| 摘要 | Sm-Co 基永磁体因为高居里温度，高单轴磁晶各向异性场和高饱和磁化强度具有高温应用前景，但它的矫顽力偏低。因此，为了改善其室温矫顽力从而提高使用温度，采用机械合金化方法制备了过渡金属掺杂和替代的Sm-Co 基永磁材料，对它们的结构、磁性和相转变进行了系统研究。结果表明，少量Cr促进了Th2Ni17型 2:17相在SmCo7-xCrx 磁体中的形成。增加Cr含量扩大了该六方相的稳定温度范围，并且显著提高了磁体矫顽力。根据初始磁化曲线和小回线的测量，SmCo7-xCrx 磁体的反磁化过程被认为由不均匀畴壁钉扎控制。少量Mn 添加有利于Th2Zn17型2:17相在SmCo6.7-xMnxCr0.3磁体中形成，而且明显增加了磁体矫顽力。通过合金退火，观察到了Th2Ni17型向Th2Zn17-type 型的结构相转变。增加Mn 还提高了Co 从2:17相析出的临界温度，增强了硬磁2:17相的热稳定性。除此之外，少量Cu 掺杂促进了在SmCo6.7-xCuxCr0.3 磁体中形成Th2Ni17型2:17 相/Co 纳米复合结构。Cu添加以单质形式析出，阻碍了畴壁移动，削弱了纳米晶间交换耦合，从而大大提高了磁体矫顽力。通过回复曲线测量，分析了磁体从可逆到不可逆的反磁化过程。
在机械合金化制备的Pr14Tb2Fe76-xCoxC6B2, Pr16Fe76-xCoxC6B2 和Pr16-xTbxFe66Co10C6B2磁体中获得了接近单相的硬磁Pr2Fe14C-型碳化物，避免了长时间复杂热处理。Co 替代减少了软磁-Fe 析出，从而在一定范围内同时提高了矫顽力和剩余磁化强度。矫顽力增强由纳米晶铁磁体有效各向异性常数 的变化进行了解释。而且，Tb 替代还降低了2:14:1碳化物的相转变温度，减少了-Fe 析出，促进了2:14:1碳化物形成，显著提高了磁体矫顽力。
采用解析方法分析了单轴取向纳米晶铁磁体的有效各向异性场和耦合畴壁结构。理论分析表明，提高晶体取向度增强了磁矩对局域易磁化轴的偏离，增加了晶间交换耦合对磁晶各向异性的削弱作用，从而减小了有效各向异性场。耦合畴壁的平均厚度和能量随取向度增加而增加。这加深了对各向异性纳米晶铁磁体矫顽力机制的理解，并预示可以通过提高晶体取向改善纳米晶软磁材料的性能。
从扩展分区模型(extended SZM)的观点研究和分析了垂直膜面取向Nd-Fe-B 膜的生长机制。由于表面粒子迁移和扩散随膜生长而减弱，Nd-Fe-B 膜的生长模式由zone II依次转变为 zone Ic 。这样，整个Nd-Fe-B 膜就由[00l] 垂直膜面取向变得越来越混乱取向。系统探讨了Nd-Fe-B 膜生长过程中取向演化和矫顽力变化的关系。通过对反磁化过程的分析，矫顽力变化被认为由形核场变化引起。而且，对垂直取向Nd-Fe-B 膜的磁粘滞进行了系统研究。采用热激活反磁化机制和Néel 的散场理论，对其反常的非单调和非线性磁粘滞系数随温度和膜厚的变化关系进行了解释。分析表明了磁粘滞行为和薄膜应力导致的散场区域的可能联系。此外，计算了垂直取向Nd-Fe-B 膜的涨落场和激活体积。
在垂直各向异性(Nd, Dy)-(Fe, Co)-B/α-Fe 纳米复合薄膜中实现了晶间交换耦合和剩磁增强。通过低温不均匀反磁化、XRD 及XPS 的分析证实了软磁α-Fe 分布在取向硬磁2:14:1 相的基体上。相对于取向单相薄膜，垂直各向异性(Nd, Dy)-(Fe, Co)-B/α-Fe 纳米复合薄膜表现了不均匀的磁畴结构和各向异性的反磁化行为。 |
| 其他摘要 | Sm-Co based hard magnets have potential high-temperature application due to their high Curie temperature, high uniaxial magnetocrystalline anisotropy field and high saturation magnetization, but with the limited coercivity. Therefore, several transition-metal elements are doped in the Sm-Co based magnets by mechanical alloying, for enhancing their room-temperature coercivities and thus the performance temperature. Their strucutures, magnetic properties and phase transformation are also systematically studied. It is revealed that a small amount of Cr favors formation of Th2Ni17-type 2:17 phase in SmCo7-xCrx magnets. Increasing the Cr content can extend temperature range of stabilizing the hexagonal 2:17 phase and significantly enhancing the coercivities. Their magnetization reversal processes are dominated by inhomogeneous domain wall pinning, according to measurements of initial magnetization curves and minor hysteresis loops. Moreover, it is shown that Mn addition facilitates formation of Th2Zn17-type 2:17 phase and pronouncedly improves the intrinsic coercivities and the maximum energy products of SmCo6.7-xMnxCr0.3 alloys. The phase transformation from Th2Ni17-type to Th2Zn17-type structure is observed in annealing the alloys. More Mn addition also increases thermal stability of the hard magnetic 2:17 phase by enhancing the critical annealing temperature above which soft magnetic Co phase appears. Besides, a small amount of Cu addition favors formation of Th2Ni17-type 2:17 phase/Co nanocomposite structure in SmCo6.7-xCuxCr0.3 alloys. The increased Cu doping causes more Cu precipitation separating the grains, which significantly enhances coercivity and weakens exchange coupling between the magnetically hard and soft phases. The reversible to irreversible reversal processes are also analyzed, upon recoil curves measurements.
