Exchange interaction and demagnetization process of high-abundance rare-earth magnets sintered using dual alloy method

The 2:14:1-type rare-earth(RE)-Fe-B permanent magnets prepared by the dual alloy method have been found to possess much superior magnetic properties to those prepared by the single alloy method,providing an appealing route to promote the utilization of high-abundance RE elements Ce and La and balance the use of the RE source.However,the relationship between magnetic interactions among different 2:14:1 main phases and superior magnetic properties is still unclear.In this study,we investigated the magnetic interactions and reversal field distribution in these magnets using first-order reversal curve(FORC)images.The FORC images showed that(Nd,Pr)27.8(La,Ce)2.7FebalM1.4B1.0(S-9)and(Nd,Pr)19.5(La,Ce)11.0FebalM1.4B1.0(S-36)have the characteristics of multiple main phases.The reverse magnetic fields corresponding to the soft and hard main phases,as well as the associated exchange coupling,were highly dependent on the La Ce content.The higher the La Ce content,the weaker the exchange coupling and the more asynchronous the demagnetization process.In addition,the FORC images indicated that the magnetization reversal process also varies with La Ce content,where the nucleation and propagation of reversed domains dominant in the S-9 magnet,while the domain propagation in the S-36 magnet is considerably suppressed.Additional micromagnetic simulations also revealed that the coercivity and exchange coupling of multi-main-phase magnets decrease with increasing La Ce content,correlating well with the experimental results.These findings may not only contribute to a better understanding of the complex magnetic interactions between the soft and hard phases and how they affect macroscopic magnetic properties but also help in improving the magnetic performance of the RE-Fe-B magnets with high La Ce content.     

Magnetic Microstructure of Sintered Nd-Fe-B Magnets Made from Casting Strips

The magnetic microstructures of two Dy-AI substituted sintered Nd-Fe-13 magnets with the different nominal compositions of Nd12.2Vy0.6Fe80.4Al0.7B6 (at.%) (composition-A,C-A) and Nd13.7Dy0.6Fe78.8Al0.7B6.2(at.%)(composition-B,C-13) prepared by strip casting technique have been revealed by using a magnetic force microscope. The magnetic properties of sintered C-B magnets are worse than that of C-A sintered magnets. In particular, the value of density products (BH)max for sintered C-A magnets is about 32% higher than that of C-B magnets, which is reflected by their quite different magnetic microstructures. We believe that for the C-B samples, the inappropriate composition and thus the redundant Nd2Fe17(B) phase of the casting strips make its final magnetic microstructures worse than the C-A, and then deteriorates the performance of the C-B magnets.     

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.     

Influence of misch metal content on microstructure and magnetic properties of R–Fe–B magnets sintered by dual alloy method

MM_(14)Fe_(79.9)B_(6.1)/Nd_(13.5)Fe_(80.5)B_6 magnets were fabricated by dual alloy method(MM, misch metal). Some magnets have two Curie temperatures. Curie temperatures T_(c1)corresponds to the main phase which contains more La Ce, and T_(c1) decreases from 276.5?C to 256.6?C with the content of MM increasing from 30.3 at.% to 50.6 at.%. The variation of Br with the increase of MM indicates the existence of inter-grain exchange coupling in the magnets. When MM/R ≤ 30.3 at.%,the magnetic properties can reach the level of the intrinsic coercivity Hcj≥ 7.11 kOe and the maximum energy product(BH)max≥ 41 MGOe. Compared with Nd, La and Ce are easier to diffuse to the grain boundaries in the sintering process,and this will cause the decrease of H_(cj) Due to the diffusion between the grains, the atomic ratio of La, Ce, Pr, and Nd in each grain is different and the percentage of Nd in all grains is higher than that in misch metal.     

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.     

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.     

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.     

