Size effects in the spin-flop transition of 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. Both the Morin transition temperature (T-M) and the spin-flop transition field (H-sf) have been found to increase for increasing particle size. However, the size dependence of H-sf (T = 0) is faster than that observed for T-M, indicating the occurrence of a modification of both the anisotropy and exchange fields induced by thermal treatments. By contrast, the increase of T-M with increasing size should be mainly ascribed to the effect of microstructure on the sole anisotropy field.