Dissertation > Industrial Technology > General industrial technology > Materials science and engineering > Smart materials

Effects of Magnetic Thermally Induced Anisotropy on Giant Magnetostrictive Materials

Author ZhangChangSheng
Tutor YanMi; MaTianYu
School Zhejiang University
Course Materials Physics and Chemistry
Keywords Giant magnetostrictive materials Magnetic annealing Induced anisotropy Magnetic domains Domain rotation model
Type PhD thesis
Year 2012
Downloads 45
Quotes 0
Download Dissertation

Giant magnetostrictive TbDyFe alloy is a kind of advanced intelligent material, which can rapidly convert magnetic energy into mechanical energy or vice versa. Therefore, this material has been widely applied in transducers, actuators, sensors, and so on. However, the complicated non-linear magnetoelastic behaviors of this material, which bring difficulties for precise device control, are currently the major concerns for practical applications. In the present work, magnetothermally induced anisotropy is used to optimize the initial magnetic domain distributions and the magnetization rotation paths, suggesting a novel technique to solve the non-linearity. Experimentally, effects of magnetic annealing on microstructures, initial magnetic domain distributions, and magnetoelastic behaviors for TbDyFe<110> oriented materials were systemically investigated. Theoretically, mechanisms of how the magnetothermally induced anisotropy influences the initial domain redistribution and magnetoelastic behaviors are revealed by proposing a modified anisotropic domain rotation model. Main achievements are as follows:Linearity and magnetostrictive performance of<110> oriented TbDyFe materials are significantly improved through transverse or noncoaxial magnetic field annealing,(abbreviated as TFA and NFA, respectively), through which redistribution of initial magnetic domain along special easy axes is realized and90°domain rotation is favored during magnetization. TFA and NFA denote that a magnetic field of240kA/m is applied perpendicular and with35°to the axis of the cylinder rod sample when cooling through its Curie temperature, respectively. No obvious changes occur in preferred crystal orientation and crystallographic morphologies, but the initial domain configurations are remarkably changed after magnetic annealing. In comparison with the thermal-demagnetized material, both of the magnetically annealed samples possess significantly enhanced saturation magnetostrictions (λs) without applying any compressive pre-stress. Meanwhile, they still exhibit obvious magnetostriction "jump" effect when applying a uniaxial compressive pre-stress. A record high λs of2680ppm for the TFA material and a satisfactory λs of2330ppm for the NFA one are achieved under a pre-stress of30MPa. The linear magnetostriction λm rises from1037ppm for the thermal-demagnetized material up to1620ppm for TFA and1358ppm for NFA, respectively. Additionally, magnetostrictive coefficient d33under the same bias condition is also enhanced after magnetic annealing. In comparison to the TFA, both the optimum pre-stress magnitude and critical bias field to reach maximum d33are simultaneously reduced by NFA, indicating a much better magnetostrictive performance could be achieved at lower stress magnitudes and smaller switching fields.The mechanisms of magnetothermally induced anisotropy influences on the magnetization movements and corresponding anisotropic magnetostrictions are revealed. Through investigating the anisotropic magnetostrictions for the<110> oriented TbDyFe materials with different demagnetized status, it is found that TFA can enhance the critical magnetization angle where the magnetostriction starts to "drop" and efficiently reduce the "dropped" value. Coaxial field annealing (CFA) can improve the axial magnetostriction dramatically from-640ppm to-1669ppm under a transverse field of640kA/m, which means a giant negative magnetostriction can be realized. For the first time, a magnetic domain rotation model is established by taking both the demagnetization effect and<110> crystal growth mechanism into account, through which the anisotropic magnetostrictions can be precisely predicted and are found to agree well with the experimental data. To understand more clearly the influences of magnetothermally induced anisotropy on anisotropic magnetostrictions, the moment movements for different demagnetized status are deduced based on the magnetization "rotation" and "jump" mechanisms. Meanwhile, the evolutions of magnetic domain fraction during different magnetization processes are also deduced by clarifying them into9kinds of domains according to the crystallographic orientation, which also explains well the anisotropic magnetostrictions.Direct evidence for the change of initial domain configurations after magnetic annealing is provided via magnetic force microscopy (MFM) study. Magnetic annealing changes obviously the initial magnetic force distribution (domain configurations) for TbDyFe<110> oriented materials from irregular and stripe-like domains into straightly arranged ones (parallel to each other), which indicates that the magnetic moments are tending to realign along one special easy axis or the annealing field direction from their initially equivalent distribution along eight<111> easy axes. After TFA, most of the domain moments are redistributed within the plane perpendicular to the<110> axis, which results in the significant improvement in axial magnetostriction performance. CFA tends to induce the moments realign near<110> axis (i.e. the annealing field direction), hence "anomalous" magnetostrictions can be obtained. MFM study suggests that magnetic annealing tends to induce a uniaxial anisotropy in GMMs with preferred crystal orientation, which explains well the observed phenomena in the present work and furnishes an important basis for exploring the correlations between magnetothermally induced anisotropy and magnetoelastic behaviors.Mechanism of how the strength of magnetothermally induced anisotropy influences on magnetoelastic behaviors in TbDyFe<110> oriented crystals is revealed quantitatively. Based on domain rotation theory and minimum energy principle,3D free energy distribution surfaces are simulated by taking different induced anisotropy energies into account. Initial magnetization alignment processes are established through minimizing the total free energy, and the corresponding magnetostrictions are calculated. Unlike the stress-induced anisotropy in the perpendicular plane, magnetothermally induced anisotropy inclines the initially equivalent magnetization distribution along eight<111> easy axes into a special one. When the induced anisotropy reaches a critical value (Ku,c) of36kJ/m3, the intrinsic cubic anisotropy turns into a purely uniaxial one. That is, the ideal90°initial state is formed. When the induced anisotropy is lower than Ku,c,<110> oriented crystal still exhibits magnetostriction "jump" effect. After the induced anisotropy reaches or exceeds Ku,c,"jump" effect disappears, but the saturation magnetostriction is obviously increased without compressive pre-stress, which explain well the experimental observations. Meanwhile, combined contribution of the magnetothermally induced anisotropy and compressive pre-stress on magnetostriction is also revealed, through which approaches for further improving the magnetostrictive performance under high-magnitude loadings are suggested.

Related Dissertations
More Dissertations