  Magnetic nanoparticles of La0.67Sr0.33MnO3 (LSMO) manganite with mean
particle sizes of 13, 16, 18 and 21 nm were prepared by the sol-gel method. The
crystal structure and mean particle size of the synthesized powders were
estimated by X-ray diffraction (XRD) analysis using rietveld refinement and
transmission electron microscopy (TEM). Fourier transform infrared (FTIR)
transmission spectroscopy revealed that stretching and bending modes are
influenced by calcinations temperature. Dc magnetization versus magnetic field
of the samples was carried out at room temperature. Magnetic dynamics of the
samples was studied by the measurement of ac magnetic susceptibility versus
temperature at different frequencies and ac magnetic fields. A
frequency-dependent peak was observed in ac magnetic susceptibility versus
temperature which is well described by Vogel-Fulcher and critical slowing down
laws, and empirical and parameters. By fitting the experimental data with
Vogel-Fulcher and critical slowing down laws, the relaxation time,
characteristic temperature, magnetic anisotropy energy, effective magnetic
anisotropy constant and critical exponent zv have been estimated. The obtained
values of c1, c2, T0 and t0 from the Vogel-Fulcher law support of the presence
of strong interaction between magnetic nanoparticles. The values of zv and t0
obtained from critical slowing down fit suggest the presence of superspin glass
behavior in LSMO nanoparticles of different sizes.

Mesoscale and Nanoscale Physics;Materials Science