Exchange-spring behavior of hard/soft magnetic multilayers: optimization study of the nanostructure

We investigate the magnetization reversal process in two-phase hard/soft (h/s) magnetic multilayers with exchange-spring behavior using a mean-field approach to determine the equilibrium configuration. Starting from a trilayer made of 50h/100s/50h layers, we separately consider the effect of increasing nanostructuration (while maintaining constant the overall h/s ratio) and decreasing the thickness of the hard phase (while keeping constant that of the soft phase) on the maximum energy product (BH)_max. In this way, the optimal composition for the multilayer exchange-spring magnet can be determined.     

Effect of the soft layer thickness on magnetization reversal process of exchange-spring nanomagnet patterns

A systematic study of the magnetization reversal behavior in the regular arrangement of L1₀-FePt based exchange-spring nanomagnets with different thicknesses of the Co soft magnetic layer is presented. The magnetic property of the hard magnet is compared to two tuned exchange-spring magnets: its systems of 20 nm L1₀-FePt/3 nm, and 7 nm Co. In particular, we focus on the switching field distribution. The exchange coupling showed narrower SFD, in spite of the decoupled part switches earlier. The magnetization switching mechanism of exchange-spring nanomagnets patterns has been revealed with a first-order reversal curves technique and the switching field distribution. Further, the microscopic results using magnetic force microscopy show that the spin rotation of the non-interacting part in the thicker soft layered exchange-spring magnet. The part influences the magnetization reversal process. According to the experimental results, exchange coupling strength can be tuned by the thickness of the soft magnetic layer.     

Phase distribution and computed magnetic properties of high-remanent composite magnets

Numerical micromagnetic calculations using finite-element techniques allow a quantitative treatment of the correlation between the microstructure and the basic magnetic properties of two-phase permanent magnets such as the remanence, the coercive field and the maximum energy product. For the investigation of (A) the role of the amount of the soft magnetic phase, and (B) the effect of grain shape, realistic three-dimensional grain arrangements have been used. The numerical results show that both short-range exchange and long-range magnetostatic interactions determine the magnetic properties. The optimal microstructure of an isotropic nanocrystalline permanent magnet was found to consist of soft magnetic particles with a large spontaneous magnetization embedded between hard magnetic grains. Exchange interactions than enhance the remanence of isotropic, composite magnets of Nd₂Fe₁₄B and -Fe by about 60%. Because of exchange hardening the soft magnetic phase can be increased up to 50% without a significant loss of coercivity. A uniform grain structure suppresses strong demagnetizing fields and this increases coercivity by 30% as compared with irregular shaped particles.     

Transition from reversible to irreversible magnetic exchange-spring processes in antiferromagnetically exchange-coupled hard/soft/hard trilayer structures

The demagnetization processes of antiferromagnetically exchange-coupled hard/soft/hard trilayer structures have been studied based on the discrete one-dimensional atomic chain model and the linear partial domain-wall model. It is found that, when the magnetic anisotropy of soft layer is taken into account, the changes of the soft layer thickness and the interfacial exchange coupling strength may lead a transition of demagnetization process in soft layer from the reversible to the irreversible magnetic exchange-spring process. For the trilayer structures with very thin soft layer, the demagnetization process exhibits typical reversible exchange-spring behavior. However, as the thickness of soft layer is increased, there is a crossover point t_c, after which the process becomes irreversible. Similarly, there is also a critical interfacial exchange coupling constant A_sh^c, above which the exchange-spring process is reversible. When A_sh<A_sh^c, the irreversible exchange-spring process is achieved. The phase diagram of reversible and irreversible exchange-spring processes is mapped in the plane of the interfacial exchange coupling A_sh and soft layer thickness N_s.     

Pinpointing chiral structures with front-back polarized neutron reflectometry

A new development in spin-polarized neutron reflectometry enables us to more fully characterize the nucleation and growth of buried domain walls in layered magnetic materials. We applied this technique to a thin-film exchange-spring magnet. After first measuring the reflectivity with the neutrons striking the front, we measure with the neutrons striking the back. Simultaneous fits are sensitive to the presence of spiral spin structures. The technique reveals previously unresolved features of field-dependent domain walls in exchange-spring systems and has sufficient generality to apply to a variety of magnetic systems.     

