Soft X-ray Magnetic Circular Dichroism Research Articles

Tuning In-Plane Magnetic Anisotropy and Interfacial Exchange Coupling in Epitaxial La2/3Sr1/3CoO3/La2/3Sr1/3MnO3 Heterostructures.

Controlling the in-plane magnetocrystalline anisotropy and interfacial exchange coupling between ferromagnetic (FM) layers plays a key role in next-generation spintronic and magnetic memory devices. In this work, we explored the effect of tuning the magnetocrystalline anisotropy of La2/3Sr1/3CoO3 (LSCO) and La2/3Sr1/3MnO3 (LSMO) layers and the corresponding effect on interfacial exchange coupling by adjusting the thickness of the LSCO layer (tLSCO). The epitaxial LSCO/LSMO bilayers were grown on (110)o-oriented NdGaO3 (NGO) substrates with a fixed LSMO (top layer) thickness of 6 nm and LSCO (bottom layer) thicknesses varying from 1 to 10 nm. Despite the small difference (∼0.2%) in lattice mismatch between the two in-plane directions, [001]o and [11̅0]o, a pronounced in-plane magnetic anisotropy was observed. Soft X-ray magnetic circular dichroism hysteresis loops revealed that for tLSCO ≤ 4 nm, the easy axes for both LSCO and LSMO layers were along the [001]o direction, and the LSCO layer was characterized by magnetically active Co2+ ions that strongly coupled to the LSMO layer. No exchange bias effect was observed in the hysteresis loops. In contrast, along the [11̅0]o direction, the LSCO and LSMO layers displayed a small difference in their coercivity values, and a small exchange bias shift was observed. As tLSCO increased above 4 nm, the easy axis for the LSCO layer remained along the [100]o direction, but it gradually rotated to the [11̅0]o direction for the LSMO layer, resulting in a large negative exchange bias shift. Therefore, we provide a way to control the magnetocrystalline anisotropy and exchange bias by tuning the interfacial exchange coupling between the two FM layers.

Texture, electronic and magnetic states of Mn atoms in polycrystalline MnSi thin films facing a c-sapphire substrate studied by soft X-ray magnetic circular dichroism

Polycrystalline MnSi is a highly efficient magnetic skyrmionic material and information carriers for future spintronic devices, deposited on a c-cut sapphire substrate by magnetron sputtering with a low base pressure. Due to their fascinating crystal structures and spin textures these Mn-based silicides are most extensively studied now a days. In order to study the appearance of any impure phases like MnxSiy, hysteresis behavior and the inception of ferromagnetic/superparamagnetic domains present in the MnSi film, we have performed X-ray diffraction (XRD), XRD pole figures in stereographic projection, vibrating sample magnetometry and powerful element specific X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism techniques. The room temperature hysteresis curve points towards the phenomenon of long-range ferromagnetic ordering. The Mn L2,3 edge XAS spectra did not show a Mn2+-like multiplet structure. The overall magnetization increases with the increase in applied magnetic field from 0.1 T to 2 T on MnSi thin film. The observed spin and orbital magnetic moments ∼ 0.03 ̶ 0.1 μB and 0.01 to 0.04 μB, respectively.

Origin of Enhancement of Orbital Magnetic Moment in SiO2-Coated Fe3O4 Nanocomposites Studied by X-ray Magnetic Circular Dichroism.

