Institute for Experimental Condensed Matter Physics, Ruhr-University Bochum, D-44780 Bochum, Germany.
Nanotechnology. 2012 Feb 10;23(5):055707. doi: 10.1088/0957-4484/23/5/055707. Epub 2012 Jan 11.
We have investigated the structure and magnetism of self-assembled, 20 nm diameter iron oxide nanoparticles covered by an oleic acid shell for scrutinizing their structural and magnetic correlations. The nanoparticles were spin-coated on an Si substrate as a single monolayer and as a stack of 5 ML forming a multilayer. X-ray scattering (reflectivity and grazing incidence small-angle scattering) confirms high in-plane hexagonal correlation and a good layering property of the nanoparticles. Using polarized neutron reflectivity we have also determined the long range magnetic correlations parallel and perpendicular to the layers in addition to the structural ones. In a field of 5 kOe we determine a magnetization value of about 80% of the saturation value. At remanence the global magnetization is close to zero. However, polarized neutron reflectivity reveals the existence of regions in which magnetic moments of nanoparticles are well aligned, while losing order over longer distances. These findings confirm that in the nanoparticle assembly the magnetic dipole-dipole interaction is rather strong, dominating the collective magnetic properties at room temperature.
我们研究了自组装的 20nm 直径氧化铁纳米粒子的结构和磁性，这些纳米粒子被油酸壳覆盖，以仔细研究它们的结构和磁性相关性。纳米粒子被旋涂在 Si 衬底上，形成单层和 5 层 ML 的多层。X 射线散射（反射率和掠入射小角散射）证实了纳米粒子具有高的面内六方相关性和良好的层状性质。使用偏振中子反射率，我们还确定了平行和垂直于层的长程磁性相关，除了结构相关之外。在 5kOe 的磁场中，我们确定了大约 80%的饱和磁化值。在剩磁中，总磁化接近于零。然而，偏振中子反射率揭示了在纳米粒子组装中，纳米粒子的磁矩很好地对齐，而在较长距离上失去了有序性。这些发现证实了在纳米粒子组装中，磁偶极子-磁偶极子相互作用相当强，在室温下主导着集体磁性性质。
