EFFECTS OF PARTICLE SIZE ON THE PROPERTIES OF La_0.67Ca_0.33MnO_3±d NANOPARTICLES PREPARED BY THE SOL-GEL METHOD

C. Vázquez-Vázquez; M.A. López-Quintela; R.D. Sánchez* and J.Rivas*

Departments of Physical Chemistry and Applied Physics*, University of Santiago de Compostela. E-15706 Santiago de Compostela (Spain)

We have prepared nanoparticles of La_0.67Ca_0.33MnO_3±d via sol-gel technology, using urea as gelificant agent and starting with a solution of the metallic nitrates. After decomposing the gel at 250^oC, the samples were calcined at temperatures ranging between 300 and 1000^oC. Crystallization begins at 500^oC and it is completed at about 600^oC.

The samples were characterized by x-ray diffraction and the Rietveld method was used in order to extract structural and microstructural information. Crystallite sizes were also determined using the Debye-Scherrer equation and compared with those obtained by Fourier methods.

Systematic variations on lattice parameters, crystallite sizes and microstrains were observed in function of the calcination temperature. Cell parameters show an increase between 600 and 700^oC, being more or less constant above this temperature. Crystallite sizes increase from ~ 20 to 130 nm when the calcination temperature increases, while lattice microstrains decrease.

Calcination at different temperatures leads to samples of different particle sizes, as it was proved by transmission electron microscopy (TEM); however, TEM micrographs show elongated particles which polar (long) axis size increases from 40 to 300 nm as the calcination temperature increases.

Magnetic properties show two behaviours: the smaller particles are superparamagnetic, while the larger ones are ferromagnetic. We have related it with mono- and multi-domain particles. Larger magnetorresistances were observed for La_0.67Ca_0.33MnO_3+d nanoparticles in comparison to conventional ceramic routes.