In this study, magnetic Fe3O4 nanoparticles (NPs) were dispersed uniformly by varying the thickness of the SiO2 coating, and their electronic and magnetic properties were investigated. X-ray diffraction confirmed the structural configuration of monophase inverse-spinel Fe3O4 NPs in nanometer size. Scanning electron microscopy revealed the formation of proper nonporous crystallite particles with a clear core-shell structure with silica on the surface of Fe3O4 NPs. The absorption mechanism studied through the zeta potential indicates that SiO2-coated Fe3O4 nanocomposites (SiO2@Fe3O4 NCs) possess electrostatic interactions to control their agglomeration in stabilizing suspensions by providing a protective shield of amorphous SiO2 on the oxide surface. High-resolution transmission electron microscopy images demonstrate a spherical morphology having an average grain diameter of ∼11-17 nm with increasing thickness of SiO2 coating with the addition of a quantitative presence and proportion of elements determined through elemental mapping and electron energy loss spectroscopy studies. Synchrotron-based element-specific soft X-ray absorption spectroscopy and X-ray magnetic circular dichroism (XMCD) techniques have been involved in the bulk-sensitive total fluorescence yield mode to understand the origin of magnetization in SiO2@Fe3O4 NCs. The magnetization hysteresis of Fe3O4 was determined by XMCD. At room temperature, the magnetic coercivity (Hc) is as high as 1 T, which is about 2 times more than the value of the thin film and about 5 times more pronounced than that of NPs. For noninteracting single-domain NPs with the Hc spread from 1 to 3 T, the Stoner-Wohlfarth model provided an intriguing explanation for the hysteresis curve. These curves determine the different components of Fe oxides present in the samples that derive the remnant magnetization involved in each oxidation state of Fe and clarify which Fe component is responsible for the resultant magnetism and magnetocrystalline anisotropy based on noninteracting single-domain particles.

Element-specific insight into ferromagnetic stability in UCoGe revealed by soft x-ray magnetic circular dichroism

Element-specific insight into ferromagnetic stability in UCoGe revealed by soft x-ray magnetic circular dichroism

Direct observation of antiferromagnetic domains and field-induced reversal in Pt/Cr2O3/Pt epitaxial trilayers

Antiferromagnet does not show the net magnetization, whereas the finite uncompensated moment can residue at the surface. On the surface of the magnetoelectric antiferromagnet, the finite boundary magnetization can acquire the magnetic response. In this paper, we address the magnetic response of the boundary magnetization in the Pt/magnetoelectric Cr2O3/Pt epitaxial trilayer based on the anomalous Hall effect (AHE) and the soft x-ray magnetic circular dichroism (XMCD). Decreasing the Cr2O3 thickness down to 15 nm, the film acquired the magnetic responsiveness, which manifested as the rectangular hysteresis in the magnetic field dependence of the AHE. The sizable XMCD intensity and the rectangular magnetic field dependence of the XMCD intensity revealed that the magnetic response was attributed to the interfacial Cr moment. The detailed investigation of AHE and XMCD revealed that the domain wall motion dominated the reversal process of the boundary magnetization, which was directly visualized by the scanning XMCD microscope.

Demagnetization processes of Nd-Fe-B sintered magnets and ferrite magnets as demonstrated by soft X-ray magnetic circular dichroism microscopy

The demagnetization processes of Nd-Fe-B sintered magnets and ferrite magnets are investigated using X-ray magnetic circular dichroism microscopy (XMCD). The average XMCD demagnetization curves are compared with the demagnetization curves from magnetic measurements. It is found that the magnetization reversal of the magnets proceeds in two steps. First, the grains are discretely demagnetized in a weak demagnetization field. When the strength of the demagnetization field increases, the magnetization of the grains adjacent to the discretely reversed grains is also reversed to form a cluster of reversed grains, and this cluster grows. Coercivity is achieved through the growth of such clusters. The demagnetization curves from XMCD and the demagnetization curves from the magnetic measurements agree well with each other when the self-demagnetization field and the magnetic after-effect in the XMCD measurement are taken into account. This result shows that the information obtained from the XMCD reflects the bulk information.

