Crystal growth and dislocations

The article attempts to review the present stage of the density functional theory for noncollinear magnetic states and its application to particular physical problems. The discussion starts with basic theorems of the theory and derivation of the Kohn-Sham equation for a noncollinear magnet. Special features of solving this equation are illustrated using the augmented-spherical-wave method generalized to the noncollinear magnetic structures as an example. Particular attention is devoted to the symmetry of the problem. It is shown that a traditional approach of the space group theory fails in the case of a noncollinear magnetic state. A generalized approach based on the notion of the spin space groups is presented which allows a consequent treatment of the symmetry properties of both nonrelativistic and relativistic problems. This approach allows the development of an exact procedure for a first-principles calculation of an incommensurate spiral structure and gives a sound basis for the calculation and physical interpretation of the noncollinear magnetic structures caused by the effects of relativity. Application of the theory to the first-principles determination of the complex noncollinear magnetic structures of various systems are discussed. Applications include the spiral structure of fcc Fe, the uncompensated magnetic structure in U3X4 and compensated magnetic structure in U2P2Sn, and two different types of weak ferromagnetism in Fe2O3 and Mn3Sn. It is shown how the calculation technique can be applied to studies of unenhanced and enhanced non-uniform magnetic susceptibilities as well as susceptibilities in fields noncollinear to the atomic moments. Illustrating results of the calculation of the susceptibility of various systems are presented. The last part of the review is devoted to applications of the theory of noncollinear magnetism to studies of temperature effects in the electronic properties of itinerant magnets.     

Chiral-like critical behavior in the antiferromagnet cobalt glycerolate

Critical exponents closely matching those of the N=2 chiral universality class have been obtained for the layered magnetic system cobalt glycerolate using muon spin relaxation. This class was originally introduced to represent geometrically frustrated triangular stacked-layer XY magnets with chiral noncollinear spin structures. Since the present magnetic system is a canted XY system without geometrical frustration or chiral degeneracy, the results indicate that the order parameter for canting in this system plays a similar role to the chiral order parameter in the geometrically frustrated systems, strongly suggesting that both types of noncollinear system share the same universality class.     

Spin current and magnetoelectric effect in noncollinear magnets

A new mechanism of the magnetoelectric effect based on the spin supercurrent is theoretically presented in terms of a microscopic electronic model for noncollinear magnets. The electric polarization P(ij) produced between the two magnetic moments S(i) and S(j) is given by P proportional e(ij) X (S(i) X S(j)) with e(ij) being the unit vector connecting the sites i and j. Applications to the spiral spin structure and the gauge theoretical interpretation are discussed.     

Nonequilibrium quantum criticality in open electronic systems

A theory is presented of quantum criticality in open (coupled to reservoirs) itinerant-electron magnets, with nonequilibrium drive provided by current flow across the system. Both departures from equilibrium at conventional (equilibrium) quantum critical points and the physics of phase transitions induced by the nonequilibrium drive are treated. Nonequilibrium-induced phase transitions are found to have the same leading critical behavior as conventional thermal phase transitions.     

Functional-integral study of spin fluctuations in small Fe clusters

Finite temperature magnetic properties of small Fe_N clusters (N ≤6) are determined in the framework of a spin-fluctuation itinerant-electron theory based on a functional integral formulation of the canonical partition function and derived statistical averages. The free energy associated to each configuration of the exchange fields throughout the cluster are calculated by using Haydock-Heine-Kellys recursion method. The statistical averages of physical interest are obtained by performing parallel-tempering Monte Carlo simulations. Representative results are discussed for the average magnetization per atom as a function of temperature. The interplay between local environment and magnetization curves is analyzed by considering the low-temperature limit of the local spin-fluctuations energies ΔF_l(ξ) at different atoms l. The electronic calculations are contrasted with the predictions of simple of phenomenological Heisenberg-like models.     

粉末粒度离散性对NdFeB磁性能的影响

制备了一种NdFeB永磁材料,探讨了制粉前处理方式对粉末离散性、粒径及成品NdFeB永磁体结构与性能的影响。原材料经熔炼、粗破碎后,采用高速风选和高压氢爆2种制粉前处理方式,然后采用同样的成型、烧结和回火工艺制备得到(PrNd)32.3Nb0.70Al0.60Cu0.20B1.03Fe65.17合金,并系统研究了该合金的显微结构和磁性能。结果表明:高压氢爆处理后得到的粉末粒度呈单分散性,粉末粒径较细,所制备得到的磁体样品的微观结构均匀性也较好;高压氢爆处理后制得的磁体样品剩余磁通密度比高速风选制粉方式得到的磁体样品高0.047 T,内禀矫顽力高 193.4 kAm－1,最大磁能积高24.6 kJm－３。     

