The coercive field of sintered and melt-spun NdFeB magnets

The magnetic hysteresis loops have been investigated in the temperature range between 4.2 and 575 K for aligned sintered permanent magnets of nominal composition Nd₁₅Fe₇₇B₈ and for isotropic melt-spun ribbons of composition Nd₁₅Fe₇₆B₉. The measured temperature and field dependence of the coercive field is analysed within the framework of theoretical results for nucleation fields of the ideal Nd₂Fe₁₄B matrix and, of disturbed surface regions of Nd₂Fe₁₄B grains. Furthermore the pinning of domain walls at thin soft magnetic grain boundary phases is considered for the high temperature range. It is concluded that for both types of NdFeB magnets the relevant magnetic hardening mechanisms at lower temperatures are nucleation processes in magnetically inhomogeneous regions whereas at higher temperatures the pinning of domain walls at grain boundaries predominates. The critical temperature where the change of nucleation hardening to pinning hardening occurs depends sensitivity on the crystal anisotropy, the grain boundary microstructure and the macroscopic grain- and multi phase arrangements.     

MAGNETIC PROPERTIES AND CRYSTALLIZATION OF THE RAPIDLY QUENCHED (Fe1-xNdx) 81.5B18.5 ALLOYS

Magnetic properties and crystallization behaviors of amorphous (Fe_1-xNd_x) _81.5B_18.5 alloys were studied. The crystallization temperature is found to rise at first and then drop monotonically with x, having a maximum value of 976K at x=0.11 (9at% Nd). The (Fe_1-xNd_x) _81.5B_18.5 alloys prepared at a quenching rate of v_s = 6.6m /s are amorphous, and exhibit good glass formability. Both the coercive field H_c and energy product (BH)_max depend strongly on Nd concentration. Amorphous (Fe_1-xNd_x) _81.5B_18.5 alloys with higher Nd concentration have a high coercive field at low temperature, due to the large random uniaxial anisotropy of Nd. The room-temperature H_c and (BH)_max obtained on optimal annealing con-ditions show two maxima as a function of Nd concentration x. The highest room-temperature coercive field H_c =22 kOe within the Nd concentrations around x=0.368 and the maximum energy product(RH)_max= 13.3 MG·Oe at x =0.055 are observed. The hard magnetic properties of these crystallized samples are related to the presence of the bard magnetic Nd₂Fe₁₄B phase.     

Phase monitoring during Nd15Fe77B8 preparation by the calciothermic reduction-diffusion process

We have examined the effect of various parameters involved in the preparation of the Nd₁₅Fe₇₇B₈ alloy by the calciothermic reduction-diffusion process. Both Nd₂O₃ and NdCl₃ were utilized as starting raw materials. Shorter reaction times and lower heating temperatures are required for the chloride than for the oxide. Among the characterization techniques used to evaluate the final product, Mössbauer spectroscopy was invaluable for the determination of the (Nd_1.1Fe₄B₄)/(Nd₂Fe₁₄B) ratio, which should be as low as possible.     

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：     

Angular dependence of coercivity of grains in nanocrystalline permanent magnets

In this paper magnetization remanence curves were studied for nanocrystalline Pr8Fe87B5, Pr12Fe82B6 and Pr15Fe77B8. Initially the sample was at remanence following saturation along z-axis. After rotating the magnet by 5n degrees （n=0, 1, ..., 18） a field H was applied along z-axis and then decreased to zero, and the remanence Jr^n was measured as a function of H. The curves were compared with those calculated based on the nucleation of reverse domain model and domain wall pinning model. The latter model succeeds in simulation much better than the former, and it is concluded that the magnetization reversal is dominated by domain wall pinning for all the samples. The nucleation mechanism contribution, while remains small, increases with the increase of Pr content.     

