Spin structures in antiferromagnetic nanoparticles

In this thesis magnetic structures of antiferromagnetic nanoparticles are studied as a function of particle size and aggregation. In nanoparticles the magnetic structure can be different from that of the corresponding bulk system due to the following reasons: a) a significant surface contribution to the magnetic anisotropy, b) the low symmetry environment of surface atoms or defects in the interior of particles leading to non collinear spin structures, and c) exchange interactions between neighbouring particles. Determining the spin structures of antiferromagnetic particles is difficult, however a detailed knowledge of it can be important for applications of antiferromagnetic nanoparticles for example combined with ferromagnetic nanoparticles in nanocomposite devices. In this thesis the magnetic structure, in particular the orientation of the spins in the antiferromagnetic sublattices, is investigated in systems of magnetic nanoparticles using a variety of experimental techniques.
The spin structure in systems with spin canting, due to magnetic atoms in low symmetry surroundings, is studied in a theoretical model that is able to quantitatively explain observations of anomalous temperature dependence of the magnetisation in certain nanoparticle systems, as welll bulk systems with spin canting due to defects. In accordance with this model magnetisation measurements on goethtie (a-FeOOH) nanoparticles are presented, showing a low temperature increase in the magnetisation.
The spin orientation in plate-shaped NiO nanoparticles with thicknesses down to 2.0 nm is investigated with the XY Z-neutron polarisation analysis technique. This provides an effective way of separating the different scattering contributions (magnetic, nuclear and spin incoherent), and thus significantly improve the earlier experimental data from unpolarised neutron diffraction. The spin orientation is found to be close to the particle plane, which is the (111) plane of the FCC structure of NiO for particles with thickness ranging from 2.2 nm to bulk (= 200 nm) particles. In the smallest particles, with a thickness of 2.0 nm, we find a reorientation of the spin to point 30? out of the plane.
Recovery of the spin reorientation, known as the Morin transition, in hematite (a-Fe2O3) nanoparticles was studied as function of particle growth and aggregation. Growth and aggregation of hematite particles with an initial size of ˜ 9 nmb in aqueous suspension was controlled by a hydrothermal treatment and by changing the ionic strenght of the suspension. Interestingly addition of NaCl to the suspension resulted in the particles aggregating in long linear chains, with neighbouring particles aligned along a common [001] axis of the hexagonal structure. The magnetic structure was investigated with Mössbauer spectroscopy, revealing a partial recovery of the Morin transition in samples with significant particle growth. The aggregation in crystallographically aligned linear chains did not introduce a Morin transition, but the addition of NaCl had the effect of partially suppressing superparamagnetic relaxation.
The spin orientation in the mineral hemo-ilmenite (a-Fe2O3-FeTiO3) consisting of nanoscale lamellar intergorwths of hematite and ilmenite was studied with uniaxial neutron polarisation analysis to determine if the unusually high magnetisation in this antiferromagneticparamagnetic mineral can be ascribed to uncompensated spins in contact layers between the lamellae. From the response of the hematite spins to an applied magnetic field we confirm that uncompensated spins as well as canted spins are important in the system. This supports the hypothesis of lamellar magnetism, proposed to explain the unusual magnetic properties of the mineral.
In summary the thesis have demonstrated methods for investigation of spin structures in magnetic nanoparticles. In particular, the classical model of the temperature dependence of canted spin structures sucessfully explains many experimental observations of anomalous temperature dependence in nanoparticle and bulk systems. Morover, XY Z neutron polarisation analysis have been demonstrated to be an effective way of investigating the magnetic properties of antiferromagnetic nanoparticles, significantly improving the unpolarised neutron powder diffraction data usually obtained in investigations of magnetic nanoparticles.