Effect of annealing time on the exchange coupling interactions and microstruc-ture of nanocomposite Pr_(7.5)Dy_1Fe_(71)Co_(15)Nb_1B_(4.5) ribbons

The influence of annealing time on the magnetic properties and microstructure of nanocomposite Pr7.5Dy1Fe71Co15Nb1B4.5 ribbons was systematically investigated by the methods of vibrating sample magnetometer （VSM）, X-ray diffraction （XRD） and high resolution transmission electron microscopy （HRTEM）. Interaction domains derived from strong exchange coupling interactions between hard and soft magnetic grains were imaged using magnetic force microscopy （MFM）. Maximum remanence, intrinsic coercivity, and maximum energy product values were obtained in the ribbons annealed at 700℃ for 15 min, which were composed of Pr2（Fe, Co）14B, α-（Fe, Co）, and slight Pr2（Fe, CO）17 phases. Although Jr, Hci, and （Bn）max decreased gradually with further increase of annealing time, it is emphasized that comparatively high Jr and Hci and （BH）max were obtained in a wide annealing time period of 15 to 360 min. The shape of initial magnetization curves and hysteresis loops change as a function of annealing time, indicating different magnetization reversal routes, which can be fully explained by the corresponding microstructure.

Dependence of Effective Anisotropy on Grain Size in Nanocrystalline Nd_2Fe_(14)B Hard Magnetic Material

Taking nanocrystalline Nd_2Fe_(14)B as a typical sample, based on Herzer′s random anisotropy theory and the cubic grain model, the partial exchange-coupling interaction model was established and the dependence of effective anisotropy constant K_(eff) on grain size was investigated. Calculation results reveal that the exchange-coupling interaction enhances and the effective anisotropy of material K_(eff) decreases with the reduction of grain size. The variation of K_(eff) is basically the same as that of coercivity. The decrease of effective anisotropy is the main reason of the reduction of coercivity for nanocrystalline Nd_2Fe_(14)B permanent magnetic material.

Grain Size Dependence of Exchange-Coupling Interaction between Magnetically Soft-Hard Grains and Effective Anisotropy

Taking α-Fe and Nd_2Fe_(14)B grains as example, the grain size dependence of the exchange-coupling interaction and effective anisotropy and also their variations depending on the ratio of magnetically soft and hard grain sizes, D_s∶D_h, were investigated. When grain size D>L_(ex), the grain’s anisotropy is the statistic value of the coupled and uncoupled part. The anisotropy constant of uncoupled part is the common value K_1 and that of coupled part varies with the distance to the grain surface. The effective anisotropy constant between magnetically soft and hard grains, K_(eff), can be expressed as the sum of the products of volume fractions for soft and hard grains, respectively, and the corresponding mean anisotropy constants. The calculation results indicate that the exchange-coupling interaction is enhanced with the reduction of grain size, and the effective anisotropy decreases with reducing grain size and increasing ratio of D_s∶D_h. In order to get high effective anisotropy constant, K_(eff), in composite magnetically soft-hard grains, the hard grain size should be larger than 30 nm and the soft grain size should be about 10 nm.

Computer Simulation of Effect of Intergrain Exchange Interaction on Magnetic Properties of Nanocomposite Magnets

Effects of the intergrain exchange interaction on magnetic properties of nanocomposite magnets were investigated by using the computer simulation based on the micromagnetic theory. The simulation was carried out under the assumptions that the strength of the intergrain exchange interaction is weaker than that of the intragrain exchange interaction, that inhomogeneous nanostructures result in the distribution of the strength of the intergrain exchange interaction, and that there exists nonmagnetic intergranular phase (NMIP) between grain boundaries. The distribution of the strength of the intergrain exchange interaction was simulated by the lognormal distribution with the standard deviation of σ.The calculations for Nd 2Fe 14B/α-Fe nanocomposite magnets reveal that a suitably weak intergrain exchange interaction and small grain size enable us to improve magnetic properties. It is also found that a Nd 2Fe 14B/α-Fe nanocomposite magnet has a potential of a (BH) max value exceeding 300 kJ·m -3. On the other hand, the calculations for Nd 2Fe 14B/Fe 3B nanocomposite magnets reveal that the distribution of the strength of the intergrain exchange interaction deteriorates magnetic properties significantly. Particularly, this tendency is remarkable, when the grain size L is larger than its optimum value, 11 nm. The existence of nonmagnetic boundary layers accelerats this tendency. At σ=0.2, the calculated demagnetization curve for the model magnet composed of Nd 2Fe 14B(36%)/Fe 3B(54%)/NMIP(10%) (Valume fraction) grains (L=15 nm) agrees with that obtained experimentally for a Nd 2Fe 14B/Fe 3B nanocomposite magnet. These results suggest importance of refinement of grain size, suppression of a nonmagnetic intergranular phase, and preparation of homogeneous nanostructure for superior magnetic properties.

