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From core/shell to hollow Fe/gamma-Fe2O3 nanoparticles: evolution of the magnetic behavior

机译：从核/壳到中空的Fe /γ-Fe2O3纳米颗粒：磁行为的演变

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摘要

High quality Fe/gamma-Fe2O3 core/shell, core/void/shell, and hollow nanoparticles with two different sizes of 8 and 12 nm were synthesized, and the effect of morphology, surface and finite-size effects on their magnetic properties including the exchange bias (EB) effect were systematically investigated. We find a general trend for both systems that as the morphology changes from core/shell to core/void/shell, the magnetization of the system decays and inter-particle interactions become weaker, while the effective anisotropy and the EB effect increase. The changes are more drastic when the nanoparticles become completely hollow. Noticeably, the morphological change from core/shell to hollow increases the mean blocking temperature for the 12 nm particles but decreases for the 8 nm particles. The low-temperature magnetic behavior of the 12 nm particles changes from a collective super-spin-glass system mediated by dipolar interactions for the core/shell nanoparticles to a frustrated cluster glass-like state for the shell nanograins in the hollow morphology. On the other hand for the 8 nm nanoparticles core/shell and hollow particles the magnetic behavior is more similar, and a conventional spin glass-like transition is obtained at low temperatures. In the case of the hollow nanoparticles, the coupling between the inner and outer spin layers in the shell gives rise to an enhanced EB effect, which increases with increasing shell thickness. This indicates that the morphology of the shell plays a crucial role in this kind of exchange-biased systems.
机译：合成了高质量的Fe /γ-Fe2O3核/壳，核/空隙/壳和两种尺寸分别为8和12 nm的中空纳米粒子，形态，表面和有限尺寸对其磁性能的影响包括系统研究了交换偏差（EB）效应。我们发现这两个系统的总体趋势是，随着形态从核/壳变到核/空/壳，系统的磁化强度减弱，粒子间的相互作用变弱，同时有效各向异性和EB效应增加。当纳米颗粒变得完全中空时，变化更加剧烈。值得注意的是，从核/壳到中空的形态变化增加了12 nm颗粒的平均阻断温度，但降低了8 nm颗粒的平均阻断温度。 12 nm粒子的低温磁行为从核/壳纳米粒子的偶极相互作用介导的集体超级自旋玻璃系统变为空心形态的壳纳米粒子的沮丧簇状玻璃状状态。另一方面，对于8 nm纳米粒子的核/壳和中空粒子，其磁性能更相似，并且在低温下可获得常规的自旋玻璃状转变。在中空纳米颗粒的情况下，壳中内外旋层之间的偶联导致增强的EB效应，该效应随壳厚度的增加而增加。这表明壳的形态在这种基于交换的系统中起着至关重要的作用。

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