Magnetic study of Mg0.95Mn0.05Fe2O4 ferrite nanoparticles

The magnetic properties of Mg0.95Mn0.05Fe2O4 ferrite samples with an average particle size of similar to 6.0 +/- 0.6 nm have been studied using X-ray diffraction, Mossbauer spectroscopy, dc magnetization and frequency dependent real chi'(T) and imaginary chi'(T) parts of ac susceptibility measurements. A magnetic transition to an ordered state is observed at about 195 K from Mossbauer measurements. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization have been recorded at low field and show the typical behavior of a small particle system. The ZFC curve displays a broad maximum at T-mean = 195 +/- 5 K, a temperature which depends upon the distribution of particle volumes in the sample. The FC curve was nearly flat below T-mean, as compared with monotonically increasing characteristics of non-interacting superparamagnetic systems indicating the existence of strong interactions among the nanoparticles. A frequency-dependent peak observcd in chi'(T) is well described by Vogel-Fulcher law, yielding a relaxation time tau(0) = 5.8 x 10(-12) s and an interaction parameter T-0 = 195 +/- 3 K. Such values show the strong interactions and rule out the possibility of spin-glass (SG) features among the nanoparticle system. On the other hand fitting with the Neel-Brown model and the power law yields an unphysical large value of tau(0) (similar to 6 x 10(-69) and 1.2 x 10(-22) s respectively). (c) 2006 Elsevier Ltd. All rights reserved.141420320