反铁磁耦合硬磁-软磁-硬磁三层膜体系的不可逆交换弹性反磁化过程

采用一维原子链模型研究了反铁磁耦合的硬磁/软磁/硬磁三层膜体系的反磁化过程. 研究结果表明,当考虑了软磁层的磁晶各向异性能后,软磁层厚度和界面交换耦合强度的改变都有可能导致软磁层的交换弹性反磁化过程由可逆过程转变为不可逆过程. 对软磁层很薄的体系,其反磁化过程是典型的可逆交换弹性反磁化过程. 然而,当软磁层厚度超过某一临界厚度t_c时,反磁化过程转变为不可逆的交换弹性反磁化过程. 软磁-硬磁界面交换耦合强度A_sh对反磁化行为也有很大的影响. 对于软磁层厚度小于临界厚度t_c的体系,也存在一个临界界面交换耦合强度A_sh^c. 当A_sh大于A_sh^c时,软磁层的反磁化过程是可逆的交换弹性反磁化过程;而当A_sh小于A_sh^c时,这一过程变为不可逆. 给出了体系的可逆与不可逆交换弹性反磁化过程随软磁层厚度和界面交换耦合强度变化的磁相图. 同时还研究了偏转场随软磁层厚度的变化关系. 关键词： 反铁磁耦合三层膜 交换弹性反磁化过程 反磁化机理 磁相图     

高性能纳米复合永磁材料的模拟计算研究

构造了在软磁相基体中析出规则形状硬磁相的各向同性和各向异性纳米复合永磁材料Pr2Fe14Bα-Fe.利用微磁学的有限元法，模拟计算了样品的磁滞回线.通过对剩磁、矫顽力和最大磁能积与软磁相厚度的关系分析，发现在软磁相厚度为0—12nm的范围内，复合磁体的矫顽力随软磁相厚度的增加而单调下降，而最大磁能积则是在适当的矫顽力和剩磁下达到最高.预计纳米复合Pr2(Fe,Co)14Bα-(Fe,Co)的各向同性磁体的最大磁能积最高可达248kJ/m3，而各向异性磁体则高达784kJ/m3. 关键词： 纳米复合永磁 矫顽力 剩磁 磁能积     

Hard–soft composite magnets

A thorough micromagnetic analysis of the exchange-spring problem is reported with special emphasis on multilayers constituted by hard–soft exchange-coupled phases. The developed one—dimensional micromagnetic model leads to a complete magnetic phase diagram in terms of layer thicknesses. Both perpendicular and parallel configurations are considered. The phase diagram provides information on the type of demagnetization processes and the critical fields at which nucleation and reversal take place, depending on the intrinsic properties of the chosen soft and hard materials. The model has been applied to a variety of hard magnetic phases (e.g., FePt, CoPt, SmCo and NdFeB), coupled to different soft materials (e.g., Fe, FeCo, FeRh or permalloy) both in the form of bilayers and multilayers. The most significant results will be highlighted.     

Interdiffusion in bilayer CoPt/Co films: potential for tailoring the magnetic exchange spring

The magnetic and microstructural properties and magnetic domain configuration of exchange-spring model bilayer samples based on L1₀-type magnetically-hard CoPt and magnetically soft Co were examined and analyzed. Bilayers of varying thicknesses and annealing conditions were examined with room-temperature and elevated-temperature SQUID magnetometry, X-ray diffraction, transmission electron microscopy and magnetic force microscopy. While lower-temperature (300°C ⩽T⩽400°C) annealing treatments produced little change in the domain configuration, it did produce subtle change in the microstructure and a noticeable increase in the degree of exchange coupling of the bilayers. Higher-temperature (T=550°C) annealing treatments produced profound changes in all parameters: the magnetic reversal behavior, the remanence ratio and the magnetic domain configuration. These changes were accompanied by distinct changes in the bilayer phase constitution and proportions of hard and soft phases produced by interdiffusion of the Co and CoPt layers which altered the overall anisotropy and associated magnetic behavior of the system. To support these conjectures micromagnetic modeling of different conditions of the bilayer properties showed that changing the relative proportions of the hard and soft layers could indeed lead to changes in the magnetic behavior similar to those observed in the experimental systems. Both the experimental and modeling-derived results of this work demonstrate that the bulk technical properties of a hard/soft magnetic nanocomposite material depend on the relative proportions of the phases present, the degree of exchange coupling across the interface between those phases as well as on the physical and magnetic properties of those phases. Changing the physical properties of the phases in systematic ways allows the magnetic properties of the ensemble to be tailored.     

