Study On The Micromagnetic Simulations For The Reversal Magnetization Of Ce-containing Magnets At Nanometer Level

In modern society,as the rare earth permanent magnet with the best comprehensive magnetic properties,neodymium-iron-boron?Nd-Fe-B?plays an important role in the progress of science and technology.The rapid increase of Nd-Fe-B production leads to the large consumption of rare earth element Nd and the overstocking of high abundance rare earth element,such as La and Ce.From the perspective of sustainable development,balanced utilization of rare earths is very significant.In this thesis,single-grain Nd-Fe-B magnets and single-grain Ce-containing magnets were studied at nanometer level,and the effects of different factors on the magnetization reversals were investigated via micromagnetic simulations.The main contents are as follows:Individual grains with diverse dimensional parameters were introduced to investigate the magnetization reversals in anisotropic Nd₂Fe₁₄B magnets.With the same bottom area and height,analysis results show that the coercive fields for different bottom shapes are of similar values.Designed as a cubic grain,the coercive field presents descending tendency as grain volume ascends.Under constant grain volume,with aspect ratio increasing,the coercive field decreases in the beginning and increases soon.Based on the demagnetization field vector,the nucleation point is chosen to discuss.Its synthetic field and reversal field are calculated.The synthetic field equal to the reversal field is defined as critical field,which always shows the same tendency as the coercive field for all cases of this study.It can be concluded that critical field is qualified to be a reference index to measure the magnitude of coercive field.Single-grain models with different cerium content or structural parameters were introduced to investigate the reversal magnetization behaviors in cerium-containing magnets.As for single?Nd,Ce?₂Fe₁₄B type grain,the coercivity decreases monotonously with the increase of cerium content.Four types of grain structure were compared.It is found that core(?Nd,Ce?₂Fe₁₄B)-shell(Nd₂Fe₁₄B)type grain with2nm thick shell always presents the largest coercivity under the same total cerium content.Furthermore,the relationship between the coercivity and the shell thickness t in core(?Nd,Ce?₂Fe₁₄B)-shell(Nd₂Fe₁₄B)type grain was studied.When the total cerium content is kept at 20.51at.%,the analyzed results show that as t varies from1nm to 7nm,the coercivity ascends gradually in the beginning,then descends quickly after reaching the maximum value when t=5nm.From perspective of the positions of nucleation points,the reasons why t affects the coercivity are discussed in detail.The effects of core size,shell thickness and shell distribution on the coercivity of single-grain Core((Nd_0.7,Ce_0.3)₂Fe₁₄B)-shell(Nd₂Fe₁₄B)magnets were studied.The results show that the coercivity decreases with the increase of core size when the shell thickness is constant.When the core size is unchanged,as the shell thickness increases gradually,the coercivity increases firstly and then decreases.With kept core size and shell volume,when the shell is distributed on the two easy-axis planes?planes perpendicular to the easy axis?of the core,the coercivity of the magnet reaches the largest value.Via magnetocrystalline anisotropy field,demagnetization energy,nucleation point and so on,the reasons for the change of coercivity in above cases can be explained.