粉末粒度离散性对NdFeB磁性能的影响

制备了一种NdFeB永磁材料,探讨了制粉前处理方式对粉末离散性、粒径及成品NdFeB永磁体结构与性能的影响。原材料经熔炼、粗破碎后,采用高速风选和高压氢爆2种制粉前处理方式,然后采用同样的成型、烧结和回火工艺制备得到(PrNd)32.3Nb0.70Al0.60Cu0.20B1.03Fe65.17合金,并系统研究了该合金的显微结构和磁性能。结果表明:高压氢爆处理后得到的粉末粒度呈单分散性,粉末粒径较细,所制备得到的磁体样品的微观结构均匀性也较好;高压氢爆处理后制得的磁体样品剩余磁通密度比高速风选制粉方式得到的磁体样品高0.047 T,内禀矫顽力高 193.4 kAm－1,最大磁能积高24.6 kJm－３。     

Electron back scattered diffraction study of SmCo magnets

The remanence and energy product of permanent magnets is a strong function of their crystallographic texture. Electron back scattered diffraction (EBSD) is a tool for texture analysis providing information about the atomic layers up to 50 nm below the surface of the material. This paper discusses experimental requirements for performing EBSD measurements on rare-earth permanent magnets and presents results on commercial SmCo magnet material. EBSD measurements proved to be very sensitive to misaligned grains and were sensitive to texture in good agreement with information provided by X-ray diffraction scans. Results for nanostructured Sm(CoFeCuZr)_z magnets are also discussed.     

Magnetic anisotropy,exchange coupling and Dzyaloshinskii–Moriya interaction of two-dimensional magnets

The two-dimensional (2D) magnets provide novel opportunities for understanding magnetism and investigating spin related phenomena in several atomic thickness. Multiple features of 2D magnets, such as critical temperatures, magnetoelectric/magneto-optic responses, and spin configurations, depend on the basic magnetic terms that describe various spins interactions and cooperatively determine the spin Hamiltonian of studied systems. In this review, we present a comprehensive survey of three types of basic terms, including magnetic anisotropy that is intimately related with long-range magnetic order, exchange coupling that normally dominates the spin interactions, and Dzyaloshinskii−Moriya interaction (DMI) that favors the noncollinear spin configurations, from the theoretical aspect. We introduce not only the physical features and origin of these crucial terms in 2D magnets but also many correlated phenomena, which may lead to the advance of 2D spintronics.     

CSNS校正磁铁积分场均匀性测量研究

中国散裂中子源(CSNS)加速器主要由一台直线加速器、一台1.6 GeV快循环加速器以及低能、高能输运线组成。其中有十二种类型校正磁铁,共计74台。校正磁铁是加速器运行的重要组成部件,必须测量其积分场及均匀性以确保加速器性能。为了快速和高效地完成磁铁测量任务,使用了旋转线圈测量系统。对测量原理、数据处理方法以及系统之间的测量结果对比进行了介绍。这对批量磁铁高效率测量是有帮助的。     

Studies on the magnetic properties for single-phase nanocrystalline FePt magnets with different orientation degrees

The single-phase nanocrystalline FePt magnets composed of 343 irregular-shaped grains are built. The demagnetization curves are simulated by micromagnetic finite element method. The remanence, coercivity and maximum energy product of the magnets decrease with deteriorating grain alignment. The characteristics of variation of magnetic properties with the degree of orientation are closely related to the average grain size of nanocrystalline magnets. The contribution of intergrain exchange coupling (IGEC) to remanence enhancement is associated to the degree of orientation, and decreases with improved grain alignment. With decreasing grain size, coercivity increases for anisotropic nanocrystalline magnets, which is completely different from that of isotropic nanocrystalline magnets.     

Electron correlations in a C20 fullerene cluster

The ground-state properties of C₂₀ fullerene clusters are determined in the framework of the Hubbard model by using lattice density-functional theory (LDFT) and scaling approximations to the interaction-energy functional. Results are given for the ground-state energy, kinetic and Coulomb energies, local magnetic moments, and charge-excitation gap, as a function of the Coulomb repulsion U/t and for electron or hole doping δ close to half-band filling (|δ| ≤1). The role of electron correlations is analyzed by comparing the LDFT results with fully unrestricted Hartree-Fock (UHF) calculations which take into account possible noncollinear arrangements of the local spin-polarizations. The consequences of the spin-density-wave symmetry breaking, often found in UHF, and the implications of this study for more complex fullerene structures are discussed.     

Anisotropic nanocomposite soft/hard multilayer magnets

Experimental and theoretical researches on nanostructured exchange coupled magnets have been carried out since about 1988. Here, we review the structure and magnetic properties of the anisotropic nanocomposite soft/hard multilayer magnets including some new results and phenomena from an experimental point of view. According to the different component of the oriented hard phase in the nanocomposite soft/hard multilayer magnets, three types of magnets will be discussed:1) anisotropic Nd_2Fe_(14)B based nanocomposite multilayer magnets, 2) anisotropic SmCo_5 based nanocomposite multilayer magnets, and 3) anisotropic rare-earth free based nanocomposite multilayer magnets. For each of them, the formation of the oriented hard phase, exchange coupling, coercivity mechanism, and magnetic properties of the corresponding anisotropic nanocomposite multilayer magnets are briefly reviewed, and then the prospect of realization of bulk magnets on new results of anisotropic nanocomposite multilayer magnets will be carried out.     