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：     

3D and 1D micromagnetic calculation for hard/soft bilayers with in-plane easy axes

Macroscopic hysteresis loops and microscopic magnetic moment distributions have been determined by three-dimensional (3D) as well as one-dimensional (1D) micromagnetic models for exchange-coupled Nd₂Fe₁₄B/α–Fe bilayers and carefully compared with each other. It is found that the results obtained from the two methods are consistent with each other, where the nucleation and coercive fields decrease monotonically as the soft layer thickness L^s increases whilst the largest maximum energy products (roughly 600 kJ/m³) occur at L^s=5 nm. Moreover, the calculated angular distributions in the thickness direction for the magnetic moments are similar. Nevertheless, the calculated critical fields and energy products by 3D OOMMF are systematically smaller than those given by the 1D model, mainly due to the local demagnetization fields, which are taken into account in the 3D calculation and ignored in the 1D calculation. It is demonstrated by the 3D calculation that the large demagnetization fields in the corners of the soft layers reduce the nucleation fields and thus facilitate the magnetic reversal. Such an effect enhances as L^s increases. When L^s=20 nm, the differences between the coercivity is as large as 30%, while the nucleation fields obtained by the two methods have opposite signs.     

Magnetic properties enhancement of Nd2Fe14B/α-Fe nanocomposites by Ta substitution

Nanostructured Nd_9.5Fe_84−xB_6.5Ta_x (x=0, 0.5, 1, 1.5, and 2) ribbons composed of Nd₂Fe₁₄B and α-Fe phases with a high coercivity and maximum energy product are fabricated by direct melt spinning. The effects of Ta addition on the structures and magnetic properties of melt-spun Nd_9.5Fe_84−xB_6.5Ta_x (x=0, 0.5, 1, 1.5, and 2) ribbons have been investigated. Compared with addition-free ribbons, small addition of Ta is found to reduce the grain sizes of the samples and improve their magnetic properties due to a strong exchange coupling between the Nd₂Fe₁₄B hard phase and α-Fe soft phase. A coercive field of 750 kA/m and a maximum energy product of 158 kJ/m³ in melt-spun Nd_9.5Fe_82.5B_6.5Ta_1.5 ribbons are obtained at room temperature.     

Magnetic and structural properties of the Nd2(Fe100−x Nb x )14B system prepared by arc melting

In this work the magnetic and structural properties are investigated by Mössbauer spectrometry, Vibrating Sample Magnetometry and X-ray diffraction of Nd₂(Fe_100?x Nb _x )₁₄B powdered alloys with x?=?0, 2 and 4 prepared by arc melting. The Mössbauer spectra of the samples were fitted with several contributions from: Nd₂Fe₁₄B, α-Fe and a paramagnetic phase associated with Nd_1.1Fe₄B₄ for x?=?0 and additionally from NbFeB and Nd₂Fe₁₇ for x?=?2 and x?=?4. The relative fractions of α-Fe and Nd₂Fe₁₄B are smaller for x?=?4 than for x?=?0, indicating that the amount of these two phases is reduced with increasing Nb content, while the relative fraction of Nd₂Fe₁₇ increases. The α-Fe grain size slightly decreases while that of the Nd₂Fe₁₄B phase is increasing, when the Nb content increases. The hysteresis loops indicate that these samples behave as hard ferromagnets, with a coercive field which decreases when the Nb content increases, but with rather low remanent magnetization.     

Magnetic properties of nanocrystalline composite in the Nd--Fe--B system

Using ⁵⁷Fe Mössbauer spectroscopy, X-ray diffraction and magnetization measurements, magnetic properties of melt-spun Nd_4.5Fe₇₇B_18.5 ribbon have been investigated after the annealing treatments which make nanocrystalline composite from the as-prepared amorphous state. Specimen ribbon which shows coercivity and is suitable for a permanent magnet consists of the mixture of Fe₃B and Nd₂Fe₁₄B. Relative fraction of these phases in the ribbon have been determined.     

Mössbauer study of intergranular phase in Nd 8 Fe86 − x Nb x B6(x = 0, 1, 2, 3) nanocomposite magnet

Nd₈Fe_86???x Nb _x B₆ (x = 0, 1, 2, 3) nanocomposite magnet has been studied by Mössbauer spectroscopy and nanostructure observation. It was found that intergranular phase formed between α-Fe and Nd₂Fe₁₄B phase in NdFeNbB alloys plays a significant role on the magnetic properties. By the addition of Nb into Nd₈Fe₈₆B₆ composition, coercivity was found to increase by 25% due to the grain refinement of both the soft and hard magnetic phases which was decreased from 50 nm of virgin Nd₈Fe₈₆B₆ to 25 nm in Nd₈Fe₈₅Nb₁B₆ alloys. The role of Nb addition was confirmed to stabilize the Nd₂Fe₁₄B lattice preventing from thermal vibration of the corresponding sites at where Fe atoms are substituted by Nb in the Nd₂Fe₁₄B lattice. The enhanced coercivity was originated from the exchange hardening of soft and amorphous phases surrounding the hard magnetic Nd₂Fe₁₄B crystal.     

