Abstract
In this study, the superhydride(LiB(Et)3H) reducing agent was used to reduce CrCl3, MnCl2, Fe(acac)2, Co(acac)2 and Ni(acac)2 at high temperature to synthesize the MnFe, MnFeCo, MnFeCoNi, and CrMnFeCoNi magnetic nanoparticles, and nanoparticles were dispersed by oleic acid in hexane. The crystal structure of nanoparticles were identified by XRD. It showed that as-prepared nanoparticles were amorphous besides MnFe which appeared peaks of unknown structure. After annealing, MnFe nanoparticles and MnFeCo nanoparticles became the BCC and MnO2 phases. The structure of annealed MnFeCoNi, Mn-rich and Mn-poor nanoparticles were FCC. However, some unknown peaks were observed in Mn-rich nanoparticles annealed at 600°C. The sizes of nanoparticles increased with annealing temperature. According to the LSW theory, the cubed diameter is proportional to the exponential of-Q/RT. For MnFe and Mn-rich nanopartcle, the active energys of the coarsening are 15.8(KJ/mol) and 24.5(KJ/mol) which suit for the surface diffusion. Below the 500°C, the active energy is 13.3(KJ/mol) for Mn-poor nanoparticles, and above the 500°C, it is 311.8(KJ/mol). It is clear that Mn-poor nanoparticles coarsen by surface diffusion at low temperature and by bulk diffusion at high temperature. Form the measurement, magnetic nanoparticles showed low saturated magnetization(Ms), low cohesive field(Hc), and normal susceptibility of the paramagnetic phase in the superparamagnetic region. In the single domain region, there were normal Ms, high Hc, and low susceptibility. In the multi-domain region, there were high Ms, normal Hc and high susceptibility of superparamagnetic phase.