Doping effects from Fe and Ge for Mn in La1-xCaxMnO3 (x=0.15-0.6)

Effects from 4% Fe or Ge for Mn in La1-xCaxMnO3 (x = 0.15-0.6) are studied experimentally. All the compounds exhibit orthorhombic structure, unaffecte d by the introduction of Fe and Ge. The magnetic ordering temperature (T-c) and, accordingly, the metal-insulator transition decrease systematically d ue to the presence of secondary ions at the Mn lattice. The doping effects depend on dopant and Ca content, and the typical reduction in T-c, compared to the undoped counterparts, is similar to 60 K. Ge is more effective in s uppressing the magnetic coupling for x less than or equal to 0.35, as indic ated by the similar to 25 K lower Curie temperature in the Ge-doped compoun ds. When x exceeds 0.35, the Fe- and Ge-doping effects reverse. The other s triking observation of the present study is the significant ferromagnetic e nhancement below similar to 130 K, characterized by a sharp jump of the mag netization under a field of 5 T, and the re-entrance of the charge ordering state below similar to 50 K in the Ce-doped La0.5Ca0.5MnO3. The different doping effects of Fe and Ge presumably originate from their different roles of charge carrier mobility. The magnetic coupling between Fe and host Mn s eems unimportant.