Nd2Fe14B化合物磁晶各向异性起源的理论研究

本文根据Nd₂Fe₁₄B化合物中存有少量巡游电子的基本实验事实,提出了在Nd₂Fe₁₄B中配位子所产生的晶场和巡游电子所产生的电场共同造成了Nd离子磁晶各向异性的理论模型。提出并解决了计算巡游电子同中心Nd离子相互作用的理论方法。在同时考虑Nd离子所受配位子的晶场和巡游电子的电场的作用情况下,用单离子模型计算了Nd₂Fe₁₄B中Nd离子所产生的磁晶各向异性及其随温度的变化。所 关键词：     

R2Fe14B型永磁材料中第二磁晶各向异性常数对反磁化过程的影响

针对低温下各向同性Pr₂Fe₁₄B永磁材料的最小形核场问题，用数值 计算法和近似解 析解研究了第二磁晶各向异性常数K₂对最小形核场的影响.研究发现，尽管对于 Nd₂ Fe₁₄B永磁材料一级近似的解析解与数值计算结果很接近，但是对于低温下各向 同性P r₂Fe₁₄B永磁材料则至少要用二级近似下的解析解才能与数值计算 结果相接近.用有 关最小形核场的计算结果很好地解释了低温时各向同性Pr₂Fe₁₄B永 磁材料的矫顽力与最小形核场的关系. 关键词： 第二磁晶各向异性常数 形核场 矫顽力     

热处理对非晶态合金Nd4Fe96-xBx磁性的影响

本文研究了用单辊急冷方法制备的非晶态合金Nd₄Fe_96-xB_x的晶化,以及热处理对其硬磁性和相组成的影响,发现非晶态合金Nd₄Fe_96-xB_x的晶化温度比相同B含量的非晶态合金Fe_100-xB_x高120—190K,X射线衍射和热磁测量表明,15≤x≤25的样品晶化相是由Nd₂Fe₁₄B(T 关键词：     

Ti掺杂Nd2Fe14B/α-Fe纳米双相复合永磁体晶化动力学

纳米双相复合稀土永磁材料,利用硬磁相高磁晶各向异性和软磁相高饱和磁化强度的优点,通过铁磁交换耦合作用获得优异的磁性能.但是如何解决软硬磁双相纳米微结构不匹配的问题,控制软硬磁相同时达到理想的纳米尺度复合是关键.本文研究了掺杂合金元素Ti对熔体快淬法制备的Nd₂Fe₁₄B/α-Fe快淬薄带晶化过程的影响.结果表明,掺杂合金元素Ti能影响Nd₂Fe₁₄B/α-Fe交换耦合磁体整个晶化动力学过程,使α-Fe相的晶化激活能升高,抑制其从非晶相中析出.同时,降低1∶7亚稳相的晶化激活能,起到稳定亚稳相的作用.而且随着晶化温度的进一步提高, α-Fe和Nd₂Fe₁₄B两相由1∶7亚稳相分解产生,从而有效避免了α-Fe相的优先析出.显微组织观察表明,掺杂Ti的样品晶粒细小、分布均匀,平均晶粒尺寸在20 nm左右,没有特别大的α-Fe粒子出现.当Ti的掺杂量原子百分数为1.0%时,获得了最佳磁性能(BH)_max=12 MG·Oe(1 G=10     

取向Pr2Fe14B与Nd2Fe14B多晶磁化曲线的 计算

基于单离子晶场模型,提出了计算稀土-Fe(Co)金属间化合物取向多晶样品磁化曲线的方法.用此方法计算了取向Pr₂Fe₁₄B和Nd₂Fe₁₄ B多晶的高场磁化曲线,计算中使用了拟合化合物单晶磁化曲线得到的交换场与晶场参数.计 算曲线与实验曲线相符合. 关键词： 磁化曲线 晶场 2Fe₁₄B')" href="#">R₂Fe₁₄B     

Si对Nd2Fe14B四方相结构和磁性的影响

本文对Si加入Nd₂Fe₁₄B四方相后所形成的合金相的结构和磁性进行了研究。结果表明,Si加入Nd₂Fe₁₄B四方相后并不破坏它的结构(Si的含量可达B的两倍),而形成Nd₂(Fe,Si)₁₄B赝三元金属间化合物。随着Si含量的增加,晶格常数和饱和磁化强度随之减小,但居里温度则随之增加。Si的加入,并不改变Nd₂Fe₁₄B的室温各向 关键词：     

