Magnetic Microstructure Research Articles

Effects of Thermal Demagnetization in Air on the Microstructure and Organic Contamination of NdFeB Magnets

Demagnetization is an essential step for the demounting and safe handling of end-of-life NdFeB. Thermal demagnetization in air is a straightforward option to demount adhesive-fixed or segmented magnets. However, this process is suspected to increase the uptake of contaminants like O, C and Zn from coatings and adhesives, potentially degrading the recyclate quality. This study tests the effects of thermal demagnetization in air at 400 °C for 15 to 240 min on variously coated samples with different initial oxidation levels. Furthermore, the possible reversal of the contaminant uptake is explored. Samples with low previous oxidation levels showed significant uptake in oxygen with a minimal diffusion depth, while the uptake depended on the used coating. The best protectiveness was achieved with NiCuNi with an increase in oxygen of only around 30%. Epoxy (up to ~130% O uptake) and Zn coatings (up to ~80% O uptake) disintegrated during the treatment and offered less protection but still made a difference compared to uncoated samples (up to ~220% O uptake). Samples with high initial oxidation levels show no clear tendency towards further oxygen uptake and the carbon uptake is generally low, likely due to contemporary epoxy coatings featuring a passivation underneath as a barrier layer. Zn infiltration, which carried organic debris, was observed. Short demagnetization times proved to be favorable for limiting the depth of the diffusing contaminants. Mechanical coating removal after thermal demagnetization in air can mitigate the contaminant uptake, producing clean, recyclable end-of-life material.

Finite element based micromagnetic simulations of heterogeneous magnetic microstructures

AbstractMagnetic materials play a crucial role in our everyday lives. As permanent magnets, they are core components in electric motors such as generators for wind turbines, or, as magnetocaloric magnets, they have the potential to change the cooling technology of tomorrow. All of this makes these materials key contributors to “green” energy conversion technology. Accordingly, pushing the development of these materials with a special focus on the performance, efficiency, and harmlessness of the material components contained for humans and the environment is of crucial importance. This work deals with the finite element‐based simulations within the framework of the micromagnetic theory to numerically support the quest of characterizing novel magnetic materials. The main focus remains on the numerical modeling of permanent magnetic polycrystalline materials and the influence of grain boundaries on the effective magnetization. For this purpose, large‐scale simulations of permanent magnetic neodymium‐iron‐boron () samples are carried out and the influence of different mesh sizes on the effective magnetization behavior is investigated.

Multiferroic Microstructure Created from Invariant Line Constraint

AbstractFerroic materials enable a multitude of emerging applications, and optimum functional properties are achieved when ferromagnetic and ferroelectric properties are coupled to a first‐order ferroelastic transition. In bulk materials, this first‐order transition involves an invariant habit plane, connecting coexisting phases: austenite and martensite. Theory predicts that this plane should converge to a line in thin films, but experimental evidence is missing. Here, the martensitic and magnetic microstructure of a freestanding epitaxial magnetic shape memory film is analyzed. It is shown that the martensite microstructure is determined by an invariant line constraint using lattice parameters of both phases as the only input. This line constraint explains most of the observable features, which differ fundamentally from bulk and constrained films. Furthermore, this finite‐size effect creates a remarkable checkerboard magnetic domain pattern through multiferroic coupling. The findings highlight the decisive role of finite‐size effects in multiferroics.

Microstructure, phase compositions, and coercivity evolution in micron-thick SmCo-based permanent magnetic films

