Bulk Magnetization Research Articles

Origins of multi-sublattice magnetism and superexchange interactions in double–double perovskite CaMnCrSbO6

The multi-sublattice magnetism and electronic structure in double–double perovskite compound CaMnCrSbO6 is explored using density functional theory. The bulk magnetization and neutron diffraction suggest a ferrimagnetic order ( TC∼ 49 K) between between Mn2+ and Cr3+ spins. Due to the non-equivalent Mn atoms (labelled as Mn(1) and Mn(2) which have tetrahedral and planar oxygen coordinations, respectively) and the Cr atom in the centre of distorted oxygen octahedron in the unit cell, the exchange interactions are more complex than that expected from a two sublattice magnetic system. The separations between the on-site energies of the d-orbitals of Mn(1), Mn(2) and Cr obtained from Wannier function analysis are in agreement with their expected crystal field splitting. While the DOS obtained from non spin-polarized calculations show a metallic character, starting from Hubbard U = 0 eV the spin-polarized electronic structure calculations yield a ferrimagnetic insulating ground state. The band gap increases with Ueff (U − J), thereby showing a Mott–Hubbard nature of the system. The inclusion of anti-site disorder in the calculations show decrease in band-gap and also reduction in the total magnetic moment. Due to the ∼90∘ superexchange, nearest neighbour exchange constants obtained from DFT are an order of magnitude smaller than those reported for various magnetic perovskite and double-perovskite compounds. The Mn(1)–O–Mn(2) (out of plane and in-plane), Mn(1)–O–Cr and Mn(2)–O–Cr superexchange interactions are found to be anti-ferromagnetic, while the Cr–O–O–Cr super-superexchange is found to be ferromagnetic. The Mn(2)–O–Cr superexchange is weaker than the Mn(1)–O–Cr super-exchange, thus effectively resulting in ferrimagnetism. From a simple 3-site Hubbard model, we derived expressions for the antiferromagnetic superexchange strength JAFM and also for the weaker ferromagnetic JFM . The relative strengths of JAFM for the various superexchange interactions are in agreement with those obtained from DFT. The expression for Cr–O–O–Cr super-superexchange strength ( J~SS ), which has been derived considering a 4-site Hubbard model, predicts a ferromagnetic exchange in agreement with DFT. Finally, our mean field calculations reveal that assuming a set of four magnetic sub-lattices for Mn2+ spins and a single magnetic sublattice for Cr3+ spins yields a much improved T C , while a simple two magnetic sublattice model yields a much higher T C .

Design and Analysis of New Type of Magnetically Controlled Reactor

In recent years, various new types of magnetic materials have emerged, especially in the field of nanotechnology. The application of material composite technology has formed a new type of dual-phase composite magnetic material. Under the control of an external magnetic field, the material can achieve functional conversion between the hard magnetic phase and the soft magnetic phase, with magnetic permeability, non-magnetic permeability, and excitation ability. In this paper, it is proposed to use this material as the basic material for the magnetic state control of the reactor core, break through the bondage of the traditional reactor core to the performance of the controllable reactor, and innovatively form a new type of reactor structure. By adding a detection and control system, according to the working state of the power grid, the function conversion of the nano adjustable magnetic material is effectively adjusted to realize the automatic adjustment of the magnetic state of the reactor. Firstly, the preparation and material properties of dual-phase composite magnetic materials were studied. The design and preparation process from magnetic powder to bulk magnet were given, and the magnetic properties were measured. Secondly, a new 380 V/100 kVar magnetically controlled reactor was designed by using dual-phase composite magnetic material to form a composite core of magnetically controlled reactor on silicon steel sheet. The simulation analysis of electromagnetic characteristics and volt-ampere characteristics was carried out to verify the correctness of the proposed scheme.

