The Magnetic Microstructure And Property Of Magnetic Nanoparticles Researched By Electron Holography

Magnetic nanoparticles can be widely applied in magnetic fluids, chemical catalysts, data storage and biomedical technologies. As to characterization, researchers generally focus on the performance of nanoparticle clusters, such as hysteresis loop, the measurement result is an average value affected by many factors. The performance of nanoparticles highly dependent on the shape, size, crystal structure and constituent of materials, so there exist a challenge to extract the physical essence. Lorentz transmission electron microscopy and electron holography can provide more information of magnetic nanoparticles due to its ultra-high spatial resolution, such as magnetic domain structure of the nanoparticles, magnetostatic interaction between the magnetic particles, furthermore the exchange coupling interaction in shell structure of soft and hard magnetic phase can be discussed. It will have a considerable significance concerning the basic research and application of the magnetic nanoparticles.In this thesis, the magnetic structure of magnetic nanoparticles was quantitatively measured in the nanometer scale with Lorentz transmission electron microscopy and electron holography.The magnetic structure of cobalt nanowires with a diameter of 15 nm indicates it is ferromagnetic, it is similar in cobalt and copper multilayer nanowires. It is available to study the magnetization state and magnetization reversal mechanism of magnetic nanoparticles by off-axis electron holography technique. How the surface structures affect the performance of microwave absorption in cobalt-nickel alloy microspheres was studied by electron holography technique. The thorns and petals as dipoles interact with the incident electromagnetic wave, stray magnetic fields is produced consequently, therefore, the electromagnetic energy is converted into other forms of energy to improve the microwave absorption capacity. It can be seen electron holography technique is a effective method to achieve the microwave absorption properties of materials.In this work the magnetic structure of Fe3O4 magnetic nanoparticles with different shapes was determined by morphology, crystal structure and interactions between the particles, it is correspond to the state of minimum energy in ferromagnetic materials. The mechanism why the specific absorption rate of Fe3O4 nanodisk is higher than the isotropic nanoparticles was explained by electron holography, it is, the nanodisks rotate parallelly to the direction of external field due to unique shape anisotropy when the external field changes direction.Finally, an experiment was carried out by exposuring a nanofiber in external magnetic field, it illustrated that Lorentz transmission electron microscopy equipped with electron holography has become a powerful tool to understand and in-situ observe the magnetic reversal of magnetic nanoparticless.