Rare earth permanent magnets constitute a mature technology, but the shock of the 2011 rare earth crisis led to the re-evaluation of many ideas from the 1980s and 1990s about possible new hard ...magnets containing little or no rare earth (or heavy rare earth). Nd–Fe–B magnets have been painstakingly and skillfully optimized for a wide range of applications in which high performance is required at reasonable cost. Sm–Co is the material of choice when high-temperature stability is required, and Sm–Fe–N magnets are making their way into some niche applications. The scope for improvement in these basic materials by substitution has been rather thoroughly explored, and the effects of processing techniques on the microstructure and hysteresis are largely understood. A big idea from a generation ago—which held real potential to raise the record energy product significantly—was the oriented exchange-spring hard/soft nanocomposite magnet; however, it has proved very difficult to realize. Nevertheless, the field has evolved, and innovation has flourished in other areas. For example, electrical personal transport has progressed from millions of electric bicycles to the point where cars and trucks with electrical drives are becoming mainstream, and looks ready to bring the dominance of the internal combustion engine to an end. As the limitations of particular permanent magnets become clearer, ingenuity and imagination are being used to design around them, and to exploit the available mix of rare earth resources most efficiently. Huge new markets in robotics beckon, and the opportunities offered by additive manufacturing are just beginning to be explored. New methods of increasing magnet stability at elevated temperature are being developed, and integrated multifunctionality of hard magnets with other useful properties is now envisaged. These themes are elaborated here, with various examples.
•Nanocrystalline Sm-Co alloys with the tetragonal 1:12 main phase were prepared for the first time.•The V doping and nanostructuring stabilize the metastable SmCo12 phase.•Stacking faults and ...high-density grain boundaries contribute to high coercivity of the SmCo12-based alloys.
The intermetallic compounds based on the tetragonal ThMn12 prototype crystal structure have exhibited great potential as advanced rare-earth-lean permanent magnets due to their excellent intrinsic magnetic properties. However, the trade-off between the phase stability and the magnetic performance is often encountered in the ThMn12-type magnets. This work was focused on the effects of V doping and nanostructuring on the phase stability and magnetic properties of ThMn12-type Sm-Co-based magnets. Novel SmCo12-based nanocrystalline alloys with the SmCo12 main phase were prepared for the first time. The prepared alloys from the optimal design achieved obviously higher coercivity than the isotropic SmFe12-based alloys, together with comparable performance of other magnetic features. The enhancement in the coercivity was ascribed to the pinning of domain walls by the nanocrystalline grain boundaries and stacking faults. First-principles calculations and magnetic structure analysis disclosed that V substitution can stabilize the SmCo12 lattice and elevate its magnetocrystalline anisotropy. This study provides a new approach to developing stabilized metastable structured rare-earth-lean alloys with high magnetic performance.
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Two different transformation routes, quenching followed by heat treatment (route 1), and cooling at intermediate rate (route 2), were used to obtain the L10 phase in Mn-Al alloys. The resulting ...materials showed remarkable differences in microstructure and magnetic properties. In addition, fully transformed material was cold-worked and then recovered by heat treatment. The microstructure of both cold-worked and recovered samples appeared similar in backscattered electron images, however, the magnetic properties differed greatly. Large numbers of electron backscatter diffraction (EBSD) patterns were recorded from all the materials and the pattern quality was quantified, yielding a measure of the dislocation density. The results indicated that both the dislocation density and the coercivity of the cold-worked sample were much higher than in the recovered sample. In addition to other microstructural changes, the route 1 sample had higher dislocation density and coercivity compared to the route 2 sample. For the as-transformed materials, the results were supported by EBSD misorientation mapping and strain analysis of x-ray diffraction data. The continuum theory of dislocations was used to show that the local stress fields of dislocations will cause distortions in the crystal structure leading to local changes in the intrinsic magnetic properties. Such features are likely to act as pinning centres for magnetic domain walls and therefore a higher dislocation density implies a higher coercivity. This conclusion was additionally supported by initial magnetisation curves, which showed evidence for an active pinning mechanism in the cold-worked sample but not for the recovered sample.
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•Dislocation density of τ-MnAl depends on transformation route or mechanical working.•High coercivity after cold working is attributed to increased dislocation density.•Initial magnetisation curve indicates pinning of domain walls.•Domain walls pinned by dislocations due to locally changed intrinsic properties.