Permanent magnetic thick films are increasingly used in Micro-Electro-Mechanical Systems (MEMS) devices, but the interplay between magnetic properties, microstructure, and film thickness remains underexplored. We deposited SmCo-based films with thicknesses ranging from 350 nm to 1300 nm using magnetron sputtering at room temperature, followed by in-situ annealing. Our study examined the evolution of microstructure, phase composition, magnetic properties, and domain structures as thickness increases. The films displayed dense, fibrous structures typical of the zone T-type region, with 3D morphologies forming a network structure and an arc-shaped surface that expands with thickness. At thinner levels (around 350 nm), an amorphous SmCo phase predominates, prone to oxidation, while at greater thicknesses, the SmCo5 phase dominates but maintains a grain size of 12–15 nm. Coercivity decreases significantly from 33 kOe to 8 kOe as thickness increases from 630 to 1300 nm due to the diminishing pinning effect and dominance of the reversal domain nucleation mechanism. Analysis using magnetic force microscopy and micromagnetic simulations indicates that this reduction in coercivity is mainly due to the decomposition of the SmCo5 phase and a reduction in magnetic domain size.

Imaging magnetic transition of magnetite to megabar pressures using quantum sensors in diamond anvil cell

High-pressure diamond anvil cells have been widely used to create novel states of matter. Nevertheless, the lack of universal in-situ magnetic measurement techniques at megabar pressures makes it difficult to understand the underlying physics of materials’ behavior at extreme conditions, such as high-temperature superconductivity of hydrides and the formation or destruction of the local magnetic moments in magnetic systems. Here, we break through the limitations of pressure on quantum sensors by modulating the uniaxial stress along the nitrogen-vacancy axis and develop the in-situ magnetic detection technique at megabar pressures with high sensitivity (~1μT/Hz\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim 1{{{\\rm{\\mu }}}}{{{\\rm{T}}}}/\\sqrt{{{{\\rm{Hz}}}}}$$\\end{document}) and sub-microscale spatial resolution. By directly imaging the magnetic field and the evolution of magnetic domains, we observe the macroscopic magnetic transition of Fe3O4 in the megabar pressure range from ferrimagnetic (α-Fe3O4) to weak ferromagnetic (β-Fe3O4) and finally to paramagnetic (γ-Fe3O4). The scenarios for magnetic changes in Fe3O4 characterized here shed light on the direct magnetic microstructure observation in bulk materials at high pressure and contribute to understanding magnetism evolution in the presence of numerous complex factors such as spin crossover, altered magnetic interactions and structural phase transitions.

Comprehensive quantifying of the Fe-Ti-B film magnetic microstructure

Investigation and quantifying of the parameters of the phase and structure state, static magnetic properties, magnetic microstructure formed over the entire volume of the film and in the near-surface layer, the surface roughness of Fe72.4Ti5.4B19.2O3.0 film were carried out using the combination of the methods such as x-ray diffraction, magnetic force microscopy, atomic force microscopy, vibrating-sample magnetometry, and the correlation magnetometry. The Fe72.4Ti5.4B19.2O3.0 films on glass substrates were produced by magnetron deposition followed by the vacuum annealing at 200 °С for 1 h. The interrelation between the investigated parameters was highlighted.

Electromagnetic modeling and analysis based on multilayer domain structure for GMI behavior in high frequency

Existing theoretical models on the frequency dependence of the magnetoimpedance (MI) in ferromagnetic microwires primarily describe the MI phenomenon at the limiting cases of lower MHz (<several hundred MHz) or higher GHz (>several GHz) ranges. However, in the intermediate region between these two ranges, known as the transition region, MI curves undergo complex transformations. These transformations have been documented in the literature, but their underlying causes remain poorly understood. Unambiguous knowledge of the domain structure and its correlation with MI properties is essential for elucidating this behavior. In this study, we have, for the first time, observed the inner core magnetic structure of Co-based microwires and revealed its relationship with the high-frequency MI effect. A distinct magnetic structure comprising longitudinal domains in the inner core (IC), circular domains in the outer shell (OS), and a transition region (TR) has been identified. This structure originates from compositional gradients and residual stresses during microwire fabrication. The IC/TR/OS structure manifests in the complex transformations of the MI behavior, exhibiting a turning point at GHz frequencies before the characteristic double MI peak. We developed a multilayer planar model that considers this more realistic magnetic structure, including the TR layer. This model successfully reproduces the key features of the MI curves and provides deeper insights into the high-frequency MI phenomenon. Our findings pave the way for optimizing the sensing capabilities of Co-based ferromagnetic microwires and demonstrate the potential of using high-frequency MI measurements to map their magnetic microstructures.

