An in-situ magnetising holder achieving 1.5 T in-plane field in 200 kV transmission electron microscope.

A strong in-plane magnetic field is required for Lorentz transmission electron microscopy (LTEM) to observe the evolution of the magnetic domain structure of materials with high coercivity, particularly for research on rare-earth permanent magnets. However, the maximum field of the present in-situ magnetising holder applied in 200-kV or 300-kV TEM does not exceed 0.1 T. In this study, the reason for the low field was analysed, and the field strength was significantly elevated by reducing the field application area of the field generator. From finite element method calculations and experimental measurements, a 1.5 T in-plane field was achieved by our new holder in a 200-kV TEM, and images with good quality could still be obtained. Using the newly developed holder, the magnetisation process of hot-pressed NdFeB magnets was observed. The in-situ magnetising holder can be used in research on a wide variety of magnetic materials.

Thermal Stability of Skyrmion Tubes in Nanostructured Cuboids.

Magnetic skyrmions in bulk materials are typically regarded as two-dimensional structures. However, they also exhibit three-dimensional configurations, known as skyrmion tubes, that elongate and extend in-depth. Understanding the configurations and stabilization mechanism of skyrmion tubes is crucial for the development of advanced spintronic devices. However, the generation and annihilation of skyrmion tubes in confined geometries are still rarely reported. Here, we present direct imaging of skyrmion tubes in nanostructured cuboids of a chiral magnet FeGe using Lorentz transmission electron microscopy (TEM), while applying an in-plane magnetic field. It is observed that skyrmion tubes stabilize in a narrow field-temperature region near the Curie temperature (Tc). Through a field cooling process, metastable skyrmion tubes can exist in a larger region of the field-temperature diagram. Combining these experimental findings with micromagnetic simulations, we attribute these phenomena to energy differences and thermal fluctuations. Our results could promote topological spintronic devices based on skyrmion tubes.

Significant Progress for Hot-Deformed Nd-Fe-B Magnets: A Review.

High-performance Nd-Fe-B-based rare-earth permanent magnets play a crucial role in the application of traction motors equipped in new energy automobiles. In particular, the anisotropic hot-deformed (HD) Nd-Fe-B magnets prepared by the hot-press and hot-deformation process show great potential in achieving high coercivity due to their fine grain sizes of 200-400 nm, which are smaller by more than an order of magnitude compared to the traditional sintered Nd-Fe-B magnets. However, the current available coercivity of HD magnets is not as high as expected according to an empirical correlation between coercivity and grain size, only occupying about 25% of its full potential of the anisotropy field of the Nd2Fe14B phase. For the sake of achieving high-coercivity HD magnets, two major routes have been developed, namely the grain boundary diffusion process (GBDP) and the dual alloy diffusion process (DADP). In this review, the fundamentals and development of the HD Nd-Fe-B magnets are comprehensively summarized and discussed based on worldwide scientific research. The advances in the GBDP and DADP are investigated and summarized based on the latest progress and results. Additionally, the mechanisms of coercivity enhancement are discussed based on the numerous results of micromagnetic simulations to understand the structure-property relationships of the HD Nd-Fe-B magnets. Lastly, the magnetization reversal behaviors, based on the observation of magneto-optic Kerr effect microscopy, are analyzed to pinpoint the weak regions in the microstructure of the HD Nd-Fe-B magnets.

Comparisons of Dy Utilization Efficiency by DyHx Grain Boundary Addition and Surface Diffusion Methods in Nd-Y-Fe-B Sintered Magnet.

Using the heavy rare earth Dy element to improve coercivity is the most common solution for hindering the reduction in magnetic properties in the Nd-Fe-B magnet, and the effective utilization of Dy has become the focus of research in industrial society. In this work, we investigated the influence of DyHx addition and diffusion methods on the microstructure, magnetic performance, and thermal stability of the Nd-Y-Fe-B magnet with a Y-rich core structure. The coercivity of the DyHx addition magnet increases from 9.45 kOe to 15.51 kOe when adding 1.6 wt.% DyHx, while the DyHx diffusion magnet increases to 15.15 kOe. According to the analysis of the microstructure and elemental distribution, both Dy-rich shells were basically formed due to the diffusion process of Dy atoms. The Dy-rich shell in the DyHx addition magnet was similar with the original core-shell structure in the Nd-Y-Fe-B magnet. However, the distinct dual-shell structure consisting of a thinner Dy-rich shell and a Y-lean shell was constructed in the DyHx diffused magnet, contributing to the superior coercivity increment and Dy utilization efficiency. Furthermore, the remanence of the DyHx diffused magnet is up to 12.90 kG, which is better than that of the DyHx addition magnet (12.59 kG), due to fewer Dy atoms entering the 2:14:1 matrix grain to cause the antiferromagnetic coupling with Fe atoms. Additionally, the thermal stability of the DyHx diffusion magnet is also better than that of the DyHx addition magnet, owing to the elevated coercivity at room temperature, which expands the application range of the Nd-Y-Fe-B magnet to a certain extent.

