Due to their non-toxicity and their ability to be functionalized, magnetite (Fe₃O₄) nanoparticles (NPs) are good candidates for a variety of biomedical applications. To better implement their applications, it is crucial to well understand the basic structural and magnetic properties of the NPs in correlation with their synthesis method. Here, we show interesting properties of Fe₃O₄ NPs of various sizes ranging from 5 to 100 nm and the dependence of these properties on particle size and preparation method. One synthetic method based on heating Fe(acac)₃ with oleic acid consistently gives 5 ± 1 nm NPs. A second method using the thermal decomposition of Fe(oleate)₃ in oleic acid led to larger NPs, greater than 8 nm in size. Increasing the amount of oleic acid caused the average NP size to slightly increase from 8 to 10 nm. Increasing both the reaction temperature and the reaction time caused the NP size to drastically increase from 10 to 100 nm. Powder X-ray diffraction and electron-microscopy imaging show a pure single crystalline Fe₃O₄ phase for all NPs smaller than 50 nm and spherical in shape. When the NPs get larger than 50 nm, they notably tend to form faceted, FeO core–Fe₃O₄ shell structures. Magnetometry data collected in various field-cooling conditions show a pure superparamagnetic (SPM) behavior for all NPs smaller than 20 nm. The observed blocking temperature, $T_{B}$ , gradually increases with NP size from about 25–150 K. In addition, the Verwey transition is observed with the emergence of a strong narrow peak at 125 K in the magnetization curves when larger NPs are present. Our data confirm the vanishing of the Verwey transition in smaller NPs. Magnetization loops indicate that the saturating field drastically decreases with NP size. While larger NPs show some coercivity ( $H_{c}$ ) up to 30 mT at 400 K, NPs smaller than 20 nm show no coercivity ( $H_{c} = 0$ ), confirming their pure SPM behavior at high temperature. Upon cooling below $T_{B}$ , some of the SPM NPs gradually show some coercivity, with $H_{c}$ reaching 45 mT at 5 K for the 10 nm NPs, indicating emergent interparticle couplings in the blocked state.

中文翻译：

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5 至 100 nm Fe3O4 纳米粒子的尺寸相关结晶和磁性特性：超顺磁性、Verwey 转变和 FeO-Fe3O4 核壳形成

由于它们的无毒和功能化的能力，磁铁矿 (Fe ₃ O ₄ ) 纳米粒子 (NP) 是各种生物医学应用的良好候选者。为了更好地实现它们的应用，充分了解与其合成方法相关的 NPs 的基本结构和磁性特性是至关重要的。在这里，我们展示了从 5 到 100 nm 的各种尺寸的 Fe ₃ O ₄ NP 的有趣特性，以及这些特性对粒径和制备方法的依赖性。一种基于用油酸加热 Fe(acac) _{3 的}合成方法始终产生 5 ± 1 nm 的纳米颗粒。使用 Fe(oleate) ₃热分解的第二种方法在油酸中导致更大的纳米颗粒，尺寸大于 8 nm。增加油酸的量导致平均 NP 尺寸从 8 nm 略微增加到 10 nm。增加反应温度和反应时间导致 NP 尺寸从 10 nm 急剧增加到 100 nm。粉末 X 射线衍射和电子显微镜成像显示所有小于 50 nm 和球形的 NP 均具有纯单晶 Fe ₃ O ₄相。当纳米颗粒大于 50 nm 时，它们特别倾向于形成多面的 FeO 核-Fe ₃ O ₄壳结构。在各种场冷却条件下收集的磁力测量数据显示所有小于 20 nm 的 NP 都具有纯超顺磁性 (SPM) 行为。观察到的阻塞温度， $T_{B}$ ，随着 NP 尺寸从大约 25-150 K 逐渐增加。此外，当存在较大的 NP 时，观察到 Verwey 转变，在磁化曲线中的 125 K 处出现一个强窄峰。我们的数据证实了较小 NP 中 Verwey 转变的消失。磁化环表明饱和场随着 NP 尺寸急剧减小。虽然较大的 NP 显示出一定的矫顽力（ $H_{c}$ ) 在 400 K 时高达 30 mT，小于 20 nm 的 NPs 没有表现出矫顽力（ $H_{c} = 0$ )，证实了它们在高温下的纯 SPM 行为。下面冷却后 $T_{B}$ , 一些 SPM NPs 逐渐表现出一定的矫顽力, 与 $H_{c}$ 对于 10 nm NP，在 5 K 时达到 45 mT，表明在阻塞状态下出现了粒子间耦合。