交换耦合磁性Fe（3-delta）O4 @ CoFe2O4 @Fe（3-delta）O4纳米粒子洋葱结构的详细研究
A Detailed Investigation of the Onion Structure of Exchanged Coupled Magnetic Fe(3-delta)O4@CoFe2O4@Fe(3-delta)O4 Nanoparticles
摘要：
纳米粒子（NP）结合了几个磁相，因为它们的磁特性的高度模块化，为尖端应用提供了广阔的前景。除了增加每个相固有的磁特性之外，从核-壳以及进一步从洋葱结构产生的界面导致协同特性，例如磁交换耦合。这种现象对于克服阻碍潜在应用的氧化铁NP的超顺磁性极限具有高度的意义。在这篇手稿中，我们报告的设计与洋葱状结构的纳米粒子已经很少报道。这些纳米颗粒由Fe（3-delta）O 4核组成，其被CoFe 2 O 4的第一壳和Fe（3-delta）O 4的第二壳覆盖。它们通过多步种子介导生长方法合成。虽然TEM显微照片清楚地显示了每个壳从铁氧化物核的生长，但核尺寸和壳厚度与尺寸增加所暗示的明显不同。我们在进行高分辨率（扫描）TEM成像和GPA时非常精确地研究了NPs的结构。用EELS研究了原子的化学组成和空间分布。用穆斯堡尔谱、软X射线吸收光谱和X射线微区衍射研究了阳离子的化学环境和氧化态。这些技术使我们能够估计核@壳结构的氧化铁核中的Fe 2+含量的增加以及核@壳@壳结构中的钴铁氧体壳厚度的增加，而氧化铁壳似乎比预期薄得多。因此，化学组成的改变以及Fe（3-δ）O 4核的尺寸和钴铁氧体壳的厚度对磁性能具有很大影响。此外，氧化铁壳的生长也显著地改变核-壳纳米颗粒的磁性。
英文摘要：
Nanoparticles (NPs) which combine several magnetic phases offer wide perspectives for cutting edge applications because of the high modularity of their magnetic properties. Besides the addition of the magnetic characteristics intrinsic to each phase, the interface that results from core-shell and, further, from onion structures leads to synergistic properties such as magnetic exchange coupling. Such a phenomenon is of high interest to overcome the superparamagnetic limit of iron oxide NPs which hampers potential applications. In this manuscript, we report on the design of NPs with an onion-like structure which have been scarcely reported yet. These NPs consist in a Fe(3_delta)O4 core covered by a first shell of CoFe2O4 and a second shell of Fe(3-delta)O4. They were synthesized by a multi-step seed mediated growth approach. Although TEM micrographs clearly show the growth of each shell from the iron oxide core, core sizes and shell thicknesses markedly differ from what is suggested by the size increase. We investigated very precisely the structure of NPs in performing high resolution (scanning) TEM imaging and GPA. The chemical composition and spatial distribution of atoms were studied by EELS. The chemical environment and oxidation state of cations were investigated by Mössbauer spectrometry, soft XAS and XMCD. These techniques allowed us to estimate the increase of Fe2+ content in the iron oxide core of the core@shell structure and the increase of the cobalt ferrite shell thickness in the core@shell@shell one, while the iron oxide shell appears to be much thinner than expected. Thus, the modification of the chemical composition as well as the size of the Fe(3-delta)O4 core and the thickness of the cobalt ferrite shell have a high impact on the magnetic properties. Furthermore, the growth of the iron oxide shell also markedly modifies the magnetic properties of the core-shell NPs.
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