Size dependence of the spin-flop transition in hematite nanoparticles

The spin-flop transition of acicular hematite nanoparticles of different size synthesized by chemical route and annealed at different temperatures has been investigated as a function of temperature. Transmission electron microscopy TEM measurements show that particles have an ellipsoidal shape, with a major axis of 330+/-50 nm and a minor axis of 70+/-10 nm. TEM and x-ray diffraction experiments show that nanoparticles are made of hematite crystallites, which grow on increasing the annealing temperature. Both the Morin transition temperature (TM) and the spin-flop transition field (Hsf ) have been found to increase for increasing crystallite size (d)for instance, for d=36 nm particles TM=164 K and the value of Hsf extrapolated at T=0 (Hsfo) is 1.7 T, whereas for bulk hematite TM=263 K and Hsfo=6.5 T. Both Hsfo and TM follow a 1/d dependence (at a faster rate for Hsfo), indicating that their variation is mainly driven by surface effects.