Magnetic Neutron Scattering of Nanoparticles: Failure of The Superspin Model

It is known that beyond a certain size magnetic nanoparticles exhibit a nonuniform internal spin structure.This feature, which is of relevance for many problems of practical interest (e.g., biomedical imaging and drug delivery), is commonly ignored when it comes to the analysis of magnetic neutron scattering data on nanoparticle ensembles.By means of numerical micromagnetic computations we study the transition from single-domain to multi-domain behavior in nanoparticles and its implications for the ensuing elastic magnetic small-angle neutron scattering (SANS) cross-section.Above the critical single-domain size we find that the magnetic SANS cross-section and the related correlation function cannot be described anymore with the uniform particle model, resulting e.g. in deviations from well-known Guinier Law. Independent of particle concentration, we identify a clear signature for the occurrence of a vortex-like spin structure at remanence [¹]. The micromagnetic approach to magnetic SANS offers greater flexibility than the structural-model-based superspin approach and allows one to access the contributions of the individual Fourier components to the cross-section and to obtain a deeper understanding of magnetic SANS.
Ref : [1] L. G. Vivas, et al, Phys. Rev. Lett. 125, 117201 (2020)