The nearly single Pr2Fe14C-type carbides are realized in mechanically alloyed Pr14Tb2Fe76-xCoxC6B2, Pr16Fe76-xCoxC6B2 and Pr16-xTbxFe66Co10C6B2 alloys, without long-period heat treatment. Co substitution is found to reduce -Fe fraction in the alloys, which increases both coercivity and remanence in a certain composition range. This coercivity increase is interpreted in terms of the effective anisotropy constant of the magnets. Moreover, Tb substitution facilitates 2:14:1 carbides formation via decreasing the 2:14:1 carbides transformation temperature and inhibiting α-Fe precipitation, thus increasing the magnets coercivities.
By considering effects of grain alignment on intergrgrain exchange coupling, effective anisotropy field and interaction domain wall structure are analytically investigated for the single-phase and anisotropic nanostructured ferromagnets with uniaxial magnetocrystalline anisotropy. It is revealed that the effective anisotropy field decreases with the grain alignment by increased deviation of magnetization from easy-axis and hence enhanced suppressing effect of the intergrain interaction on the anisotropy. The averaged interaction domain wall thickness and energy density increase, as the nanocrystalline ferromagnet becomes more aligned. It provides more insight to coercivity mechanism in aligned nanomagnets and suggests that soft magnetic properties could be improved by improving the crystallographic orientation.
The growth mechanism for the out-of-plane orientation of Nd-Fe-B films is investigated in terms of the extended structure zone model. The growth mode changes sequentially from zone II to zone Ic, with the lowered thermally-induced adparticles’ mobility on the surface as the film grows. As such, the whole Nd-Fe-B film evolves from [00l] out-of-plane orientation to more randomly out-of-plane alignment. With the growth of the out-of-plane oriented Nd-Fe-B film, the correlation between the magnetic properties and the alignment evolution has been studied. The coercivity variation with the film growth results from the nucleation field change, which is understood by analyzing the nucleation-dominant reversal processes for the out-of-plane oriented Nd-Fe-B films. Furthermore, magnetic viscosity in perpendicularly anisotropic Nd-Fe-B films is investigated. The anomalous temperature dependence and the thickness dependence of the maximum viscosity coefficients are observed and interpreted based on the thermally activated reversal processes and Néel’s disperse field theory. The analysis implies the possible correlation between the magnetic viscosity and the disperse field regions in the reversal processes, which is set up by the internal strain in the films. The fluctuation fields and the activation volumes are also deduced for the films.
Exchange coupling and remanence enhancement are observed in (Nd, Dy)-(Fe, Co)-B/α-Fe films with perpendicular magnetic anisotropy. The soft α-Fe presence in the matrix of aligned 2:14:1 hard magnetic phase is verified by the two-step reversal process at low temperatures, XRD and XPS analyses. Compared with its single-phase counterpart, the anisotropic (Nd, Dy)-(Fe, Co)-B/α-Fe film displays more inhomogeneous magnetic - domain structure. The aligned nanocomposite film demonstrates the anisotropic demagnetization behavior, with the homogeneous reversal process becoming inhomogeneous as the field deviates from perpendicular to parallel to the film plane. |
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