Superparamagnetism and interparticle interactions in ZnFe2O4nanocrystals

Abstract

ZnFe₂O₄ particles ranging from 4 to 19 nm have been prepared by a solvothermal method. They exhibit superparamagnetic behavior above the blocking temperature which takes a value from 14 to 62 K, magnetization values from 30 to 60 emu g⁻¹, an anisotropy constant that ranges from 1.1 × 10⁵ to 3.4 × 10⁵ erg cm⁻³ and an effective superparamagnetic moment from 7.0 × 10² to 8.6 × 10³μ_B. The superparamagnetic behavior is affected by the presence of dipolar interparticle interactions which are deeply influenced by the synthesis conditions. These interactions yield a wider ZFC maximum and a higher blocking temperature. Moreover, other kinds of interparticle interactions have been found to take place at low temperature and occur through frozen surface spins. Whereas the dipole interactions reduce the anisotropy of the system, the surface spin interactions tend to increase it and the material becomes magnetically harder.