Sm–Co high-temperature permanent magnet materials

Permanent magnets capable of reliably operating at high temperatures up to ~450?C are required in advanced power systems for future aircrafts, vehicles, and ships. Those operating temperatures are far beyond the capability of Nd–Fe–B magnets. Possessing high Curie temperature, Sm–Co based magnets are still very important because of their hightemperature capability, excellent thermal stability, and better corrosion resistance. The extensive research performed around the year 2000 resulted in a new class of Sm_2(Co, Fe, Cu, Zr)_(17)-type magnets capable of operating at high temperatures up to 550?C. This paper gives a systematic review of the development of Sm–Co permanent magnets, from the crystal structures and phase diagrams to the intrinsic magnetic properties. An emphasis is placed on Sm_2(Co, Fe, Cu, Zr)_(17)-type magnets for operation at temperatures from 300?C to 550?C. The thermal stability issues, including instantaneous temperature coefficients of magnetic properties, are discussed in detail. The significance of nanograin structure, nanocrystalline, and nanocomposite Sm–Co magnet materials, and prospects of future rare-earth permanent magnets are also given.     

Electromagnetic wave absorbing properties and hyperfine interactions of Fe–Cu–Nb–Si–B nanocomposites

The Fe–Cu–Nb–Si–B alloy nanocomposite containing two ferromagnetic phases(amorphous phase and nanophase phase) is obtained by properly annealing the as-prepared alloys. High resolution transmission electron microscopy(HRTEM) images show the coexistence of these two phases. It is found that Fe–Si nanograins are surrounded by the retained amorphous ferromagnetic phase. M¨ossbauer spectroscopy measurements show that the nanophase is the D03-type Fe–Si phase, which is employed to find the atomic fractions of resonant57 Fe atoms in these two phases. The microwave permittivity and permeability spectra of Fe–Cu–Nb–Si–B nanocomposite are measured in the frequency range of 0.5 GHz–10 GHz. Large relative microwave permeability values are obtained. The results show that the absorber containing the nanocomposite flakes with a volume fraction of 28.59% exhibits good microwave absorption properties. The reflection loss of the absorber is less than-10 dB in a frequency band of 1.93 GHz–3.20 GHz.     

The magnetization reversal behaviour for SmCo6.8Zr0.2 and SmCo6.8Zr0.2a-（Fe,Co） nanocrystalline magnets at low temperature

This paper reports that the SmCo6.8Zr0.2 nanocrystalline permanent magnets and SmCo6.8Zr0.2/a-（Fe,Co） nanocomposite permanent magnets are successfully produced by mechanical alloying and subsequently annealing at 700℃ for 10 minutes. The x-ray diffraction results show that the phase structure of SmCo6.8Zr0.2 nanocrystalline permanent magnets is composed of SmCo7 phase and SmCo6.8Zr0.2/a-（Fe,Co） nanocomposite permanent magnets is composed of SmCo7 and a-（Fe,Co） phases. The mechanism of magnetization reversal is mainly controlled by inhomogeneous domain wall pinning in SmCo6.8Zr0.2 and SmCo6.8Zr0.2/a-（Fe,Co） magnets. The inter-grain exchange interaction at low temperature is investigated, which shows that the inter-grain exchange interaction of SmCo6.8Zr0.2/a-（Fe,Co） magnets increases greatly by the decrease of the measured temperature. According to Amirr-H/Hcj, Amrev-H/Hcj and Xirr-H/Hcj curves at room temperature and 100 K, the changes of irreversible and reversible magnetization behaviours of SmCo6.8Zr0.2 and SmCo6.8Zr0.2/a-（Fe,Co） magnets with the decreasing temperature are analysed in detail. The magnetic viscosity and the activation volume of SmCo6.8Zr0.2 and SmCo6.8Zr0.2/a-（Fe,Co） magnets at different temperatures are also studied.     

Effects of terbium sulfide addition on magnetic properties,microstructure and thermal stability of sintered Nd-Fe--B magnets

To increase coercivity and thermal stability of sintered Nd–Fe–B magnets for high-temperature applications, a novel terbium sulfide powder is added into(Pr_(0.25)Nd_(0.75))_(30.6)Cu_(0.15)Fe_(bal)B_1(wt.%) basic magnets. The effects of the addition of terbium sulfide on magnetic properties, microstructure, and thermal stability of sintered Nd–Fe–B magnets are investigated.The experimental results show that by adding 3 wt.% Tb_2S_3, the coercivity of the magnet is remarkably increased by about 54% without a considerable reduction in remanence and maximum energy product. By means of the electron probe microanalyzer(EPMA) technology, it is observed that Tb is mainly present in the outer region of 2:14:1 matrix grains and forms a well-developed Tb-shell phase, resulting in enhancement of HA, which accounts for the coercivity enhancement.Moreover, compared with Tb_2S_3-free magnets, the reversible temperature coefficients of remanence(α) and coercivity(β) and the irreversible flux loss of magnetic flow(hirr) values of Tb_2S_3-added magnets are improved, indicating that the thermal stability of the magnets is also effectively improved.     