Confining single Er3+ ions in sub-3 nm NaYF4 nanoparticles to induce slow relaxation of the magnetisation

Molecular systems known as single-molecule magnets (SMMs) exhibit magnet-like behaviour of slow relaxation of the magnetisation and magnetic hysteresis and have potential application in high-density memory storage or quantum computing. Often, their intrinsic magnetic properties are plagued by low-energy molecular vibrations that lead to phonon-induced relaxation processes, however, there is no straightforward synthetic approach for molecular systems that would lead to a small amount of low-energy vibrations and low phonon density of states at the spin-resonance energies. In this work, we apply knowledge accumulated over the last decade in molecular magnetism to nanoparticles, incorporating Er3+ ions in an ultrasmall sub-3 nm diamagnetic NaYF4 nanoparticle (NP) and probing the slow relaxation dynamics intrinsic to the Er3+ ion. Furthermore, by increasing the doping concentration, we also investigate the role of intraparticle interactions within the NP. The knowledge gained from this study is anticipated to enable better design of magnetically high-performance molecular and bulk magnets for a wide variety of applications, such as molecular electronics.

Fundamental Study on Recovery of Spilled Oil by Magnetic Pickering Emulsification

Heavy oil spills from stranded oil tankers are a global problem. The recovery of spilled oil must have a low environmental impact and must be capable of processing large quantities at high speeds. In this study, we investigated the application of magnetic separation for the recovery of spilled oil. Pickering emulsions were formed by heavy oil and magnetite, and recovery was carried out using a drum‐type permanent magnet. Pickering emulsions were successfully formed by stirring heavy oil A and magnetite, and magnetic separation experiments showed that up to 90% of the heavy oil A could be recovered. To recycle magnetite, a small amount of heavy oil was added to the recovered Pickering emulsions, and then a strong magnetic field of a superconducting bulk magnet was applied, which resulted in easy solid–liquid separation. The possibility of recycling magnetite after separation was also demonstrated. Finally, a small‐scale system using drum‐type permanent magnets was developed, and magnetic separation experiments were performed. The results showed that approximately 77% of the heavy oil A could be recovered. Particle trajectory simulations also demonstrated the recoverability of heavy oil at the sea surface. © 2024 Institute of Electrical Engineers of Japan and Wiley Periodicals LLC.

Robustness of magnetocrystalline anisotropy and coercivity in Fe–Co–B

As a potential candidate for critical chemical element free permanent magnet, (Fe0.7Co0.3)2B, shows promising intrinsic magnetic properties, including large magnetization, high Curie temperature and moderate magnetocrystalline anisotropy (MCA). Still, the coercivity that has been achieved is relatively low. To understand and improve coercivity, we study the robustness of MCA related to the fluctuation of chemical composition and the structural distortion and its effect on coercivity in (Fe0.7Co0.3)2B from first principles DFT (density functional theory) calculation and micromagnetic simulation. A fluctuation of cobalt content of 10%, or 1% isotropic lattice strain, or 2% tetragonal distortion reduces MCA energy up to 30%. By considering randomly distributed local MCA energy reduction up to 30%, the calculated coercivity of anisotropic bulk magnets reduced from 3.6 kOe to 2.2 kOe. Rational microstructure design, such as grain size refinement and development of strong crystal texture of MCA easy-axis (i.e., degree of alignment for (Fe0.7Co0.3)2B grains), can enhance coercivity and mitigate the effect of local MCA reduction. In addition to acceptable intrinsic magnetic properties, the promising hard magnetic phase needs good MCA robustness to facilitate the development of practical magnets.

Production of Sm2Fe17N3 Bulk Magnets

Sm2Fe17N3 powder exhibits excellent magnetic properties but is unstable and decomposes into α-Fe and SmN phases at high temperatures. Therefore, the key to producing Sm2Fe17N3 bulk magnets is to reduce the deterioration of Sm2Fe17N3 powder during sintering. Herein, Sm2Fe17N3 bulk magnets were made using the spark plasma sintering (SPS) method with the addition of zinc stearate powder and zinc powder. Adding small amounts of zinc stearate powder and zinc powder improved the magnetic anisotropy and the coercivity of the magnets, respectively. The magnets produced by the SPS method using zinc stearate powder and zinc powder exhibited enhanced magnetic properties almost comparable to those of Sm2Fe17N3 powder.

