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.     

Angular dependence of coercivity in Pr---Fe---B sintered magnets

Hysteresis loops were measured for varying angles of the applied field at different temperatures in oriented sintered magnets of compositions Fe₇₅Pr₁₇B₈ and Fe₅₃Pr₁₇B₃₀. While at elevated temperatures the coercive field increases monotonically with the angle of the applied field, a minimum in the angular dependence is found below room temperature. The temperature dependence of coercivity for the field applied parallel and with an angle of 45° to the alignment direction is well described by a nucleation model in the whole temperature range. The results are compared to Fe---Nd---B magnets, showing that the occurrence of a minimum in the angular dependence is related to the ratio of the experimental to the theoretical coercive fields.     

纳米晶永磁材料晶间交换耦合作用的模拟计算研究

利用微磁学有限元法，模拟计算了单相和复相各向同性纳米晶磁体的起始磁化曲线、退磁曲线和回复曲线.验证了用δm(H)曲线的正峰值来衡量纳米晶磁体晶间交换耦合作用的有效性.计算 结果表明，纳米晶单相和复相磁体的晶间交换耦合作用都随晶粒尺寸的增加而降低，当晶粒尺寸过大时 复相磁体表现出两相行为，其δm(H)曲线出现了两个正峰值.分析表明，外场较小的正峰值是软磁相与硬磁相晶粒之间交换耦合作用的结果，而外场较大的正峰值是硬磁相晶粒之间交换耦合作用的结果. 关键词： 纳米晶永磁 晶间交换耦合 δm(H)曲线     

晶粒大小分布对纳米晶磁体Pr2Fe14B剩磁和矫顽力的影响

用微磁学有限元法计算了纳米晶磁体Pr2Fe14B的退磁曲线，文中模拟了三种具有不同晶粒大小分布的样品。计算结果表明，剩磁增强与磁体的晶粒尺寸分布情况无关而依赖于平均晶粒直径。晶粒尺寸分布对矫顽力的影响与磁体中的晶间交换耦合作用密切相关。     

纳米晶永磁中面缺陷对畴壁钉扎机理的研究

畴壁钉扎模型的矫顽力可以表示为Hc=αpink2K1/μ0Ms-NeffMs，计算了微结构参数αpink随面缺陷内磁性参数A′和K′1的变化情况.结果表明，αpink可以在很宽的范围里取值.结合纳米单相Nd2Fe14B磁体，研究了晶粒边界的磁性参数和晶界厚度对αpink的影响，当A′/A=05，K′1/K1=01，以及晶界厚度r0＝332nm时，αpink最大.同时研究了纳米复相Nd2Fe14B/α-Fe磁体的αpink随α-Fe晶粒尺寸r0的变化情况，当r0＝7nm时，αpink最大. 关键词： 面缺陷 矫顽力 钉扎机理 纳米晶     

The dependence of coercivity on anisotropy field in sintered R-Fe-B permanent magnets

To investigate the dependence of intrinsic coercivity of R-Fe-B permanent magnets (R = rare earths) on the anistropy field of R₂Fe₁₄B, the temperature dependence of the intrinsic coercivity of Pr₁₅Fe₇₇B₈ and Nd₁₅Fe₇₇B₈ have been measured from 4.2 K to the Curie temperature of the magnets. Study of the interrelation between coercivity and anisotropy field by modifying the anisotropy field through atomic replacement in the rare earth sublattice has been also attempted in pseudobinary alloys (Nd_1−xR′_x)₁₅Fe₇₇B₈ with R′ = Tb, Dy and Er. A coercivity versus anisotropy field and composition. The R₁₅Fe₇₇B₈ has been constructed from the observed dependence of the coercivity on both temperature and composition. The role of boundary phases is discussed by comparing R₁₅Fe₇₇B₈ with SmCo₅ magnets.     

晶粒易轴取向度对纳米晶永磁Pr2Fe14B磁性的影响

构造了立方和不规则形状晶粒的各向异性纳米晶单相Pr₂Fe₁₄B磁体 .利用微磁学的有限元法，模拟计算了样品的磁滞回线.计算结果表明，随着磁体晶粒易轴取向度的变差， 磁体的剩磁、矫顽力均随之下降.不同晶粒尺寸的纳米晶单相Pr₂Fe_{14B磁体，其磁 性能随取向度的变化快慢不同，原因在于磁体中的晶间交换作用 (IGEC) 的强弱不同.随着 晶粒取向度的提高，纳米晶单相磁体的矫顽力逐渐增加，这完全不同于烧结磁体. 关键词： 纳米晶磁体 矫顽力 剩磁}     

