Bulk Magnetization Research Articles

Migration of N element and evolution of microstructure in spark plasma sintered bulk γ′-Fe4N

Magnetic bulk γ′-Fe4N alloys were successfully fabricated by spark plasma sintering (SPS) using high purity γ′-Fe4N powder at 500 °C and 550 °C under 100 MPa and 150 MPa, respectively. The density of the bulk γ′-Fe4N was 7.092 g/cm3 with 98.8 % of the theoretical density, which exhibit the high saturation magnetization 199.6 emu/g. The morphology, microstructure, sintering process and magnetic properties of the bulk samples is explored. It is found that the resistivity and hardness which closely corresponds to the density at the center and margin of the bulk sample are different for the temperature and pressure distribution. With the temperature and pressure increases in the SPS process, N element gradually migrates from the central region of high temperature and high pressure to the margin region of low temperature and low pressure, resulting in the appearance of pores and the decreased density. Moreover, ε-Fe3N phase in the margin region is higher than that in the central region. The phase distribution in the SPS process and the related electrical properties provide insight into the formation and evolution of the phase in the SPS sintering process, which can help to obtain bulk samples with good properties and uniform composition in practical applications.

Engineering microstructure to improve coercivity of bulk MnBi magnet

MnBi is a candidate material for high-temperature magnets because of its increasing coercivity with increasing temperatures up to 255 °C. However, most efforts in fabricating bulk MnBi magnets have run into the problem of preserving the coercivity (Hcj) of its feedstock powders. About 70% of powder's Hcj would be lost during the densification process. Our micromagnetic modeling shows that the coercivity mechanism of the MnBi bulk magnet is controlled by nucleation of the reversal magnetization domains, and the large Hcj loss that occurred during the powder consolidation process can be attributed to the inter-grain magnetic coupling. To attain a high Hcj, the grains in the MnBi bulk magnet must be separated with a non-magnetic grain boundary phase (GBP). To validate this GBP hypothesis, we engineered MnBi bulk magnets with two different types of GBP. The first type of GBP was created in-situ by precipitating excessive Bi from the grains; the second type was created ex-situ by coating silicates on the feedstock powders before the consolidation. While both GBP work, the ex-situ approach resulted in a better Hcj due to a more uniform GBP distribution. The Hcj loss was reduced from 70% to 15%, and the (BH)max of a warm sintered bulk magnet reached 8.9 MGOe.

Bulk second-order optical nonlinearities in centrosymmetric materials

The semiclassical theory of electrical susceptibility predicts null even-order susceptibilities for all centrosymmetric materials. However, second harmonic generation and other even-order nonlinear optical phenomena are often seen in them. Contributions due to magnetic dipoles, bulk quadrupoles, or symmetry ruptures describe many of these effects, but not all of them, as can be seen in some experiments recalled here. This work presents an extension of the theory that renders more general expressions for susceptibilities. According to these expressions, bulk even-order susceptibilities may eventually transform as pseudo-tensors and, therefore, may have nonzero values in centrosymmetric structures. We show examples of how this fact provides an alternative or simpler explanation of the conflicting experiments. Our result may have important implications in lasing, communications, imaging, quantum optics, quantum computing, and other fields, as it widens the family of materials in which to look for nonlinear effects such as second harmonic generation, the Pockels effect, spontaneous parametric downconversion, and other phenomena.

Exploiting different morphologies of non-ferromagnetic interacting precursor’s for preparation of hexaferrite magnets