Cross-sectional surface analysis of magnetic domains, microstructures, and magnetic properties of the low-temperature phase MnBi prepared by low-temperature vacuum sintering

In this work, the low-temperature phase MnBi prepared by a low-temperature vacuum sintering process at 325 °C was studied. We found a significant increase in the energy product from 2.63 MGOe in the 12-h sintered sample to 3.64 MGOe in the 48-h sintered sample. This improvement is attributed to the solid-liquid diffusion process. Cross-sectional scanning electron microscopy (SEM) reveals that MnBi forms at the external surface of Mn particles and along interior surfaces, notably within cracks. Transmission electron microscopy further demonstrates that the Mn ratio increases and Bi decreases with distance from the crack. The selected area diffraction showed variations in the Mn ratio with distance from cracks and identified both Bi and MnBi phases in the MnBi layer. Magnetic force microscopy (MFM) analysis exhibited large phase shifts indicating repulsive or attractive forces in single ferromagnetic domains. This provides valuable insight into magnetic domains in the MnBi regions near Mn cracks. The MnBi formation model, developed for the vicinity of single cracks with uniform MnBi content, partly explains the magnetic interactions and phase shifts observed near these cracks. These findings provide significant insights into the MnBi microstructural and magnetic properties, potentially useful in tailoring and engineering magnetic structures.

Effect of 10 MeV electron irradiation on structural and magnetic properties of Ti- and Al- substituted strontium hexaferrite SrFe11.3Ti0.4Al0.3O19

The irradiation stability of Ti- and Al-substituted SrFe11.3Ti0.4Al0.3O19 strontium hexaferrite to 10 MeV electron irradiation with the fluences of 1014 and 2·1017 cm−2 was investigated using various characterization techniques such as scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectrum microanalysis, atomic force microscopy, Raman spectroscopy, vibrating sample magnetometry and magnetic force microscopy. Notable changes in lattice vibrations, magnetic domain microstructure and magnetization behavior were obtained after the electron irradiation with the fluence of 2·1017 cm−2. Phonon hardening observed by Raman spectroscopy confirms the increasing disorder in crystalline structure and the presence of spin-phonon coupling in the samples after electron irradiation with the fluence 2·1017 cm−2. It was found that the strongest spin-phonon interactions correspond to A1g vibration modes at trigonal bipyramidal 2b, 2a octahedral, 4f1 tetrahedral sites and E1g mode at 4f2 octahedral site in electron-irradiated SrFe11.3Ti0.4Al0.3O19 hexaferrite, while the most radiation-resistant phonon mode is A1g vibration of the Fe–O bonds at the 4f2 octahedral site. No significant change was observed in magnetic saturation magnitude after electron irradiation. The increase in coercivity and significant decrease of magnitude of the dM/dH derivative of magnetization curves with increasing radiation fluence is attributed to the increase in uniaxial magnetic-crystalline anisotropy. It is found that the fractal dimension of hierarchical magnetic domain structure in SrFe11.3Ti0.4Al0.3O19 continuously decreases with increasing fluence of 10 MeV electron irradiation, which can be caused by the distortion in uniaxial magnetocrystalline anisotropy and irradiation-induced defects.

Ultrafast manipulations of nanoscale skyrmioniums

The advancement of next-generation magnetic devices depends on fast manipulating magnetic microstructures at the nanoscale. A universal method is presented for rapid and reliable generating, controlling, and driving nano-scale skyrmioniums, through high-throughput micromagnetic simulations. Ultrafast switches are realized between skyrmionium and skyrmion states and rapidly change their polarities in monolayer magnetic nanodisks by perpendicular magnetic fields. The transition mechanism by alternating magnetic fields differs from that under steady magnetic fields. New skyrmionic textures, such as flower-like and windmill-like skyrmions, are discovered. Moreover, this nanoscale skyrmionium can move rapidly and stably in nanoribbons using weaker spin-polarized currents. Explicit discussions are held regarding the physical mechanisms involved in ultrafast manipulations of skyrmioniums. This work provides further physical insights into the manipulation and application of topological skyrmionic structures for developing low-power consumption and nanostorage devices.

