Small-angle neutron scattering (SANS) is one of the most important techniques for microstructure determination, being utilized in a wide range of scientific disciplines, such as materials science, physics, chemistry, and biology. The reason for its great significance is that conventional SANS is probably the only method capable of probing structural inhomogeneities in the bulk of materials on a mesoscopic real-space length scale from roughly 1 to 300 nm. Moreover, the exploitation of the spin degree of freedom of the neutron provides SANS with a unique sensitivity to study magnetism and magnetic materials at the nanoscale. As such, magnetic SANS ideally complements more real-space and surface-sensitive magnetic imaging techniques, e.g., Lorentz transmission electron microscopy, electron holography, magnetic force microscopy, Kerr microscopy, or spin-polarized scanning tunneling microscopy. This review summarizes the recent applications of the SANS method to study magnetism and magnetic materials. This includes a wide range of materials classes from nanomagnetic systems such as soft magnetic Fe-based nanocomposites, hard magnetic Nd-Fe-B-based permanent magnets, magnetic steels, ferrofluids, nanoparticles, and magnetic oxides to more fundamental open issues in contemporary condensed matter physics such as skyrmion crystals, noncollinear magnetic structures in noncentrosymmetric compounds, magnetic or electronic phase separation, and vortex lattices in type-II superconductors. Special attention is paid not only to the vast variety of magnetic materials and problems where SANS has provided direct insight, but also to the enormous progress made regarding the micromagnetic simulation of magnetic neutron scattering.
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We thank Giordano Benacchio, C. Franz, D. Kim, Artem Malyeyev, Mike Manno, Eric McCalla, Jeff Walter, and D. Zákutná for a critical reading of the manuscript. A. M. thanks the National Research Fund of Luxembourg for financial support. E. A. P. acknowledges ABB for its continuous support. S. E. and D. B. acknowledge financial support from the Deutsche Forschungsgemeinschaft (Project No. BE 2464/10-3), the EU-FP7 project “NANOPYME” (310516), and the EU Horizon-2020 project “AMPHIBIAN” (720853). M. R. E. was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Award No. DE-SC0005051. F. B. thanks Andreas Ulbricht for fruitful discussions over the many years. C. L. was funded by the U.S. Department of Energy through the University of Minnesota Center for Quantum Materials under DE-FG02-06ER46275 and DESC-0016371. S. D. acknowledges financial support from the German Research Foundation (DFG Emmy Noether Grant No. DI 1788/2-1).
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