Superparamagnetism and interparticle interactions in ZnFe2O4 nanocrystals

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 x 10(5) to 3.4 x 10(5) erg cm(-3) and an effective superparamagnetic moment from 7.0 x 10(2) to 8.6 x 10(3) mu(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.