Magnetic properties and phase compositions of rapidly quenched Nd4Fe96−x B x after crystallization

Permanent magnets, composition Nd₄Fe_96?x B _x (x=10–21), were made by rapid quenching followed by crystallization. The effects of B concentration and annealing time on the magnetic properties and phase compositions were studied by Mössbauer spectroscopy and magnetic measurements. The stripping method was adopted to obtain the subspectral area of each Fe phase. Optimum magnetic properties are obtained for the composition Nd₄Fe₇₇B₁₉ annealed at 670°C for 3 minutes. Then the remanence is 12.0 kGs, the intrinsic coercivity 3.2 kOe, and the maximum energy product 12.6 MGOe. The crystalline phases are Fe₃B, Nd₂Fe₁₄B and α-Fe with volume percentages of 60%, 36%, and 4%, respectively.     

Effect of the intergrain interactions on the coercivity and its angular dependence for Nd(FeCo)B sintered magnets

Effects of interactions between grains with different alignment degrees on the coercivity and its angular dependence for Nd₁₆(Fe_0.8Co_0.2)₇₈B₆ sintered magnets have been studied. The experiments show that the intrinsic coercivity _jH_c decreases with enhancing grain alignment (decreasing alignment coefficient σ), the coercivity _jH_c(θ) increases with increasing angle θ between the applied field and the texture axis of the magnets and the variation ratio is larger for the magnets with better grain alignment. The coercivity of the magnets should be determined by the critical field making the moment of individual grains reverse and the interactions between the grains. For the sintered magnets composed of the grains with μm size, the magnetostatic interaction between the grains is stronger than the exchange coupling interaction and it makes the coercivity of magnet increase with increasing alignment coefficient σ. Taking into account the intergrain interactions, the starting field theory of coercivity is in good agreement with the experimental results for Nd₁₆(Fe_0.8Co_0.2)₇₈B₆ sintered magnets.     

Inducing anisotropy in bulk Nd-Fe-Co-Al-B nanocrystalline alloys by quenching in magnetic field

We have observed magnetic anisotropy in bulk Nd_55−xCo_xFe₃₀Al₁₀B₅ (x=10, 15 and 20) alloys prepared by copper mold suction casting method with a presence of external magnetic field (quenching field) μ₀H=0.25 T. By changing direction of the measuring field from perpendicular to parallel one in comparison with that of the quenching field, coercive force of the alloys slightly decreases while remanent magnetization and squareness of hysteresis loop increase more clearly. It is also found that the higher Co-concentration in the alloys the larger magnetic anisotropy is induced. The structure analyses manifest nanocrystalline particles embedded in residual amorphous matrix of the alloys. The size of the particles is in range of 10-30 nm and their crystalline phases consist of Nd₂(Fe,Co)₁₄B, Nd₃Co, Nd₃Al, NdAl₂ and Nd.     

Effect of element substitution on nanostructure and hard magnetism of rapidly quenched Nd2Fe14B

The structural and magnetic properties of Nd₁₂Fe₈₂B₆ and Nd₁₀M₂Fe₈₂B₆ (M = Nb, Ti, Zr, Cr) alloys prepared using arc melting and melt spinning have been investigated. All the samples are found to crystallise with a tetragonal Nd₂Fe₁₄B phase without any alloy or elemental impurities. There is a small decrease in the unit cell volume of Nd₂Fe₁₄B due to transition metal (M) addition. The substitution of Nb and Ti refines and homogenises the nanostructure of the alloys, promoting intergrain exchange coupling leading to an increase in the remanence and energy product. For example, the remanence and energy product of Nd₁₂Fe₈₂B₆ and Nd₁₀Nb₂Fe₈₂B₆ are 8.4 kG and 15 MGOe, and 9.9 kG and 20 MGOe, respectively. The J(T) curves are similar to those of a single phase ferromagnetic material suggesting no segregation of ferromagnetic impurities. The observed structural and magnetic properties are consistent with the fact that the substitutional transition metal atoms occupy the Nd site of the tetragonal Nd₂Fe₁₄B crystal lattice. The improvement of magnetic properties of nanocrystalline Nd₂Fe₁₄B alloys with the decrease in Nd concentration may be beneficial for the application of this material in bonded magnets.     

