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.     

Influence of magnetic field sweep rate on the hysteresis loops of Ni0.8Fe0.2/Fe0.5Mn0.5 exchange bias bilayer

The influence of the magnetic field sweep rate on the hysteresis loops of exchange bias Ni0.8Fe0.2/Fe0.5Mn0.5 bilayers has been investigated with a vibrating sample magnetometer. It was found that the sweep rate of 13.6 kA/4πms is high enough to bring about obvious changes in the hysteresis loops of the exchange bias bilayer. High sweep rate in the magnetization reversal stage enlarges the coercivity of the sample, while high sweep rate in the saturation state reduces the coercivity. The above phenomena were attributed to magnetic viscosity in the ferromagnetic layer enhanced by the interface exchange interaction and domain magnetization reversals assisted by thermal fluctuation in the antiferromagnetic layer respectively.     

On the convergence of a regularization method for variational inequalities

For variational inequalities in a finite-dimensional space, the convergence of a regularization method is examined in the case of a nonmonotone basic mapping. It is shown that a fairly general sufficient condition for the existence of solutions to the original problem also guarantees the convergence and existence of solutions to perturbed problems. Examples of applications to problems on order intervals are presented.     

High-coercivity nanocomposite permanent magnet based on Nd–Fe–B–Ti–C with Cr addition for high-temperature applications

We have investigated influences of various elements (Ti, Nb, Mo, Ta, Hf, W, V, Cr) on magnetic properties of Fe–B/Nd₂Fe₁₄B-based Nd–Fe–B–Ti–C nanocomposite magnets in order to obtain larger coercivity required for high-temperature applications. As a result, addition of Cr was found to be most effective among additive elements investigated to enhance coercivity. Thermal flux losses of high-coercivity (H_cJ=1609 kA/m) Nd–Fe–B–Ti–C–Cr nanocomposite magnet at 200 °C are less than 2%.     

Coercivity Conditions for Equilibrium Problems

The study of the existence of solutions of equilibrium problems on unbounded domains involves usually the same sufficient assumptions as for bounded domains together with a coercivity condition. We focus on two different conditions: the first is obtained assuming the existence of a bounded set such that no elements outside is a candidate for a solution; the second allows the solution set to be unbounded. Our results exploit the generalized monotonicity properties of the function f defining the equilibrium problem. It turns out that, in both the pseudomonotone and the quasimonotone setting, an equivalence can be stated between the nonemptyness and boundedness of the solution set and these coercivity conditions. In the pseudomonotone case, we compare our coercivity conditions with various coercivity conditions that appeared in the literature.We thank an anonymous referee for valuable comments and suggestions.     

Enhanced Magnetic Hardness in a Nanoscale Metal–Organic Hybrid Ferrimagnet

A new layered metal–organic hybrid compound, namely, [Co₃(μ₃‐OH)₂(BTP)₂] ( 1 ; BTP=4‐(3‐bromothienyl)phosphonate), is reported. The inorganic layer can be viewed as a pseudo‐Kagomé lattice composed of corner‐sharing irregular triangles of Co₃(μ₃‐OH), with the cavities filled with the PO₃ groups. The interlayer space is occupied by the 3‐bromothienyl groups of BTP^2?. The bulk sample of compound 1 experiences a long‐range ferromagnetic ordering below 30.5 K, with a coercivity (H_c) of 5.04 kOe at 5 K. A systematic study on the size‐dependent magnetic coercivity of 1 reveals that the coercivity of 1 increases with reduced particle size from the micrometer to the nanometer scale. When the particle size is about 50–200 nm, the coercivity reaches 24.2 kOe at 5 K. The results demonstrate that compound 1 can vary from a soft magnet to one of the hardest molecule‐based magnets, simply by reducing the particle size to nanoscale region.     

不同易轴取向下对Nd_2Fe_(14)B/Fe_(65)Co_(35)磁性双层膜的微磁学模拟

运用三维数值模拟计算方法,计算了膜面外不同易轴取向下Nd2Fe14B/Fe65Co35磁性双层膜的磁滞回线、角度分布、成核场、矫顽力和磁能积等,并与实验结果进行了细致比较.计算结果表明:只有当易轴与外场之间的夹角β=0?时,才有明显的成核现象,其成核场和矫顽力均随着软磁相厚度Ls的增加而降低;随着易轴偏角β的增大,剩磁逐渐减小,磁滞回线的方形度降低,从而磁能积减小,在Ls=1 nm,β=0?时磁能积(561.61 kJ/m3)最大.理论计算所得的磁滞回线与实验磁滞回线符合得很好,剩磁和矫顽力的理论值与实验值相差很小.     

沉积在液相基底表面磁性薄膜的形成机理和特性研究

采用气相沉积方法在硅油基底表面成功制备了一种具有近似自由支撑的新型铁薄膜系统，并研究了其生长机制、内应力分布以及低温磁特性．实验发现，此类铁薄膜的生长机制与沉积在液相基底表面非磁性薄膜的情况类似，即服从二阶段生长模型．在固定基底温度的条件下，当沉积速率较小时，可制得近似透明的连续铁薄膜，薄膜中呈现明显的特征尺寸达10^2μm数量级的带状准周期有序结构，它是由铁薄膜样品中内应力释放时所引起的薄膜板块间相互挤压而逐渐形成的．当沉积速率较大时，制得的连续铁薄膜呈金属色．实验发现，在临界温度Tc=10—15K附近，具有金属色的铁薄膜样品的矫顽力Hc有一明显的极大值峰．研究表明，这一奇异的矫顽力特性与液相基底表面铁薄膜中的原子团簇尺寸分布、无序的薄膜表面磁各向异性以及团簇间的磁性相互作用等因素有关．     

Coercivity enhancement of sintered Nd-Fe-B magnets by grain boundary diffusion with Pr80-xAlxCu20 alloys

A grain boundary diffusion (GBD) process with Pr_80-xAl_xCu₂₀ (x = 0, 10, 15, 20) low melting point alloys was applied to commercial 42M sintered Nd-Fe-B magnets. The best coercivity enhancement of a diffused magnet was for the Pr₆₅Al₁₅Cu₂₀ GBD magnet, from 16.38 kOe to 22.38 kOe. Microstructural investigations indicated that increase in the Al content in the diffusion source can form a continuous grain boundary (GB) phase, optimizing the microstructure to enhance the coercivity. The coercivity enhancement is mainly due to the formation of a continuous GB phase to separate the main phase grains. Exchange decoupling between the adjacent main phase grains is enhanced after the GBD process. Meanwhile, the introduction of Al can effectively promote the infiltration of Pr into the magnet, which increases the diffusion rate of rare-earth elements within a certain range. This work provides a feasible method to enhance coercivity and reduce the use of rare-earth resources by partial replacement of rare-earth elements with non-rare-earth elements in the diffusion source.