Simulation And Mechanism Studies On Dynamic Magnetization Of High Abundance Cerium-based Rare Earth Materials

As a major development direction of China's strategic emerging industries,the rare earth permanent magnets?REPMs?play a crucial role in those applications of zero-energy consumption expected,such as clean energy acquisition,efficient green power output,steady-state magnetic field generation,non-contact transmission and so on.With the increasing market demand,the rare earth elements?REEs?employed for REPMs has accounted for around 40%of the total consumption of that in China nowadays.As a result,the high abundance REEs have a large backlog and the unbalanced utilization of REEs is becoming increasingly serious.In recent years,China has made remarkable progress in the research of high abundance REEs based permanent magnetic materials.A kind of RE₂Fe₁₄B?2:14:1?based sintered permanent magnet with Multi-Main-Phase-Alloy has been successfully developed and applied in commercial production,and excellent practical performance is achieved as the high abundance REE exceeds 30%of the total rare earth contented.Meanwhile,that the research on the unique magnetic physics emerged,such as the mechanism of strong coupling between two grains with completely different permanent magnetic properties and the abnormal increase of coercivity with the increase of Ce content,will not only enrich the understanding of the magnetic properties of materials and magnetic theory itself,but also contribute further to the development of high abundance REPM.Accordingly,this dissertation is motivated to focus on the investigation of 2:14:1 type rare earth magnetic materials with Ce,the highest natural abundance REE,and the influence of composition and preparation process on the dynamic processes of magnetization and demagnetization of the materials were studied systematically.It is well known that the magnetic properties of materials,especially the permanent magnetic properties,are intimately related to both the intrinsic properties and microstructure of materials.For sintered permanent magnets prepared by dual alloy method,that the element diffusion and composition reconstruction makes the microstructure correlate strongly with both the intrinsic characteristics of composition and phase formation and fabrication process.In consideration of the fact that the experimental methods generally adopted presently are far from to distinguish or verify directly the internal physical mechanism clearly,the numerical simulations are necessarily carried out for the studies combined with the experiment-designed based on characteristics of materials.By analyzing the experimental results reported previously to extract the typical microstructure characteristics,and using the numerical analysis methods of finite element and finite difference,the microstructure details which are difficult to be directly controlled or observed in the experiment are simulated,and the dynamic magnetization process of Ce-based REPMs was studied especially.Moreover,the simulated results were verified in comparison with several actual materials designed.The results show that Ce-based magnetic materials are of manifold magnetic properties.Depending on the composition and preparation process settled on purpose,Ce-based magnetic materials may perform as either permanent magnetic materials or non-trivial topological magnetic ones.The major research content and results obtained in this dissertation are summarized as follows:The modeling and analysis of single and multi-main-phase(Ce_xNd_1-x2)Fe₁₄B magnets were carried out respectively by micromagnetism method.The influence of model size,grain size,and width and intrinsic properties of grain boundaries on coercivity was investigated.The simulation results show that the abnormal increase of coercivity within the range of 20%³0%Ce content in the experiment results from the effect of grain boundary phase,no matter with the model size and grain size,respectively.The grain boundary phase existed pins the reversed magnetization domain leading to the increase of coercivity.Whereas,the coercivity decreases with the increase of Ce content monotonously for the case without grain boundary phase included.By fixing the proportion of the grain boundary phase in the magnet,the coercivity increases with the decrease of the exchange integral constant?A?,and decreases with the decrease of the magnetocrystalline anisotropy constant?K?of the grain boundary phase.However,when the value of A or K was too large,the abnormal phenomena of coercivity disappear.These numerical results are helpful to clarify the physical mechanism of abnormal increase in coercivity of the Ce-contained magnets.It is found that the grain boundary phase with a certain thickness can weaken the exchange coupling between grains and play a crucial role in improving coercivity.Compared with ferromagnetic grain boundary phase,non-magnetic grain boundary phase can further improve the coercivity of magnets.However,for core-shell particles,the thicker the Nd or Ce rich shell is not the better.Excessive thick grain shell will eliminate the chemical heterogeneity of surface and core of grains.Similarly,it is found that the coercivity of magnets with core-shell structure does not decrease monotonously with the increase of Ce content,indicating that core-shell structure is not the decisive factor for the abnormal increase of coercivity.The effects of Ce content and phase composition of grain boundary on the magnetic properties are studied from the experimental perspective.It is ensured experimently that the suppression of the coercivity reduction with Ce concentration of the magnets benefits from the formation of continuous grain boundary phase,which can effectively reduce the exchange coupling between adjacent grains.These results show that the theoretical model here used is reasonable and meaningful for comprehensively understanding the coercivity mechanism in multi-main-phase RE-Fe-B magnets.The equilibrium magnetic domain structure,dynamic response frequency,and energy state of Ce-Fe-B disk samples in diameter of D and thickness of T)is simulated by the finite difference method in terms of micromagnetism for vaious restricted sizes,initial magnetization states,and magnetocrystalline anisotropy.The as-quenched Ce_13.5Fe_79.5-xCo_xB₇?x=0,3,6,9,12?melt spun ribbons are confirmed to be uniform amorphous by X-ray diffraction,selective area's electron diffraction and magnetic measurements.The simulation results show that,the amorphouse phase?K=0 J/m³?with T/D<1 prefers to a stable magnetic vortices independent of the initial magnetization states.This result is consistent with that of the magnetic domain patterns,observed by the Lorentz transmission electron microscopy,in the amorphous Ce_13.5Fe_79.5-xCo_xB₇?x=0,3,6,9,12?ribbons,for which a stable magnetic vortex state was found to be formed spontaneously.When K increases gradually to a certain critical value,the magnetic vorticity state undergoes transforming into a Bloch-like Skyrmion.As K continues to increase,the non-trivial topological magnetic domain structure is destroyed,and the magnetic moment tends to be distributed along the uniaxial anisotropic direction of the magnetic crystal,forming a magnetic bubble structure.Materials with non-trivial topological magnetic domain structure has potential application prospects in the field of information storage.The results in this dissertation provide us new ideas for the research and development and preparation of novel functional materials with high abundant REEs,and make great contribution to the expansion of new applications of high abundant rare earth elements.