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 (T-M) and the spin-flop transition field (H-sf) have been found to increase for increasing crystallite size (d): for instance, for d=36 nm particles T-M=164 K and the value of H-sf extrapolated at T=0 (H-sf0) is 1.7 T, whereas for bulk hematite T-M=263 K and H-sf0=6.5 T. Both H-sf0 and T-M follow a 1/d dependence (at a faster rate for H-sf0), indicating that their variation is mainly driven by surface effects.