纳米晶磁芯恒定磁化速率磁性能参数

设计加工了一套可用于测量环形磁芯在8～12 T/s恒定磁化速率下脉冲性能的实验平台,该实验装置主要包括脉冲形成网络、放电开关、匹配负载及被测磁芯。基于该平台对国产的铁基纳米晶磁芯的磁化曲线进行了测试,得到了铁基纳米晶磁芯在200～300 ns范围内的损耗和非饱和脉冲导磁率。实验结果表明:铁基纳米晶磁芯的相对脉冲导磁率随着磁感应增量增大及工作脉宽减小而减小,磁芯损耗则逐渐增大。     

纳米晶磁芯恒定磁化速率磁性能参数

设计加工了一套可用于测量环形磁芯在8～12 T/s恒定磁化速率下脉冲性能的实验平台,该实验装置主要包括脉冲形成网络、放电开关、匹配负载及被测磁芯。基于该平台对国产的铁基纳米晶磁芯的磁化曲线进行了测试,得到了铁基纳米晶磁芯在200～300 ns范围内的损耗和非饱和脉冲导磁率。实验结果表明：铁基纳米晶磁芯的相对脉冲导磁率随着磁感应增量增大及工作脉宽减小而减小,磁芯损耗则逐渐增大。     

Angular dependence of the coercive force of textured rare earth permanent magnets

On the basis of model calculations experimental results on the angular dependence of the coercivity H_c and the remanence coercivity H_R of hard magnetic materials of the type SmCo₅, Sm₂(Co, Fe, Cu, Zr)₁₅ and Nd₂Fe₁₄B are discussed. In the model coherent rotation as well as incoherent magnetization jumps (e.g. 180°-Bloch walls) are included. The texture is described by an axial symmetric distribution of the easy axes with only oneparameter. For Sm₂(Co, Fe, Cu, Zr)₁₅ the model explains irreversible (H_R (θ)-curves) as well as reversible (H_R(θ)−H_c(θ)) magnetization processesin good agreement with the experiments, whereas stronger deviations exist for SmCo₅ and Nd₂Fe₁₄B, especially in the H_c(θ)-curves. These deviations should be caused by other reversible magnetization processes     

基于永磁恒定磁场激励的起始磁化曲线测量

现有起始磁化曲线测量系统需绕制励磁线圈和感应线圈,在线应用受限.为此,本文提出了一种基于永磁恒定磁场激励的起始磁化曲线测量原理并搭建了相应测量系统.该系统采用永磁磁化器作为激励磁源,以对称磁化方法在圆柱棒状构件上激励出随轴向位置变化的恒定磁场作为激励磁场;采用阵列霍尔探头测量构件表面不同提离下的轴向和法向磁感应强度;并基于多项式外推法和磁场高斯定理外推法,推算构件与空气分界面上的轴向和法向磁感应强度;进一步地,根据分界面上的磁感应强度获取构件的起始磁化曲线.系统测量结果表明,在永磁恒定磁场激励下,无须励磁线圈和感应线圈即可方便地获取棒状构件的起始磁化曲线,测量误差小于10%,测量误差标准差小于0.01,重复性较好.该系统可为便捷地在线测量棒状构件起始磁化曲线提供新途径.     

Sm_2Co_(17)型永磁合金的辐照效应研究

Sm_2Co_(17)型永磁合金大量使用在上海同步辐射光源储存环的永磁型波荡器上,在受到长期辐照后会发生磁性能损失的现象,进而影响同步辐射光的品质.为了探索其潜在的微观机理,本文首先对Sm_2Co_(17)型永磁合金所处混合辐射场的粒子及相关物理量进行了计算分析,确定引发磁性能损失的主要粒子是中子.然后采用Ar离子模拟中子辐照损伤的方法对其进行辐照,采用透射电镜对其辐照前后的微观形貌及微观结构进行了研究探讨,采用振动样品磁强计对永磁合金辐照前后的饱和磁化强度进行了分析对比,并讨论了微观结构演化与宏观磁性能变化的联系.结果表明,Ar离子辐照后Sm_2Co_(17)型永磁合金饱和磁化强度的不可逆损失与微观结构变化有直接的关系,其2:17相从单晶结构转变为非晶结构是造成其磁性能损失可能的微观机制.     