Effect of optimized aging processing on properties of the sintered Dy-doped Nd–Fe–B permanent magnet

We investigate the effect of the optimized aging processing on magnetism and mechanical property of the sintered Dydoped Nd–Fe–B permanent magnet. The experimental results show that the magnetism, especially intrinsic coercivity, of the optimized aged Dy-doped Nd–Fe–B magnet is more excellent than that of the sintered one, but the former's strength and hardness are lower than that of the latter. It was observed that the optimized aged Dy-doped Nd–Fe–B magnet have more uniform grain size, thinner(Nd, Dy)-rich boundary phase. By means of the EBSD technology, the number of larger angle grain boundaries in the optimized aged Dy-doped Nd–Fe–B magnet is more than that of the sintered one. The reasons for the increased intrinsic coercivity and decreased mechanical properties of the optimized aged Dy-doped Nd–Fe–B magnet are also discussed.     

57Fe Mössbauer spectrometry: A powerful technique to analyze the magnetic and phase characteristics in RE–Fe–B permanent magnets

This review summarizes the recent advances on the application of ⁵⁷Fe M?ssbauer spectrometry to study the magnetic and phase characteristics of Nd–Fe–B-based permanent magnets. First of all, the hyperfine structures of the Ce₂Fe₁₄B,(Ce,Nd)₂Fe₁₄B and MM₂Fe₁₄B phases are well-defined by using the model based on the Wigner-Seitz analysis of the crystal structure. The results show that the isomer shift δ and the quadrupole splitting ?EQ of those 2:14:1 phases show minor changes with the Nd content, while the hyperfine field Bhfincreases monotonically with increasing Nd content and its value is influenced by the element segregation and phase separation in the 2:14:1 phase. Then, the hyperfine structures of the low fraction secondary phases are determined by the ⁵⁷Fe M?ssbauer spectrometry due to its high sensitivity. On this basis,the content, magnetic behavior, and magnetization of the REFe₂ phase, the amorphous grain boundary(GB) phase, and the amorphous worm-like phase, as well as their effects on the magnetic properties, are systematically studied.     

The magnetization reversal behaviour for SmCo6.8Zr0.2 and SmCo6.8Zr0.2/α-（Fe,Co） nanocrystalline magnets at low temperature

This paper reports that the SmCo 6.8 Zr 0.2 nanocrystalline permanent magnets and SmCo 6.8 Zr 0.2 /α-(Fe,Co) nanocomposite permanent magnets are successfully produced by mechanical alloying and subsequently annealing at 700 C for 10 minutes.The x-ray diffraction results show that the phase structure of SmCo 6.8 Zr 0.2 nanocrystalline permanent magnets is composed of SmCo 7 phase and SmCo 6.8 Zr 0.2 /α-(Fe,Co) nanocomposite permanent magnets is composed of SmCo 7 and α-(Fe,Co) phases.The mechanism of magnetization reversal is mainly controlled by inhomogeneous domain wall pinning in SmCo 6.8 Zr 0.2 and SmCo 6.8 Zr 0.2 /α-(Fe,Co) magnets.The inter-grain exchange interaction at low temperature is investigated,which shows that the inter-grain exchange interaction of SmCo 6.8 Zr 0.2 /α-(Fe,Co) magnets increases greatly by the decrease of the measured temperature.According to Δm irr-H/H cj,Δm rev-H/H cj and χ irr-H/H cj curves at room temperature and 100 K,the changes of irreversible and reversible magnetization behaviours of SmCo 6.8 Zr 0.2 and SmCo 6.8 Zr 0.2 /α-(Fe,Co) magnets with the decreasing temperature are analysed in detail.The magnetic viscosity and the activation volume of SmCo 6.8 Zr 0.2 and SmCo 6.8 Zr 0.2 /α-(Fe,Co) magnets at different temperatures are also studied.     

