Nonmagnetic Phase Research Articles

Synchronous enhancement mechanism of magnetic property for dual-main-phase Ce magnets by grain boundary diffusion with Pr-Al alloy

The Pr80Al20 (at%) alloy was diffused into the dual-main-phase Ce magnets (Ce/TRE = 40 wt.%, TRE: total rare earth) with different diffusion time by grain boundary diffusion process (GBDP). The magnetic property and the microstructure of a series of magnets with different diffusion time were investigated. The coercivity gradually increased as the diffusion time increased, while the remanence remained basically constant. The improvement of coercivity originated from the synergistic contribution of modified grain boundaries with decoupling effect and Ce-rich main phase containing Pr with improved magnetocrystalline anisotropy. The percentage of the irreversible portion and the nucleation field of the reverse domain calculated from recoil loops curves further confirmed that the diffusion of Pr80Al20 alloy enhanced the coercivity. The enhanced anisotropy of the Ce-rich grains compensates for the decay of the remanence carried by the diffusion-induced increase in the nonmagnetic grain boundary phase, which ultimately results in the remanence remaining essentially constant. The above phenomena further broaden the room-temperature applications of dual-main-phase magnets with high Ce content without heavy rare-earth grain boundary diffusion.

Tripling in coercivity (Hc) for the thermal and structural highly stabilized MnAlBi-C rare earth free permanent magnet via Bi doping

The limited coercivity (Hc) and metastability of the ferromagnetic τ phase MnAl-C magnet have constrained its magnetic performance and application potential. Bi was doped into the Mn54Al44-xBixC2 alloy system in this study. The doped Bi could effectively stabilize the metastable τ phase and inhibit the non-magnetic Al-rich phase formation. The ferromagnetic τ phase ratio increased dramatically as the doped Bi increase, with a peak Ms of 81 emu/g was achieved for the post-annealed Mn54Al43.6Bi0.4C2 alloy at 19 kOe. High Hc ferromagnetic LTP MnBi phase forms within the Mn54Al44-xBixC2 (x = 0.2, 0.4, 0.6) alloys when it been annealed at 300℃ for 24 h, the samples tripled or even quadrupled in Hc compared with the initial synthesized alloy. The doped Bi dissolved within lattice caused the τ phase thermally stability, structural stability improves substantially; 0.4 Bi doped Mn54Al43.6Bi0.4C2 alloy exhibited a combination of high Ms and Hc even after been ball milled for a long period of 15 and 20 h. The (Ms, Hc) achieved a value of (59.6 emu/g, 3526 Oe) for the 15 h milled sample and (51.5 emu/g, 4546 Oe) for the 20 h milled sample at 19 kOe, respectively; which is desirable for its application as a high-performance rare earth free permanent magnet.

Microstructure and magnetic properties of grain boundary diffused Nd-Fe-B magnets with Tb-Al-Ga alloy

The coercivity of Nd-Fe-B magnets can be effectively improved by grain boundary diffusion using low melting point alloy as diffusion source. In this work, Tb-Al-Ga alloy was used as diffusion source for grain boundary diffused Nd-Fe-B magnets. Tb-rich shell structure, potential synergistic effect of Al-Ga, interface between diffusion source and the magnets were analyzed. When the magnets were processed at 875 °C for 10 h, the coercivity increased by 123 % from 10.38 kOe to 23.15 kOe and the remanence and the maximum magnetic energy product only decreased by 3 %. Tb element mainly formed shell structure with high magneto-crystalline anisotropy field surround the main phase grains. Non-magnetic Al element was distributing around the main phase grains as network structure, which could well isolate the main phase grains. The Tb-rich shell with high magneto-crystalline anisotropy field and the non-magnetic phase with magnetic exchange decoupling between the adjacent grains contributed to substantial increase in the coercivity of the grain boundary diffused magnets. The grain boundary phase was wetted by Al element, which promoted Tb element diffused into the magnets more deeply.

Magnetic bond-order potential for iron-cobalt alloys

For large-scale atomistic simulations of magnetic materials, the interplay of atomic and magnetic degrees of freedom needs to be described with high computational efficiency. Here we present an analytic bond-order potential (BOP) for iron-cobalt, an interatomic potential based on a coarse-grained description of the electronic structure. We fitted BOP parameters to magnetic and non-magnetic density functional theory (DFT) calculations of Fe, Co, and Fe-Co bulk phases. Our BOP captures the electronic structure of magnetic and nonmagnetic Fe-Co phases. It provides accurate predictions of structural stability, elastic constants, phonons, point and planar defects, and structural transformations. It also reproduces the DFT-predicted sequence of stable ordered phases peculiar to Fe-Co and the stabilization of B2 against disordered phases by magnetism. Our Fe-Co BOP is suitable for atomistic simulations with thousands and millions of atoms.

Multiple surface states, nontrivial band topology, and antiferromagnetism in GdAuAl4Ge2

Magnetic topological states of matter provide a fertile playground for emerging topological physics and phenomena. The current main focus is on materials whose magnetism stems from 3d magnetic transition elements, e.g., MnBi2Te4, Fe3Sn2, and Co3Sn2S2. In contrast, topological materials with the magnetism from rare earth elements remain largely unexplored. Here we report rare earth antiferromagnet GdAuAl4Ge2 as a candidate magnetic topological metal. Angle resolved photoemission spectroscopy (ARPES) and first-principles calculations have revealed multiple bulk bands crossing the Fermi level and pairs of low energy surface states. According to the parity and Wannier charge center analyses, these bulk bands possess nontrivial Z 2 topology, establishing a strong topological insulator state in the nonmagnetic phase. Furthermore, the surface band pairs exhibit strong termination dependence which provides insight into their origin. Our results suggest GdAuAl4Ge2 as a rare earth platform to explore the interplay between band topology, magnetism and f electron correlation, calling for further study targeting on its magnetic structure, magnetic topology state, transport behavior, and microscopic properties.