Soft X-ray spectroscopy study of Fe impurities in Mn[formula omitted]CoxGa Heusler films

Co-substituted Mn3−xCoxGa Heusler films, exhibiting the magnetic anisotropy transition with x, are considered as promising spintronic materials. We have investigated the local electronic structures of Mn3−xCoxGa films as well as the chemical distributions of the substituted ions employing photoemission spectroscopy (PES), scanning photoelectron microscopy (SPEM), soft X-ray absorption spectroscopy (XAS), and soft X-ray magnetic circular dichroism (XMCD). SPEM measurements show that Co ions are distributed homogeneously in Mn3−xCoxGa films. On the other hand, some Fe impurities are detected in the capping layers of Mn3−xCoxGa films. Despite the existence of ferromagnetic metallic Fe impurities, however, it is found that the magnetic anisotropy transition in Mn3−xCoxGa is determined mainly by the substituted Co ions but hardly by Fe impurities. This work demonstrates that, in searching for new functional materials, special attention should be paid on the magnetic Fe impurities that might affect their magnetic properties.

Origin of orbital magnetic moment in e-beam evaporated SrRuO3 thin films studied by soft X-ray magnetic circular dichroism

Depending on the epitaxial strain, the ferromagnetic thin film SrRuO3 (SRO) prepared by e-beam evaporation, exhibits variable easy magnetization axes, and shows a metal-insulator transition as the thickness of the thin film decreases. We investigated the magnetic properties of SRO thin films produced on SrTiO3 (001) substrates in a series of thicknesses utilizing soft x-ray magnetic circular dichroism at the Ru M2,3 edge. Results have demonstrated that when film thickness decreases to around five monolayers, the film transitions from ferromagnetic to nonmagnetic, consistent with measurements of the magnetization. The spin magnetic moment also decreases with decreasing film thickness. A method for obtaining the orbital magnetic moment is discussed using the electronic structure of the comparatively strained SRO thin film.

Effect of annealing temperature on the structural, electronic and magnetic properties of Co doped TiO2 nanoparticles: An investigation by synchrotron-based experimental techniques

The present study examines the effect of annealing temperature on the structural, electronic and magnetic properties of Co-doped TiO2 (TCO) nanoparticles (NPs) synthesized by the sol-gel method. X-ray powder diffractograms of TCO NPs annealed at 400, 600 and 800 °C exhibited the monophasic anatase, diphase (anatase and rutile) and monophasic rutile structures respectively. A morphological investigation using high-resolution scanning electron microscopy (HR-SEM) reveals the presence of nanoparticles with diameters ranging from ∼11 to ∼51 nm. The widening and shifting of the most intense Eg peak in Raman studies and SEM images of TCO NPs suggest Co incorporation in the TiO2 matrix and the formation of oxygen vacancies. Magnetic measurements of TCO at room temperature demonstrate ferromagnetic behaviour which is caused by the existence of oxygen vacancies and the doped Co content. Element specific technique, soft X-ray magnetic circular dichroism (XMCD) was used to study the magnetism in Co doped TiO2 samples. The estimated total magnetic moment by XMCD was in the range of ∼1.3 ± 0.1–2.3 ± 0.2 μB/Co. This huge magnetization was first time observed by the surface-sensitive Total Electron Yield (TEY) mode. The estimated saturation magnetic moment is in the range of 1.0–1.5 μB/Co, also consistent with XMCD analysis and cluster model calculation. The results obtained by XMCD recommend an intrinsic behaviour of ferromagnetism. The X-ray absorption spectra (XAS) of Co-doped TiO2 nanoparticles at Ti L2,3 and Co L2,3 -edge in TEY mode concludes that the valence states of Co and Ti ions are in 2 + and 4 + respectively. Cluster model calculation also confirmed that Co ions are in a 2 + valence state with D2 h high spin crystal-field symmetry at the surface while a random crystal field symmetry in bulk has been observed which is well consistent with experimental data.

