Magnetic dynamics of small alpha-Fe2O3 and NiO particles studied by neutron scattering

We have studied the magnetic dynamics in nanocrystalline samples of alpha-F e2O3 (hematite) and NiO by inelastic neutron scattering. By measuring aroun d the structural and the antiferromagnetic reflections, we have probed unif orm and staggered magnetic oscillations, respectively. In the hematite part icles, we observed a clear double peak in the energy distribution of the an tiferromagnetic signal, in addition to a quasi-elastic peak. We interpret t he double peak to represent collective magnetic excitations. Broadening of the central quasi-elastic peak with increasing temperature is interpreted a s a sign of superparamagnetic relaxation. Studies of the antiferromagnetic signal from NiO also show evidence of collective magnetic excitations, but with a higher energy of the precession states than for hematite. The inelas tic signal at the structural reflection of NiO presents evidence for unifor m magnetic oscillations very similar to the antiferromagnetic signal, as is expected for a simple antiferromagnet. The hematite sample did not show an y signs of uniform oscillations, although these have been predicted theoret ically.