Sintered cold compacted hexaferrite magnets with appreciable magnetic properties and crystallite alignment were made from non-magnetic precursors without applying an external magnetic field. This work presents a novel approach employing non-ferromagnetic interacting precursors comprising of platelet shaped six-line ferrihydrite and needle shaped goethite nanoparticles. A hydrothermal synthesis route was employed to produce platelet shaped six-line ferrihydrite of ~5 nm thickness. Needle shaped goethite nanoparticles were likewise prepared by hydrothermal synthesis with apparent dimensions of ~10 × 27 × 10 nm 3 extracted from X-ray powder diffraction data. The powder diffraction Rietveld modelling also revealed the presence of an amorphous phase in the six-line ferrihydrite and a SrCO 3 impurity. The presence of needle shaped goethite nanoparticles improves the alignment of magnets, while retaining the coercivity ( H c ), in contrast to hexaferrite magnets prepared from six-line ferrihydrite by spark plasma sintering (SPS). The non-ferromagnetically interacting precursors were directly converted to the SrFe 12 O 19 magnets by pressing them with conventional compaction technique followed by subsequent sintering of the pellets. Decoupling the pressing and sintering step is interesting for industrial production of magnets. The hexaferrite magnets prepared displayed good combination of saturation magnetization M s = 70 Am 2 /kg and coercivity H c = 297 kA/m with some degree of alignment of the crystallites M r / M s = 0.71. This procedure exploits the anisotropic shape of the crystallites and compaction using uniaxial pressure followed by sintering into aligned bulk magnets. Two sets of hexaferrite bulk magnets were prepared by sintering at 900 °C and held for 2 h and 1050 °C with a holding time 0 min. The hexaferrite magnets sintered at 1050 °C were subjected to transmission pole figure analysis. The texture index for each pellet were extracted from the pole figure analysis. Employing needle shaped goethite nanoparticles actually enhanced the alignment of the hexaferrite magnets. The magnet obtained from only six-line ferrihydrite displayed only a slightly improved texture index when compared with mixture of six-line ferrihydrite and goethite nanoparticles. • Two different morphologies comprising platelets and needles were employed in synthesizing SrFe 12 O 19 magnets. • Simple conventional compaction were performed to produce highly aligned SrFe 12 O 19 magnets. • This method requires no magnetic field to align the crystallites along the easy axis. • Magnet with saturation magnetization, M s of 70 Am 2 /kg and coercivity H c of 297 kA/m with 18 kJ/m 3 were produced.

Multi-Fluid MHD Simulations of Europa's Plasma Interaction: Effects of Variation in Europa's Atmosphere.

Europa's plasma interaction is inextricably coupled to its O2 atmosphere by the chemical processes that generate plasma from the atmosphere and the sputtering of magnetospheric plasma against Europa's ice to generate O2. Observations of Europa's atmosphere admit a range of possible densities and spatial distributions (Hall et al., 1998, https://doi.org/10.1086/305604). To better understand this system, we must characterize how different possible configurations of the atmosphere affect the 3D magnetic fields and bulk plasma properties near Europa. To accomplish this, we conducted a parameter study using a multi‐fluid magnetohydrodynamic model for Europa's plasma interaction (Harris et al., 2021, https://doi.org/10.1029/2020ja028888). We varied parameters of Europa's atmosphere, as well as the conditions of Jupiter's magnetosphere, over 18 simulations. As the scale height and density of Europa's atmosphere increase, the extent and density of the ionosphere increase as well, generating strong magnetic fields that shield Europa's surface from impinging plasma on the trailing hemisphere. We also calculate the precipitation rate of magnetospheric plasma onto Europa's surface. As the O2 column density increased from (1–2.5) × 1014 cm−2, the precipitation rate decreased sharply then leveled off at 2 × 1024 ions/s for simulations with low magnetospheric plasma density and 6.4 × 1024 ions/s for simulations with high magnetospheric plasma density. These results indicate that the coupling between Europa's plasma populations and its atmosphere leads to feedback that limits increases in the ionosphere density.

Review of synthesis of high volumetric density, low gravimetric density MgB2 bulk for potential magnetic field applications

This review paper summarizes and analyses the current literature on the synthesis of dense MgB2 bulk with high density and by both the modified solid reaction method and the Mg infiltration method. MgB2 bulk shows Telsa-level magnetic field-trapping ability in small volumes (1–5 cm3) at 15–20 K, which is beneficial for multiple applications, particularly where light weighting is beneficial. The review also compares the properties of MgB2 to rare earth barium cuprate bulk magnets and NbTi solenoid magnets and discuess potential applications such as magnetic lenses, compact NMR, and magnetic shielding.

NMR Relaxometry Using Outer Field of Single-Sided HTS Bulk Magnet Activated by Pulsed Field

We have succeeded in getting <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> H NMR signals at 47.8 MHz (1.12 T) from a single-sided HTS bulk magnet activated by pulse-field magnetization. Diffusion NMR using an inhomogeneous outer field was also demonstrated on the surface. This kind of NMR relaxometry can not get the information of molecular structures, which conventional NMR using a homogeneous magnetic field can acquire. However, the relaxation of nuclear spin magnetization is still observable. A higher magnetic field gradient generated by the small HTS bulk magnet, 67 T/m at a 5 mm distance from the surface, can be effectively used to observe the self-diffusion coefficient of liquids in higher spatial resolution, 44 μm. The easy accessibility to the strong field gradient is one of the unique properties ofcompact HTS bulk magnets. We believe that single-sided (unilateral) diffusion NMR will become the new application of HTS bulk magnets.