145Nd NMR in Nd15Fe77B8H x

The effect of absorbed hydrogen on the¹⁴⁵Nd NMR in Nd₁₅Fe₇₇B₈ has been investigated at helium temperatures. Hydrogen absorption is accompanied by a drop of NMR frequencies and changes in the quadrupole splitting. The drop of NMR frequencies is related to the lattice expansion. The influence of the hydrogen on the spin reorientation can be responsible for the changes in the quadrupole splitting. This work was supported by Project CPBPU1, U4     

Ferromagnetic resonance in nanocrystalline Fe73.5CuNb3Si13.5B9 (Finemet)

The ferromagnetic resonance linewidth of Fe_73.5CuNb₃Si_13.5B₉ melt-spun ribbons has been investigated as a function of annealing temperature. Fe_73.5CuNb₃Si_13.5B₉ possesses an ultrafine grain structure which can be altered by suitable annealing to exhibit a combination of excellent soft magnetic characteristics and high saturation induction. It is a ferromagnetic metal consisting of crystallites whose anisotropy axes are randomly oriented and which may interact with each other via exchange or dipolar fields. When annealed at moderate temperatures, Fe_73.5CuNb₃Si_13.5B₉ is characterized by an extremely low coercive field, and is useful in magnetic cores and memories. At a critical temperature (∼600°C), the grain size rapidly increases with annealing temperature. This onset of crystallization is accompanied by a relatively abrupt increase in the FMR linewidth, the magnetic anisotropy field, and the coercive field. We extend the scaling arguments of Herzer to explain these obviously related phenomena.     

Magnetization reversal mechanism of anisotropic HDDR Nd2Fe14B-based magnet powder

The coercivity mechanism of anisotropic Nd₂Fe₁₄B-based magnetic powders prepared by hydrogenation–decomposition–esorption–recombination process was studied. Polarization and corresponding differential susceptibility curves of the powders in its thermally demagnetized state were measured. Microstructure and constituents of the powders were investigated by means of transmission electron microscope and scanning electron microscope with energy dispersive X-ray detector. In addition, theoretical calculation of the intrinsic coercive force of the magnetic powders was performed. It is concluded that the magnetic hardening mechanism of the powders is the pinning of domain walls at grain boundaries of the Nd₂Fe₁₄B main phase and Nd-rich phase that distributes homogeneously around some conglomerations composed of fine Nd₂Fe₁₄B grains. The coercive force of the powders is mainly determined by the pinning of domain walls by Nd-rich boundary phase.     

MAGNETIC VISCOSITY AND DEMAGNETIZATION BEHAVIOUR IN ISOTROPIC NANOCRYSTALLINE Pr12Fe82B6 RIBBONS

We present the experimental results of the magnetic viscosity, demagnetization curve and recoil loop for isotropic nanocrystalline Pr₁₂Fe₈₂B₆ ribbons prepared by melt-spinning. The thermal fluctuation field, activation volume and irreversible demagnetization are discussed. The coercivity mechanism is mainly determined by the inhomogeneous nucleation rather than a simple nucleation of reverse domain.     

Corrosion characteristics of NdFeB sintered magnets containing various alloying elements

The effect of small additions (1 at%) of some elements (P, Cr, Ti, Zr, Pb or Sn) on the corrosion behaviour and magnetic properties of Nd₁₅Fe₇₇B₈ sintered magnets has been investigated. It was established that most advantageous was the addition of 1 at% Cr which distinctly inhibited both acid and atmospheric corrosion processes while it did not deteriorate the magnetic characteristics of the magnet. Addition of Cr accelerates, however, the dissolution of the magnet at strongly cathodic polarization.