Neutron study of magnetic excitations in 8-nm -Fe₂O₃ nanoparticles

By use of inelastic neutron scattering we have studied magnetic fluctuations in 8-nm particles of antiferromagnetic alpha-Fe2O3 (hematite) as a function of temperature and applied magnetic fields. The fluctuations are dominated by uniform excitations. Studies have been performed on both coated (noninteracting) and uncoated (interacting) particles. We have estimated the magnetic anisotropy energy and found that the data are in good agreement with the value obtained from Mossbauer spectroscopy. The energy epsilon(0) of the uniform excitations depends strongly on the uncompensated moment, which is caused by finite-size effects, and we have estimated the size of this moment from the experimental neutron data. The field dependence of epsilon(0) for the interacting nanoparticles differs strongly from that of the noninteracting nanoparticles, and this is a result of the influence of exchange interaction between the particles.