The Study Of Electron Holography On The Micromagnetic Structure Of Low-dimensional Individual Magnetic Materials

Low-dimensional magnetic material monomers are one of the hot topics in material science and condensed-matter physics not only for their important basic research values of novel physical properties but also for the extensive application prospect in high-density storage,spintronic devices and magnetic nanomedicine in recent years.In the practical researches and applications,the dimension of low-dimensional magnetic material is generally at the nanometer scale and its magnetic properties and micromagnetic structure will show unprecedented changes.Therefore,the relationship between their magnetic structure and the magnetic anisotropy,the microstructure of materials needs extensive research and recognition.In this thesis,we have systematically studied the micromagnetic structure,sample morphology,crystal structure of magnetic anisotropic materials?such as AlNiCo alloys,nanostructured barium ferrites,nickel ferrite single-particle-chain nanowire and cobalt ferrite nano-sheet?,and the relationship between each parameter.The main results are shown as follows:?1?The magnetic structure and coercivity mechanism of AlNiCo alloys dominated by shape anisotropy are studied by transmission electron microscopy and off-axis electron holography.It is found that the AlNiCo alloy is spinodally decomposed into two phases?non-magnetic?2 phase and ferromagnetic?1 phase?after the magnetic heat treatment.Rod-like?1 phase is surrounded by the matrix?2 phase.Typically,two types of AlNiCo alloys,36Co and 40Co,were investigated,indicating that the?1 phase in 36Co has a larger diameter and volume fraction than that of 40Co.The electron holography results reveal that the magnetic properties of AlNiCo are derived from the shape anisotropy of the?1 phase and the magnetic isolation function of the?2 phase.Combined with theoretical calculations and micromagnetic simulations,it is shown that the magnetization processes of 36Co and 40Co are nonuniform reversal mode.The geometry and the spacing between adjacent?1 phase is the main factors affecting the magnetic properties of AlNiCo alloys.?2?The magnetization distribution and its relationship with the nanowire shape,grain orientation and crystal structure in the anisotropic single-axis M-type barium ferrite was studied by spherical aberration transmission electron microscopy,electron holography,and micromagnetic simulation.As the model materials,the M-type barium ferrite single-particle-chain nanowires shows a high magnetocrystalline anisotropy.The commercial nanosheets and electrospinned barium ferrite single-particle chain nanowires were used for the comparison experiments.The results showed that the polycrystalline nanosheet exhibited a magnetic vortex structure,and the distribution of magnetic flux lines in the single-crystal nanosheet and single-particle-chain nanowire is along the easy axis,i.e.,the[001]zone axis.However,for uniaxial monocrystalline magnetic nanomaterials,the magnetocrystalline anisotropy determines its magnetization distribution,and shape anisotropy did not show observable effect.The above results have been verified by OOMMF.?3?The dynamic magnetization behavior and the relationships between magnetic structure,shape and crystal structure were systematically investigated in NiFe₂O₄single-particle chain nanowires,which is a representative low magnetocrystalline anisotropy material with spinel cubic structure.The dynamic magnetization reversal process of nickel ferrite nanowire was studied by electron holography,and its corresponding hysteresis loop was obtained.It displays that the coercivity of the single-particle nanowire is 450 Oe,which is much larger than that?200 Oe?of nanowires.Combined with micromagnetic simulation,it can be found that the magnetization distribution,in the nickel ferrite single-particle nanowires was determined by the shape anisotropy,while the magnetocrystalline anisotropy played a minor role.?4?The structure and properties of and The relationship between their magnetic structure,size,shape and magnetocrystalline anisotropy were studied preliminarily cubic-spinel-structure CoFe₂O₄ nanosheets with different magnetic anisotropy.In this experiment,CoFe₂O₄ nanosheets were prepared by electrospinning and characterized by XRD,VSM and TEM.It is found that the saturation magnetization M_s is 76 emu/g,the coercivity is 556 Oe and their structures are typical cubic spinel structure.And the CoFe₂O₄ nanosheets were composed with CoFe₂O₄ single crystalline nanoparticles,whose size ranging from 80 nm to 500 nm.The electron holography results indicated that the magnetic domain structure of the nanosheets was determined by its magnetocrystalline anisotropy and shape anisotropy.Meanwhile,the size and geometry of nanosheets also had a significant effect on the magnetic structure of the sample.