Laser directed energy deposition of Alnico-8H from blended elemental powders: Effect of nickel increase on magnetic properties

Alnico-8H (14 wt% Ni) and Alnico-8H (19 wt% Ni) were additively fabricated from a blend of elemental powders with compositions (in wt.%) of 38Co-29Fe-14Ni-8Al-8Ti-3Cu and 33Co-29Fe-19Ni-8Al-8Ti-3Cu, respectively, using laser-directed energy deposition technique. Post additive fabrication both compositions were subjected to homogenization heat treatment at 1250 °C for 30 min and magnetic annealed at 650 °C under 2.5 T magnetic field for 60 mins (1 hr). X-ray diffraction was employed for studying the phase evolution. Scanning and transmission electron microscopes were utilized for characterization of Alnico alloys. The targeted compositions for Alnico-8H (14 wt% Ni) and Alnico-8H (19 wt% Ni) were nearly achieved in the L-DED processed samples. homogenization at 1250 °C, followed by magnetic annealing at 650 °C, resulted in an increase in grain size. The transmission electron microscopy revealed the presence of ferromagnetic α1 and paramagnetic α2 phases in the magnetic annealed microstructure. The increase in Nickel content from 14 to 19 wt% resulted in increase in coercivity (242 to 255 Oe), remanence (4437 to 8659 G), and maximum energy product (BHmax) (0.27 to 0.53 MGOe).

Magnetic properties and microstructural of ion-implanted Gadolia doped Ceria-Barium monoferrite thin film deposited on Kapton for electromagnetic wave absorber application

Gadolia-doped Ceria-Barium Monoferrite (GDC-BaFe2O4) thin films have been deposited on Kapton sheets using the DC sputtering method as electromagnetic (EM) wave-absorbing materials. The present study has examined the structural, magnetic, and microwave absorption properties of GDC-BaFe2O4. The primary approaches used in this investigation included changing the surface form and applying EM wave-absorbing materials as surface coatings. X-ray diffraction research showed that the GDC-BaFe2O4 thin films have an orthorhombic crystal structure. The bonding investigation revealed the existence of Fe-O, Ce-O, and Ba-O-Ba functional groups at 426.87, 517.42 and 1014.84 cm−1, respectively. Vibrating-sample magnetometry confirmed that saturation magnetization decreased with increasing sputtering time and revealed the ferromagnetic behavior of the GDC-BaFe2O4 thin film. Ce, Gd, Ba, Fe, and O were evenly and similarly distributed over the Kapton surface of the GDC-BaFe2O4 thin films, according to energy-dispersive X-ray spectroscopy and scanning electron microscopy. Based on the atomic force microscopy test, the root-mean-square roughness (Rq) increased during the ion implantation procedure. With a 7.5-minute sputtering period, the microwave absorption ability reached a maximum reflection loss value of −39.6 dB (99.98 %) at 10 GHz. As a result of these findings, GDC-BaFe2O4 appears to be a promising candidate for EM wave absorption.

