New material boosts neutron imaging tools for materials research

<p>This thesis explores innovative strategies to improve <strong>Fe-based polarizing multilayer neutron optics</strong>, a cornerstone technology for neutron scattering used to investigate materials at the atomic and molecular levels. Polarization analysis plays a crucial role in uncovering insights into magnetic domain structures, molecular orientations, and protein shapes, enabling breakthroughs in materials science, physics, biology, chemistry, and cultural heritage. However, the performance of conventional multilayer optics, such as Fe/Si systems, is hindered by challenges including low reflectivity due to rough interfaces, reduced polarization caused by scattering length density (SLD) mismatches, spin-flip scattering arising from magnetic inhomogeneities, and difficulties in achieving uniform magnetic behavior, to name a few. These challenges become increasingly critical at higher scattering angles or vectors, where thinner and smoother layers are essential for high performance.</p><p>This work introduces <strong>isotope-enriched boron carbide (¹¹B₄C)</strong> as a key material to address these limitations, demonstrating its versatility through two main approaches: as interlayers and as a mixed component within Fe and/or the non-magnetic layers. Initial studies focused on Fe/¹¹B₄CTi multilayers, which outperformed traditional Fe/Si systems in both reflectivity and polarization. The incorporation of ¹¹B₄C <strong>reduced interface widths</strong>, as evidenced by sharper Bragg peaks in X-ray reflectivity measurements, and enabled <strong>polarization improvements</strong> from 61% to 78% for multilayers with 25 Å bilayer thicknesses and 20 periods. These enhancements were driven by better SLD tuning, through compositional variations, and smoother interfaces, showcasing the potential of ¹¹B₄C to optimize multilayer performance.</p><p>Further studies explored the use of ¹¹B₄C as interlayers in Fe/Si multilayers, resulting in significant performance gains. Interface widths were reduced from 9.5 Å to 5.2 Å, leading to a 125% increase in reflectivity and 15% higher polarization for multilayers with a period thickness of 15 Å and 80 periods. Acting as a barrier between Fe and Si layers, the ¹¹B₄C interlayers improved interface smoothness, suppressed intermixing, and maintained the structural and magnetic integrity of the multilayers.</p><p>The most significant advancements were achieved in multilayers where ¹¹B₄C was mixed into both Fe and Si layers. In these systems, the amorphization effect of ¹¹B₄C <strong>eliminated lateral structural correlations and magnetic coercivity</strong>, and at the same time drastically <strong>reducing spin-flip scattering</strong>. These modifications allowed the multilayers to operate at much lower external magnetic fields, making them more efficient, practical and enables possibilities of new experimental setups. Magnetic off-specular polarized neutron reflectometry (PNR) and muon spin spectroscopy measurements confirmed the uniform magnetic behavior of Fe/Si + ¹¹B₄C-containing multilayers, showing the <strong>suppression of magnetic domains even at low magnetic fields</strong>. The ability to stabilize magnetic properties and enhance polarization performance underscores the significant impact of this approach.</p><p>Additionally, the tunability of ¹¹B₄C concentrations enabled precise <strong>control over magnetic properties</strong> such as coercivity and interlayer exchange coupling. Vibrating sample magnetometry and PNR measurements revealed the emergence of low-field antiferromagnetic coupling and demonstrated how multilayer properties could be tailored for specific applications. These findings highlight the versatility of ¹¹B₄C as both a structural and magnetic modifier in multilayer systems.</p><p>This thesis establishes ¹¹B₄C as a highly effective material for Fe-based polarizing neutron optics, addressing critical challenges by improving reflectivity and polarization, reducing spin-flip scattering, and enabling better control of magnetic properties. By overcoming these limitations, ¹¹B₄C expands the capabilities of neutron scattering techniques, paving the way for advancements in materials science, magnetism, and beyond.</p>