Multi‐Spacecraft Magnetic Field Reconstructions: A Cross‐Scale Comparison of Methods

AbstractSpace plasma studies frequently use in situ magnetic field measurements taken from many spacecraft simultaneously. A useful data product of these measurements is the reconstructed magnetic field in a volume near the spacecraft observatory. We compare a standard Linear method of computing the magnetic field at arbitrary spatial points to two novel approaches: a Radial Basis Function interpolation and a time‐dependent 2D inverse distance weighted interpolation scheme called Timesync. These three methods, which only require in situ measurements of the magnetic fields and bulk plasma velocities at a sparse set of spatial points, are implemented on synthetic data drawn from a time‐evolving numerical simulation of plasma turbulence. We compare both the topology of the reconstructed field to the ground truth of the simulation and the statistics of the fluctuations found in the reconstructed field to those from the simulated turbulence. We conclude that the Radial Basis Function and Timesync methods outperform the Linear method in both topological and statistical comparisons.

Modeling the scattering polarization in the solar Ca I 4227Å line with angle-dependent PRD effects and bulk velocities

Context. Modeling the scattering polarization signals of strong chromospheric lines requires solving the radiative transfer problem for polarized radiation, out of local thermodynamic equilibrium, taking partial frequency redistribution (PRD) effects into account. This problem is extremely challenging from a computational standpoint and, so far, most studies have been carried out by either modeling PRD effects under the angle-average approximation or by considering academic models of the solar atmosphere. Thanks to a new solution strategy, applicable to atomic systems that allow for a linearization of the problem, accurate solutions can now be routinely obtained in realistic 1D models, taking angle-dependent (AD) PRD effects into account. Aims. This work is aimed at assessing the suitability and performance of this new approach to handling dynamic scenarios. At the same time, it aims to explore the joint impact of magnetic fields and bulk velocities on the scattering polarization profiles of strong resonance lines, accounting for AD PRD effects and considering more realistic atmospheric models than in previous investigations. Methods. By using a two-level atomic model for neutral calcium, we synthesized the intensity and polarization profiles of the Ca I 4227 Å line. Our calculations were performed in 1D atmospheric models, both semi-empirical and extracted from 3D magnetohydrodynamic simulations, including vertical bulk velocities and magnetic fields of arbitrary strength and orientation, both constant and varying with height. Results. We obtained accurate solutions after only a few iterations across all considered scenarios. Even when formulating the problem in the observer’s reference frame, the frequency and angular grids required for accurate results were easily manageable. The calculated profiles showed the expected signatures of bulk velocities: wavelength shifts, enhancement of the line-core polarization amplitude, and prominent asymmetries in the wing signals. The results obtained in atmospheric models with complex thermal, dynamic, and magnetic structures unveiled the broad diversity of features in the emergent radiation that can be expected from realistic scenarios. Conclusions. The presented results assess the suitability of the proposed solution strategy and its parallel implementation, thus supporting its generalization to the 3D case. Our applications in increasingly realistic atmospheric models showed the difficulty related to precisely establishing the individual weight of bulk velocities and magnetic fields in the shape of the emergent profiles. This highlights the need to account for both these physical ingredients to perform reliable inversions of observed scattering polarization profiles.

Production of Mn-Ga Magnets.

Mn-based magnets are known to be a candidate for use as rare-earth-free magnets. In this study, Mn-Ga bulk magnets were successfully produced by hot pressing using the spark plasma sintering method on Mn-Ga powder prepared from rapidly solidified Mn-Ga melt-spun ribbons. When consolidated at 773 K and 873 K, the Mn-Ga bulk magnets had fine grains and exhibited high coercivity values. The origin of the high coercivity of the Mn-Ga bulk magnets was the existence of the D022 phase. The Mn-Ga bulk magnet consolidated at 873 K exhibited the highest coercivity of 6.40 kOe.

Topological Heusler Magnets‐Driven High‐Performance Transverse Nernst Thermoelectric Generators

AbstractTopological magnets (TMs) with the coupled topology of electronic band structures and spin configuration have exhibited exotic transport properties that are overwhelmingly appealing for transverse thermoelectric applications. Despite the continuous discovery of TMs in recent years, the development of Nernst generators has much lagged. Here, high‐performance Nernst generators are developed utilizing polycrystalline topological Heusler bulk magnets. Benefiting from the robustness of topological effect to grain boundary scattering, polycrystalline Co2MnGa is found to exhibit a large Nernst thermopower of ≈6.5 µV K−1 at 300 K, which is comparable to the previously reported record value in its single crystal. The developed Nernst generators thereby exhibit excellent performance with an output voltage of 5.4 mV and power of 14.3 µW, significantly higher than that of Nernst thermopiles assembled using conventional ferromagnets. These results pave the way to advancing TMs with intrinsically large Berry curvature for transverse thermoelectric applications.

