Ideal interaction-free magnetite nanoparticles were prepared, and their magnetic properties were measured to clarify the true nature of magnetic anisotropy of individual magnetite nanoparticles at the nanoscale and to analyze the shape, surface, and crystalline anisotropy contributions. Spherical (17.7 nm), cubic (10.6 nm), and octahedral-shaped magnetite nanoparticles with average sizes ranging from 7.6 to 23.4 nm were synthesized using solution techniques. Then, these nanoparticles were coated with silica at appropriate shell thicknesses to prepare magnetic interaction-free samples, and their noninteractive nature was confirmed through first-order reversal curve diagrams. For these well-isolated nanoparticles, remanent magnetizations of the hysteresis loops are just equal to a half of the saturation magnetization. This result clearly indicates that uniaxial magnetic anisotropy is predominant in each nanoparticle. In order to clarify the details of the uniaxial magnetic anisotropy, the analysis of blocking temperature−switching field distribution diagrams is constructed based on thermal decay curves of isothermal remanent magnetization at various applied fields. The obtained effective magnetic anisotropy constant K eff is distributed around 10−20 kJ/m 3 and has insignificant size dependence. This result seems inconsistent with the inverse proportion relation of K eff with size predicted for surface magnetic anisotropy. The theoretical calculation suggested that the crystalline magnetic anisotropy plays a major role in magnetic properties of the magnetite nanoparticles at lower temperatures. However, it should be noted that K eff seems slightly different for the different shapes. The above study indicates that control size, shape, and interparticle interactions is required to strictly discuss such delicate differences of magnetic anisotropy of individual magnetite nanoparticles for the design of thermal seeds for magnetic hyperthermia.

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Present status and prospects of magnetic iron oxide nanoparticle (MION) synthesis and their properties for the development of the next generation magnetic fluid suitable for full-scale applications in engineering and medicine are reported. First, the progress in the development of monodispersed size and shape-controlled MION synthesis technology is reported. Then, the nanoscale magnetic properties and flow characteristics investigated using the fine tunned MIONs are reported. From the microscopic and macroscopic structural analysis of the needle-like condensed phase in a magnetic fluid under an external magnetic field, (a) internal spatial order free and (b) chain clustering condensation of small and large particles, respectively, were identified. On the other hand, the estimation of the magnetic anisotropy of individual MIONs was different from the inversely proportional relationship of surface magnetic anisotropy predicted from the size contribution. In the case of particles with isotropic shapes, magnetocrystalline anisotropy plays a significant role in the low-temperature magnetic properties for engineering and medical applications. Finally, based on the above findings, the guidelines for developing next-generation magnetic fluid are proposed.

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