Speaker
Description
Small-angle neutron scattering (SANS) is a powerful probe of nanoscale structure, but its full potential in magnetism is unlocked when the incident beam is polarized. This lecture introduces the physical foundations and practical implementation of polarized SANS, demonstrating how the technique provides access to information hidden from other characterization methods. SANS probes real-space structure in the 1–100 nm range. Neutrons carry an intrinsic magnetic moment and therefore scatter not only from nuclear scattering length density but also from the magnetization component perpendicular to the scattering vector Q, giving direct access to nanoscale spin textures and magnetic correlations — something X-ray scattering, electron microscopy, and bulk magnetometry cannot provide. Polarizing the beam allows nuclear and magnetic contributions to be separated; the resulting spin-dependent cross-sections, measured as a function of q and applied field, encode the nanometer-scale spatial distribution of magnetization. We walk through the key concepts, such as magnetic scattering length density, spin-dependent cross-sections, field-induced moment alignment, and the experimental implementation of polarization analysis, including sample environment, field geometry, and the separation of nuclear and magnetic scattering in a SANSPOL experiment. To ground these concepts, we apply SANSPOL to magnetic nanoparticles (NPs), which are of high interest for data storage, spintronics[1], magnetic hyperthermia, and drug delivery[2,3], and whose nanoscale magnetic morphology remains poorly understood and is inaccessible to conventional probes such as DC magnetometry and AC susceptibility. Three case studies on chemically and architecturally distinct systems are presented. In CoFe₂O₄ NPs, surface spin disorder is shown to be field-dependent: the magnetically active volume grows as disordered surface spins are progressively polarized, allowing extraction of the spin-disorder energy and surface anisotropy constant across samples from 3.1 to 12.8 nm [4,5]. In core/shell ε-Fe₃N NPs, a spatially inhomogeneous magnetic response from the oxidic shell is uncovered — entirely invisible to bulk probes. Together, these results establish polarized SANS as a cornerstone technique in nanomagnetism, making measurable what was previously hidden in plain sight.
[1] P. Bender et al. J. Phys. Chem. C 122 (2018) 3068.
[2] A. Lak, S. Disch, P. Bender Adv. Science 8 (2021) 2002682.
[3] A. Lappas et al. Phys. Rev. X 9 (2019) 041044.
[4] D. Zákutná et al. Phys. Rev. X 10 (2020) 031019.
[5] M. Gerina et al., Nanoscale Adv. 5 (2023) 4563-4570.
