Nanocrystalline Permanent Magnets Research Articles

Tunning Pr to achieve ultra-high magnetic properties of anisotropic nanocrystalline (Sm,Pr)Co5 magnets

In this study, we reported on the bulk anisotropic nanocrystalline Sm1-xPrxCo5 (x = 0.2–0.6) magnets aimed at achieving excellent energy products. Through a combination of advanced processing techniques, including melt spinning, hot pressing, and hot deformation process, bulk magnets with a fine nanocrystalline structure were successfully synthesized. The results revealed that the substitution of Pr strengthened the c-axis texture and the saturation magnetization, facilitating the enhancement of remanence (Mr) and maximum magnetic energy product [(BH)max]. However, Pr substitution decreased the coercivity (Hci) due to the reduced magnetocrystalline anisotropy field. Excellent magnetic properties are obtained for Sm0.4Pr0.6Co5 magnets with the (BH)max of 23.2 MGOe and Hci of 11.8 kOe. The prepared (Sm,Pr)Co5 magnets with different Pr contents have essentially the same microstructure with RECo5 main phase grains of about 400 nm in diameter and fine dispersed rare earth-rich nanoprecipitates. Our findings can provide reliable reference for manufacturing nanocrystalline permanent magnet materials and promote the development of rare earth permanent magnet new materials.

Image-based prediction and optimization of hysteresis properties of nanocrystalline permanent magnets using deep learning

We demonstrate the use of model order reduction and neural networks for estimating the hysteresis properties of nanocrystalline permanent magnets from microstructure. With a data-driven approach, we learn the demagnetization curve from data-sets created by grain growth and micromagnetic simulations. We show that the granular structure of a magnet can be encoded within a low-dimensional latent space. Latent codes are constructed using a variational autoencoder. The mapping of structure code to hysteresis properties is a multi-target regression problem. We apply deep neural network and use parameter sharing, in order to predict anchor points along the demagnetization curves from the magnet’s structure code. We present a proof of concept for microstructure design of nanocrystalline Nd2Fe14B permanent magnets using image-based machine learning. We show how new grain structures can be generated by interpolation between two points in the latent space and demonstrate inverse design. We show how to compute the granular microstructure that will give a demagnetization curve with a given remanence, squareness, and coercivity through optimization in latent space.

Microstructure evolution and coercivity enhancement in Nd-Zn alloy-doped Nd-Fe-B powders

The limitation of Nd-Fe-B permanent magnets is their unavoidable demagnetization at elevated temperatures. In this work, we explore the effect of Nd70Zn30 doping and the subsequent grain boundary diffusion process on the coercivity and thermal stability of nanocrystalline melt-spun Nd13.44Fe81.12B5.44 alloy. The coercivity of 11.26 kOe (∼54.5 %) was achieved with the optimized conditions of 5 wt% Nd70Zn30 doping and subsequent diffusion, while the thermal stability was also improved without significant deterioration of the remanence. A detailed investigation reveals that the enhancement of coercivity and thermal stability of nanograins originates from the suppression of exchange interaction among grains due to grain boundary phase evolution. The current findings exhibit the potential of their use as nanocrystalline permanent magnets to improve the coercivity and thermal stability of Nd-Zn low melting point alloy.

Exploring the Hysteresis Properties of Nanocrystalline Permanent Magnets Using Deep Learning

We demonstrate the use of model order reduction and neural networks for estimating the hysteresis properties of nanocrystalline permanent magnets from microstructure. With a data-driven approach, we learn the demagnetization curve from data-sets created by grain growth and micromagnetic simulations. We show that the granular structure of a magnet can be encoded within a low-dimensional latent space. Latent codes are constructed using a variational autoencoder. The mapping of structure code to hysteresis properties is a multi-target regression problem. We apply deep neural network and use parameter sharing, in order to predict anchor points along the demagnetization curves from the magnet's structure code. The method is applied to study the magnetic properties of nanocrystalline permanent magnets. We show how new grain structures can be generated by interpolation between two points in the latent space and how the magnetic properties of the resulting magnets can be predicted.

Bulk Nanocrystalline Permanent Magnets by Selective Laser Melting

AbstractFe-Nd-B powders were processed by additive manufacturing using laboratory scale selective laser melting to produce bulk nanocrystalline permanent magnets. The manufacturing process was carried out in a specially developed process chamber under Ar atmosphere. This resulted in novel types of microstructures with micrometer scale clusters of nanocrystalline hard magnetic grains. Owing to this microstructure, a maximum coercive field strength (coercivity)μ0Hcof 1.16 T, a remanenceJrof 0.58 T, and a maximum energy product(BH)maxof 62.3 kJ/mm3could, for example, be obtained for the composition Nd16.5-Pr1.5-Zr2.6-Ti2.5-Co2.2-Fe65.9-B8.8.

