properties of La0.7Ca0.3Mn1-xM'xO3 (M'=Al,Ti) are studied. The dilution of the Mn lattice 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 La0.7Sr0.3Mn1-xM'xO3 [Phys. Rev. B 73, 184403 (2006)] is no longer obtained. Extrapolations of the Tc(np) linear curves to Tc=0 give np values much smaller than 1. These results suggest that, according to a molecular-field approximation, antiferromagnetic superexchange between Mn ions is significant in La0.7Ca0.3MnO3, in contrast to what observed in La0.7Sr0.3MnO3. Additionally, structural data of the Al-substituted samples suggest that variations of the eg-electron bandwidth W cannot explain the decrease of Tc in magnetically diluted manganites.