•Coercivity of NdFeB thin magnet has been significantly enhanced by Pr-Al-Cu diffusion.•A 49% enhancement of coercivity in block magnets is obtained by the diffusion.•Optimized microstructure after ...grain boundary diffusion is observed and confirmed.•Diffusion behavior and diffusion coefficients of Pr, Al and Cu are investigated.
Low melting point Pr-Al-Cu alloys were employed in grain boundary diffusion for sintered NdFeB magnets. The coercivity of the 7 × 7 × 2 mm magnet was enhanced from 1000 kA/m to 1360, 1615, and 1714 kA/m by diffusing Pr70Al10Cu20, Pr70Al15Cu15 and Pr70Al20Cu10 alloys, respectively, at 800 °C for 2 h followed by 500 °C for 3 h. The optimized microstructure with strip-like grain boundary phase was evident in diffused magnets, which helps to reduce the magnetic exchange interaction between the hard magnetic grains and improve the coercivity. Under the same diffusion process, the coercivity of the diffused magnets decreased with increasing sample thickness, but 27% coercivity enhancement can still be obtained in the magnet with 6 mm in height. A Φ10×10 mm block magnet was diffused by Pr70Al20Cu10 alloy at 800 °C for 2–10 h. Up to 49% enhancement in coercivity was achieved, which confirms the diffusion ability of Pr-Al-Cu alloy. The diffusion behavior of Pr, Al and Cu elements has been analyzed by Fick’s law. Based on the diffusion coefficients obtained at 800 and 900 °C, Pr diffuses faster than Cu and Al along the grain boundary. The current results demonstrated that Pr-Al-Cu alloys can effectively optimize the grain boundary structure and improve the coercivity of NdFeB magnets.
Two-dimensional van der Waals materials have demonstrated fascinating optical and electrical characteristics. However, reports on magnetic properties and spintronic applications of van der Waals ...materials are scarce by comparison. Here, we report anomalous Hall effect measurements on single crystalline metallic Fe3GeTe2 nanoflakes with different thicknesses. These nanoflakes exhibit a single hard magnetic phase with a near square-shaped magnetic loop, large coercivity (up to 550 mT at 2 K), a Curie temperature near 200 K and strong perpendicular magnetic anisotropy. Using criticality analysis, the coupling length between van der Waals atomic layers in Fe3GeTe2 is estimated to be ~5 van der Waals layers. Furthermore, the hard magnetic behaviour of Fe3GeTe2 can be well described by a proposed model. The magnetic properties of Fe3GeTe2 highlight its potential for integration into van der Waals magnetic heterostructures, paving the way for spintronic research and applications based on these devices.
The atomic thickness of two-dimensional materials provides a unique opportunity to control their electrical1 and optical2 properties as well as to drive the electronic phase transitions3 by ...electrostatic doping. The discovery of two-dimensional magnetic materials4–10 has opened up the prospect of the electrical control of magnetism and the realization of new functional devices11. A recent experiment based on the linear magneto-electric effect has demonstrated control of the magnetic order in bilayer CrI3 by electric fields12. However, this approach is limited to non-centrosymmetric materials11,13–16 magnetically biased near the antiferromagnet–ferromagnet transition. Here, we demonstrate control of the magnetic properties of both monolayer and bilayer CrI3 by electrostatic doping using CrI3–graphene vertical heterostructures. In monolayer CrI3, doping significantly modifies the saturation magnetization, coercive force and Curie temperature, showing strengthened/weakened magnetic order with hole/electron doping. Remarkably, in bilayer CrI3, the electron doping above ~2.5 × 1013 cm−2 induces a transition from an antiferromagnetic to a ferromagnetic ground state in the absence of a magnetic field. The result reveals a strongly doping-dependent interlayer exchange coupling, which enables robust switching of magnetization in bilayer CrI3 by small gate voltages.