Phase composition and magnetic properties in nanocrystalline permanent magnets based on misch-metal

The magnetic properties and phase composition of magnets based on misch-metal (MM) with nominal composition of MM13+xFe84−xB6.5 with x=0.5, 1, 1.5, 2 and 2.5 using melt-spinning method were investigated. For x=1.5, it could exhibit best magnetic properties (Hcj=753.02kAm−1, (BH)max=70.77kJm−3). X-ray diffraction and energy dispersive spectroscopy show that the multi hard magnetic phase of RE2Fe14B (RE=La, Ce, Pr, Nd) existed in the magnets. The domain wall pinning effect and the exchange coupling interaction between grains are dependent on the abnormal RE-rich phase composition. Optimizing the phase constitution is necessary to improve magnetic properties in MM-Fe-B magnets for utilizing the rare earth resource in a balanced manner.

Micromagnetic simulation for the magnetization reversal process of Nd-Fe-B hot-deformed nanocrystalline permanent magnets

We numerically demonstrated the magnetization reversal process inside a hot-deformed nanocrystalline permanent magnet. We performed large-scale micromagnetics simulation based on the Landau–Lifshitz–Gilbert equation with 0.1 billion calculation cells. The simulation model for the hot-deformed nanocrystalline permanent magnet consists of 2622 tabular grains that interact with each other by inter-grain exchange and dipole interactions. When the strength of the external field approached a coercive force, nucleation cores were created at the grain surface. The magnetization reversal was propagated by the inter-grain and dipole interactions. When the grains had overlapping regions parallel to the external field, the magnetization reversal propagated quickly between the grains due to the dipole interaction. In contrast, the motion of the magnetic domain wall was inhibited at interfaces between the grains perpendicular to the external field. Reversal magnetic domains had a pillar-shaped structure that is parallel to the external field. In the perpendicular direction, the reversal magnetic domain expanded gradually because of the inhibition of the domain wall motion.

Magnetization reversal processes of isotropic permanent magnets with various inter-grain exchange interactions

We performed a large-scale micromagnetics simulation on a supercomputing system to investigate the properties of isotropic nanocrystalline permanent magnets consisting of cubic grains. In the simulation, we solved the Landau–Lifshitz–Gilbert equation under a periodic boundary condition for accurate calculation of the magnetization dynamics inside the nanocrystalline isotropic magnet. We reduced the inter-grain exchange interaction perpendicular and parallel to the external field independently. Propagation of the magnetization reversal process is inhibited by reducing the inter-grain exchange interaction perpendicular to the external field, and the coercivity is enhanced by this restraint. In contrast, when we reduce the inter-grain exchange interaction parallel to the external field, the coercivity decreases because the magnetization reversal process propagates owing to dipole interaction. These behaviors show that the coercivity of an isotropic permanent magnet depends on the direction of the inter-grain exchange interaction.

Crystallographic alignment and magnetic anisotropy inducement from as-deformed disproportionated Nd(Fe,Co)B alloy with different deformation ratios

The process of cold deformation and desorption-recombination vacuum annealing was proposed to produce anisotropic nanocrystalline Nd(Fe,Co)B magnet by using as-milled disproportionated nano-structured alloy powders as the precursor material. The effect of deformation ratio on crystal alignment and magnetic anisotropy was experimentally studied using X-ray diffraction and transmission electron microscopy, and the underlying mechanisms were clarified by referring to correlation of critical driving energy with nucleation activation energy for the reaction. Cold deformation plays an essential role in the formation of anisotropic magnet via the induction of desired texture by vacuum annealing treatment, composed of oriented Nd2(Fe,Co)14B nano-crystals with well defined c-axis alignment, maintaining a coherency relationship with as-deformed disproportionated α-(Fe,Co) precursor described by [110]α-(Fe,Co)∥[001] Nd2(Fe,Co)14B. The formation of [00l] Nd2(Fe,Co)14B orientation stems from preferential nucleation and growth of Nd2(Fe,Co)14B crystals induced by (110) texture of α-(Fe,Co) phase during desorption-recombination process. Increasing the deformation ratio results in the improvement of remanence and energy product, but moderate loss in coercivity, and the former reason can be ascribed to stronger crystallographic texture along [00l] direction (c-axis alignment), while the latter is related to the absence of intergranular phase, or stray field and weaker exchange coupling caused by grain coarsening, which was further elaborated by magnetization behavior study. The optimum magnetic properties of (BH)max: 140.91 kJ/m3, Hci: 650 kA/m and Br: 0.94 T were achieved by vacuum annealing as-deformed samples with deformation ratio of 70% at 780 °C for 30 min, due to pronounced anisotropy and a fully recombined microstructure with uniform grains of about 45 nm in average size. The evident anisotropy and enhanced remanence of nano-grained magnet prove advantageous for the fabrication of textured nanocrystalline permanent magnet.