Dy在Nd2Fe14B晶格中的占位及其对Fe原子磁矩影响的第一性原理计算

基于第一性原理投影缀加波和梯度矫正局域密度近似(PAW-GGA), 研究了Nd₂Fe₁₄B和Dy₂Fe₁₄B的基态晶格属性, 进而对Dy在Nd₂Fe₁₄B晶格中的掺杂进行了研究, 并采用GGA+U的方式进行了原子磁矩计算, 并与自旋轨道耦合 (SOI) 计算结果进行了对照. 置换计算表明, Dy原子倾向于置换Nd₂Fe₁₄B晶格中4f位的稀土原子. 磁矩计算表明, 在R₂Fe₁₄B (R: 稀土元素) 晶格中, 4f位的稀土元素与Fe原子作用更强, 对磁性能的影响更大. 稀土原子与Fe的作用与距离呈正相关. 关键词： 2Fe₁₄B')" href="#">Nd₂Fe₁₄B 晶格占位 形成能 原子磁矩     

Nd2Fe14B的价电子结构分析和磁性计算

应用固体与分子经验电子理论计算了Nd₂Fe₁₄B的价电子结构、磁矩和居里温度,计算结果与实验值相符.计算表明:该合金的磁性与3d磁电子数成正比.从Fe(c)晶位到Fe(k₂)晶位磁矩增加,其机理源于价电子、哑对电子和3d磁电子之间的转化,有78%的哑对电子和18%的3d共价电子转化成了磁电子.居里温度和磁矩与Fe原子配位数成正比,与加权等同键数I^σ成反比,Nd原子 关键词： 2Fe₁₄B')" href="#">Nd₂Fe₁₄B 价电子结构 居里温度     

(Nd1－xGdx)3Fe27.31Ti1.69化合物的结构和磁性

通过X射线衍射和磁性测量等手段研究了(Nd_1-xGd_x)₃Fe_27.31Ti_1.69(0≤x≤0.6)化合物的结构和磁性.X射线衍射测量结果表明Gd替代后并未改变Nd₃(Fe,Ti)₂₉化合物的晶体结构，但引起了晶胞体积收缩.随着Gd含量的增加，化合物的居里温度T_C和室温磁晶各向异性场B_a单调增加，而自旋重取向 关键词： 1-xGd_x)₃Fe_27.31Ti_1.69化合物')" href="#">(Nd_1-xGd_x)₃Fe_27.31Ti_1.69化合物 磁晶各向异性 自旋重取向 磁相图     

快淬Nd3.6Pr5.4Fe83Co3B5薄带的反磁化行为和磁黏滞性

用电弧熔炼法制备了Nd_3.6Pr_5.4Fe₈₃Co₃B₅合金铸锭,然后利用熔旋快淬法在铜辊转速V=20m/s下制备了Nd_3.6Pr_5.4Fe₈₃Co₃B₅薄带.快淬带主要由软磁相α-Fe和Nd₂Fe₁₄B型的硬磁相组成.采用直流退磁剩磁曲线方法分析了样品在反磁化过程中的可逆与不可逆磁化部分,并研究了软磁相和硬磁相的反磁化行为,得到样品的不可逆磁化形核场H_no约为440kA/m.同时研究了样品的磁黏滞性,结果表明由于软磁相的存在使得热激活体积较大. 关键词：     

Research on preparing compact bulk nanocomposite Nd2Fe14B/α-Fe magnetic materials by hot extrusion

In this paper, compact bulk nanocomposite Nd₂Fe₁₄B/α-Fe magnetic materials were prepared by hot extrusion of amorphous and nanocrystalline powders, which were prepared by high-energy ball-milling (HEBM) of the Nd₂Fe₁₄ B-type hard magnetic phase with 20 vol% of α-Fe as soft magnetic phase. The extrusion temperature has important influence on magnetic properties and microstructure of magnetic materials. The results show that the grain size of Nd₂Fe₁₄B and α-Fe phase increases steadily with increasing extrusion temperature. Furthermore, optimal extrusion temperature of 1223 K occurs, at which the highest magnetic properties and relative density can be obtained.     

