Vivas Laura G, Yanes Rocio, Michels Andreas
Physics and Materials Science Research Unit, University of Luxembourg, 162A avenue de la Faiencerie, Luxembourg, L-1511, Luxembourg.
Department of Applied Physics, University of Salamanca, Plaza de los Caidos, Salamanca, 37008, Spain.
Sci Rep. 2017 Oct 12;7(1):13060. doi: 10.1038/s41598-017-13457-2.
Magnetic small-angle neutron scattering (SANS) is a powerful technique for investigating magnetic nanoparticle assemblies in nonmagnetic matrices. For such microstructures, the standard theory of magnetic SANS assumes uniformly magnetized nanoparticles (macrospin model). However, there exist many experimental and theoretical studies which suggest that this assumption is violated: deviations from ellipsoidal particle shape, crystalline defects, or the interplay between various magnetic interactions (exchange, magnetic anisotropy, magnetostatics, external field) may lead to nonuniform spin structures. Therefore, a theoretical framework of magnetic SANS of nanoparticles needs to be developed. Here, we report numerical micromagnetic simulations of the static spin structure and related unpolarized magnetic SANS of a single cobalt nanorod. While in the saturated state the magnetic SANS cross section is (as expected) determined by the particle form factor, significant deviations appear for nonsaturated states; specifically, at remanence, domain-wall and vortex states emerge which result in a magnetic SANS signal that is composed of all three magnetization Fourier components, giving rise to a complex angular anisotropy on a two-dimensional detector. The strength of the micromagnetic simulation methodology is the possibility to decompose the cross section into the individual Fourier components, which allows one to draw important conclusions regarding the fundamentals of magnetic SANS.
磁性小角中子散射（SANS）是研究非磁性基质中磁性纳米颗粒组装体的一种强大技术。对于此类微观结构，磁性SANS的标准理论假定纳米颗粒均匀磁化（宏观自旋模型）。然而，有许多实验和理论研究表明这一假设并不成立：偏离椭球形颗粒形状、晶体缺陷或各种磁相互作用（交换作用、磁各向异性、静磁作用、外场）之间的相互作用可能导致自旋结构不均匀。因此，需要建立纳米颗粒磁性SANS的理论框架。在此，我们报告了单个钴纳米棒的静态自旋结构及相关非极化磁性SANS的数值微磁模拟。在饱和状态下，磁性SANS截面（如预期的那样）由颗粒形状因子决定，但在非饱和状态下会出现显著偏差；具体而言，在剩磁状态下，会出现畴壁和涡旋状态，这导致磁性SANS信号由所有三个磁化傅里叶分量组成，在二维探测器上产生复杂的角各向异性。微磁模拟方法的优势在于能够将截面分解为各个傅里叶分量，这使得人们能够就磁性SANS的基本原理得出重要结论。