The Effect of Grain Size on the Diffusion Efficiency and Microstructure of Sintered Nd-Fe-B Magnets by Tb Grain Boundary Diffusion.

The grain boundary diffusion process (GBDP) of heavy rare earth Tb is an effective method to improve the coercivity of Nd-Fe-B magnets, and the matrix grain size has a crucial effect on the diffusion efficiency and depth of the Tb element. In this work, magnets with different grain sizes have been fabricated using powder metallurgy to investigate the effect of grain size on Tb diffusion efficiency and the microstructure of Nd-Fe-B-type magnets. After the Tb diffusion process, the coercivity increment of the magnet with 4.9 µm large grain is 8.60 kOe, which is much higher than that of the magnet with 3.0 µm small grain (~5.90 kOe), which clearly demonstrates that the coercivity increment decreases as the grain size decreases. Microstructure analysis suggested that grain refinement significantly increases the total surface area, resulting in narrowing and discontinuity of the grain boundary phase (GBP). Therefore, as the channel for diffusion, the narrowing and discontinuity of the GBP are unfavorable for diffusion, resulting in a decrease in diffusion efficiency.

Enhancement of Domain Wall Pinning in High-Temperature Resistant Sm2Co17 Type Magnets by Addition of Y2O3.

In this study, the effects of Y2O3 addition on the magnetic properties, microstructure and magnetization reversal behavior of Sm(Co0.79Fe0.09Cu0.09Zr0.03)7.68 magnet were investigated. By addition of Y2O3, the coercivity was increased from 21.34 kOe to 27.42 kOe at 300 K and from 5.14 kOe to 6.27 kOe at 823 K. A magnet with a maximum magnetic energy product of 9.86 MGOe at 823 K was obtained. With the interdiffusion of Y and Sm after appropriate addition, the Cu content within the cell boundary phase close to the oxide was detected to be nearly twice as high as that away from the oxide. We report for the first time that a collection of lamellar phases were formed on both sides of the inserted oxide, providing a strong pinning field against magnetic domain wall motion based on in-situ Lorentz TEM observation. Furthermore, the ordering process of the original magnet was delayed after Y2O3 addition, resulting in the refinement of cellular structure, which can also enhance the domain wall pinning ability of cellular structures based on micromagnetic simulation. However, excessive addition of Y2O3 led to large Cu-rich phase and Zr-rich impurity phase precipitated at the edge of the oxide, resulting in the destruction of cellular structures and a significant reduction in coercivity. This study provides a new technical approach to regulate the microstructure of Sm2Co17 type magnets. Addition of Y2O3 is expected to play a significant role in improvement of high temperature magnetic properties.

The evolution of phase constitution and microstructure in iron-rich 2:17-type Sm-Co magnets with high magnetic performance.

Iron-rich 2:17-type Sm-Co magnets are important for their potential to achieve high coercivity and maximum magnetic energy product. But the evolution of phase structure, which determines magnetic properties, remains an unsolved issue. In this study, the phase constitution and microstructure of solution-treated 2:17-type Sm-Co alloys are studied. The increase of Fe content promotes the ordering transformation from the 1:7H phase to partially ordered 2:17R and lamellar Zr-rich 1:3R phase. This ordering transformation is mainly due to the competitive atoms occupation of Zr, Fe and Sm in the 1:7H phase. To ease this competition, we modify Sm content in iron-rich 2:17-type Sm-Co magnets. Different solution precursors and corresponding cellular structures are observed. Solution precursor with 1:7H, partially ordered 2:17R, 2:17H and 1:3R phase evolves into uneven and incomplete cellular structures, while solution precursor with partially ordered 2:17R phase forms larger cell size with less lamellar phase, thus poor coercivity and magnetic energy product. However, solution precursors with single 1:7H phase evolve into uniform cellular structures and perform high coercivity and magnetic energy product. Our results indicate if a single 1:7H phase could be obtained in solution-treated 2:17-type Sm-Co magnets with higher iron content, much higher magnetic properties could be achieved.

Correction: Well protected SmCo nanoclusters: fabrication and transformation to single crystals.

[This corrects the article DOI: 10.1039/C7RA10158A.].

Magnetic Properties Improvement of Die-upset Nd-Fe-B Magnets by Dy-Cu Press Injection and Subsequent Heat Treatment.

