Nanoscale size effect on surface spin canting in iron oxide nanoparticles synthesized by the microemulsion method
Nanoscale size effect on surface spin canting in iron oxide nanoparticles synthesized by the microemulsion method
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DOI：
10.1088/0022-3727/45/19/195001
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发表时间：
2012-05-16
影响因子：
3.4
通讯作者：
Wende, Heiko
中科院分区：
文献类型：
--
作者：
Darbandi, Masih;Stromberg, Frank;Wende, Heiko
采用微乳法在室温条件下高产率合成了大小均匀、晶粒均匀的尖晶石结构的氧化铁纳米颗粒（IONPs），其平均直径分别为3,6和9nm。有机表面活性剂分子既可以作为微乳液体系的稳定剂，又可以作为纳米颗粒表面的覆盖层，从而使纳米颗粒在原位稳定。通过精心调整制备条件，实现了NP大小的控制。采用x射线衍射、透射电镜和扫描电镜对其结构、形貌和NP尺寸分布进行了研究。在4.2 K条件下，利用穆斯堡尔谱研究了纳米粒子的大小和封盖对其磁性结构的影响。由于表面体积比的增加，Fe自旋的平均倾斜角（相对于外加场方向）随着NP尺寸的减小而增加。通过比较相同直径的裸NPs和封顶NPs，我们验证了自旋倾斜不受有机封顶的影响。这意味着裸核粒子和带帽核粒子的磁性取向几乎相同。同时，封盖材料能够防止颗粒直接接触时产生的团聚效应。利用核壳模型，我们发现自旋倾斜起源于NPs的表面壳层。此外，穆斯堡尔谱参数提供了离子表面中存在高分数Fe3O4（磁铁矿）的证据。（有些数字可能只在在线期刊中以彩色显示）
Uniformly sized and crystalline iron oxide nanoparticles (IONPs) with spinel structure and mean diameters of about 3, 6 and 9 nm were synthesized in high yield using the microemulsion route at room temperature. The nanoparticles (NPs) were stabilized in situ by organic surfactant molecules which acted both as a stabilizer of the microemulsion system and as a capping layer of the NP surface. NP size control was attained by careful adjustment of the preparation conditions. The structure, morphology and NP size distribution were investigated by x-ray diffraction, transmission electron microscopy and scanning electron microscopy. A particular effort was devoted in this work to study the effect of size and capping of these NPs on their magnetic structure by in-field Mossbauer spectroscopy at 4.2 K. The mean canting angle (relative to the applied field direction) of the Fe spins was observed to increase with decreasing NP size due to the enhanced surface-to-volume ratio. Comparing bare and capped NPs of the same diameter, we verified that the spin canting was not affected by the organic capping. This implied almost identical magnetic orientations of bare and capped NPs. Simultaneously, the capping material was capable of preventing agglomeration effects which can occur in case of direct particle contact. Using a core/shell model, we showed that spin canting originated from the surface shell of the NPs. Furthermore, the Mossbauer spectral parameters provided evidence for the existence of a high fraction of Fe3O4 (magnetite) in the IONP. (Some figures may appear in colour only in the online journal)