Research On Rare-earth Permanent Nanomagnets By Micromagnetic Simulations And Preparation By Ball-milling

Magnetism is a fundamental property of matter,and also an important branch of physics.Nowadays,permanent magnets,which can be divided into magnetic film and magnetic grain structures,hold a significant market share worldwide.Thanks to their excellent properties,such as their high storage density and high performance,they are currently the most investigated magnetic materials.This thesis investigates Sm Co5/Fe nanocomposite films theoretically,and describes as well the preparation of Nd Fe B nanoparticles.The influence of the soft layer thickness of the nanocomposite films on the demagnetization process and coercivity was investigated,as well as the influence of the ball-mill temperature on the particle size distribution,shape,structure and coercivity of Nd Fe B nano-particles.The main results of this thesis are as follows:(1)We made micromagnetic simulations and obtained the demagnetization process of Sm Co5/Fe/Sm Co5 nanocomposites.The parameters used for the simulations are closed to experimental ones,and our results are consistent with the measured ones.Through the analysis of our results,we obtained the influence of the soft layer thickness on the coercivity.Especially,this thesis explains the evolution of the soft layer thickness under various applied magnetic fields,which can help us analyse the discrepancy between experimental and theoretical values.Our results demonstrate that with the increase of the soft layer thickness,the coercivity mechanism of the Sm Co5/Fe/Sm Co5 nanocomposite is firstly nucleation,then self-pinning,and finally pinning.(2)We controlled the ball-mill temperature to observe its influence on the particle size distribution,shape,structure and coercivity of Nd Fe B nano-particles.Firstly,we observed the evolution over time of ball-mill at low temperature,which helped us confirmthe temperature range of ball-mill,an important step as it can improve the reliability of our results.The observations show that there is a strong influence of ball-mill structure on the particle size distribution,shape,structure and coercivity of Nd Fe Bnano-particles.Low temperature ball-milling is more homogeneous,particles are smaller and properties are improved in comparison with the situation at room temperature.At low temperature,ball-milling tends to make less clusters than at room temperature,but after a given time,ball-milling will reach a saturation state and no further refining of the particles will be possible.Various low temperatures will have different influences on the refining of NdFe B nanoparticles.