Ultrafine-grained die-upset Nd-Fe-B magnets are of importance because they provide a wide researching space to redesign the textured structures. Here is presented a route to obtain a new die-upset magnet with substantially improved magnetic properties. After experiencing the optimized heat treatment, both the coercivity and remanent magnetization of the Dy-Cu press injected magnets increased substantially in comparison with those of the annealed reference magnets, which is distinct from the reported experimental results on heavy rare-earth diffusion. To study the mechanism, we analyzed the texture evolution in high-temperature annealed die-upset magnets, which had significant impact on the improvement of remanent magnetization. On basis of the results, we find that the new structures are strongly interlinked with the initial structures. With injecting Dy-Cu eutectic alloy, an optimized initial microstructure was achieved in the near-surface diffused regions, which made preparations for the subsequent texture improvement. Besides, the Dy gradient distribution of near-surface regions of the Dy-Cu press injected magnets was also investigated. By controlling the initial microstructure and subsequent diffusion process, a higher performance magnet is expected to be obtained.

Enhanced magnetic refrigeration properties in Mn-rich Ni-Mn-Sn ribbons by optimal annealing.

The influence of annealing time on temperature range of martensitic phase transition (ΔT(A-M)), thermal hysteresis (ΔThys), magnetic hysteresis loss (ΔMhys), magnetic entropy change (ΔS(M)) and relative refrigeration capacity (RC) of the Mn-rich Ni43Mn46Sn11 melt spun ribbons have been systematically studied. By optimal annealing, an extremely large ΔS(M) of 43.2 J.kg(-1)K(-1) and a maximum RC of 221.0 J.kg(-1) could be obtained respectively in a field change of 5 T. Both ΔT(A-M) and ΔThys decreases after annealing, while ΔMhys and ΔS(M) first dramatically increase to a maximum then degenerates as increase of annealing time. A large effective cooling capacity (RC(eff)) of 115.4 J.kg(-1) was achieved in 60 min annealed ribbons, which increased 75% compared with that unannealed ribbons. The evolution of magnetic properties and magnetocaloric effect has been discussed and proved by atomic ordering degree, microstructure and composition analysis.

Growth mechanism and magnetic properties of monodisperse L1(0)-Co(Fe)Pt@C core-shell nanoparticles by one-step solid-phase synthesis.

In this report, we present a novel one-step solid-phase reaction method for the synthesis of L10-CoPt@C core-shell nanoparticles (NPs) using organic metal precursors without surfactants. The obtained CoPt@C NPs have a good face-centered tetragonal single crystal structure and regular shape. The mean size of CoPt is 14 nm with a uniform carbon shell. The evolution of the core-shell structure during the synthesizing process is investigated in detail. Firstly organic metal precursors are decomposed, followed by the formation of grains/clusters in a metal-carbon intermediate state. Then the metal-carbon small grains/clusters agglomerate and recrystallize into single crystal metal alloy NPs covered with a carbon layer. The carbon shell is effective in preventing the coalescence of L10-CoPt NPs during high temperature sintering. The prepared L10-FePt nanoparticles have a high coercivity of up to 12.2 kOe at room temperature. This one-step solid-state synthesizing method could also be employed for the preparation of other types of nanostructures with high crystallinity, monodispersity and chemically ordered phase.

Growth mechanisms and size control of FePt nanoparticles synthesized using Fe(CO)x (x < 5)-oleylamine and platinum(ii) acetylacetonate.

By using Fe(CO)x-OAm (oleylamine, x < 5) as the Fe precursor to slow down the formation rate of FePt nanoparticles (NPs), a time dependence of the NPs' nucleation and growth process was observed by transmission electron microscopy (TEM). The complexing temperature of OAm and Fe(CO)5 at which Fe(CO)x-OAm was formed has a strong influence on the nucleation rate and growth process of the NPs. TEM analyses indicated that the NPs with isotropic shape were single crystalline throughout the synthesis and were formed by a diffusion-controlled Ostwald-ripening (OR) growth mechanism. The nanorod particles were first formed via joining of arbitrarily oriented single crystals and the two crystals formed a uniform particle afterwards, as described by the oriented-attachment (OA) mechanism. The ratio of OAm to Fe(CO)5 used in the preparation of Fe(CO)x-OAm has a significant influence on the growth process, and subsequently the shape, size and size distribution of the FePt NPs. By adjusting the ratio and its complexing temperature, single-crystal FePt NPs with controllable size and isotropic shape were obtained. The insight into the exploration of the specific roles of the reaction conditions and the formation mechanisms provided important information for controlling the morphology of the nanoparticles.