This paper deals with global integrability for solutions to quasilinear elliptic systems involving N$N$ equations of the form
...−∑i=1nDi∑β=1N∑j=1nai,jα,β(x,u(x))Djuβ(x)=fα(x),inΩ,u(x)=0,on∂Ω,$$\begin{equation*} {\begin{cases} \displaystyle -\sum _{i=1}^n D_i {\left(\sum _{\beta =1}^N \sum _{j=1}^n a^{\alpha, \beta } _{i,j} (x,u(x)) D_j u^\beta (x) \right)} =f^\alpha (x), & \mbox{ in } \Omega, \\10pt \displaystyle u(x)=0, &\displaystyle \mbox{ on } \partial \Omega, \end{cases}} \end{equation*}$$where Ω$\Omega$ is an open bounded subset of Rn$\mathbb {R}^n$, n>2$n>2$, u=(u1,u2,…,uN):Ω⊂Rn→RN$u=(u^1,u^2,\ldots,u^N):\Omega \subset \mathbb {R}^n \rightarrow \mathbb {R}^N$, N≥2$N\ge 2$. Under degenerate coercivity condition of the diagonal coefficients and proportional condition of the off‐diagonal coefficients, we obtain some global integrability results.
•Coercivity enhancement of thicker sintered NdFeB magnets by grain boundary diffusion can be obtained.•Tb55Ce20Cu25 alloys infiltrate into deeper interior of the magnets, and the optimized ...microstructure after GBD result in coercivity enhancement.•10-mm-thick magnets diffused with Tb55Ce20Cu25 optimizes coercivity enhancement of 7.4 kOe as well as improves the thermal stability.
Coercivity enhancement of thicker sintered NdFeB magnets with 5–10 mm in thickness by grain boundary diffusion (GBD) with low-melting Tb75−xCexCu25 alloy powders (x = 0–45) is demonstrated. For 5-mm-thick magnets, after GBD with Tb75−xCexCu25 alloys (x = 0–45), a significant iHc increment (ΔiHc) of 8.2–11.5 kOe with a slight decrease in Br is found. Although the magnet GBD treated with Tb75Cu25 exhibits the highest ΔiHc of 11.5 kOe, the coercivity enhancement per wt% Tb usage (ΔiHc/wt.% Tb) could be increased by about 2-folds with increasing x from 0 to 45. As for the 10-mm-thick magnets, large ΔiHc of 8.4 kOe and 7.4 kOe can also be attained by GBD with Tb75Cu25 and Tb55Ce20Cu25 powders, respectively. The optimized microstructure and the effective distribution of Tb and Ce at grain boundary after GBD with Tb75−xCexCu25 are the main factors for the coercivity enhancement. As a result, the magnetic properties, as well as the thermal stability of the sintered NdFeB magnets with 5–10 mm in thickness, are improved considerably. This work provides a cost-effective way to enhance the coercivity of the thicker sintered NdFeB magnets.
Spinel cubic ferrites have huge applications in memory and high frequency devices. For the improvement of these modern devices, the magnetic coercivity, permeability, and dielectric properties of a ...ferrite are the important issues. This article focuses on improving the magnetic coercivity, magnetic permeability, and dielectric properties of Co0.2Zn0.3Ni0.5EuxFe2–xO4 ferrites, where x = 0.00, 0.06, and 0.10. The X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDX), vibrating sample magnetometer (VSM), and an impedance analyzer were used to characterize the structural, magnetic, and dielectric properties of the samples. The XRD patterns indicate the formation of spinel cubic structure of the samples with a secondary peak (EuFeO3) for Eu doped samples. The densities and porosities of the samples follow an inverse trend, where the doped samples’ lattice parameters are increased with the increment of rare earth Eu concentration. The FTIR analysis also proves the spinel cubic phase of the samples. The average grain size of the ferrites is obtained via FESEM images, and it is increased from 121 to 198 nm. VSM analysis confirms that doping of the Eu content also changes other hysteresis loop properties of Co0.2Zn0.3-Ni0.5EuxFe2–xO4 ferrites. Particularly, the coercivity of the Eu doped samples is greater than that of the mother alloy (x = 0.00). The EDX study shows that there is no impurity contamination in the ferrites. The permeability and dielectric measurements show an improved quality factor of the Eu-doped samples with low magnetic and dielectric losses. Frequency dependent resistivity and impedance analysis also show the improved nature. From the observed properties of the samples, all the investigated ferrites might be strong candidates for potential applications in memory devices, magnetic sensors, and high frequency applications.
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