Nickel oxide (NiO) and hematite (a-Fe2O3), both antiferromagnets, have magnetic properties which at nanoscale differ from those of the bulk materials. With emphasis on NiO nanoparticles and comparisons with a-Fe2O3 nanoparticles these magnetic propertiesare studied by a range of experimental techniques: elastic and inelastic neutron scattering, Mössbauer spectroscopy, x-ray diffraction, transmission electron microscopy and vibrating sample magnetometry. Knowledge of the size and shape of thenanoparticles is an often neglected prerequisite for studies of their magnetic properties. The NiO nanoparticles are found to be plate shaped with the (111) planes as plate faces, a thickness of about 2.3 nm and a diameter of about 13 nm. The magneticstructure is similar to that of bulk NiO, with the spins confined in the (111) planes. Measurements of the spin dynamics reveal a value of the magnetic anisotropy aligning the spins along an easy axis within the planes significantly larger than the bulkvalue. The agglomeration state of the particles has an important influence on the magnetic properties of the particles. In both materials exchange interactions between the particles strongly affect the spin dynamics, increasing the resonance energy ofzero wave vector (q = 0) spin waves and suppressing the thermally activated relaxation of the spins, known as superparamagnetism. A significant reduction of this interaction is surprisingly achieved by simple mechanical treatments, such as grinding in amortar by hand. Nanoparticles of antiferromagnetic materials will have an uncompensated magnetic moment, arising at finite particle sizes because of a surplus of spins in one sublattice. This uncompensated moment is quantified in the NiO nanoparticles andfound to be independent of the aggregation state. Use of the recently implemented monochromatic imaging mode of the RITA-II triple axis neutron spectrometer to measure inelastic neutron scattering data from the NiO nanoparticle samples is described. Theadvantages of using such a multi-blade mode are demonstrated.
Energiteknologier på vej; Risø-PhD-30(EN); Risø-PhD-30