Effect of ferrofluid concentration on electrical and magnetic properties of the Fe3O4/PANI nanocomposites

Magnetic nanocomposites can be controlled and tailored to provide the desired mechanical, physical, chemical, and biomedical properties depending on the final applications. The coating of ferrite nanoparticles with polymers affords the possibility of minimizing agglomeration in large-scale commercial synthesis of nanocomposite materials. The process of coating not only provides effective encapsulation of individual nanoparticles, but also controls the growth in size, thus, yielding a better overall size distribution. In this paper, in-situ polymerization of aniline was carried out in different concentration of the ferrofluid with the aim to obtain agglomerate free nanocomposites. The role of the ferrite concentration was investigated by the spectral, morphological, conductivity, and magnetic properties of Fe₃O₄/polyaniline (PANI) nanocomposites. XRD revealed the presence of spinel phase of Fe₃O₄ and the particle size was calculated to be 14.3 nm. Spectral analysis confirmed the formation of PANI encapsulated Fe₃O₄ nanocomposite. Conductivity of the nanocomposites was found to be in the range of 0.001–0.003 S/cm. Higher saturation magnetization of 3.2 emu/g was observed at 300 K, revealing a super paramagnetic behavior of this nanocomposite.     

Synthesis, structural and magnetic properties of the nanocomposite Fe63B23Nd7Y3Nb3Cr1 magnets

The Fe₆₃B₂₃Nd₇Y₃Nb₃Cr₁ nanocomposite magnets in the form of sheets have been prepared by copper mold casting technique. The phase evolution, crystal structure, microstructural and magnetic properties have been investigated in the as-cast and annealed states. The as-cast sheets show magnetically soft behaviors which become magnetically hard by thermal annealing. The optimal annealed microstructure was composed of nanosize soft magnetic α-Fe (19-29 nm) and hard magnetic Nd₂Fe₁₄B (45-55 nm) grains. The best hard magnetic properties such as intrinsic coercivity, _jH_c of 1119 kA/m, remanence, B_r of 0.44 T, magnetic induction to saturation magnetization ratio, M_r/M_s=0.61 and maximum energy product, (BH)_max of 55 kJ/m³ was obtained after annealing at 680 °C for 15 min. The annealing treatment above 680 °C results in non-ideal phase grains growth, which degrade the magnetic properties.     

Synthesis,structures and magnetic properties of Pr-lean Pr2Fe14B/Fe3B nanocomposite alloys

The lean rare-earth Pr_4.5Fe_77−xTi_xB_18.5 (x=0, 1, 4, 5) nanocomposite alloys were prepared by melt spinning method and subsequent thermal annealing. The effect of Ti content and annealing temperature on the magnetic properties and the microstructure of these magnets were investigated. The enhancing coercivity H_c from 211.4 to 338.2 kA/m has been observed at the optimal annealing temperature of 700 °C by the addition of 5 at% Ti in Pr₂Fe₁₄B/Fe₃B alloys. It was also found that increasing Ti content leads to marked grain refinement in the annealed alloys, resulting in strong exchange-coupling interaction between the hard and the soft phases in these ribbons. In addition, the magnetization reversal behaviors of Pr₂Fe₁₄B/Fe₃B nanocomposites were discussed in detail.     

Die-upset Nd11.5Fe72.4Co9Nb1B6.1 magnets with additions of Zn, Al and Sn

The magnetic properties and microstructure were studied for bulk Nd_11.5Fe_72.4Co₉Nb₁B_6.1 magnets synthesized by hot-pressing and subsequent die-upsetting the melt-spun ribbons with additions of three kinds of low-melting-point metal (Zn, Al and Sn). Die-upset Nd_11.5Fe_72.4Co₉Nb₁B_6.1 magnets have low magnetic properties since they have an inhomogeneous microstructure with many coarse grains. The microstructure of die-upset magnets remains almost unchanged with Al and Sn additions, which only have negative effects on the magnetic properties. Different from Al and Sn additions, Zn addition changes the phase composition of the starting melt-spun powers due to the reaction of Zn and Nd₂Fe₁₄B during hot-pressing and hot-deforming and enhances the development of the desired [0 0 1] texture and improves the microstructure of die-upset magnets. As a result, an anisotropic magnet with good maximum energy product (221 kJ/m³) and high coercivity (670 kA/m) is obtained by adding 2 wt% Zn to the Nd_11.5Fe_72.4Co₉Nb₁B_6.1 alloy.     

Fe65B22Nd9Mo4 bulk nanocomposite permanent magnets produced by crystallizing amorphous precursors

The Fe₆₅B₂₂Nd₉Mo₄ nanocomposite permanent magnets in the form of a rectangular cross sectioned rod have been prepared by annealing the amorphous precursors. The thermal behavior, structure and magnetic properties of the magnets have been investigated by differential scanning calorimetry, X-ray diffractometry, electron microscopy and magnetometry techniques. The as-cast Fe₆₅B₂₂Nd₉Mo₄ alloy showed soft magnetic properties, which changed into magnetically hard after annealing. Results provoke that the magnetic properties of the alloy are sensitive to thermal processing conditions. The optimum hard magnetic properties with a remanence (B_r) of 0.56 T, coercivity (_iH_c) of 920.7 kA/m and maximum energy product (BH)_max of 50.15 kJ/m³ were achieved after annealing the alloy at 983 K for 10 min. The good magnetic properties of Fe₆₅B₂₂Nd₉Mo₄ magnets are ascribed to the exchange coupling between the nano-scaled soft α-Fe, Fe₃B and hard Nd₂Fe₁₄B magnetic grains.     