A Numerical Evaluation of Magnetizing Characteristic of Bulk Magnet Excited by Pulsed-Field Magnetization With Different Shaped Soft-Iron Yokes

We aim to improve magnetizing efficiency while maintaining the trapped magnetic field of REBCO bulk magnet excited by pulsed-field magnetization (PFM). In PFM, a soft-iron yoke is generally used in order to exposing the bulk to as much magnetic field as possible, and we investigate an influence of the shape and size of the yoke on the trapped field performance experimentally. In this paper, we performed numerical analysis using the 3D simulation model based on our PFM experimental system, and estimated the magnetizing characteristics when using the disk-, ring- and cross-shaped yokes. The analysis results showed the same tendency as the experimental results.

Microscopic probing of the doping effects of In ions in Fe3O4

Minute examination of local lattice structures in matter affected by impurity doping is of special importance for the development of functional materials. In order to obtain microscopic information on spinel ferrites, in the present work, we introduced nonmagnetic In3+ ions in Fe3O4 and probed their site selectivity and the doping effect on the local lattice structures and bulk magnetism by means of 57Fe Mössbauer spectroscopy and positron annihilation spectroscopies. The Mössbauer parameters of the area intensity and isomer shift (IS) show that In3+ ions predominantly reside in the tetrahedral A site, especially at low doping level. With increasing concentration of In ions, however, they gradually occupy the octahedral B site replacing Fe3+ ions. Along with the site information, the IS values confirmed that the introduced In ions squeeze the B-site Fe ions at their nearest neighbors. Supporting results were obtained from positron annihilation lifetime spectroscopy; positron lifetimes become shorter with increasing In concentration, signifying that the oxygen ions are pressed by the introduced In ions resulting in lowering the volume of the adjacent lattice vacancies. The results of Doppler broadening spectroscopy also support the squeezing effect; the positrons in the vacancies adjacent to In ions are more likely to annihilate with the inner shell electrons of the surrounding oxygen ions as a result of a reduction in the vacancy volume.

Rapid screening of magnetic properties in several Fe-X-Ni systems via combinatorial materials chip method

Fe-X-Ni (X = Cr, W and V) combinatorial thin-film (∼100 nm thick) materials chips covering the full composition range of ternary systems were fabricated. The crystal structure distribution was mapped by micro-beam X-ray diffractometers (XRD) and the magnetic hysteresis loops over the chip were characterized by a high-throughput magneto-optical Kerr effect (HT-MOKE) system to establish the composition-phase-magnetic properties relationships. The results showed that saturation magnetization for all systems has a strong dependency on alloying composition, and decreases with increasing dopped elements content as a general trend. Although the trend of saturation magnetization in bulk is in good agreement with that from thin films, all bulk samples show almost no coercivity, attributable to the much smaller grain size, and stronger texture in thin-film samples. Comparing the Fe-X-Ni systems under a similar condition, in the out-of-plane, Cr alloying obtained the largest coercivity (∼400 mT) followed by W alloying (∼300 mT) and then V alloying (∼200 mT). We suggest that alloying with different elements leads to the diverse orientation and crystallinity of the fcc phase resulting in different magnetic properties. Meanwhile, the effect of heat treatment on magnetic properties indicates that saturation magnetization is more closely related to the duration of heat treatment.

Short-range antiferromagnetic interaction and spin-phonon coupling in La2CoMnO6 double perovskite

Weak antiferromagnetic (AFM) interaction in the ferromagnetic (FM) partially ordered La2CoMnO6 (LCMO) was detected by Raman spectroscopy by monitoring spin-phonon coupling. Because of the sensibility to probe short-range disorder and lattice modifications, the Raman spectroscopy showed to be an useful tool to indicate less remarkable magnetic transitions in LCMO compound. Apart from the expected spin-phonon coupling due to the long-range FM superexchange (Tc ∼230 K), phonons parameters pointed out an additional spin-phonon coupling related to the short-range AFM interaction at Tc ∼135 K, which was not detected from the magnetic bulk response. These results reinforce the Raman spectroscopy as a powerful technique to detect antisite disorder into A2B′B’’O6 magnetic double perovskites, whose magnetic properties are driven by superexchange interactions.

Magnetic and spin-phonon coupling studies of magneto-electric SrFe8Co2Ti2O19 M−type hexaferrite

Structural, magnetic, in-field 57Fe Mössbauer and Raman studies of SrFe8Co2Ti2O19 polycrystalline magneto-electric hexaferrite are reported in this article. From bulk magnetization measurements, a transition is observed at 330 K, which is ascribed to a conical like magnetic transition. The population of spin-up and spin-down sites at tetrahedral and octahedral sites are calculated from in-field 57Fe Mössbauer spectroscopic studies. Furthermore, the observed cation distribution suggests that the Co2+ ions occupy the spin-down 4f2 octahedral site, which is reported to stabilize the conical magnetic structure and is expected to play an important role in the magneto-electric properties. Temperature-dependent Raman spectroscopy studies confirmed the spin-phonon coupling in the prepared sample.