Magnetic Force Microscopy Study of Zr2Co11‐Based Nanocrystalline Materials: Effect of Mo Addition

The addition of Molybdenum was used to modify the nanostructure and enhance coercivity of rare‐earth‐free Zr2Co11‐based nanocrystalline permanent magnets. The effect of Mo addition on magnetic domain structures of melt spun nanocrystalline Zr16Co84-xMox (x = 0, 0.5, 1, 1.5, and 2.0) ribbons has been investigated. It was found that magnetic properties and local domain structures are strongly influenced by Mo doping. The coercivity of the samples increases with the increase in Mo content (x ≤ 1.5). The maximum energy product (BH) max increases with increasing x from 0.5 MGOe for x = 0 to a maximum value of 4.2 MGOe for x = 1.5. The smallest domain size with a relatively short magnetic correlation length of 128 nm and largest root‐mean‐square phase shift Φrms value of 0.66° are observed for the x = 1.5. The optimal Mo addition promotes magnetic domain structure refinement and thus leads to a significant increase in coercivity and energy product in this sample.

Hf Doping Effect on Hard Magnetism of Nanocrystalline Zr$_{18\hbox{-}{\rm x}}$Hf$_{\rm x}$Co$_{82}$ Ribbons

The effects of substituting Zr by Hf on the structural and the magnetic properties of the nanocrystalline rapidly solidified Zr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">18-x</sub> Hf <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">82</sub> ribbons (x = 0, 2, 4, and 6) have been studied. X-ray diffraction and thermomagnetic measurement results indicated that upon rapid solidification processing four magnetic phases occur: rhombohedral Zr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> , orthorhombic Zr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> , hcp Co, and cubic Zr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">23</sub> phases. Microstructure analysis results showed the reduction in the percentage of the soft-magnetic phase (Co) compared to the hard-magnetic phase (Zr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> (rhombohedral)) with the increase in the Hf concentration. All the samples under investigation have ferromagnetic nature, at 4.2 K and at room temperature. The coercive force (H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) and the saturation magnetization are (M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> ) found to linearly increases with x (x ≤ 2), then H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> slightly increases and M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> slightly decreases with increasing x. The maximum energy product (BH) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> at room temperature is found to increases with increasing x reaching a maximum value for x = 4. The magnetocrystalline anisotropy parameter of these samples are calculated to be K = 1.1 MJ/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> and independent of Hf concentration. The above results indicate that the replacement of Zr by Hf improves the hard-magnetic properties of this class of rear-earth-free nanocrystalline permanent magnet materials.

Directional annealing studies on rapidly solidified Sm–Co–Nb–C alloys

In this paper, a process that develops texture in nanocrystalline permanent magnet alloys is presented. An originally isotropic material is passed through a high up-temperature gradient, inducing directional grain growth. Texture development by directional annealing of melt-spun Sm12Co88, (Sm12Co88)99Nb1, (Sm12Co88)99C1, and (Sm12Co88)98Nb1C1 alloys was examined. Samples directionally annealed were compared with conventionally annealed samples. Strong (006) in-plane texture was observed by X-ray diffraction in Sm12Co88 and (Sm12Co88)99Nb1 alloys and the anisotropy was corroborated by magnetic measurements (magnetic texture ∼20–53%). Directional annealing produced only slight texture in the (Sm12Co88)99C1 and (Sm12Co88)98Nb1C1 alloys. The development of texture is critically dependent on annealing temperature, the up-temperature gradient, translational velocity, and alloy composition. The activation energy for anisotropic grain growth was estimated to be ∼28 and ∼42kJmol−1 for Sm12Co88 and (Sm12Co88)99Nb1, respectively. These results indicate that directional annealing as a route to texture development in nanocrystalline permanent magnet alloys is a feasible process.

