Micromagnetic Simulation And Demagnetizing Field Analysis Of NdFeB-based Permanent Magnets

Rare earth permanent magnets play an indispensable role in various fields of the modern society.Currently the research and development of of permanent magnetic materials mainly focused on the properties enhancement of NdFeB magnets by grain boundary regulation and microstructure optimization,and search for the next generation of high performance permanent magnetic materials,such as the nanocomposite permanent magnets.Magnetism theoretical analysis is helpful for the experimental study of the permanent magnetic materials.Micromagnetic simulation is one of the most important theoretical tools,by which the magnetization distribution,energy change and demagnetizing field change during the magnetization and demagnetization process of the magnets can be systematically analyzed.In this thesis work,based on scientific problems encountered in the current experimental research on the permanent magnetic materials,the grain boundary phase and grain boundary diffusion,the core-shell structure in the dual-main phase magnets and the nanocomposite magnets with different shapes of soft magnetic phases are studied by micromagnetic simulation,with a special focus on the demagnetizing field analysis.A simplified analysis method based on micromagnetic simulation is also proposed to study the demagnetizing field of a permanent magnet.Firstly,the anisotropic effects of the grain boundaries on the demagnetization process are studied by micromagnetic simulation.Regardless of the macroscopic demagnetizing field,the demagnetizing fields in the grain boundary layers parallel to of the magnetization direction are large along the reversed direction,hence the influence of these layers on the magnetization process and magnetic properties is very significant.The simulation results show that reducing the thickness and the saturation magnetization of these layers can effectively retard the demagnetization process.The demagnetizing field in the grain boundaries perpendicular to the magnetization direction is along the magnetization direction,hence these layers have less effect on the demagnetization process and magnetic properties.Taking the macroscopic demagnetizing field into account,the demagnetizing fields in the regions near the pole surfaces of the magnets are large,and the reversal magnetic domains tend to nucleate in these regions.Simulation results show that if the grain boundary diffusion is carried out from the two pole surfaces,the magnetic hardening shells will form in these regions,and this is the most efficient grain boundary diffusion approach.This simulation result provides a significant guidance for the grain boundary diffusion process.Secondly,the core-shell structures in the dual-main-phase magnets are studied by micromagnetic simulation.The coercivity of the dual-main-phase magnet without core-shell structure is much lower than that of the single-main-pase magnet with the same composition.The reason can be attributed to the low anisotropic field of the Ce-rich main phases,and the demagnetization process begins in these Ce-rich phases.The increased Nd content in the shells of the Ce-rich main phases will increase the anisotropic field.This kind of shells is similar to the magnetic hardening shell produced by grain boundary diffusion and it is the main reason of the magnetic properties enhancement.Ce diffusion into the Ce-free main phases leads to the formation of the shells with reduced anisotropic field.Hence the magnetic moments will revcerse in a lower applied magnetic field.The change of the demagnetizing field in these two kinds of shells will partly counteract the influence of the change of anisotropic field in these shells.However,the influence of the change of anisotropic field is still the decisive factor.In addition,the influence of the shell thickness is less than that of the shell composition.With high Nd content in the shells of the Ce-rich main phases,increasing the thickness of these shells helps to increase the coercivity further.The simulation results are in good agreement with the experimental results.Thirdly,the nanocomposite permanent magnets with different shapes and directions of soft magnetic phase are studied by micromagnetic simulation.The shape and size of the soft magnetic phase can influence the exchange coupling effect with hard magnetic phase.The reason can be attributed to the specific surface area influenced by the shape and size.Reducing the size and changing the shape to nanowire or nanofilms can increase the specific surface area of the soft magnetic phases,and hence strengthen the exchange coupling effect.The soft magnetic phases of nanowire arrays with different directions will give nanocomposite magnets with different properties.With the nanowires parallel to the easy axis of the hard phase,the coupling effects,including exchange coupling and dipolar coupling,can be enhanced.However,it does not result in the enhancement of the coercivity.With the nanowires perpendicular to the easy axis of the hard phase,the reversal magnetization direction is perpendicular to the nanowires,which is also the hard magnetization direction of the shape anisotropy.It means the complete reverse of the moments in the soft magnetic phases needs a large applied magnetic field,and the coercivity of the nanocomposite magnets increases.Similar to the nanowire arrays,high coercivity can be obtained by placing soft magnetic phase of nanofilms perpendicular to the easy axis of the hard phases.This simulation results provide a promising method to increase the coercivity of the nanocomposite magnets.Lastly,a simplified analysis method based on micromagnetic simulation is proposed to study the demagnetizing field of a permanent magnet.Based on the additivity law of the demagnetizing field,the demagnetizing field in a magnet distributed with nonmagnetic particles can be separated into two parts,one generated by an ideal magnet without nonmagnetic particle,and another one is generated by a reversed magnet with the same shape as the nonmagnetic particle.Therefore,the complicated demagnetizing field of a real magnet could be analyzed by only focusing the stray field of the reserved magnet.Using this simplified approach,the influence of different nonmagnetic phases on the demagnetization field of the permanent magnet have been discussed.The results show that the maximum value of demagnetization field is not significantly affected by the particle size,but the area of the influence region is proportional to the size of the particles.The large particles will produce a large area overlapped with those influenced by other nonmagnetic particles,which would reduce the coercivity.The pole surfaces of the nonmagnetic particles would generate a larger demagnetizing field than the side surfaces.With the increasing length to diameter ratio along the magnetization direction,the demagnetization field on the pole surface will increase.Furthermore,if the shape of the pole surface has a convex corner,it will increase the demagnetization field.The demagnetizing field near the nonmagnetic particle will be further increased by the large macroscopic demagnetizing field near the pole surfaces.The above results are meaningful to improve the microstructure and magnetic properties of the permanent magnets.In conclusion,based on micromagnetism,this thesis provides some references for the process control of sintered magnets and nanocomposite magnets from the point of view of micro-structure.The current work has certain theoretical and practical significance for the development of high-performance rare earth permanent magnets.