Three-and two-dimensional calculations for the interface anisotropy dependence of magnetic properties of exchange-spring Nd2Fe(14)B/α-Fe multilayers with out-of-plane easy axes

Hysteresis loops,energy products and magnetic moment distributions of perpendicularly oriented Nd2Fe(14)B/α-Fe exchange-spring multilayers are studied systematically based on both three-dimensional(3D)and one-dimensional(1D)micromagnetic methods,focused on the influence of the interface anisotropy.The calculated results are carefully compared with each other.The interface anisotropy effect is very palpable on the nucleation,pinning and coercive fields when the soft layer is very thin.However,as the soft layer thickness increases,the pinning and coercive fields are almost unchanged with the increment of interface anisotropy though the nucleation field still monotonically rises.Negative interface anisotropy decreases the maximum energy products and increases slightly the angles between the magnetization and applied field.The magnetic moment distributions in the thickness direction at various applied fields demonstrate a progress of three-step magnetic reversal,i.e.,nucleation,evolution and irreversible motion of the domain wall.The above results calculated by two models are in good agreement with each other.Moreover,the in-plane magnetic moment orientations based on two models are different.The 3D calculation shows a progress of generation and disappearance of vortex state,however,the magnetization orientations within the film plane calculated by the 1D model are coherent.Simulation results suggest that negative interface anisotropy is necessarily avoided experimentally.     

复合磁场引导的小型化二极管初步设计

设计了一种非浸没式小型化轴向无箔二极管,其阴极发射区位于螺线管中心孔以外,采用螺线管、永磁体和软磁体构成的复合引导磁场系统。采用CST优化设计二极管结构,使其满足800 kV电压下绝缘要求; 优化设计磁场系统结构及物理参数,使其满足引导强流相对论电子束高效率传输的要求。采用粒子模拟(PIC)软件仿真电子束产生及传输过程,验证其高传输效率。设计的阳极筒直径比原结构缩小约40%,在产生同样均匀区轴向磁场强度情况下,引导磁场重量和功耗比原结构降低约40%,仿真结果显示主引导磁场0.85 T下,8 kA电流能够实现100%传输效率。     

Ni对Fe_3Si软磁体磁性能影响的微观组织结构和电子结构研究

采用实验和第一性原理方法,从微观组织结构和电子结构研究了Ni浓度和退火时间对Fe_3Si软磁体磁性能的影响.结果表明Ni含量为3.125at.%时,软磁体具有最大的饱和磁化强度和最低的矫顽力值.但当浓度超过9.375at.%后,软磁体的饱和磁化强度减小,而矫顽力增大,说明适当添加Ni可提高Fe_3Si软磁体的磁性能.Ni元素从微观组织结构和电子结构两方面均对软磁体的磁性能产生较大的影响.     

Effect of γ-ray irradiation on the magnetic properties of NdFeB and Fe–Cr–Co permanent magnets

The effect of γ-ray irradiation on the magnetic properties of NdFeB and Fe–Cr–Co permanent magnets has been investigated. The magnetic flux loss of two kinds of magnets before and after irradiation was measured. Results show that the effect of γ-ray irradiation on the magnetic properties of sintered NdFeB is not so obvious as that on Fe–Cr–Co magnet. Irradiation-induced damage from γ-ray for the Fe–Cr–Co magnets was characterized for the first time. The decline of permanent magnetic properties of Fe–Cr–Co magnet induced by γ-ray irradiation is reversible except for the maximum energy product (BH)_max. The difference of coercivity mechanism between these two kinds of permanent magnets is responsible for the different dependence of magnetic properties loss induced by γ-ray irradiation.     

基于磁化电流法的双稳压电悬臂梁磁力精确分析

双稳压电悬臂梁结构常常用于振动能量采集系统,其中的非线性磁力与系统势函数和动力学方程的建立紧密相关,非线性磁力的正确分析和精确计算对系统振动响应和能量采集效果的准确预测至关重要.本文采用形状函数分析方法,通过悬臂梁弯曲斜率的整体积分计算,得到了悬臂梁末端的运动轨迹及其末端磁铁精确的位置与姿态,并由此根据磁化电流理论建立了双稳压电悬臂梁能量采集系统的磁力计算模型,给出了末端磁铁受到的水平轴向磁力和竖直纵向磁力及其合磁力的变化规律.数值模拟发现,随着末端磁铁竖直纵向位移逐渐增大,磁铁受到的水平轴向磁力和竖直纵向磁力都会依次由排斥力转变为吸引力,从而导致磁力合力的方向会随磁铁位移发生跨越两个象限的大幅度变化.实验验证表明,磁力计算结果与实验测量结果符合良好,其精确度优于现有文献方法的精度,因此本文的方法可以准确预测双稳压电悬臂梁振动过程的磁力变化规律.     