Inverse Faraday Effect in an Optomagnonic Waveguide

Single-mode high-index-contrast waveguides have been ubiquitously exploited in optical, microwave, and phononic structures for achieving enhanced wave-matter interactions. Although microscale optomechanical and electro-optical devices have been widely studied, optomagnonic devices remain a grand challenge at the microscale. Here, we introduce a planar optomagnonic waveguide platform based on a ferrimagnetic insulator that simultaneously supports single transverse mode of spin waves (magnons) and highly confined optical modes. The colocalization of spin and light waves gives rise to an enhanced inverse Faraday effect, and as a result, magnons are excited by an effective magnetic field generated by interacting optical photons. Moreover, the strongly enhanced optomagnonic interaction allows us to observe such an effect using low-power (milliwatt level) light signals in the continuous-wave form, as opposed to high-intensity (megawatt peak power) light pulses that are typically required in magnetic bulk materials or thin films. The optically driven magnons are detected electrically with preserved phase coherence, showing the feasibility for launching spin waves with low-power continuous optical fields.

Relating spin-polarized STM imaging and inelastic neutron scattering in the van der Waals ferromagnetFe3GeTe2

Information about the magnetic properties of van der Waals ferromagnets comes so far largely from macroscopic techniques, with much less being known about the magnetism at the microscopic level. By comparing atomic-scale imaging of domain walls at the surface of the van der Waals ferromagnet Fe${}_{3\ensuremath{-}x}$GeTe${}_{2}$ with inelastic neutron scattering measurements of the magnon dispersion in the bulk, the authors elucidate here the interplay between the bulk and surface magnetism and image the low-energy electronic structure.

Magnetic separation of nickel-plating waste liquid using a high temperature superconducting bulk magnet

Ni is one of the important metal resources. Because Ni-containing waste liquid is drained after several plating turns in the factories, an effective recycling technique should be developed. A unique magnetic separation technique using high temperature superconducting bulk magnet has succeeded in collecting Ni-sulfate crystals, which were fabricated from the Ni-plating waste liquid. Pulsed-field magnetizing method was employed to activate the bulk magnet up to 2.80 T, which produced a field space of 1.40 T on the surface of the waste channel. Green coarse crystals were attracted from the flowing stream of Ni-saturated liquid containing weakly-magnetic particles of Ni-compounds. The magnetically-collected particles were identified as paramagnetic NiSO4/6H2O crystals, and slight differences in Ni concentration and grain size were observed between the particles attracted and not-attracted to the 1.8 T magnetic pole. In both cases, the large grains were found to consist of a single phase. The compound can be used as a raw material in the Ni-recycle process. This preferential extraction suggests a novel recycling method of Ni resource.

Hard x-ray photoemission spectroscopy of the ferrimagnetic series Gd6(Mn1−xFex)23

We study the evolution of the electronic structure of the intermetallic series ${\mathrm{Gd}}_{6}{({\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x})}_{23}, x=0.0--0.75$, which shows nonmonotonic ferrimagnetic ordering temperatures ${T}_{C}$ but with a systematic reduction of the total bulk magnetization upon increasing Fe content, $x$. We have carried out hard x-ray photoemission spectroscopy to elucidate the relation between electronic structure and properties of the series. The Gd $3d$ and Gd $4d$ core-level spectra indicate trivalent ${\mathrm{Gd}}^{3+}$ multiplets in the intermediate-coupling scheme with features due to $L\text{\ensuremath{-}}S$ and $j\text{\ensuremath{-}}J$ coupling. The Fe $2p$ core levels show asymmetric single peak metal-like spectra, while the Mn $2p$ core levels show asymmetric doublet peaks. The relative intensities of the Mn $2p$ doublets as a function of $x$ indicate occupancy changes of distinct crystallographic sites associated with Mn up-spin and down-spin states. The valence band spectra identify the Gd $4f$ states at high binding energies ($\ensuremath{\sim}7.4$ eV). The Mn $3d$ states occur at the Fermi level and as a broad feature between 2 and 5 eV binding energy in ${\mathrm{Gd}}_{6}{\mathrm{Mn}}_{23}$. Upon substitution, the Fe $3d$ states show up as small shifts to higher binding energies compared to Mn $3d$ states. The Fe $3s$ and Mn $3s$ spectra show exchange split peaks, allowing an estimate of the Mn and Fe magnetic moments using a Van Vleck analysis, which also provides a quantification of occupancy changes with $x$. The overall results are consistent with the bulk net magnetization, indicating that Mn up-spin sites become Fe down-spin sites on substitution, while the nonmonotonic ${T}_{C}$ originates in a change from Mn sublattice to Fe sublattice derived ordering.

