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

The magnetic properties of Mg0.95Mn0.05Fe2O4 ferrite samples with an average particle size of ∼6.0±0.6 nm have been studied using X-ray diffraction, Mössbauer spectroscopy, dc magnetization and frequency dependent real χ′(T)χ′(T) and imaginary χ″(T)χ″(T) parts of ac susceptibility measurements. A magnetic transition to an ordered state is observed at about 195 K from Mössbauer 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 Tmean=195±5K, a temperature which depends upon the distribution of particle volumes in the sample. The FC curve was nearly flat below Tmean, as compared with monotonically increasing characteristics of non-interacting superparamagnetic systems indicating the existence of strong interactions among the nanoparticles. A frequency-dependent peak observed in χ′(T)χ′(T) is well described by Vogel–Fulcher law, yielding a relaxation time τ0=5.8×10−12s and an interaction parameter T0=195±3K. 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 Néel–Brown model and the power law yields an unphysical large value of τ0τ0 (∼6×10−69 and 1.2×10−22 s respectively).