Synthesis and properties of nanograined La-Ca-manganite-Ni-ferrite composites

Composites of La0.67Ca0.33MnO3 (LCMO), a colossal magnetoresistance perovskite and NiFe2O4 (NF), an insulating magnetic oxide have been prepared in situ using microwave-refluxing technique. Microstructural studies of the composites show that the two phases are uniformly distributed and the grain size of both the phases is identical, in the range 10-40 nm. The NF appears as a separate phase for concentrations x > 0.10 M, where M is the molecular weight of NF in the starting precursor solution. For concentrations < 0.1 M, the X-ray diffraction studies indicate the presence of only the LCMO phase. Pure LCMO exhibits an insulator-metal transition T-MI at 215 K whereas the magnetic transition T-C occurs at 250 K. The transition temperatures T-C of this LCMO phase decreases from 250 to 125 K for the addition of 0.15 M NF while T-MI decreases from 215 to 90 K for the addition of just 0.02 M NF to the precursor solution. The magnetoresistance (MR) magnitude or increase in electrical conductivity of LCMO in the presence of an external magnetic field decreases progressively with the addition of NF. The MR of the composite with 0.01 M NF increases with decreasing temperature to similar to 16% at 85 K in the presence of 8.5 kOe external field. The suppression of electrical and magnetic transitions in LCMO is found to be due to substitution of Mn by either or both Ni and Fe. The increasing MR response of the composite is due to the nanocrystalline grain size of the two phases and also partly due to the substitution phenomenon. (c) 2007 Elsevier B.V. All rights reserved.