Multimodal analysis of Zr substitution effects on magnetic and crystallographic properties in (Sm1−xZrx)(Fe0.8Co0.2)12 compounds with ThMn12 structure

SmFe12-based compounds containing Zr atoms have been recognized as candidates for novel high-performance permanent magnet materials with a high saturation magnetization. Partial substitution of Sm with Zr in (Sm1−xZrx)(Fe0.8Co0.2)12 has been known to enhance the magnetization of the compound. This study focused on (Sm1−xZrx)(Fe0.8Co0.2)12 single-crystalline films to evaluate the effects of Zr substitution for Sm on their structural and magnetic properties via systematic multimodal analysis. X-ray absorption fine structure (XAFS) and scanning transmission electron microscopy indicated a selective occupation of the Sm site by Zr atoms without disturbing the Fe sublattices. The chemical shift in the Zr-K edge XAFS spectra suggested electron transfer from Zr to the Fe band. Soft X-ray magnetic circular dichroism analysis proved that the magnetic moment of Sm was two orders of magnitude smaller than the magnetic moments of Fe and Co, and its contribution to the total magnetization was negligible. The partial substitution of Sm with Zr resulted in a volumetric expansion and c-axis length elongation despite Zr having a smaller atomic radius, which is unique to (Sm1−xZrx)(Fe0.8Co0.2)12 without structure-stabilizing elements such as Ti, V, and Si. This anisotropic expansion is explained by an increase in the magnetic moment and magnetic interactions in the Fe8f site, which is induced by the electron transfer from Zr to Fe8f atoms. Thus, we conclude that the electron transfer from Zr to the Fe8f atoms principally drives magnetization enhancement in (Sm1−xZrx)(Fe0.8Co0.2)12 single-crystalline films.

Effect of Cr-substitution on vanadium dioxide thin films studied by soft X-ray magnetic circular dichroism

The tunability of magnetic properties of VO 2 has fascinated substantial attention in the first generation of spintronics-related applications such as read head sensors for hard disk drives and magnetic random-access memory. In this work, we focus on Cr doped VO 2 , V 1− x Cr x O 2 (0 ≤ x ≤ 0 . 3) (VCO) thin films and study the effect of Cr-substitution on the structural, electronic and magnetic properties. The VCO films were deposited on R-sapphire substrates using a pulse laser deposition method. Soft X-ray absorption spectroscopy (XAS) and soft X-ray magnetic circular dichroism (XMCD) studies of VCO thin films were performed to reveal and understand the origin of maTablegnetization and their enhancement with Cr-substitution. The XAS spectra suggest that the valences of V ions are in tetra and pentavalent states and Cr ions in the trivalent state. The XMCD spectra imply that the V and Cr ions are ferromagnetic at room temperature. These results also indicate that the VCO films are in the ferromagnetic insulating state and charge-ordered at the V and Cr sites. These results are consistent with the first principle density functional theory calculations. • XRD results suggest that a gradual structural change from the monoclinic M1 phase (0 ≤ x < 0.05) to triclinic T phase (0.05 ≤ x ≤ 0.20 to monoclinic M2 phase ( x > 0.20). • First Study to understand the origin of magnetization. • The VCO films are in the ferromagnetic insulating state and charge-ordered at the V and Cr sites, consistent with DFT. • The XMCD spectra indicate that the V and Cr ions are ferromagnetic at room temperature.

Electronic and magnetic properties of FeCr2O4 nanoparticles by advanced synchrotron based soft X-ray magnetic circular dichroism

Multifunctional materials are now being considered for a variety of technical uses. The iron-chromite spinel FeCr2O4 NPs generated by the sol-gel process has been studied experimentally in this paper. The material crystallized into a cubic form, according to structural analysis based on x-ray diffraction. The Vibrating Sample Magnetometer, Soft-x-ray absorption spectroscopy (XAS) and soft-x-ray magnetic circular dichroism (XMCD) were used to explore the electronic and magnetic structures of FeCr2O4 NPs. Cr ions have trivalent Cr3+ and Fe ions have divalent Fe2+ states present here. The Fe 3d states have a high degree of hybridization with the O 2p states. The Fe and Cr 2p XMCD analysis shows antiparallel alignment of Cr and Fe spins, as well as remarkable hybridization of Fe 3d and O 2p electrons.