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 SmCo6.8Zr0.2 nanocrystalline permanent magnets and SmCo6.8Zr0.2/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 SmCo6.8Zr0.2 nanocrystalline permanent magnets is composed of SmCo7 phase and SmCo6.8Zr0.2/α-(Fe,Co) nanocomposite permanent magnets is composed of SmCo7 and α-(Fe,Co) phases. The mechanism of magnetization reversal is mainly controlled by inhomogeneous domain wall pinning in SmCo6.8Zr0.2 and SmCo6.8Zr0.2/α-(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/α-(Fe,Co) magnets increases greatly by the decrease of the measured temperature. According to Δmirr–H/Hcj, Δmrev–H/Hcj and χirr–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/α-(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/α-(Fe,Co) magnets at different temperatures are also studied.

The physical origin of open recoil loops in nanocrystalline permanent magnets

The numerical simulation of the open recoil loops has been carried out using micromagnetic finite element method. By giving an example for this issue, the magnetization behaviors during the recoil processes of nanocomposite Pr2Fe14B∕α-Fe magnets have been analyzed. It is the strong intergrain exchange coupling that results in the magnetization reversal in some hard grains during the recoil processes, which leads to the large opening of recoil loops. The magnetization reversal of α-Fe grain follows that of its neighboring hard grains. Consequently, the opening of recoil loops is enhanced due to the presence of α-Fe grains.

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 Jnr 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.

Influence of dynamic crystallization on exchange-coupled NdFeB nanocrystalline permanent magnets

Dynamic crystallization was introduced to improve the magnetic properties of NdFeB nanocrystalline permanent magnets by optimizing microstructure. The microstructure was studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). It has been determined that, compared with the conventional heat treatment, dynamic crystallization can shorten the crystallization time. Moreover, dynamic crystallization can refine grains, enhance the exchange-coupled interaction among grains, and promote the magnetic properties. As a result, the optimal magnetic properties of Nd 10.5(FeCoZr) 83.4B 6.1 ( B r = 0.685T, H ci = 732 kA·m −1, H cb = 429 kA·m −1, (BH) m = 75 kJ·m −3) are obtained after dynamic crystallization heat treatment at 700 °C for 10 min.

Micromagnetic Analysis on Demagnetization Process of Single-Phase Nanocrystalline Permanent Magnets with Different Degrees of Orientation

A three-dimentional finite element micromagnetic algorithm was developed to study the magnetization reversal of Pr 2Fe 14B single-phase nanocrystalline permanent magnets. A single-phase nanocrystalline Pr 2Fe 14B magnets composed of 216 irregular shaped grains was built. The magnetic hysteresis loops were simulated by micromagnetic finite element method. The contribution of intergrain exchange coupling (IGEC) to remanence enhancement is considered related to the alignment degree in oriented magnets, and decreased with improved grain alignment. For the magnets with perfectly crystallo-graphic alignment of grains, the contribution of IGEC to remanence enhancement is nearly zero. The shape of demagnetization curve is not only dependent on grain alignment degree but also on the strength of IGEC in magnets.

Modern nanocrystalline/nanostructured hard magnetic materials

Abstract Nanocrystalline permanent magnets (pms) are based on the nucleation mechanism in the case of RE2Fe14B-based pms, whereas in the high-temperature Sm2(Co,Cu,Fe,Zr)17-type magnets repulsive and attractive pinning as well as nucleation mechanisms become effective with increasing temperature.

Investigation of intergrain exchange coupling interaction of nanocrystalline permanent magnets by numerical simulation

The initial curves, demagnetization curves and recoil curves of Pr2Fe14B and Pr2Fe14B-α Fe nanocrystalline permanent magnets have been calculated by micromagnetic finite_element method. The validity of δm(H)curves, which can be used to determine the strength of intergrain exchange coupling interaction (IGEC), is approved by the calculated results. It is found that IGEC weakens with increasing grain size D for single-phase nanocrystalline and nanocomposite permanent magnets. Thus, the demagnetization curve shows a two-phase behavior and the δm(H) curve shows two peaks for the nanocomposite sample with D=20nm.The peak at small field can be attributed to the exchange interaction between the magnetic hard and soft grains, while the peak at large field may be due to the IGEC among hard grains.

Investigation on the coercivity and remanence of single-phase nanocrystalline permanent magnets by micromagnetic finite-element method

The demagnetization curves for nanocrystalline permanent magnets are calculated using micromagnetic finite-element method. The calculated results are compared with the experimental results on PrFeB ribbons. The dependence of coercivity on temperature has been discussed with the change of anisotropy and exchange coupling. The revision of coercivity form is discussed for the grain size in nanometer scale. Usually, remanence enhancement is mainly determined by the competition between anisotropy and exchange coupling. When dipolar interaction cannot be neglected in comparison with exchange coupling, the decrease of remanence is found. The dependence of hard magnetic properties on grain boundary character is analyzed, and the way to improve energy product is proposed.