An optimized magnet for magnetic refrigeration

A magnet designed for use in a magnetic refrigeration device is presented. The magnet is designed by applying two general schemes for improving a magnet design to a concentric Halbach cylinder magnet design and dimensioning and segmenting this design in an optimum way followed by the construction of the actual magnet. The final design generates a peak value of 1.24 T, an average flux density of 0.9 T in a volume of 2 L using only 7.3 L of magnet, and has an average low flux density of 0.08 T also in a 2 L volume. The working point of all the permanent magnet blocks in the design is very close to the maximum energy density. The final design is characterized in terms of a performance parameter, and it is shown that it is one of the best performing magnet designs published for magnetic refrigeration.     

纳米复合永磁材料中软磁性相交换硬化的研究

本文就纳米复合永磁材料中软磁相被交换硬化问题,从一维模型和三维模拟计算进行了分析研究. 一维和三维各向异性样品研究表明,在相同微结构下,当硬磁相的各向异性降低时,除矫顽力降低外,在磁矩全部反转之前退磁曲线是一样的. 因此,硬磁相各向异性的降低不会导致最大磁能积(BH)_max增大和剩磁增加. 对于三维各向同性样品的模拟计算表明,降低硬磁相的各向异性会使剩磁和(BH)_max都明显降低. 因此,增强硬磁相的各向异性并增大硬磁相晶粒尺寸是提高 关键词： 纳米复合永磁 矫顽力 剩磁 磁能积     

Effect of process on the magnetic properties of bonded NdFeB magnet

The effects of magnetic separation, coupling treatment, lubricating treatment, preform and biaxial molding on the density and magnetic properties of bonded NdFeB magnet were investigated. The results demonstrate that magnetic separation separates the powders with low coercive force; coupling treatment improves the interfaces between the powders and the binders; decrease in volume fraction of the binder increases magnetic properties of the magnet; granular arrangement improves both the magnetic and mechanical properties when powders are arranged in certain size; lubricating treatment improves the formability of the magnet and preform and biaxial molding improves both density and magnetic properties greatly. Combining these methods, the density of the bonded NdFeB magnet can reach 6.52 g/cm³ and the maximum energy product can reach 114 kJ/m³.     

On synchrotron radiation in non-uniform magnetic fields

We study the conditions in which a charged particle in an inhomogeneous magnetic field (particularly at the edges of a “long” uniform magnet or in a “short” magnet) can emit synchrotron radiation with a spectrum extending beyond the “critical frequency”. We suggest that this effect should be clearly visible (and also useful) in the case of very high energy proton storage rings.     

Synthesis of magnetic systems producing field with maximal scalar characteristics

A method of synthesis of the magnetic systems (MSs) consisting of uniformly magnetized blocks is proposed. This method allows to synthesize MSs providing maximum value of any magnetic field scalar characteristic. In particular, it is possible to synthesize the MSs providing the maximum of a field projection on a given vector, a gradient of a field modulus and a gradient of a field energy on a given directing vector, a field magnitude, a magnetic flux through a given surface, a scalar product of a field or a force by a directing function given in some area of space, etc. The synthesized MSs provide maximal efficiency of permanent magnets utilization. The usage of the proposed method of MSs synthesis allows to change a procedure of projecting in principal, namely, to execute it according to the following scheme: (a) to choose the sizes, a form and a number of blocks of a system proceeding from technological (economical) reasons; (b) using the proposed synthesis method, to find an orientation of site magnetization providing maximum possible effect of magnet utilization in a system obtained in (a). Such approach considerably reduces a time of MSs projecting and guarantees maximal possible efficiency of magnets utilization. Besides it provides absolute assurance in “ideality” of a MS design and allows to obtain an exact estimate of the limit parameters of a field in a working area of a projected MS.The method is applicable to a system containing the components from soft magnetic material with linear magnetic properties.     

Influence of the phenomena occurring at the interface between L1<Subscript>0</Subscript>-ordered-FePt and Fe on the coercivity behavior

L1₀-ordered FePt/Fe thin bi-layers were grown using a molecular beam epitaxy onto (100)-MgO substrates changing the soft Fe layer thickness. The study of the intermixing phenomena occurring at the hard/soft interfaces was carried out using surface Mössbauer spectroscopy. The magnetic properties of the samples were analyzed with a magneto-optical Kerr effect magnetometer. The surface morphology and the magnetic domains were analyzed with an UHV atomic and magnetic force microscopy in tapping and lift mode respectively.The present work clearly demonstrates that the degree of interface intermixing and reactions is the responsible for the coercivity behavior in exchange-spring magnets.