Synthesis and magnetic properties of Fe100-xMox alloy nanowire arrays

The Fe100-xMox(13≤x≤25) alloy nanowire arrays are synthesized by electrodeposition of Fe 2+ and Mo 2+ with different ionic ratios into the anodic aluminum oxide templates.The crystals of Fe100-xMox alloy nanowires gradually change from polycrystalline phase to amorphous phase with the increase of the Mo content and the nanowires are of amorphous structure when the Mo content reaches 25 at%,which are revealed by the X-ray diffraction and the selected area electron diffraction patterns.As the Mo content increases,the magnetic hysteresis loops of Fe100-xMox alloy nanowires in parallel to the nanowire axis are not rectangular and the slopes of magnetic hysteresis loops increase.Those results indicate that the magnetostatic interactions between nanowires and the magnetocrystalline anisotropy both have significant influences on the magnetization reversal process of the nanowire arrays.     

Solitons and soliton molecules in two nonlocal Alice–Bob Sawada–Kotera systems

Two nonlocal Alice–Bob Sawada–Kotera(ABSK) systems, accompanied by the parity and time reversal invariance are studied. The Lax pairs of two systems are uniformly written out in matrix form. The periodic waves, multiple solitons, and soliton molecules of the ABSK systems are obtained via the bilinear method and the velocity resonant mechanism. Though the interactions among solitons are elastic, the interactions between soliton and soliton molecules are not elastic.In particular, the shapes of the soliton molecules are changed explicitly after interactions.     

Equivalent circuit model including magnetic and thermo sources for the thermo–magneto–electric coupling effect in magnetoelectric laminates

The nonlinear thermo–magneto–mechanical magnetostrictive constitutive and the linear thermo–mechanical-electric piezoelectric constitutive are adopted in this paper. The bias magnetic field and ambient temperature are equivalent to a magnetic source and a thermo source, respectively. An equivalent circuit, which contains a magnetic source and a thermo source at the input, for the thermo–magneto–electric coupling effect in magnetoelectric(ME) laminates, is established. The theoretical models of the output voltage and static ME coefficient for ME laminates can be derived from this equivalent circuit model. The predicted static ME coefficient versus temperature curves are in excellent agreement with the experimental data available both qualitatively and quantitatively. It confirms the validity of the proposed model. Then the models are adopted to predict variations in the output voltages and ME coefficients in the laminates under different ambient temperatures, bias magnetic fields, and the volume ratios of magnetostrictive phases. This shows that the output voltage increases with both increasing temperature and increasing volume ratio of magnetostrictive phases; the ME coefficient decreases with increasing temperature; the ME coefficient shows an initial sharp increase and then decreases slowly with the increase in the bias magnetic field, and there is an optimum volume ratio of magnetostrictive phases that maximize the ME coefficient.This paper can not only provide a new idea for the study of the thermo–magneto–electric coupling characteristics of ME laminates, but also provide a theoretical basis for the design and application of ME laminates, operating under different sensors.     

Magnetic and mechanical properties of Ni–Mn–Ga/Fe–Ga ferromagnetic shape memory composite

A ferromagnetic shape memory composite of Ni–Mn–Ga and Fe–Ga was fabricated by using spark plasma sintering method. The magnetic and mechanical properties of the composite were investigated. Compared to the Ni–Mn–Ga alloy,the threshold field for magnetic-field-induced strain in the composite is clearly reduced owing to the assistance of internal stress generated from Fe–Ga. Meanwhile, the ductility has been significantly improved in the composite. A fracture strain of 26% and a compressive strength of 1600 MPa were achieved.     

Remanence Properties and Magnetization Reversal Mechanism of Fe Nanowire Arrays

Remanence properties and magnetization reversal mechanism of Fe nanowire arrays with diameters 16nm and 130nm are studied. Isothermal remanent magnetization curves show that the contribution of irreversible magnetization decreases when the diameter changes from 16nm to 130hm. The remanence coercivities of these nanowires obtained in dc-demagnetization curve are about 2400 Oe and 800 Oe, respectively. The magnetization reversal mechanism is different in these two samples. For the nanowire array with diameter 16nm, both the nucleation and the pinning have effects on magnetization reversal mechanism, and the pinning field (about 2500 Oe) is larger than the nucleation field (about 2200 Oe). However, for the nanowire array with diameter 130nm, the magnetization reversal mechanism is dominated by the pinning effect of domain walls.