The origin of graduated demagnetization curves of NdFeB magnets

We present an analysis of the magnetization process of NdFeB-type magnets which exhibit a constriction or a shoulder of the demagnetization curve near the remanence point B_r. This constriction which drastically reduces the maximum energy product of the permanent magnet is discussed in terms of i) a siperposition of a soft and a hard magnetic phase, ii) an oxidation process of surface layers, iii) and the spin reorientation which changes the magnetic structure from easy c-axis to easy cone below 135 K. We demonstrate that the effect of surface oxidation and of a soft magnetic phase are of considerable influence over the entire temperature range while the influence of the spin reorientation is dominant below 150 K. The appearance of the cone structure prevents the application of NdFeB magnets in a magnetic circuit below 150 K.     

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.     

Initial magnetization curve and hardening mechanism in rapidly quenched NdFeB magnets

Rapidly quenched NdFeB alloys were prepared with varying grain sizes well below and above the critical size for single domain particles. The contributions of the single and multi-domain particles to the initial magnetization curve are analyzed. By changing the quenching speed, i.e., the average grain size, the shape of the initial magnetization curve changes characteristically. The volume fraction of single domain grains is determined from the initial magnetization curves. From scanning electron micrographs the grain size distribution is evaluated and the critical size for single domain particles in the bulk material is deduced from a quantitative analysis of the magnetization curves. It reaches twice the value of the theoretical value for isolated spherical particles. A low temperature treatment following the thermal demagnetization lowers the initial susceptibility in underquenched ribbons as well as in an MQIII magnet. This effect reflects the irreversibility of the transition from the multi to the single domain particle state during the cooling. The temperature dependence of the single domain particle size is deduced from the initial magnetization curves of low temperature treated samples. It is shown that these experimental results are consistently explained assuming the nucleation mechanism to apply for rapidly quenched NdFeB magnets. The results are compared to the behaviour of hard ferrites.     

Texture in NdFeB magnets analysed on the basis of the determination of Nd2Fe14B single crystals easy growth axis

We have shown by X-ray analysis and magnetic measurements, that the easy growth axis of Nd₂Fe₁₄B crystals corresponds to the “a” axis of the tetragonal structure while the easy magnetization axis is the “c” axis at temperatures above 135K. This correlation allows to understand some interesting features in NdFeB magnets:

• •-anisotropic contraction during sintering in magnets obtained by classical powder metallurgy,
• •-orientation mechanism during hot pressing (“die upset”) of magnets based on melt spun ribons.

     

Effects of oxidation of DyH_3 in Nd–Fe–B sintered magnets

The effects of oxidation of Dy H3 with respect to dysprosium addition to Nd–Fe–B sintered magnets are examined.Samples sintered with the addition of freshly milled dysprosium hydride, dysprosium hydride exposed to air at room temperature for 15 min and dysprosium hydride exposed to air at 100°C for 3.5 hours are studied from the aspects of magnetic properties, microstructures, and their degradation, respectively. It is found that some oxidized dysprosium is distributed in the Nd-rich phase; hence, the decrease of remanence occurred. The degradation results indicate that preoxidised dysprosium can be a major factor in increasing the corrosion rate. The microstructures and corrosion acceleration test suggested that the oxidation is detrimental to remanence.     

Domain structures of Nd13Fe80B7 magnets during HDDR process

The domain structures of Nd13Fe80B7 alloy at different stages of the HDDR process have been revealed using a magnetic force microscope. In the as-cast samples, the columnar crystals with easy axis perpendicular to one another are clearly characterized by their different domain structures. For the insufficient and sufficient HD treatment, an obvious change of domain structure occurs, which is related to the variation of composition and crystalline microstructure during the HD process. And for the samples after sufficient DR processing, it is confirmed that the configuration of the columnar crystals is retained by the detected domain structures.     

Plastic deformation modeling of backward extrusion process for Nd–Fe–B ring magnets

3D finite element-based software (3D DEFORM) was used to simulate the thermal extrusion process of nanocrystalline magnetic ring. The effective stresses and effective strains for a ring magnet at different stages of the extrusion process were determined by simulation. The effective strains at different stages are displayed. The effective stresses on the cross section are determined by simulation. The test results of magnetic properties were of good validation of the three-dimensional finite element analysis for nanocrystalline backward extruded ring. 3D finite element-based plastic deformation simulation is proved to be an effective way to analyze the hot extrusion process of nanocrystalline magnetic ring, and to provide guiding for the mold design of thermal extrusion.     

Magnetic domain structures of overquenched Nd–Fe–B permanent magnets studied by electron holography

Microstructures and magnetic domain structures of overquenched Nd–Fe–B permanent magnets have been investigated in detail by transmission electron microscopy. While magnetic domain boundaries are clarified by Lorentz microscopy, magnetization distribution in the domains is clearly observed by electron holography. In the as-quenched magnet, the size of the magnetic domains is in the range from 200 to 500 nm and the direction of the magnetic lines of force changes gradually in wide region, while in the annealed one having the crystalline phase of Nd₂Fe₁₄B, the direction of the magnetic lines of force changes drastically especially at the grain boundaries. Furthermore, the direction of the magnetic lines of force changes more drastically in the specimen annealed at 893 K than the specimen annealed at 843 K. This result clearly indicates that the magnetocrystalline anisotropy is enhanced with the increase of annealing temperature, resulting in strong domain wall pinning.     

