Dennis C L, Jackson A J, Borchers J A, Gruettner C, Ivkov R
Material Measurement Laboratory, NIST, Gaithersburg, MD 20899, United States of America.
Nanotechnology. 2018 May 25;29(21):215705. doi: 10.1088/1361-6528/aab31d. Epub 2018 Mar 1.
We show the effects of a time-invariant magnetic field on the physical structure and magnetic properties of a colloid comprising 44 nm diameter magnetite magnetic nanoparticles, with a 24 nm dextran shell, in water. Structural ordering in this colloid parallel to the magnetic field occurs simultaneously with the onset of a colloidal uniaxial anisotropy. Further increases in the applied magnetic field cause the nanoparticles to order perpendicular to the field, producing unexpected colloidal unidirectional and trigonal anisotropies. This magnetic behavior is distinct from the cubic magnetocrystalline anisotropy of the magnetite and has its origins in the magnetic interactions among the mobile nanoparticles within the colloid. Specifically, these field-induced anisotropies and colloidal rearrangements result from the delicate balance between the magnetostatic and steric forces between magnetic nanoparticles. These magnetic and structural rearrangements are anticipated to influence applications that rely upon time-dependent relaxation of the magnetic colloids and fluid viscosity, such as magnetic hyperthermia and shock absorption.
我们展示了时不变磁场对一种胶体的物理结构和磁性的影响，该胶体由直径为44纳米的磁铁矿磁性纳米颗粒组成，表面有一层24纳米的葡聚糖壳，分散在水中。这种胶体中与磁场平行的结构有序化与胶体单轴各向异性的出现同时发生。进一步增加外加磁场会使纳米颗粒垂直于磁场方向排列，产生意想不到的胶体单向和三角各向异性。这种磁行为不同于磁铁矿的立方磁晶各向异性，其起源于胶体中可移动纳米颗粒之间的磁相互作用。具体而言，这些场诱导的各向异性和胶体重排是由磁性纳米颗粒之间静磁力和空间位阻之间的微妙平衡导致的。预计这些磁和结构重排会影响依赖磁性胶体随时间弛豫和流体粘度的应用，如磁热疗和减震。