Magnetic properties of (Sm, Zr)(Fe, Co)10−xTix (x = 0, 0.5, 1) bulk magnets

It would be desirable if the magnetic properties of Sm–Fe alloys could be increased without the need for nitrogenation. In this experiment, Sm–Fe bulk magnets were prepared from the Sm–Fe powders by the spark plasma sintering (SPS) technique without nitrogenation. The structures and magnetic properties of the Sm–Fe bulk magnets prepared by the SPS technique were investigated. By optimization of the composition and production process, it was found that the (Sm0.7Zr0.3)(Fe0.7Co0.3)9Ti magnet with the fine Th2Zn17-type phase showed a high coercivity of 4.35 kOe.

Exploiting mechanochemical activation and molten-salt-assisted reduction-diffusion approach in bottom-up synthesis of Sm2Fe17N3

Sm2Fe17N3 powders were prepared by a novel molten-salt mechanochemically assisted reduction-diffusion (RD) approach. CaCl2, plays a critical role during mechanochemical processing as a dispersant, facilitating the RD process by dissolving the precursors in a molten flux and further assisting during the washing step by efficient removal of by-products thus minimizing the surface oxidation of Sm2Fe17N3 powder particles. Various milling times and RD temperatures were examined, and synthesis conditions were optimized to achieve pure-phase Sm2Fe17N3 powders with low aggregation of magnetic particles. Powders synthesized at 950 °C RD exhibited the highest hard-magnetic properties: coercivity Hc of 13.5 kOe, and maximum energy product (BH)max of 19.4 MGOe. This was attributed to the formation of fine single-phase Sm2Fe17N3 particles as well as minimal oxidation of particle surface. Furthermore, by densifying the Sm2Fe17N3 powders with high-pressure spark plasma sintering, a bulk magnet with (BH)max of 16.5 MGOe and a relative density of 86 % was produced indicating that the obtained Sm2Fe17N3 powders had low oxygen content.

Intrinsic ferromagnetism in Hg1-xFexSe (0.00012 ≤ x ≤ 0.0013) diluted magnetic semiconductor bulk single crystals at room temperature: Role of hybridization of impurity electronic states

We investigate room-temperature structural properties and spontaneous magnetism in a series of Hg1-xFexSe (0.00012 ≤ x ≤ 0.0013) diluted magnetic semiconductor bulk single crystals grown by the Bridgman method. In order to elucidate the experimentally observed high-temperature magnetism in the studied systems, the dependencies of magnetization on magnetic field strength for the entire sample series were studied. Their detailed analysis revealed impurity contributions typical of ferromagnetic materials, which are described by magnetization curves with the parameters characterizing the spontaneous spin magnetism of the donor conduction electrons of the outer d-shells of the impurity atoms. According to the previously-developed theoretical concept, such a magnetic ordering mechanism stems from the direct exchange coupling of donor conduction electrons due to hybridization of impurity states, rather than from inter-impurity interactions. The results of optical emission spectroscopy and X-ray diffraction analysis confirmed the observed ferromagnetism to be intrinsic and linked directly to the d-electrons of iron impurity atoms.

Investigation of the correlation between optical and γ-ray flux variations in the blazar Ton 599

ABSTRACT The correlation between optical and γ-ray flux variations in blazars reveals a complex behaviour. In this study, we present our analysis of the connection between changes in optical and γ-ray emissions in the blazar Ton 599 over a span of approximately 15 yr, from 2008 August to 2023 March. Ton 599 reached its highest flux state across the entire electromagnetic spectrum during the second week of 2023 January. To investigate the connection between changes in optical and γ-ray flux, we have designated five specific time periods, labelled as epochs A, B, C, D, and E. During periods B, C, D, and E, the source exhibited optical flares, while it was in its quiescent state during period A. The γ-ray counterparts to these optical flares are present during periods B, C, and E; however, during period D, the γ-ray counterpart is either weak or absent. We conducted a broad-band spectral energy distribution (SED) fitting by employing a one-zone leptonic emission model for these epochs. The SED analysis unveiled that the optical–ultraviolet emission primarily emanated from the accretion disc in quiescent period A, whereas synchrotron radiation from the jet dominated during periods B, C, D, and E. Diverse correlated patterns in the variations of optical and γ-ray emissions, like correlated optical and γ-ray flares, could be accounted for by changes in factors such as the magnetic field, bulk Lorentz factor, and electron density. On the other hand, an orphan optical flare could result from increased magnetic field and bulk Lorentz factor.