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 longrange 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.

Effective anisotropy and coercivity in nanocrystalline single-phase NdFeB permanent magnetic material

The effect of exchange-coupling interaction on the effective anisotropy and its varying tendency in nanocrystalline single-phase NdFeB permanent magnetic material have been investigated. The results show that the exchange-coupling interaction between grains makes the effective anisotropy of material, Keff, decrease with the reduction of grain size. The variation of Keff is basically the same as that of coercivity. The decrease in effective anisotropy is the main reason of the reduction of coercivity for nanocrystalline single-phase NdFeB permanent magnetic material. In order to get high anisotropy and coercivity in nanocrystalline single-phase NdFeB permanent material, the grain size should be larger than 35 nm.

Investigation on Magnetic Properties of Exchange Coupled Transition Metal Complexes──I. Application of the Kambe's Theory

Magnetic susceptibility of a series of dinuclear Ⅴ(Ⅳ) . Co(Ⅱ) and Mn(Ⅲ) complexes has been measured in the temperature range of 1. 5 ～300K. The isotropic Heisenberg theory has been applied to study the temperature-dependent behaviour of the magnetic susceptibility of these complexes and a corresponding program for fitting the experimental results has been set up on a VAX 11/785 computer. Conclusions can be drawn that the three complexes studied are all anti-ferromagnetically coupled with coupling constants -4. 4 , -115. 2 , and - 8. 4 cm￣(-1) for Ⅴ(Ⅳ) . Co(Ⅱ) ,and Mn(Ⅲ) complexes, respectively.

Exchange-coupling interaction and effective anisotropy in nanocomposite permanent materials

Taking Nd2Fe14B/α-Fe as example, the exchange-coupling interactions between magnetically soft and hard grains in nanocomposite permanent materials and their effects on the effective anisotropy of materials were investigated. The calculation results expressed that the exchange-coupling interactions are enhanced with the reduction of grain size, and the effective anisotropy of materials decreases with the reduction of gram size and the increase of magnetically soft phase component. The remanence and the effective anisotropy of materials possess the opposite variation trend with the change of grain size and phase ratio. The mean grain size should be in the range of 10-15 nm and the ratio of soft phase should be less than 50% for getting the magnet with high energy product.

Magnetic Properties and Intergranular Action in Bonded Hybrid Magnets

Magnetic properties and intergranular action in bonded hybrid magnets,based on NdFeB and strontium ferrite powders were investigated.The long-range magnetostatic interaction and short-range exchange coupling interaction existed simultaneously in bonded hybrid magnets,and neither of them could be neglected.Some magnetic property parameters of hybrid magnets could be approximately obtained by adding the hysteresis loops of two magnets pro rata.

Magnetic analyses of isosceles tricobalt(II) complexes containing two types of octahedral high-spin cobalt(II) ions

The observed magnetic data for two isosceles tricobalt(II) complexes have been successfully analyzed, considering the axial distortion around each cobalt(II) ion, the local spin-orbit coupling, the anisotropic exchange interactions, and the intermolecular exchange interactions. The complexes each contains two types of octahedral high-spin cobalt(II) ions (CoA and CoB) in the shape of an isosceles triangle (CoA1–CoB–CoA2), and the contribution of the orbital angular momentum is significant. The exchange interaction between the CoA and CoB ions is practically negligible (J = ~ 0), whereas the interaction between the CoA1 and CoA2 ions is ferromagnetic (J’ > 0) for both complexes.

Effect of Bath pH on the Squareness of Electroless CoNiWP Magnetic Films

Electroless CoNiWP magnetic films were prepared by varying the bath pH and then characterized by energy dispersive X-ray analysis, X-ray diffraction and magnetic force microscopy, it has been found that the microstructure and the magnetic properties of films were influenced greatly by the bath pH. At the bath pH 8.06, the grain size and coercivity of the films reach maximuml while the squareness （Mr/Ms） of MH curves reaches minimum. The Henkel plots indicates that the exchange-coupling interaction is very weak at this pH, which may be caused by phase-separation and large grain size, and then results in the lowest squareness. At pH above 8.5, obvious exchange-coupling interaction is observed because of the inexistence of phase-separation and the refinement of grain size.

