Characterization of Mossbauer and Superparamagnetic Properties in Maghemite Nanoparticles Synthesized by a Sol-Gel Method

Maghemite (& gamma;-Fe2O3) nanoparticles were synthesized via a sol-gel process with (Fe(NO3)(3) 9H(2)O) as a starting material and annealed in an Ar/H-2 (5%) balanced gas atmosphere. According to x-ray analysis, the average particle size was found to be 7.0 nm with a narrow size distribution for samples annealed at 150 & DEG;C. Transmission electron microscopy (TEM) analysis also confirmed a particle size of 7.2 nm. The structural and magnetic properties were analyzed using x-ray diffraction (XRD), vibrating-sample magnetometry (VSM), and Mossbauer spectroscopy, and were found to have a spinel structure and exhibit superparamagnetic behavior. TEM was carried out to monitor the size and morphology of the particles. The hyperfine fields at 4.2 K for the A and B sites were determined to be 509 kOe and 476 kOe, respectively. The isomer shift values were & delta;(B) = 0.36 mm/s and & delta;(A) = 0.32 mm/s, which both correspond to Fe3+. Since magnetite (Fe3O4) has Fe2+ and maghemite has only Fe3+, it can be seen from the Mossbauer result that the powder heat-treated at 150 & DEG;C is maghemite. The blocking temperature (T-B) of the superparamagnetic maghemite nanoparticles was approximately 167 & PLUSMN; 5 K. The magnetic anisotropy constant was calculated to be 1.4 x 10(6) ergs/cm(3). The coercivity value at 0 K was calculated as H-CO = 174.5 Oe.

Automated summary

Maghemite nanoparticles were synthesized via a sol-gel process and annealed in an Ar/H-2 (5%) balanced gas atmosphere. The average particle size was found to be 7.0 nm with a narrow size distribution. The nanoparticles exhibited superparamagnetic behavior with a spinel structure. The Mossbauer result confirmed that the powder heat-treated at 150 & DEG;C was maghemite.