Magnetic Small-Angle Neutron Scattering From Nanoparticles: Theory and Simulation of Surface Anisotropy and Magnetodipolar Interaction Effects Beyond the Superspin Model

Magnetic nanoparticles possess tunable properties that make them attrac-
tive for applications in medicine, materials science, and nanotechnology.
A key challenge in realizing their potential lies in understanding complex
spin structures at the nanoscale. Magnetic small-angle neutron scattering
(SANS), especially in its polarized form, is a powerful method to probe
such textures. However, interpreting SANS data remains di cult due to
its reciprocal-space nature and ensemble averaging over randomly oriented
particles. This thesis presents a combined strategy integrating atomistic
spin dynamics simulations with analytically tractable surrogate models to
investigate magnetic SANS signatures of ultra-fine nanoparticles. Atomistic
simulations o er detailed insight into spin disorder from surface anisotropy,
exchange, and dipole-dipole interactions, but their high dimensionality lim-
its direct data fitting. Analytical models are therefore developed to extract
key physical features in a reduced parameter space suitable for system-
atic comparison with experiments. The approach is first applied to spher-
ical nanoparticles with dominant surface anisotropy. Simulations reveal
characteristic inhomogeneous magnetization profiles—ranging from tangen-
tial (artichoke-like) to radial (hedgehog-like) textures—whose signatures
appear in the SANS cross section as spin-disorder-induced smearing and
anisotropy-dependent form-factor shifts. Perturbative analytical results re-
produce these features and yield interpretable expressions for the scatter-
ing intensity. The second part examines vortex-like spin structures in iron
nanoparticles via micromagnetic simulations and a novel power-series ex-
pansion. Even linear magnetization approximations capture the dominant
SANS response, suggesting symmetry-based interpretation strategies. Fi-
nally, the spin dynamics of nanoparticles is addressed, focusing on surface-
anisotropy-induced nutation. Simulations and analytical models reveal a
collective nutational mode of the net magnetic moment with characteristic
frequencies potentially detectable via inelastic neutron scattering.