Isotropic orbital magnetic moments in magnetically anisotropic SrRuO3 films

Epitaxially strained SrRuO3 films have been a model system for understanding the magnetic anisotropy in metallic oxides. In this paper, we investigate the anisotropy of the Ru 4d and O 2p electronic structure and magnetic properties using high-quality epitaxially strained (compressive and tensile) SrRuO3 films grown by machine-learning-assisted molecular beam epitaxy. The element-specific magnetic properties and the hybridization between the Ru 4d and O 2p orbitals were characterized by Ru M2,3-edge and O K-edge soft X-ray absorption spectroscopy and X-ray magnetic circular dichroism measurements. The magnetization curves for the Ru 4d and O 2p magnetic moments are identical, irrespective of the strain type, indicating the strong magnetic coupling between the Ru and O ions. The electronic structure and the orbital magnetic moment relative to the spin magnetic moment are isotropic despite the perpendicular and in-plane magnetic anisotropy in the compressive-strained and tensile-strained SrRuO3 films; i.e., the orbital magnetic moments have a negligibly small contribution to the magnetic anisotropy. This result contradicts Bruno model, where magnetic anisotropy arises from the difference in the orbital magnetic moment between the perpendicular and in-plane directions. Contributions of strain-induced electric quadrupole moments to the magnetic anisotropy are discussed, too.

Electronic structure and magnetic anisotropy transition in Mn3−xCoxGa Heusler films investigated with soft x-ray spectroscopy

Employing soft x-ray magnetic circular dichroism (XMCD) and soft x-ray absorption spectroscopy (XAS), we have investigated electronic structures and spin configurations of Co-substituted ${\mathrm{Mn}}_{3}\mathrm{Ga}$ (${\mathrm{Mn}}_{3\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{Ga}$) Heusler films, which are promising spintronic materials exhibiting perpendicular magnetic anisotropy (PMA). Co $2p$ XAS and XMCD measurements, combined with the charge-transfer multiplet analysis, show that the substituted Co ions in ${\mathrm{Mn}}_{3\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{Ga}$ consist of mainly ferromagnetic metallic Co clusters and a small amount of divalent Co ions with disordered magnetic moments. Mn $2p$ XAS and XMCD measurements show that Mn ions at both the octahedral and tetrahedral sites in ${\mathrm{Mn}}_{3\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{Ga}$ are nearly divalent with weak covalent-bonding nature, reflecting non-Jahn-Teller ions, and that the magnetic moments of Mn ions are polarized along the ordered magnetic moments of the substituted Co ions. Angle-dependent Co $2p$ XMCD data of ${\mathrm{Mn}}_{3\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{Ga}$ agree with the weak magnetic anisotropy transition from PMA to in-plane magnetic anisotropy (IMA) with increasing $x$. Spin and orbital magnetic moments of Co ions are estimated by using the sum-rule analysis, which reveal negligible Co-content and polar-angle dependencies of the Co orbital to spin magnetic moment ratio. This work reveals that the PMA-to-IMA magnetic transition in ${\mathrm{Mn}}_{3\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{Ga}$ is determined mainly by ferromagnetic metallic Co clusters.

Enhanced d−p hybridization intertwined with anomalous ground state formation in the van der Waals itinerant magnet Fe5GeTe2

