Graphic Modeling Micromagnetic Simulation Analysis And Theoretical Verification Based On Microstructure Of Permanent Magnetic Materials

Permanent magnet materials（such as NdFeB-,FePt-,SmCo-based ones）are widely used in the fields of motors,wind powers,hybrid vehicles,aerospaces and so on,because of their high resistance to external magnetic fields,high-density storable magnetic energies,and ability to maintain external magnetic fields for a long time under certain conditions.With the development of society,science and technology,the low cost and high performance requirements of permanent magnet materials are increasing day by day.However,there are still many problems can not be understood in the research process of the permanent magnet materials,such as the regulation and roles of grain boundary phases in NdFeB magnets,the regulation of magnetic anisotropy and the influence mechanism of microstructure on magnetic properties in SmCo films.The traditional methods of studying permanent magnet materials are mainly experimental preparation,characterization and analyses,which try to find better material systems and preparation processes through continuous experiments.However,their long research and development cycles,high costs and difficulties in controlling the microstructure are restricting the fast development of permanent magnet materials research.At the same time,for the phenomena existing in many experimental studies,there is still a lack of in-depth theoretical analysis and interpretation.For example,the change of magnetic domains during magnetization reversal is not clear.Micromagnetic simulation provides a more efficient possibility for this.It can quickly and intuitively reflect the changes of magnetic domains during magnetization reversal through simulation calculation,and even speculate the magnetic properties corresponding to a certain microstructure.Analysis and clarification of the mechanism of the magnetization process play an important role in theoretical guidance experiments.It is common to use the micromagnetic simulation to construct a model to explain the phenomena produced in the experiment,but the existing models are mostly composed of regular geometry,which is difficult to reflect the microscopic structure in the actual situation,resulting in a huge gap between the simulation results and the experiment ones.Therefore,it is proposed in this dissertation that a micro-structure based on experimental characterization is constructed a more detailed micro-magnetic simulation model,so as to better analyze the phenomena appearing in the experiment,and based on this,it provides guidance for the experiment through the optimization model.In this study,NdFeB-,FePt-and SmCo-based magnetic materials are used as the objects,and their intrinsic mechanisms are analyzed based on the graphical modeling of the microstructure,which verifies correctness of the model.The NdFeB-based nanocomposite magnets have been prepared by the copper-mold casting technique.The magnetic properties were calculated by the micromagnetic simulation using the magnetic force microscopy image.The influence of different saturation magnetization of amorphous grain boundary phases on the magnetization reversal process was investigated.The simulation results indicate that the coercivity increases with reducing saturation magnetization of amorphous phases when the grain size varied from 300 to 1000 nm.The magnetization reversal process demonstrates that the magnetic moments of NdFeB phase show the prior inversion at the grain boundaries.Hot-deformed NdFeB magnets have attracted considerable attention due to their high thermal stability and exceptional corrosion resistance.In this study,dependence of the magnetic properties and recoil loops on the presence of a grain boundary phase（GBP）and Nd-rich secondary phases in hot-deformed NdFeB magnets was explored using micromagnetic simulations.By adjusting parameters of the simulation model,the influence of several factors（thickness and spontaneous magnetization（M_s）of the GBP,total volume fraction and spatial distribution as well as M_s of the secondary phases）on the main and recoil loops was investigated.A change in thickness and M_s of the GBP affects the coercivity（H_c）and remanence of the main loop.In particular,an increase of M_s leads to a decrease in H_c of the magnet.The influence of the secondary phase depends on its spatial distribution.Dispersed small volume grains result in lower H_c,but the role in decreasing H_c is weaker when the smaller grains aggregate to be a large volume grain.Open recoil loops were interpreted by analyzing the distribution of calculated magnetic moments and energy terms.Loop opening is not directly related to H_c of the main loop.Instead,it is mainly due to small irreversibilities in the dipolar energy of different magnetic configurations which appear along the recoil loop.L1₀ FePt-based films have great potential for use in ultrahigh-density storage media,magnetic micro-systems,and as a coating for high coercivity magnetic force microscopy probes,due to their excellent magnetic properties and good chemical stability.In this dissertation,FePt films with a composition gradient were deposited by magnetron sputtering of a Fe target partially covered by Pt foil.The films were deposited at room temperature,on thermally oxidised 100 mm Si substrates.2D composition maps of as-deposited films were made using Energy Dispersive X-Ray analysis in an SEM.Post-deposition annealing of full substrates was carried out using a rapid thermal annealing furnace.Influence of the size and the annealing temperature and time of the Pt foil were investigated on the magnetic and structural properties of the films.High throughput magnetic characterisation was carried out using an in-house developed scanning Magneto-Optic Kerr effect system operated at room temperature.More detailed magnetic characterisation of certain sample parts was carried out using SQUID-VSM and Magnetic Force Microscopy.The great potential of high throughput film preparation and characterisation was demonstrated in the development of L1₀ FePt-based hard and hard-soft nanocomposite films.Rare earth permanent magnetic thin films,such as SmCo-based films,are promising candidates for future thermal-assisted magnetic recording media because of their supreme thermal stability and fine superparamagnetic critical size.Phase compositional distributions and crystallographic orientations can directly influence the magnetic domain evolutions and magnetic performance of SmCo films.However,these films often exist in multi-phases without well-defined distinct magnetic anisotropy,thereby causing difficulties in analyzing their functions.The effects of atomic diffusions induced by annealing on crystallization and magnetic anisotropy energy are poorly understood.In this dissertation,the influence of doping Cu atoms via introducing Cu layer on magnetic anisotropy of Cr/SmCo/（Cu）/Cr films is investigated.Introducing a Cu layer and post-annealing lead to tunable magnetic anisotropy from isotropy to anisotropy and domain structure formation.Micromagnetic simulation is used to further analyze the effects of crystallization,anisotropy field and compositional distribution on the magnetization reversal behavior and coercivity variations in SmCo films.It reveals that differences of the anisotropy field of Co and amorphous phases could affect the domain wall motions and coercivity,which shows the analogous trend as an experimental result.Moreover,the elevated anisotropy constant of Sm（Co,Cu）alloy and the small fractions of SmCo₅ boundary phases are also beneficial to the enhancement of coercivity derived by calculating domain wall energy.In-plane magnetic anisotropy is improved by the SmCo₅ phase,which has an in-plane preferred orientation of the c axis.This dissertation provides theoretical and experimental bases for the future fabrication of SmCo-based films by controlling atomic diffusion with optimized grain boundary phase distribution.