铁基超导线带材研究现状及展望

铁基超导体具有极高的上临界磁场、较小的各向异性、简单的制备工艺等优点,在高场超导磁体、核磁共振谱仪、可控核聚变装置、高能粒子加速器等领域具有重要的应用前景。为满足上述高场强电应用需求,必须制备出高性能超导线带材。文章详细介绍了多种体系铁基超导线带材的研究进展,分析当前线带材研究中存在的问题,提出改善线材传输性能的途径,并对铁基超导线材的发展趋势进行展望。     

螺管型NbTi储能磁体的优化及尺寸效应

储能系统是改善供电质量,保证工作系统可靠运行的重要装置.超导储能(SMES)的核心部件是超导磁体.储能范围1MJ～100MJ的小型或微型SMES近年得到特别的重视.本文讨论了螺管型铌钛SMES磁体的尺寸参量对储能水平和储能密度的影响,给出了小型SMES螺管磁体的尺寸与储能水平的关系曲线,提出了该类磁体在较大储能范围内快速确定设计方案的简便方法.     

上海同步辐射装置储存环注入冲击磁铁的设计

开展了上海同步辐射装置储存环注入冲击磁铁的结构和脉冲电源的技术设计,并进行了磁场分布和励磁电流波形的模拟计算。还给出了陶瓷真空盒镀膜的设计参数,提出了计算陶瓷真空盒镀膜接受同步光功率的公式,研究了防止镀膜和波纹管受到同步光照射的机械布局设计。     

用于宽带啁啾脉冲光参量放大系统精确同步的新方法

基于非共线光参量放大,以波长800 nm飞秒激光的倍频光为抽运光,以激光二极管640 nm连续波激光为信号光,产生了与800 nm飞秒激光精确同步的无直流本底的1064 nm脉冲光。实验结果显示该1064 nm的光脉冲在脉冲能量和空间光束质量上均可作为1064 nm波长脉冲放大器的种子光。仅通过一级简单紧凑的非共线参量放大就可实现光周期量级的光参量啁啾脉冲放大系统的抽运激光和种子光的精确同步。     

Rare earth-cobalt permanent magnets

This paper reviews the historical background and the development of rare earth-cobalt-based permanent magnets from basic science studies on rare earth-transition metal alloys in the 1960's to today's broad spectrum of commercial magnet types and their applications. It puts the RE-Co magnets in perspective relative to older magnet types and also traces the path to the subsequent development of the related Nd-Fe-B magnets. The treatment is qualitative, with emphasis on the relationship between fundamental properties of the compounds and the interaction between microstructure and magnetic domain walls that makes high coercivity and the exceptional hard magnetic properties of the rare-earth magnets possible. The various kinds of RE-Co magnets in production and use today, some of their engineering properties, and economic aspects governing their applicability, cost and availability are also discussed. Many references provide a guide to the special literature regarding the physics, metallurgy, manufacture, product selection and properties of rare earth-cobalt magnets.     

上海同步辐射装置储存环注入冲击磁铁的设计

 开展了上海同步辐射装置储存环注入冲击磁铁的结构和脉冲电源的技术设计,并进行了磁场分布和励磁电流波形的模拟计算。还给出了陶瓷真空盒镀膜的设计参数,提出了计算陶瓷真空盒镀膜接受同步光功率的公式,研究了防止镀膜和波纹管受到同步光照射的机械布局设计。     

A multi-stable energy harvester: Dynamic modeling and bifurcation analysis

A theoretical investigation is conducted on the dynamic and energetic characteristics of a multi-stable bimorph cantilever energy harvester that uses magnetic attraction effect. The multi-stable energy harvester under study is composed of a bimorph cantilever beam with soft magnetic tip and two externally fixed permanent magnets that are arranged in series. With this configuration, the magnetic force and the moment that are exerted on the cantilever tip tend to be highly dependent on the magnetic field induced by the external magnets. Such an energy harvester can possess multi-stable potential functions, ranging from mono-stable to penta-stable. The mechanism that governs the formation of this multi-stability is thoroughly identified and examined thorough a bifurcation analysis performed on the system?s equilibrium solutions. From this analysis, it is found that the transitions between these multi-stable states occur through very complicated bifurcation scenarios that include degenerate pitchfork bifurcations and mergers of pitchfork bifurcations or saddle-node bifurcations. Bifurcation set diagram is obtained, which is composed of five separate parametric regions, from mono- to penta-stability. The resulting stability map satisfactorily describes the multi-stable characteristics of the present energy harvester. In addition, the dynamic and energetic characteristics of the present multi-stable energy harvester are more thoroughly examined using its potential energy diagrams and a series of numerical simulations, and the obtained results are compared with those for the equivalent bi-stable cases.