${\mathrm{Fe}}_{5}{\mathrm{GeTe}}_{2}$ is a van der Waals (vdW)-coupled unconventional ferromagnetic metal with a high Curie temperature (${T}_{\mathrm{C}}$) exceeding 300 K. The formation of an anomalous ground state significantly below ${T}_{\mathrm{C}}$ has received considerable attention, resulting in increased interest in understanding the spin-polarized electronic state evolution near the Fermi energy (${E}_{\mathrm{F}}$) as a function of temperature. Despite recent extensive studies, a microscopic understanding of the spin-polarized electronic structure around ${E}_{\mathrm{F}}$ has not yet been established owing to the intrinsic complexity of both the crystal and band structures. In this study, we investigate the temperature dependence of element-specific soft x-ray magnetic circular dichroism (XMCD). A systematic temperature evolution in the XMCD signal from both magnetic Fe and its ligand Te is clearly observed. More importantly, the enhancement in the hybridization between the Fe $3d$ and Te $5p$ states at low temperature in the zero-magnetic field limit is revealed. We discuss the implications of our observation in line with possible emergence of an exotic magnetic ground state in ${\mathrm{Fe}}_{5}{\mathrm{GeTe}}_{2}$.

Photon correlation spectroscopy with heterodyne mixing based on soft x-ray magnetic circular dichroism

Many magnetic equilibrium states and phase transitions are characterized by fluctuations. Such magnetic fluctuation can in principle be detected with scattering-based x-ray photon correlation spectroscopy (XPCS). However, in the established approach of XPCS, the magnetic scattering signal is quadratic in the magnetic scattering cross section, which results not only in often prohibitively small signals but also in a fundamental inability to detect negative correlations (anticorrelations). Here, we propose to exploit the possibility of heterodyne mixing of the magnetic signal with static charge scattering to reconstruct the first-order (linear) magnetic correlation function. We show that the first-order magnetic scattering signal reconstructed from heterodyne scattering now directly represents the underlying magnetization texture. Moreover, we suggest a practical implementation based on an absorption mask rigidly connected to the sample, which not only produces a static charge scattering signal but also eliminates the problem of drift-induced artificial decay of the correlation functions. Our method thereby significantly broadens the range of scientific questions accessible by magnetic x-ray photon correlation spectroscopy.

Observing the orbital angular momentum of Fe and Co in chiral magnet Fe0.75Co0.25Si using soft x-ray magnetic circular dichroism

The intermetallic compound Fe1−xCoxSi has a helical magnetic order for 0.05&amp;lt;x&amp;lt;0.8, whereas its orbital angular momentum contributing to the occurrence of a Dzyaloshinskii–Moriya interaction has not yet been verified. We applied soft x-ray magnetic circular dichroism spectroscopy on the Fe L2,3 and Co L2,3 edges below Tc for Fe0.75Co0.25Si such that their orbital magnetic moments morb were evaluated independently. The dichroic signals provide direct experimental evidence that morb for both Fe and Co is coupled in a parallel manner with each spin counterpart mspin. The ratio of morb to mspin is independently estimated as morb/mspin∼3% for Fe and 9% for Co. By comparing the average of the two mspin values with the saturated magnetization using a commercial superconducting quantum interference device (SQUID) magnetometer, the hole number in the d bands for both Fe and Co is roughly estimated as approximately 1.5. This suggests that Fe and Co should be arranged closely for stabilizing the ferromagnetic moments.

Origin of Magnetization in Silica-coated Fe3O4 Nanoparticles Revealed by Soft X-ray Magnetic Circular Dichroism