Magnetic microstructure of nanocrystalline Fe-Nb-B alloys as seen by small-angle neutron and x-ray scattering

Magnetic microstructure of nanocrystalline Fe-Nb-B alloys as seen by small-angle neutron and x-ray scattering

Effects of magnetic field heat treatment on magnetic properties and microstructure of Sm2Co17-type sintered magnets

Sm2Co17-type sintered magnets have attracted widespread attention for their excellent magnetic properties and thermal stability. The magnetic properties largely depend on the cellular structure and Cu, Fe distributions within these magnets. This paper systematically studies the effect of magnetic field heat treatment (MFHT) on the magnetic properties and microstructure of Sm2Co17-type sintered magnets. MFHT improves the room-temperature magnetic properties and remanence temperature stability of magnets but slightly deteriorates the temperature coefficient of intrinsic coercivity. A microstructure analysis shows that MFHT improves the cell boundary phase content and optimizes the cellular structure and Cu, Fe distributions of magnets. A high-density short lamellar phase, possibly induced by MFHT, provides easy diffusion paths for Cu and Fe, thereby improving the magnetic properties of magnets. Compared with conventional aging treatment, this paper proposes a promising method to improve the magnetic properties of Sm2Co17-type sintered magnets under the same heat treatment time and temperature.

Phase structure evolution and its effect on magnetic and mechanical properties of B-doped Sm2Co17-type magnets with high Fe content

Abstract The unique cellular microstructure of Fe-rich Sm2Co17-type permanent magnets is closely associated with the structure of the solid solution precursor. We investigate the phase structure, magnetic properties, and mechanical behavior of B-doped Sm2Co17-type magnets with high Fe content. The doped B atoms can diffuse into the interstitial vacancy, resulting in lattice expansion and promote the homogenization of the phase organizational structure during the solid solution treatment in theory. However, the resulting second phase plays a dominant role to result in more microtwin structures and highly ordered 2 : 17R phases in the solid solution stage, which inhibits the ordering transformation of 1 : 7H phase during aging and affects the generation of the cellular structure, and to result in a decrease in magnetic properties, yet the interface formed between it and the matrix phase hinders the movement of dislocations and enhances the mechanical properties. Hence, the precipitation of high flexural strain grain boundary phase induced by B element doping is also a new and effective way to improve the flexural strain of Sm2Co17-type magnets. Our study provides a new understanding of the phase structure evolution and its effect on the magnetic and mechanical properties of Sm2Co17-type magnets with high Fe content.

Effect of antioxidants on the efficiency of jet milling and the powder characteristics of Sm2Co17 permanent magnets

Abstract This study investigated the effect of antioxidants on the grinding efficiency, magnetic powder characteristics, microstructure, and magnetic properties of 2:17 type SmCo permanent magnet materials. The results show that adding antioxidants helps improve the dispersion among magnetic powders, leading to a 33.3% decrease in jet milling time and a 15.8% increase in magnet powder production yield. Additionally, adding antioxidants enhances the oxidation resistance of the magnetic powders. After being stored in a constant temperature air environment at 25 °C for 48 h, the O content in the powder decreased by 33% compared to samples without antioxidants. While in the magnet body, the O content decreased from 0.21 wt.% to 0.14 wt.%, which helps increase the effective Sm content and domain wall pinning uniformity in the magnet. Excellent magnetic properties were obtained in the magnet with added antioxidants: B r = 11.6 kGs, SF = 79.6%, H cj = 16.8 kOe, and (BH)max = 32.5 MGOe.

Effect of Pressure on Ce-Substituted Nd-Fe-B Hot-Deformed Magnets in the Hot-Pressing Process.

