本文研究了三氧化二铁磁性纳米颗粒在不同频率交变磁场下的组装研究。结果发现，磁性纳米颗粒的组装结构与外场频率和颗粒驰豫时间的差有关，对于10nm左右的γ-Fe2O3纳米颗粒而言，较高的频率有利于线性组装结构的形成。另外，交变磁场诱导的组装和颗粒表面修饰密切相关，较厚的表面包覆层会抑制组装。这可能是由于较厚的包覆层削弱了颗粒间磁相互作用，从而使得作用在单位颗粒上的磁场力变得较弱的原因。我们的研究表明，交变磁场诱导磁性纳米颗粒形成纤维状组装结构的机理和静磁场是不一样的，交变磁场诱导组装的过程是一个驰豫的过程。本研究有利于扩展对基于外场组装纳米颗粒的基础研究和实际应用。
Jianfei Sun, Yunxia Su, Chunyu Wang, and Ning Gu,
The investigation of frequency response for the magnetic nanoparticulate assembly induced by time-varied magnetic field ,Nanoscale Research Letters 2011, 6:453
Figure 2 SEM images of bare g-Fe2O3 nanoparticles after solvent drying. In absence of the alternating magnetic field (a) and in presence of alternating magnetic field with different frequency (1 kHz (b), 5 kHz (c), 10 kHz (d), 50 kHz (e), 100 kHz (f), and 20 Hz (g)). The concentration of sample was 12.5 μg/ml. The naturally drying sample showed amorphous aggregates, while the field-treated samples showed more or less onedimensional orientation. With the frequency increasing, the chain-like assembly was more and more obvious. However, for the 20-Hz alternating magnetic field, the field-treated sample re-showed the amorphous aggregates to some extent, meaning that the alternating magnetic field of he frequency had not induced the assembly of γ-Fe2O3 nanoparticles.
Figure 4 Schematic illustration of assembly mechanism based on the field periods and the colloidal relaxation time. If the relaxation time is above the period of field, the assembly can occur. If the relaxation time is below the period of field, there is no attractive force to drive the assembly.
