Grain size effect on the Neel temperature and magnetic properties of nanocrystalline NiFe2O4 spinel

Nanocrystalline NiFe2O4 spinel ferrites with various grain sizes have been synthesized by ball milling the bulk NiFe2O4. The average grain sizes were estimated from the X-ray line broadening of the (3 1 1) reflection. The Neel temperatures of NiFe2O4 for various grain sizes were determined by magneto thermogravimetric method. The magnetic behaviour has been explained by combining the effects of changes in cation distribution on milling and finite size scaling. The shift in B-H loops has been correlated to the surface spin effects. The high coercivities observed here may be due to high anisotropies of the milled samples. The Hopkinson peak observed just below the Neel temperature has been explained by the mathematical formalism given by the Stoner Wohlfarth model. (C) 2002 Elsevier Science B.V. All rights reserved.