Pre-orientation sintering: an alternative process to prepare high-performance anisotropic Nd–Fe–B HDDR magnets

Hydrogen–disproportionation–desorption–recombination (HDDR) Nd–Fe–B magnetic powders are promising to prepare bulk anisotropic magnets, but high magnetic performance has not been achieved due to the absence of an Nd-rich phase in the powder and the low degree of orientation of the bulk magnets. In this study, an alternative process of pre-orientation sintering via magnetic alignment followed by spark plasma sintering was proposed to prepare the precursor of hot-deformation (HD) magnets, and a high maximum energy product of 294 kJ m−3 was achieved in the HD magnet with a relatively low height reduction of 35%, then an improved coercivity of 1107 kA m−1 could be obtained followed by a grain boundary diffusion of Pr40Tb30Al20Cu10. Microstructure analysis indicates that pre-orientation of HDDR powders facilitates grain rotation and alignment during the HD process, thereby reducing the minimum deformation ratio. It helps to obtain the deformed grains with lower shape anisotropy and smaller grain size, enabling a good compatibility of magnetic and mechanical behaviors. In-situ Lorentzian transmission electron microscopy results show that the magnetic domains have been strongly fixed by the thick intergranular RE-rich phase and the fully Tb-diffused grains, which contributes to the improved coercivity after grain boundary diffusion. This study provides a guiding significance for the preparation of the anisotropic Nd–Fe–B HDDR magnets with optimized performance.

Design of High-Remanence Nd-Fe-B Hot-Pressed Magnets by Manipulating Coercivity of Hydrogenation-Disproportionation-Desorption-Recombination Treated Anisotropic Precursors.

We propose a method of manipulating the coercivity of anisotropic hydrogenation-disproportionation-desorption-recombination (HDDR) powders to fabricate high-remanence and fine-grained Nd-Fe-B magnets using only hot-pressing without a subsequent hot-deformation process. By reducing the Nd content of anisotropic HDDR precursors such that their coercivity (Hcj) is lowered, the c-axis of each HDDR particle is well-aligned parallel to the direction of the applied magnetic field during the magnetic alignment step. This is because the magnetic repulsive force between adjacent particles, determined by their remanent magnetization, decreases as a result of the low coercivity of each particle. Therefore, after hot-pressing the low-Hcj HDDR powders, a significantly higher remanence (11.2 kG) is achieved in the bulk than that achieved by hot-pressing the high-Hcj HDDR powders (8.2 kG). It is clearly confirmed by the large-scale electron backscatter diffraction (EBSD) analysis that the alignment of the c-axis of each anisotropic HDDR particle in the bulk is improved when low-Hcj HDDR powders are used to fabricate hot-pressed magnets. This coercivity manipulation of HDDR powders can be a helpful method to expand the use of HDDR powders in fabricating anisotropic Nd-Fe-B bulk magnets.

Surface‐Sensitive Detection of Magnetic Phase Transition in Van Der Waals Magnet CrSBr

Abstract2D magnets have recently drawn enormous interest. As an air‐stable A‐type van der Waals antiferromagnet (AFM), CrSBr has attracted great attention but has also led to controversies about its large‐span ordering temperatures. Herein, a systematic study of the magnetic phase transition in single‐crystalline CrSBr with ultrahigh‐quality through surface‐sensitive X‐ray magnetic linear dichroism and X‐ray magnetic circular dichroism measurements combined with vibrating sample magnetometry for characterization of bulk magnetization is reported. The interlayer AFM order of both surface and bulk CrSBr is revealed to maintain a similar Néel temperature within the range of 132–142 K. However, the intralayer ferromagnetic (FM) order of surface CrSBr is found to sustain up to ≈238 K, 70 K higher than the value obtained from the bulk CrSBr, demonstrating a dramatically different surface and bulk Curie temperature in CrSBr. Moreover, a half‐filled t2g electronic state for Cr3+ ions with magnetic moment of ≈3 µB/Cr in CrSBr is clearly identified. These results enrich the understanding of the electronic structure and magnetism in CrSBr, providing this material as a promising building block for future spintronic devices.

