Magnetic properties and coercivity mechanism of precipitation-hardened Gd－Co based ribbons

Gd(Co_{0.88-x}Cu_xFe_{0.09}Zr_{0.03})_z ribbons with x=0.075-0.200 and z=6.4－7.7 have been prepared by a melt-spinning technique. A cellular microstructure consisting of 2:17 cells surrounded by the 1:5 cell boundary phase is obtained after precipitation hardening. The dependence of room temperature coercivity on the heat treatment process suggests that the long-time isothermal aging is not helpful for the development of magnetic properties. Positive temperature coefficient of remanence from room temperature to about 673K is typical for all samples, while positive temperature coefficient of coercivity is obtained only in ribbons with low Cu content. The coercivity mechanism of the precipitation-hardened ribbons at different temperatures is also discussed.     

非线性椭圆型边值问题解的存在性

Hilbert空间方法被用于一类二阶半线性椭圆型边值问题并得出了某些条件下的解的存在性.     

Carbon Encapsulation of High Entropy Alloy Nanoparticles with Extraordinary Coercivity and Saturation at Room Temperature

Owing to their unique properties and technological potential, high entropy alloys (HEAs) have become the subject of great interest in the materials science community. HEAs consist of more than four principle elements in equimolar ratio so their configurational entropy is intrinsically greater than one-principle element based. The increasing surface energy and chemical tendency toward clustering of like atoms at low dimension, however, make production of HEA-nanoparticles (HEA-NPs) extremely difficult. A facile production of HEA-NPs inside carbon nanotubes and nanoparticles is demonstrated in this work. Electron microscopic and elemental analyses confirm encapsulated to be solution phase; some embrace carbides and form multidomains with chemical composition ranging from quaternary to quinary phase. Multidomains and nonmagnetic centers create hardening thus promoting coercivity significantly at room temperature. Alloying induces electron redistribution into high spin states, accounting for observed high saturation. Configurational entropy of encapsulated HEA-NPs lies on a range comparable with bulk.     

Effects of orientation and interaction of grains on coercivity for sintered NdFeB magnets

The coercivity of NdFeB magnets is determined by the coercivity of individual grains and the interaction between the grains composed of the magnets. The coercivity of individual grains and the intergrain interaction depend on the degree of the grain alignment, “tanθ type” Gaussian function is applied to describing the degree of the grain alignment. According to different coercivity mechanisms, there are different formula on the coercivity and the angular dependence of coercivity. The interaction between grains can be classified as the long-range magnetostatic interaction and the exchange-coupling interaction of neighboring grains. For the sintered magnet, the grain size is large and the grain boundaries are mostly separated by the non-magnetic phase. So, the long-range magnetostatic interaction is much stronger than the exchange coupling interaction and it makes the coercivity of the magnet composed of misaligned grains be bigger than that of the magnet composed of ideally aligned grains. The effects of coercivity of individual grains and the intergrain interactions are taken into account, and the starting field theory is in agreement with the experimental result for the coercivity of sintered NdFeB magnets. Project supported by the National Natural Science Foundation of China (Grant No. 59571017).     

Sm(CobalFexCu0.049Zr0.024)7.5(x=0.102～0.282)的结构和磁性能

采用XRD,AFM等检测方法研究了Fe含量对Sm（CobalFexCu0.049Zr0.024）7.5（x=0.102-0.282）的磁性能和显微结构的影响。结果表明,磁体的Br先随Fe含量的增加而增加,在x=0.239时达最大值1.099 T,进一步增加Fe含量由于FeCo软磁性相的形成而导致Br下降。当Fe含量由x=0.239增加至0.282时,Hci由最大值1660.3 kA·m^-1迅速降低至979.1kA·m^-1。Fe含量对磁体的相结构没有显著的影响,主要由2∶17R相、1∶5相和2∶17H相构成。但当Fe含量x〈0.197时,Sm（CobalFexCu0.049Zr0.024）7.5的胞状组织未发生显著变化,平均尺寸约为80nm;当Fe含量x〉0.197时,胞状结构发生长大,均匀性变差,胞状结构的变化导致矫顽力的温度稳定性随Fe含量的增高而大幅降低;当x=0.282时,磁体在500℃下的Hci只有114 kA·m^-1,磁通不可逆损失达-36.7%。     

High-Resolution Transmission Electron Microscopy Studies of Sol-Gel-Derived Cobalt-Substituted Barium Ferrite

Nanocrystal specimen of cobalt (II)-doped magnetoplumbite (M-type) BaFe₁₂O₁₉ were synthesized via a sol-gel method using ethylene glycol as precursor. Hexagonal-like platelets and other morphologies of ferrites were obtained at the final stage of annealing process. The specimens were characterized using X-ray diffraction, SQUID magnetometry measurement and high-resolution transmission electron microscopy. The saturation magnetization of the specimen at 300 K was found to be greatly reduced from 48 (3) emu/g in parent ferrite to 28 (3) emu/g in the doped ferrite. A total reduction of 74% in the value of intrinsic coercivity was recorded, 276 (15) G from the doped ferrite compared to 1080 (54) G in the parent ferrite.     

纳米Nd2Fe14B永磁材料的有效各向异性随晶粒尺寸的变化

以纳米Nd2Fe14B硬磁材料为例, 从Herzer的随机各向异性理论出发, 采用立方体晶粒结构模型, 建立了纳米硬磁性晶粒之间的部分交换耦合模型, 研究了材料的有效各向异性随晶粒尺寸的变化关系.结果表明: 晶粒之间的交换耦合相互作用随晶粒尺寸的减小而增强, 材料的有效各向异性Keff随晶粒尺寸的减小逐渐下降. Keff随晶粒尺寸的变化规律与矫顽力的变化规律基本相似, 纳米Nd2Fe14B硬磁材料矫顽力的下降主要由有效各向异性的减小引起.     

Boundedness of the solutions to non-linear systems with Morrey data

We consider non-linear elliptic systems satisfying componentwise coercivity condition. The non-linear terms have controlled growths with respect to the solution and its gradient, while the behaviour in x is governed by functions in Morrey spaces. We firstly prove essential boundedness of the weak solution and then we obtain Morrey regularity of its gradient.     

Finite size effects with variable range exchange coupling in thin-film Pd/Fe/Pd trilayers

The magnetic properties of thin-film Pd/Fe/Pd trilayers in which an embedded - thick ultrathin layer of Fe induces ferromagnetism in the surrounding Pd have been investigated. The thickness of the ferromagnetic trilayer is controlled by varying the thickness of the top Pd layer over a range from 8 to . As the thickness of the top Pd layer decreases, or equivalently as the embedded Fe layer moves closer to the top surface, the saturated magnetization normalized to area and the Curie temperature decreases, whereas the coercivity increases. These thickness-dependent observations for proximity-polarized thin-film Pd are qualitatively consistent with finite size effects that are well known for regular thin-film ferromagnets. The critical exponent β of the order parameter (magnetization) is found to approach the mean field value of 0.5 as the thickness of the top Pd layer increases. The functional forms for the thickness dependences, which are strongly modified by the nonuniform exchange interaction in the polarized Pd, provide important new insights to understand nanomagnetism in two-dimensions.