Improvement of the magnetic properties of Nd9.5Fe81Zr3B6.5 nanocomposite magnets by semi-melt spinning

Nd_9.5Fe₈₁Zr₃B_6.5 ribbons are prepared by single roller melt-spinning technique at 1150 °C which is in the solid and liquid coexistence zone. The phase evolution and magnetic properties were studied by X-ray diffraction, differential scanning calorimetry, transmission electron microscopy observations, and magnetization measurements. The experimental results show that in comparison to the ribbons quenching at higher temperature, the thickness of ribbons prepared at 1150 °C are insensitive to the wheel speed and an uniform nanoscale structure with fine grains can be obtained directly from the semi-melt and the exchange coupling interaction between the grains was enhanced for the nanocomposite permanent alloy which can contributed to excellent magnetic properties.     

Annealing effect of NiO/Co90Fe10 thin films: From bilayer to nanocomposite

Exchange-biased bilayers are widely used in the pinned layers of spintronic devices. While magnetic field annealing (MFA) was routinely engaged during the fabrication of these devices, the annealing effect of NiO/CoFe bilayers is not yet reported. In this paper, the transition from NiO/Co₉₀Fe₁₀ bilayer to nanocomposite single layer was observed through rapid thermal annealing at different temperatures under magnetic field. The as-deposited and low-temperature (<623 K) annealed samples had rock salt (NiO) and face center cubic (Co₉₀Fe₁₀) structures. On the other hand, annealing at 623 K and 673 K resulted in nanocomposite single layers composed of oxides (matrix) and alloys (precipitate), due to grain boundary oxidization and strong interdiffusion in the NiO/CoFe and CoFe/SiO₂ interfaces. The structural transition was accompanied by the reduction of grain sizes, re-ordering of crystallites, incensement of roughness, and reduction of Ni²⁺. When measured at room temperature, the bilayers exhibited soft magnetism with small room-temperature coercivity. The nanocomposite layers exhibited an enhanced coercivity due to the changes in the magnetization reversal mechanism by pinning from the oxides. At 10 K, the increased antiferromagnetic anisotropy in the NiO resulted in enhanced coercivity and exchange bias in the bilayers. The nanocomposites exhibited weaker exchange bias compared with the bilayers due to frustrated interfacial spins. This investigation on how the magnetic properties of exchange-biased bilayers are influenced by magnetic RTA provides insights into controlling the magnetization reversal properties of thin films.     

Preparation of superparamagnetic Fe3O4/PMMA nano composites and their magnetorheological characteristics

Fe₃O₄/PMMA composite particles were fabricated by a simple one-pot hydrothermal method. The magnetic measurement showed that the composite particles displayed a higher saturated magnetization and superparamagnetic property. The rheological properties of the magnetorheological fluids (MRFs) based on Fe₃O₄/PMMA particles were measured on a rotational rheometer with a magnetic field generator. It was found that the MRFs exhibited better MR effect and sendimentary stability than the similar materials.     

Preparation and magnetic properties of BaFe12O19/Ni0.8Zn0.2Fe2O4 nanocomposite ferrite

Nanocomposite of hard (BaFe₁₂O₁₉)/soft ferrite (Ni_0.8Zn_0.2Fe₂O₄) have been prepared by the sol–gel process. The nanocomposite ferrite are formed when the calcining temperature is above 800 °C. It is found that the magnetic properties strongly depend on the presintering treatment and calcining temperature. The “bee waist” type hysteresis loops for samples disappear when the presintering temperature is 400 °C and the calcination temperature reaches 1100 °C owing to the exchange-coupling interaction. The remanence of BaFe₁₂O₁₉/Ni_0.8Zn_0.2Fe₂O₄ nanocomposite ferrite with the mass ratio of 5:1 is higher than a single phase ferrite. The specific saturation magnetization, remanence magnetization and coercivity are 63 emu/g, 36 emu/g and 2750 G, respectively. The exchange-coupling interaction in the BaFe₁₂O₁₉/Ni_0.8Zn_0.2Fe₂O₄ nanocomposite ferrite is discussed.