Effect of Co on the thermal stability and impact toughness of sintered Nd–Fe–B magnets

The effect of Co on the thermal stability and impact toughness of sintered Nd–Fe–B magnets has been investigated. The results showed that the addition of Co decreased the intrinsic coercivity and the temperature coefficient of remanence (α), and increased the temperature coefficient of coercivity (β) for sintered Nd–Fe–B magnets. The impact toughness of sintered Nd–Fe–B magnets with the addition of Co first decreases, reaches a minimum, and then starts to increase. The possible reasons for increasing the temperature coefficients of coercivity (β) for sintered Nd–Fe–B magnets were analyzed, and the relations between the microstructure and impact toughness of sintered Nd–Fe–B magnets were studied.     

Regulating element distribution to improve magnetic properties of sintered Nd–Fe–B/Tb–Fe–B composite magnets

Nd content was varied in Nd_(13.2-x)Fe_(80.8+x)B_6(x = 0, 0.5, 1, and 1.5) to optimize the magnetic properties of sintered Nd–Fe–B/Tb–Fe–B composite magnets, which were prepared by mixing 9 g of Nd–Fe–B with 1 g of Tb_(17)Fe_(75)B_8 powder.In conventional magnets, by reducing Nd content, the coercivity of 10.4 kOe in Nd_(13.2)Fe_(80.8)B_6 decreases to 7.2 kOe in Nd_(12.2)Fe_(81.8)B_6; meanwhile, in Nd–Fe–B/Tb–Fe–B magnets the coercivity does not decrease when reducing Nd content.In the intergranular phase, the Tb content increases owing to the reducing Nd content of the Nd–Fe–B alloy in the sintered composite magnets.Therefore, the excess Tb in Tb_(17)Fe_(75)B_8 enters the intergranular phase, and more Tb atoms can substitute for Nd at the grain boundary of the Nd–Fe–B phase, leading to a more significant increase in coercivity.The remanence increases with reducing Nd content, and the energy product of 39.1 MGOe with a high coercivity of 21.0 kOe is obtained in Nd_(12.2)Fe_(81.8)B_6/Tb_(17)Fe_(75)B_8 magnets.These investigations show that magnetic properties can be further improved by regulating the element distribution in sintered composite magnets.     

Re-exchange of Fe and Cu at the interface in sintered Nd–Fe–B magnets: A method to eliminate Fe precipitation at grain boundaries

According to the decoupling hypothesis for magnetic grains, the coercivity in sintered Nd–Fe–B magnets is increased after Cu doping, which is due to the formation of non-magnetic grain boundaries. However, this method partially fails, and ferromagnetic Fe-segregation occurs at the grain boundary. We discovered both experimentally and through calculation that the Fe content at the grain boundaries can be tuned across a wide range by introducing another element of Ag. Segregated Fe at high temperature at the grain boundary re-dissolves into Nd₂Fe₁₄B grains during annealing at low temperature. Both configurable and magnetic entropies contribute a large driving force for the formation of nonmagnetic grain boundaries. Almost zero Fe content could be achieved at the grain boundaries of sintered Nd–Fe–B magnet.     

Magnetic properties of misch-metal partially substituted Nd–Fe–B magnets sintered by dual alloy method

The misch-metal(MM) partially substituted Nd–Fe–B sintered magnets were fabricated by the dual alloy method,and the crystal structure, microstructure, and magnetic properties were analyzed comprehensively. X-ray diffraction(XRD)reveals that the increasing content of the MM has an inconsiderable effect on the crystallographic alignment of the magnets.Grains of the two main phases are uniformly distributed, and slightly deteriorate on the grain boundary. Due to the diffusion between the adjacent grains, the MM substituted Nd–Fe–B magnets contain three types of components with different Ce/La concentrations. Moreover, the first-order reversal curve(FORC) diagram is introduced to analyze the magnetization reversal process, coercivity mechanism, and distribution of reversal field in magnetic samples. The analysis indicates that there are two major reversal components, corresponding to the two different main phases. The domain nucleation and growth are determined to be the leading mechanism in controlling the magnetization reversal processes of the magnets sintered by the dual alloy method.     

Effect of annealing temperature on coercivity of Nd–Fe–B magnets with TbFeAl doping by process of hot-pressing

The Nd–Fe–B magnets are pre-sintered and then processed with hot-pressing, and the resulting magnets are called the hot-pressed pretreated(HPP) magnets. The coercivity of the HPP magnets increases as the annealed temperature increases.When the annealing temperature is 900℃, the coercivity of the magnet is only 17.6 kOe(1Oe = 79.5775 A·m~(-1)), but when the annealing temperature rises up to 1060℃, the coercivity of the magnet reaches 23.53 k Oe, which is remarkably increased by 33.7%. The microstructure analysis indicates that the grain surface of the HPP magnet becomes smoother as the annealed temperature increases. The microstructure factor α is changed according to the intrinsic coercivity model formula. The α of the magnet at 900℃ is only 0.578, but it is 0.825 at 1060℃. Microstructural optimization is due mainly to the increase of coercivity of the HPP magnet.     