With the increasing demand for Nd-Fe-B magnets across various applications, the cost-effective substitution of Ce has garnered significant interest. Many studies have been conducted to achieve the high magnetic properties of Nd-Ce-Fe-B hot deformation magnets in which Nd is replaced with Ce. We propose a method to improve magnetic properties of the Ce-substituted Nd-Ce-Fe-B hot-deformed magnets by optimizing the hot-pressing process. This study investigates the microstructure and properties following hot deformation of Ce-substituted Nd-Ce-Fe-B magnets fabricated at a constant temperature and different pressures (100-300 MPa) during the hot-pressing process. The results highlight the influence of pressure from previous hot-pressing processes on grain alignment and microstructure during hot deformation. Magnets subjected to hot pressing at 200 MPa followed by hot deformation achieved superior magnetic properties, with Hci = 8.9 kOe, Br = 12.2 kG, and (BH)max = 31 MGOe with 40% of Nd replaced with Ce. Conversely, precursors prepared at 100 MPa exhibited low density due to high porosity, resulting in poor microstructure and magnetic properties after hot deformation. In magnets using precursors prepared at 300 MPa, coarsened grains and a condensed h-RE2O3 phase were observed. Incorporating Ce into the magnets led to insufficient formation of RE-rich phases due to the emergence of REFe2 secondary phases, disrupting grain alignment and hindering the homogeneous distribution of the RE-rich phase essential for texture formation. Precursors prepared under suitable pressure exhibited uniform distribution of the RE-rich phase, enhancing grain alignment along the c-axis and improving magnetic properties, particularly remanence. In conclusion, our findings present a strategy for achieving the ideal microstructure and magnetic properties of hot-deformed magnets with high Ce contents.

Analysis of mechanism for effectively coercivity increment in (Nd, Ce)-Fe-B diffusion magnets

In this work, we explore how Ce in (Nd, Ce)-Fe-B magnet optimizes the microstructure of the base magnet, and enhances Grain Boundary Diffusion (GBD) of Dy. As Ce/total rare earth (TRE) content increases from 0 wt% to 16.95 wt%，the coercivity of the base magnets decreased by 24.93 %. However, lower-melting-point Ce element tended to distribute in the grain boundaries (GBs), and the thickness of the GBs layer increased from 2.46 nm to 5.54 nm. This provided more pathways for the subsequent GBD. Importantly, the diffusion depth of Dy element increased, and more core-shell structure formed as Ce increased. It led to a higher coercivity increment. Compared to GBD magnets without Ce, the coercivity increment of GBD magnets with Ce/TRE = 16.95 wt% increased by 39.54 %.

Insight into the effect of Ce content on the magnetic properties of Ce-magnets with grain boundary diffusion Pr-Al alloys

The development of cost-effective Ce-magnets with high Ce content is urgent to realize the vision of cost reduction and performance enhancement. Dual-main-phase Ce-magnets with different Ce content (Ce accounted for the total amount of rare earths is 40%, 50% and 60%) are prepared. The Pr80Al20 (at%) alloy is selected as diffusion source for the three magnets using the grain boundary diffusion technique under the same conditions. The correlation of magnetic properties and microstructures before and after diffusion with different Ce contents are systematically analyzed. According to the EPMA results, the presence of Ce in the magnets facilitates the diffusion of Pr elements along the grain boundaries, which significantly increases the diffusion depth of Pr inside the magnets. As a consequence, the percentage increment of coercivity increase with the Ce content due to the increased diffusion depth. This study provides new insight into the preparation of dual-main-phase Ce-magnets with high Ce content of 40–60%.

Preparation of (Nd, Ce)-Fe-B Regenerated Magnets by In-Situ Restoration of Grain Boundary Structure Using Nascent Nd-Fe-B Powder.

Rare earth resource recycling is an important endeavor for environmental protection and resource utilization. This study explores the method of preparing regenerated magnets using waste magnets as raw materials based on existing processes. By utilizing existing Nd-Fe-B production equipment, various waste magnets are transformed into recycled powder. Next, nascent Nd-Fe-B powders with slightly higher rare earth content are selected as the repairing agent. The regenerated magnets are prepared by incorporating the nascent powder into the recycled powder. The focus lies in investigating the repairing effect of the nascent powder repairing agent on the microstructure of regenerated magnets and exploring the influence of sintering temperature and powder addition on the magnetic properties and microstructure of the regenerated magnets. The results showed that the nascent powder increased the proportion of grain boundary phases and effectively repaired the grain boundary structure of the regenerated magnets. In addition, the Pr element in the nascent powder replaces the Ce element in the recycled powder, which ultimately improves the magnetic properties of the regenerated magnet in a comprehensive manner. This study provides valuable insights and guidance for rare earth resource recycling and the preparation of regenerated magnets.