纳米晶复合永磁材料的交换耦合相互作用和磁性能

本文介绍了纳米磁性材料晶粒交换耦合相互作用的有关理论。采用不同模型讨论了晶粒交换耦合相互作用对纳米单相软磁材料、永磁材料及双相复合永磁材料磁性能的影响。简述了用δM(H)曲线和不可逆磁导率的变化研究晶粒交换耦合相互作用及反磁化过程的方法。讨论了合金成分配比、添加元素、制备及热处理工艺对磁体硬磁性能的影响。从理论和实验两方面分析、研究了纳米复合永磁材料的反磁化过程和矫顽力机制。

磁性材料中的晶粒相互作用

磁性材料中的晶粒相互作用对材料的宏观磁性有决定性影响。相互作用可分为晶粒之间的长程静磁相互作用和近邻晶粒之间的交换耦合相互作用。晶粒相互作用强弱取决于材料的微结构（晶粒形状、尺寸、取向及界面耦合程度）。可以从理论模型计算和实验曲线分析两种方法研究、比较两种晶粒相互作用。最后介绍了纳米软磁材料和纳米双相永磁材料中的交换相互作用对材料性能的影响。

纳米复合钕铁硼磁体的矫顽力模型及影响因素综述

总结了纳米复合钕铁硼磁体的矫顽力模型,并分析了晶间交换耦合作用、相互作用畴、静磁耦合作用及添加元素等对矫顽力的影响。当晶粒尺寸较小时,晶间交换耦合作用的增强和相互作用畴的存在不利于矫顽力的提高;静磁耦合作用对矫顽力的影响还存在争论,有待于进一步的研究。

磁学的发展与重要磁性材料的应用

回顾了磁学的研究和发展,介绍了磁性材料若干新的发现、性能及应用,同时展望了磁性材料的发展趋势和研究开发的重点。

纳米复合永磁材料的研究进展

纳米复合永磁材料兼具硬磁性相的高矫顽力和软磁性相的高饱和磁化强度,磁性能优异。本文综述了纳米复合永磁材料的磁性相晶粒间的交换耦合作用、矫顽力机制、微磁学模拟等理论方面的研究进展,同时论述了成分、添加元素、制备工艺对材料微观结构和磁性能的影响,着重介绍了最有应用价值的快淬法磁粉制备工艺。

磁性纤维随机混合媒质等效电磁参数的计算

为计及强散射偶极子间的相互作用,引入参量εh和μh,利用广义多重散射理论及MATLAB计算机软件对在2GHz^18GHz频率下的体积占空比为8%的铁纤维随机混合媒质等效磁导率的实验值进行曲线的拟合,导出了耦合系数εh和μh,并在此基础上,对体积占空比分别为30%和20%的铁纤维随机混合媒质等效磁导率进行了计算,计算结果与实验吻合良好。

交换耦合的纳米软-硬磁性晶粒的有效各向异性常数

以α-Fe、Nd2Fe14B为例，研究了纳米晶粒尺寸对交换耦合的软-硬磁性晶粒各向异性的影响。结果表明，软-硬磁性晶粒的有效各向异性常数〈Ksh〉随软磁性晶粒尺寸Ds的增加而减小，随硬磁性晶粒尺寸Dh的增加而增大。当Ds／Dh为固定值且1／1．1≥Ds／Dh≥1／1．4时，〈Ksh〉在Ds为10～20nm之间取得一极大值。当Dh〉25nm，Ds在10nm左右时，可获得较大的〈Ksh〉。

异双核X-M之间的磁耦合交换作用:Keggin型杂多酸衍生物[M(H_2O)XW_(11)O_(39)]^(7-)(X=Fe~Ⅲ,Co~Ⅲ;M=Co~Ⅱ)的密度泛函研究

采用DFT-BS方法研究异双核的Keggin型杂多酸衍生物[M(H2O)XW11O39]7-(Ⅰ:X=FeⅢ,M=CoⅡ;Ⅱ:X=CoⅢ,M=CoⅡ)的磁耦合作用,计算得到耦合常数(J)为负值,表明所研究体系具有反铁磁性;J值大小顺序为|J(Ⅰ)|<|J(Ⅱ)|,说明磁耦合作用增强;体系Ⅰ与Ⅱ相比,X由FeⅢ变成CoⅢ,M不变,桥氧原子Ob和Ob2(Ob′2)上的自旋密度增大,进一步从相关BS态的磁轨道比较得出,体系Ⅱ中轨道重叠程度大于体系Ⅰ,结果使X-M之间的反铁磁耦合作用加强.

快淬纳米复合α-Fe/Pr_2Fe_(14)B薄带的硬磁性研究

通过熔体快淬法得到α Fe/Pr2 Fe1 4 B纳米双相磁体 ,研究了不同辊速快淬带的磁性能 ,找到最佳直接快淬辊速条件 ,并比较了直接快淬得到的样品和非晶薄带晶化后得到的样品的磁性能 ,发现直接快淬得到的样品的矫顽力和机械性能优于非晶晶化样品。