纳米复合永磁材料的有效各向异性与矫顽力

研究了纳米Nd2Fe14B/α-Fe复合永磁材料中晶粒交换耦合相互作用对有效各向异性的影响和变化规律.结果表明：晶粒之间的交换耦合作用使材料的有效各向异性Keff随晶粒尺寸的减小而下降、随软磁性相成分的增加而降低. 当晶粒尺寸 减小到4nm时，Keff值减小为其各自通常各向异性常数值的1/3—1/4.有效各向异性的变化特点与矫顽力的变化规律基本相同.纳米复合永磁材料矫顽力的降低主要由于有效各向异性的减小而引起. 关键词： 纳米复合永磁材料 交换耦合相互作用 有效各向异性 矫顽力     

Study of the magnetic interaction in nanocrystalline Pr-Fe-Co-Nb-B permanent magnets

The magnetic properties of an isotropic, epoxy resin bonded magnets made from Pr-Fe-Co-Nb-B powder were investigated. The magnetization reversal process and magnetic parameters were examined by measurements of the initial magnetization curve, major and minor hysteresis loops and sets of recoil curves. From the initial magnetization curve and the field dependencies of the reversible and irreversible magnetization components derived from the recoil loops it was found that the magnetization reversal process is the combination of the nucleation of reversed domains and pinning of domain walls at the grain boundaries and the reversible rotation of magnetization vector in single domain grains. The interactions between grains were studied by means of δM plots. The nonlinear behavior of δM curve approve that the short range intergrain exchange coupling interactions are dominant in a field up to the sample coercivity.The interaction domains and fine magnetic structure were revealed as the evidence of exchange coupling between soft α-Fe and hard magnetic Nd₂Fe₁₄B grains.     

Sm3（F3,Co,Mn）29 compounds：promising materials for permanent magnets

The outstanding hard-magnetic properties are reported of Sm_3Fe_{28.1-x}Co_xMo_{0.9} compounds with x=12, 14, 16. In this alloy system, only a small amount of Mo is needed to stabilize the 3:29 structure so that the magnetic properties are not seriously affected by the presence of this nonmagnetic element. Substitution of Co for Fe leads to a significant increase of the magnetic anisotropy, and for x≥14 the easy magnetization direction changes from easy plane to the easy axis. In this alloy system, the compound Sm_3Fe_{12.1}Co_{16}Mo_{0.9} is a very promising candidate for permanent magnet applications. Its room temperature saturation magnetization (μ_0M_s=1.5 T) and anisotropy field (B_{an}=6.5 T) are comparable to the values for Nd_2Fe_{14}B (μ_0M_s=1.6 T and B_{an}=7 T). However, the Curie temperature of Sm_3Fe_{12.1}Co_{16}Mo_{0.9} is 1020 K, which is appreciably higher than that for Nd_2Fe_{14}B (T_C=588 K).     

纳米复合永磁Pr9Fe74Co12B5Snx(x=0, 0.5)的磁化行为与磁黏滞性

用熔体快淬法制备了纳米复合永磁样品Pr₉Fe₇₄Co₁₂B₅ 与Pr₉Fe₇₄Co₁₂B₅Sn_0.5，分析了样品的起始磁化、反磁化过程，测得样品的总磁化率、可逆磁化率以及样品的磁黏滞性.结果表明，两样品在室温下均表现为单一硬磁相磁化行为，在低温下表现为双相行为，且由于添加Sn后使晶粒均匀化从而导致样品低温下的双相行为更加明显.添加Sn后引起样品中软磁相含量和软磁相晶粒尺寸的增加，使磁化反转中可逆磁化部分增多，且使反磁化形核场降低.磁黏滞性研究表明，热激活体积与软磁相晶粒的大小有关. 关键词： 纳米复合永磁 磁化反转 磁粘滞     

Structure dependence of the magnetic-susceptibility behaviour at low magnetizing field

Summary The magnetic susceptibility during hysteresis cycles was measured in both amorphous and crystalline metallic ribbons. The analysis of the dependence on magnetizing field evidences a different behaviour due to the structural differences. The fundamental importance of the amplitude of the magnetizing field used for dynamic evaluation of initial susceptibility is demonstrated and related to the measured effects as well. Moreover, both temperature and frequency effects on the initial susceptibility confirm some peculiar properties related to the amorphous character.     

Angular dependence of the coercivity and remanence of ordered arrays of Co nanowires

The angular dependence of the coercivity and remanence of ordered hexagonal arrays of Co nanowires prepared using anodic aluminum oxide templates was investigated. The experimental evolution of coercivity as a function of the angle, in which the external field is applied, is interpreted considering micromagnetic simulations. Depending on the angle between the axis of the wire and the applied magnetic field direction our results show that the magnetization reversal mode changes from vortex to a transverse domain wall. Besides, we observed that the dipolar interactions cause a reduction in coercive fields, mainly in the direction of easy magnetization of the nanowires. Good agreement between numerical and experimental data is obtained.