Synergistic grain boundary modification and magnetic hardening for coercivity enhancement in Nd-Ce-Fe-B sintered magnets with Dy[sbnd]Cu addition

Obtaining high coercivity in bulk magnets is essential to broaden the prospects for large-scale applications of Nd-Ce-Fe-B magnets. In this work, the potential of low-cost Nd-Ce-Fe-B magnets is unlocked by improving their magnetic properties, and the coercivity enhancement mechanism is elucidated through systematic material characterization. First, by designing the substrate with RE-rich (31.2 wt%), coercivity up to 13.5 kOe was obtained in Nd-Ce-Fe-B magnets with Ce/RE = 30 wt% (RE: rare earth). Microstructural analysis revealed that the high coercivity was attributed to the formation of a continuous GB phase layer induced by the excess RE. Furthermore, by doping only 2.4 wt% DyCu, the coercivity was substantially enhanced to 19.5 kOe and the coercivity temperature coefficient (20–150 °C) achieved −0.529%/°C while the remanence was maintained at 12.0 kG. Elemental distribution analysis shows that Dy enters the matrix phase while Nd/Ce is expelled, and the additional RE further widens the GB thickness, which weakens the exchange coupling effect between adjacent grains. Moreover, Dy mainly replaces Ce rather than Nd on the grain surface, resulting in a Dy-rich shell, which greatly enhances the anisotropy field of the shell region. Magnetic domain analysis shows that GB modification and magnetic hardening effectively suppress the nucleation and cascade propagation of the reversal domains, and their synergistic enhancement of the coercivity is also verified by micromagnetic simulations.

Room temperature ferromagnetism in the nanolaminated MAX phase (Mn1−xCrx)2GaC

MAX phases are a class of intrinsically nanolaminated materials, which combine features of metals and ceramics, owing to the alternating metallic and covalent bonding between atomic layers. Magnetic MAX phases have been known for a decade, but ferromagnetism at room temperature in this highly anisotropic system has been elusive, limiting their value as magnets in practice. Here, we show that a MAX phase with a strong ferromagnetic response is obtained by substituting Mn with Cr on the M-site in the well-known Mn2GaC. The ferromagnetic response is observed in (Mn1−xCrx)2GaC with 0.06 < x < 0.29 up to temperatures well exceeding room temperature (489 K). The strongest magnetization is achieved with x = 0.12, reaching a saturation moment of 1.25 μB and a remanence of 0.67 μB per M-atom at 3 K and maintaining 0.90 and 0.44 μB per M-atom, respectively, at 300 K. This is the first experimental report of a significant ferromagnetic response in a MAX phase at room temperature. The results open the door to the use of MAX phases in a broad range of applications, from bulk magnets in power electronics to spintronic devices.

Radiation damping at clinical field strength: Characterization and compensation in quantitative measurements.

In any MR experiment, the bulk magnetization acts on itself, caused by the induced current in the RF receiver circuit that generates an oscillating damping field. This effect, known as "radiation damping" (RD), is usually weak and, therefore, unconsidered in MRI, but can affect quantitative studies performed with dedicated coils that provide a high SNR. The current work examined RD in a setup for investigations of small tissue specimens including a quantitative characterization of the spin-coil system. A custom-made Helmholtz coil (radius and spacing 16 mm) was interfaced to a transmit-receive (Tx/Rx) switch with integrated passive feedback for modulation or suppression of RD similar to preamplifier decoupling. Pulse sequences included pulse-width arrays to demonstrate the absence/ presence of RD and difference techniques employing gradient pulses or composite RF pulses to quantify RD effects during free precession and transmission, respectively. Experiments were performed at 3T in small samples of MnCl2 solution. Significant RD effects may impact RF pulse application and evolution periods. Effective damping time constants were comparable to typical T2 * times or echo spacings in multi-echo sequences. Measurements of the phase relation showed that deviations from the commonly assumed 90° angle between the damping field and the transverse magnetization may occur. Radiation damping may affect the accuracy of quantitative MR measurements performed with dedicated RF coils. Efficient mitigation can be achieved hardware-based or by appropriate consideration in the pulse sequence.