Tailoring magnetic nanoparticles (MNPs) by choosing a suitable combination of size, shape, and material is the basis for realizing various technological, biomedical, or environmental applications. For optimal performance in a specific application, it is crucial to interrelate their macroscopic characteristics with the structural and magnetic properties of MNPs and their ensembles. For example, disorder effects -ubiquitous in nanomaterials - crucially determine the magnetic heating performance of MNPs used for hyperthermia, magnetic particle imaging, and catalysis applications [1].
In this contribution, I will present the advantages of small-angle neutron scattering (SANS) for investigating the nanoscale distribution of spin disorder in the relevant mesoscopic size range from 1 to a few hundred nanometres and with nm resolution [2]. Firstly, I will show that the MNPs' morphology and chemical homogeneity can be revealed in great detail with SANS, which is not evident from transmission electron microscopy. Then, I will focus on the magnetic morphology of the single-phase MNPs. I will show that the classical picture that considers the single-phase MNPs as a collinearly magnetized core with a structurally and magnetically disordered surface region falls short as the more complex field-dependent magnetization processes near the surface of the MNPs were unveiled [3]. Furthermore, the particle and coherent size dependence of the surface anisotropy constant will be discussed. Additionally, I will demonstrate that the sensitivity of SANS to nanoscale density variations has the advantage of revealing the chemical morphology of core-shell MNPs consisting of a wüstite-like core and a spinel-type shell [4]. Finally, I will show that even different local magnetizations within core-shell MNPs can be fully disentangled.

References
[1] A. Lak, S. Disch, P. Bender Adv. Science 8 (2021) 2002682.
[2] D. Honecker et al., Nanoscale Adv. 4 (2022) 1026.
[3] D. Zákutná et al. Phys. Rev. X 10 (2020) 031019.
[4] D. Zákutná et al. ACS Chem. Mater. 35 (2023) 2302-2311.