Abstract
We present here a detailed investigation of the static and dynamic magnetic behavior of a Mg0.95Mn0.05Fe2O4 spinel ferrite nanoparticle system synthesized by high-energy ball milling of almost identical particle size distributions (< D > = 4.7, 5.1 and 6.0 +/- 0.6 nm). The samples were characterized by using x-ray diffraction, Mossbauer spectroscopy, dc magnetization and frequency dependent real. chi'(T) and imaginary chi ''(T) parts of ac susceptibility measurements. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization have been recorded in a low field and show a behavior typical of superparamagnetic particles above a temperature of 185 +/- 5 K, which is further supported from the temperature dependent Mossbauer measurements. The fact that the blocking temperature calculated from the ZFC magnetization and Mossbauer data are almost similar gives a clear indication of the interparticle interactions among these nanoparticle systems. This is further supported from the FC magnetization curves, which are almost flat below a certain temperature (less than the blocking temperature), as compared with the monotonically increasing behavior characteristics of non-interacting superparamagnetic particles. A shift of the blocking temperature with increasing frequency was observed in the real. chi'(T) and imaginary. chi ''(T) parts of the ac susceptibility measurements. The analysis of the results shows that the data fit well with the Vogel-Fulcher law, whereas trials using the Neel -Brown and power law are unproductive. The role of magnetic interparticle interactions on the magnetic behavior, namely superparamagnetic relaxation time and magnetic anisotropy, are discussed.