Magnetite (Fe3O4) nanoparticles (NPs) and SiO2-coated Fe3O4 nanoparticles have successfully been synthesized using co-precipitation and modified Stöber methods, respectively. The samples were characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, high-resolution transmission electron microscopy (HRTEM), vibrating sample magnetometer (VSM) techniques, X-ray absorption spectroscopy (XAS), and X-ray magnetic circular dichroism (XMCD). XRD and FTIR data confirmed the structural configuration of a single-phase Fe3O4 and the successful formation of SiO2-coated Fe3O4 NPs. XRD also confirmed that we have succeeded to synthesize nano-meter size of Fe3O4 NPs. HRTEM images showed the increasing thickness of SiO2-coated Fe3O4 with the addition of the Tetraethyl Orthosilicate (TEOS). Room temperature VSM analysis showed the magnetic behaviour of Fe3O4 and its variations that occurred after SiO2 coating. The magnetic behaviour is further authenticated by XAS spectra analysis which cleared about the existence of SiO2 shells that have transformed the crystal as well as the local structures of the magnetite NPs. We have performed XMCD measurements, which is a powerful element-specific technique to find out the origin of magnetization in SiO2-coated Fe3O4 NPs, that verified a decrease in magnetization with increasing thickness of the SiO2 coating.Graphical Magnetite (Fe3O4) nanoparticles (NPs) and SiO2-coated Fe3O4 nanoparticles have successfully been synthesized using co-precipitation and modified Stöber methods, respectively. The samples were characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, high-resolution transmission electron microscopy (HRTEM), vibrating sample magnetometer (VSM) techniques, X-ray absorption spectroscopy (XAS), and X-ray magnetic circular dichroism (XMCD). XRD and FTIR data confirmed the structural configuration of a single-phase Fe3O4 and the successful formation of SiO2-coated Fe3O4 NPs. XRD also confirmed that we have succeeded to synthesize nano-meter size of Fe3O4 NPs. HRTEM images showed the increasing thickness of SiO2-coated Fe3O4 with the addition of the Tetraethyl Orthosilicate (TEOS). Room temperature VSM analysis showed the magnetic behaviour of Fe3O4 and its variations that occurred after SiO2 coating. The magnetic behaviour is further authenticated by XAS spectra analysis which cleared about the existence of SiO2 shells that have transformed the crystal as well as the local structures of the magnetite NPs. We have performed XMCD measurements, which is a powerful element-specific technique to find out the origin of magnetization in SiO2-coated Fe3O4 NPs, that verified a decrease in magnetization with increasing thickness of the SiO2 coating.

Most Frequently Asked Questions about the Coercivity of Nd-Fe-B Permanent Magnets

Physically, the coercivity of permanent magnets should scale with the anisotropy field of ferromagnetic compounds, HA; however, the typical coercivity values of commercial polycrystalline sintered magnets are only ~0.2 HA, which is known as Brown’s paradox. Recent advances in multi-scale microstructure characterizations using focused ion beam scanning electron microscope (FIB/SEM), aberration corrected scanning transmission electron microscopy (Cs-corrected STEM), and atom probe tomography (APT) revealed detailed microstructural features of commercial and experimental Nd-Fe-B magnets. These investigations suggest the magnetism of a thin layer formed along grain boundaries (intergranular phase) is a critical factor that influence the coercivity of polycrystalline magnets. To determine the magnetism of the thin intergranular phase, soft X-ray magnetic circular dichroism (XMCD) and electron holography played critical role. Large scale micromagnetic simulations using the models that are close to real microstructure incorporating the recent microstructure characterization results gave insights on how the coercivity and its thermal stability is influenced by the microstructures. Based on these new findings, coercivity of Nd-Fe-B magnets are being improved to its limit. This review replies to most frequently asked questions about the coercivity of Nd-Fe-B permanent magnets based on our recent studies.

軟X線磁気円二色性顕微分光によるNd-Fe-B熱間加工磁石の初磁化過程および減磁過程における磁区観察

A large step on the initial magnetization curve has been widely observed in Nd-Fe-B hot-deformed magnets. This behavior has been considered as that the first- and latter-half parts of the initial magnetization curve correspond to the magnetization reversals of multi- and single-domain grains, respectively. In this study, the detailed magnetic domain structure of a Nd-Fe-B hot-deformed magnet has been studied for the initial magnetization and demagnetization processes by using a soft X-ray magnetic circular dichroism (XMCD) microscopy. The observed magnetization reversal behavior for the initial magnetization process is quite different from that previously considered. The multi-domain states move from grains to grains through the gradual displacement of domain wall passing through many grains, and then the massive domain wall displacement takes place. The latter part of domain wall displacement is similar with that found in the demagnetization process near the coercivity. Moreover, we found that there are several strong pinning sites which are identical for both the initial magnetization and the demagnetization processes.