Effect of the substitution of Pr for Nd on microstructure and magnetic properties of nanocomposite Nd2Fe14B/α-Fe magnets

Microstructure and magnetic properties of melt-spun nanocomposite magnets with nominal compositions of (Nd_1−xPr_x)₉Fe₈₆B₅ (x=0–1) were investigated. Substitution of Nd by Pr could significantly improve the hard magnetic properties of the nanocomposite magnets; the intrinsic coercivity (_iH_c) and the maximum magnetic energy product ((BH)_max) increase from 414 kA/m and 124 kJ/m³ for x=0 to 493 kA/m and 152 kJ/m³ for x=0.6, respectively. Further substituting Nd by Pr (x>0.6) strongly weakens exchange-coupling interaction between magnetically hard and soft phases.     

The magnetic properties,thermal stability and microstructure of Nd–Fe–B/Ga sintered magnets prepared by blending method

The effect of gallium added by blending method on the magnetic properties, thermal stability and microstructure of Nd_16.5Dy_16.0Fe_53.45Co_13.0B_1.05 (wt%) sintered magnets was investigated. The experimental results show that an appropriate Ga addition can markedly increase the coercivity, reduce the irreversible loss and slightly enhance the remanence. For instance, by adding 0.5 wt% Ga, the coercivity is increased from 1232 to 1819 kA/m; the irreversible loss after being exposed at 200°C for 0.5 h is reduced from above 33% to below 5%. Microstructure analyses show that the grain boundaries of the magnets with and without Ga addition are substantially different. The grain boundaries of the Ga-free magnet are meandrous. On the other hand, most of the boundaries of Ga-containing magnets are straight and smooth. These characteristics can be explained by the appearance of new phases during sintering process.     

The temperature dependence of the Mössbauer hyperfine parameters in Nd2Fe14B

We have determined the temperature dependence of the internal hyperfine field, isomer shift, and quadrupole shift at each of the six iron sites in Nd₂Fe₁₄B. The hyperfine parameters are consistent with the local iron site environments. The quadrupole and isomer shifts and their temperature dependences support our assignments of the relative ordering of the internal hyperfine fields as j₂>k₂>ck₁>j₁>c. We obtain a Mössbauer temperature of 390 K for Nd₂Fe₁₄B, which compares well with the Debye temperature of 420K for pure iron.     

Effects of post-sinter annealing on microstructure and magnetic properties of Nd–Fe–B sintered magnets with Nd–Ga intergranular addition

We investigate the effects of post-sinter annealing on the microstructure and magnetic properties in B-lean Nd–Fe–B sintered magnets with different quantities of Nd–Ga intergranular additions. The magnet with fewer Nd–Ga additions can enhance 0.2 T in coercivity, with its remanences nearly unchanged after annealing. With the further increase of the Nd–Ga addition, the annealing process leads coercivity to increase 0.4 T, accompanied by a slight decrease of remanence. With the Nd–Ga addition further increasing and after annealing, however, the increase of coercivity is basically constant and the change of remanence is reduced. Microstructure observation indicates that the matrix grains are covered by continuous thin grain boundary phase in the magnets with an appropriate Nd–Ga concentration after the annealing process. However, the exceeding Nd–Ga addition brings out notable segregation of grain boundary phase, and prior formation of part RE6 Fe13 Ga phase in the sintered magnet. This prior formation results in a weaker change of remanence after the annealing process.Therefore, the diverse changes of magnetic properties with different Nd–Ga concentrations are based on the respective evolution of grain boundary after the annealing process.     

Effect of Tb on the intrinsic coercivity and impact toughness of sintered Nd–Dy–Fe–B magnets

The effect of Tb on the coercivity and impact toughness of sintered Nd–Dy–Fe–B magnets has been investigated. The results showed that the addition of Tb enhanced the intrinsic coercivity, reduced the remanence and improved the impact toughness of sintered magnets. The optimum impact toughness of sintered magnets was achieved when 1.0 at% Tb was incorporated. The possible reasons for increasing the intrinsic coercivity and improving impact toughness of sintered magnets were analyzed, and the relations between the microstructure and impact toughness of sintered magnets were studied.     

Progress in recycling of Nd–Fe–B sintered magnet wastes

Significant efforts have been put into the recycling of bulk Nd–Fe–B sintered magnet wastes around the world in the past decade because bulk Nd–Fe–B sintered magnet wastes are valuable secondary rare-earth resources.There are two major facts behind the efforts.First, the waste magnets contain total rare-earth content as high as more than 30 wt.%, which is higher than most natural rare-earth mines.Second, the waste magnets maintain the physical and chemical properties of the original magnets even with deterioration of the properties on surfaces due to corrosion and contamination.In this review,various techniques for recycling bulk Nd–Fe–B sintered magnet wastes, the overall properties of the recycled Nd–Fe–B sintered magnets, and the mass production of recycled magnets from the wastes are reviewed.