Synthesis of hard magnetic nanoparticles for applications in permanent magnets

Abstract

Nanotechnology brings material science new opportunities for making novel materials with more attractive functions. By miniaturizing the materials into nanoscale region (≤100 nm), their thermal properties, mechanical properties, optical properties, magnetic properties and electrical properties will be significantly changed or show some novel phenomena, such as the GMR/MTJ effects, surface plasmonics, the novel properties of 2D Graphene, etc. Based on the bottom-up approach as discussed in this thesis, the nanoscale entities can be built directly from atoms or molecules in gas phase through a thermodynamic process. From the perspective of time dimension, this zoom-in process gives us the opportunity to freeze the nanoscale entities at certain stage or to combine more materials at the nanoscale region. As a result, novel material phases or nanocomposite materials that are impossible to make at bulk level, can be easily fabricated by this bottom-up nanotechnology.In this study, the magnetron-sputtering-based gas phase condensation method was used to make hard magnetic nanoparticles, including SmCo5, body-centered tetragonal (bct) Fe and α´´-Fe16N2 nanoparticles, which can be applied for making future advanced permanent magnets. Large size SmCo5 nanoparitcles with a coercivity of 3360 Oe at 5 K have been directly fabricated by this magnetron-sputtering-based gas-phase condensation method. Based on this method, we studied the thermodynamic process for the growth of SmCo5 nanoparticles. It was found that the well-crystallized SmCo5 nanoparticles tended to form a hexagonal disk shape with its easy axis perpendicular to the disk plane. More importantly, under the condition of high sputtering current, well-crystallized nanoparticles were found to be formed through a three-stage growth process: aggregation, coalescence and second crystallization.The bct-structured Fe and α´´-Fe16N2 nanoparticles have been successfully synthesized by the same gas phase condensation method. Phase formation mechanism has been studied at different thermodynamic conditions. It was found, those two metastable phases, which are hard to make at bulk level, can be formed by freezing the nanoparticles at an intermediate stage. The magnetic properties of bct Fe nanoparticles have been studied. Its magnetocrystalline anisotropy (Ku) and saturation magnetization (Ms) were found to be 5.06×106 erg/cc and 1635 emu/cc respectively at 0 K, which make it a potential candidate for synthesizing future stronger permanent magnets. Besides the study of making bct Fe nanoparticles, the feasibility of making α´´-Fe16N2 nanoparticles by this gas phase condensation method has also been studied thoroughly. The α'-Fe8N nanoparticles were directly formed in the as-deposited samples and α''-Fe16N2 nanoparticles have been successfully obtained by annealing the Fe8N nanoparticles at 250 ℃ for 48 hours. By measuring its magnetic properties, the magnetic anisotropy constant Ku of the FeN nanoparticle sample was determined to be 2.56×105 erg/cc. Currently, the applications of magnetic nanoparticles are limited by particle yield, especially for making bulk permanent magnets. The reality of low yield is also the biggest obstacle for expanding the application of this gas-phase condensation method both in scientific research and industry. In this study, a novel hollow cathode nanoparticle fabrication system has been developed to increase the particle yield by five times, while the uniformity and crystallinity of the nanoparticle are still maintained. This technique can be easily scaled up to make large amount of nanoparticles and is highly compatible to industrial production, showing the way to industrialize the manufacture of diverse nanoparticles.