Ultrafast optical control of surface and bulk magnetism in magnetic topological insulator/antiferromagnet heterostructure

Optical control of the magnetic properties in topological insulator systems is an important step in applying these materials in ultrafast optoelectronic and spintronic schemes. In this work, we report the experimental observation of photo-induced magnetization dynamics in the magnetically doped topological insulator (MTI)/antiferromagnet (AFM) heterostructure composed of Cr-(Bi,Sb)2Te3/CrSb. Through proximity coupling to the AFM layer, the MTI displays a dramatically enhanced magnetism, with robust perpendicular magnetic anisotropy. When subjected to intense laser irradiation, both surface and bulk magnetism of the MTI are weakened by laser-induced heating of the lattice, however, at the surface, the deleterious heat effect is compensated by the strengthening of Dirac-hole-mediated exchange coupling as demonstrated by an unconventional pump-fluence-dependent exchange-bias effect. Through theoretical analyses, the sizes of exchange coupling energies are estimated in the MTI/AFM bilayer structure. The fundamentally different mechanisms supporting the surface and bulk magnetic order in MTIs allow a novel and distinctive photo-induced transient magnetic state with antiparallel spin configuration, which broadens the understanding of the magnetization dynamics of MTIs under ultrashort and intense optical excitation.

Spin Waves and Magnetic Exchange Hamiltonian in CrSBr.

CrSBr is an air‐stable two‐dimensional (2D) van der Waals semiconducting magnet with great technological promise, but its atomic‐scale magnetic interactions—crucial information for high‐frequency switching—are poorly understood. An experimental study is presented to determine the CrSBr magnetic exchange Hamiltonian and bulk magnon spectrum. The A‐type antiferromagnetic order using single crystal neutron diffraction is confirmed here. The magnon dispersions are also measured using inelastic neutron scattering and rigorously fit the excitation modes to a spin wave model. The magnon spectrum is well described by an intra‐plane ferromagnetic Heisenberg exchange model with seven nearest in‐plane exchanges. This fitted exchange Hamiltonian enables theoretical predictions of CrSBr behavior: as one example, the fitted Hamiltonian is used to predict the presence of chiral magnon edge modes with a spin‐orbit enhanced CrSBr heterostructure.

Preparation of α′-Fe(N)/α′-Fe8N by γ′-Fe4N precursor

An approach by using the γ′-Fe4N precursor was proposed to prepare a bulk magnet with high α″-Fe16N2 content, and the feasibility for high N content α′-Fe(N)/α′-Fe8N was demonstrated in ribbon samples. γ′-Fe4N decomposed when it was heated to a high temperature, and then, a supersaturated γ-Fe(N) phase formed because of N atoms diffusing out from the γ′-Fe4N lattice. The short-lived supersaturated γ-Fe(N) had the face-centered cubic lattice, and its nitrogen content is beyond 3.0 wt. %. The supersaturated γ-Fe(N) acts as a precursor of the martensite transformation for a body-centered tetragonal phase. After the cryo-treatment, which was essential to complete the martensite transformation, a mixture of α′-Fe(N) and α′-Fe8N was obtained. The N content of more than 2.5 wt. % in the samples was achieved.

High Coercive RE-Fe-B Powders Recycled via HD and HDDR Process from Waste Magnets

In order to fabricate high-coercivity hard magnetic RE-Fe-B powders from the waste bulk magnets, the hydrogen decrepitation (HD) and hydrogenation-disproportionation-desorption-recombination (HDDR) processes were employed to the waste RE-Fe-B magnets. The optimum HD and HDDR conditions for the waste RE-Fe-B magnets were established through the systematic investigation on the effect of HD and HDDR treatment temperature on the magnetic and microstructural properties of final recycled powders. We found that the waste magnets were crushed into the flakes with a uniform size distribution when the HD treatment was carried at 200 ℃. By applying the HDDR process to the HD treated flakes at 920 ℃, high coercivity of 11.2 kOe could be achieved in the recycled RE-Fe-B HDDR powders. Based on the results from the microstructural characterization, the underlying reasons for the magnetic and microstructural changes of the recycled powders as a function of the HD and HDDR treatment temperature is discussed in detail.