Opportunities for near-surface small-angle neutron scattering to probe magnetic nanostructures within thin-film volumes

Abstract

Near-surface small-angle neutron scattering (NS-SANS) is a highly versatile, yet under-utilized, technique in condensed matter research. It addresses the shortcomings of transmission SANS to enable the characterization of nano-structures within extremely small sample volumes in the thin-film limit. NS-SANS stands out in its capacity to resolve 1D, 2D or 3D structural, chemical and magnetic correlations beneath the surfaces of thin films with nanometre resolution. By varying the incident angle above the critical angle of reflection, NS-SANS delivers tuneable depth sensitivity across nano-confined volumes, effectively minimizing noise contributions from substrates while surpassing the surface-sensitive capabilities of grazing-incidence SANS. This perspective highlights the future potential of NS-SANS to study condensed matter thin films and heterostructures, with a special focus on nanoscale magnetic phenomena, such as topological skyrmion lattices, superconducting vortex lattices and chiral domain walls, which are of timely interest to the magnetism and quantum materials communities.

Keywords: grazing-incidence small-angle neutron scattering; magnetism; near-surface small-angle neutron scattering; thin films.

Figure 1

Schematic of the NS-SANS geometry for a vertical reflection plane. The coordinate system is chosen such that the sample surface lies in the xy plane and the scattering plane of the detector lies in the yz plane. A beam of monochromatic and highly collimated neutrons propagates along the x direction and impinges onto the sample surface at a shallow incidence angle α_i greater than or equal to the critical angle α_c of the sample. In this geometry, the sample is fully illuminated and the bulk ordering of the sample is probed. The resulting detector image will be sensitive to lateral and vertical correlations in Q_y and Q_z, respectively.

Figure 2

Penetration depth D of neutrons as a function of α_i/α_c. The exemplar curve has been calculated for the cubic chiral magnet Fe_0.75Co_0.25Si at a neutron wavelength of 5 Å, resulting in a critical angle of α_c = 0.37°. Depending on the material species, the absolute values of D and α_i/α_c will vary and can be determined from equations (1) and (2). For α_i/α_c = 1, neutrons begin to undergo small-angle scattering within the bulk volume of the film in the NS-SANS regime. For increasingly larger incident angles, such that α_i/α_c > 1, the penetration depth of neutrons into the sample is greatly enhanced and NS-SANS measurements can be optimized towards bulk sensitivity in the range of 10⁴ nm below the sample surface.

Figure 3

Comparison of scattering patterns obtained in the (a) helical, (b) conical and (c) skyrmion lattice phases of MnSi in the transmission SANS geometry (column 1) and in the NS-SANS geometry (column 2). Adapted with permission from Causer et al. (2023a ▸).