Single crystal synthesis and low-lying electronic structure of V3S4

We report successful growth of millimeter-sized high quality single crystals of V3S4, a candidate topological semimetal belonging to a low-symmetry space group and consisting of only low atomic number elements. Using density functional theory calculations and angle-resolved photoemission spectroscopy, we show that the nonmagnetic phase of monoclinic V3S4 hosts type-II Dirac-like quasiparticles which opens a sizable gap due to spin orbit coupling, as well as theoretical multiple nodal lines that are eliminated also by spin orbit coupling. These results suggest that relativistic effects give rise to profound modifications of the topological properties even in compounds with low-weight elements.

Ab initio study of the newly proposed full Heusler alloy Co2CuAl : Insights from DFT

The Wien2k code is used to investigate the structural and electronic properties of Co2CuAl using tools of Density Functional Theory (DFT). In light of different exchange correlation potentials, we have obtained the information about optimized crystal structural, density of states (DOS), and band structure of possible non-magnetic and spin polarisation phases of this alloy. Structure optimization analysis revealed that the alloy is stable in the spin-polarised (SP) phase. Further the analysis of electronic DOS and band structure show the presence of anisotropic semi-metallic behaviour and indicate the potential of Co2CuAl to be a novel Heusler alloy.

Quantitative characterization and magnetic separation of copper pyrometallurgical tailing for molybdenum and copper pre-concentration and cleaning of environmentally hazardous elements

The comprehensive and harmless utilization of copper slag flotation tailing (CSFT) is the key to a waste-free and sustainable copper industry. Here, the mineralogy and molybdenum micro-dissemination in CSFT were quantified and characterized, and the feasibility of the comprehensive recovery of Mo, Cu and Fe resources from CSFT and hazardous elements fractionation behavior by magnetic separation was discussed. To investigate the occurrence and abundance of Mo in each phase, CSFT was classified into four types of phases, including magnetite/hematite (27.26%), silicate associations (43.37%), and metallic sulphides and oxides. Molybdenum distribution is closely related to magnetite/hematite-Fe and S. The magnetic separation results indicated that Mo and Cu were enhanced in magnetic products by 34–41 and 15–21%, respectively. Cleaner non-magnetic residues were found to decrease significantly by 37–44, 58–60, and 11–19% for Cu, As, and Cr, respectively. Mineral fractionation was observed instead of chemical changes during magnetic separation. Despite a weak separation effect on magnetic and non-magnetic phases due to their close bonds and fine-disseminated minerals, the Fe-silicate associations without magnetic phases were well separated into non-magnetic residues. The enrichment of Ca, Mg and separation of an iron-silicate component in non-magnetic residues enhanced the cementitious property and allowed the development of more pathways of reutilization.

Ferromagnetic Mn–Al–C L10 Formation by Electric Current Assisted Annealing

The ferromagnetic Mn–Al–C τ‐phase ( tetragonal structure) shows intrinsic potential to be developed as a permanent magnet; however, this phase is metastable and is easily decomposed to nonmagnetic stable phases, affecting negatively the magnetic properties. Giving the necessity to careful control of its synthesis, the use of a novel approach is investigated using electric current–assisted annealing to obtain pure τ‐phase samples. The temperature and electrical resistance of the samples are monitored during annealing and it is shown that the change in resistance can be used to probe the phase transformation. Upon increase of electric current density, the required temperature for the ferromagnetic phase formation is reduced, reaching a maximum shift of 140 °C at 45 A mm−2. Even though this noticeable shift is achieved, the magnetic properties are not affected showing coercivity of 0.13 T and magnetization of 90 Am2 kg−1, independently from the electric current density used during annealing. Microstructural investigation reveals the nucleation of the τ‐phase at the grain boundaries of the parent ε‐phase. In addition, the existence of twin boundaries upon nucleation and growth of the metastable phase for all evaluated annealing conditions is observed, resulting in similar extrinsic magnetic properties.

Topological Phase Transition and Lattice Dynamical Properties of Half‐Heusler XPtBi (X = Gd, Nd)

Topological materials ranging from insulator to semimetals depending upon the band degeneracies are attracting the interest of researchers nowadays due to their fascinating physical properties. Among these, half‐Heuslers GdPtBi and NdPtBi have been theoretically predicted to be topological semimetals. The structural stability and electronic properties of the proposed compounds are computed within the framework of density functional theory. The results of structural optimization show that the ferromagnetic phase of the considered compounds is more stable than nonmagnetic phase. Moreover, the inclusion of magnetization in both these compounds generates Weyl nodes in the compounds. The dynamical stability of XPtBi (X = Gd and Nd) is confirmed by the computed vibrational properties of the compounds. Further, the thermodynamical parameters such as specific heat, entropy, Debye's temperature, and thermal expansion have been investigated by applying Debye's framework based on quasi‐harmonic approximation in the temperature range from 0 to 1000 K and pressure range 0–5 GPa. This work will encourage experimentalists to further study these materials for practical utilization.

Eco-friendly and economical preparation of permanent ferrite magnet with ultrapure magnetite concentrate: Focus on the action mechanism of silicon and aluminum impurities

In this study, ultrapure magnetite concentrate (UMC) was used to replace high-quality iron oxide red (IOR) for permanent ferrite magnet (PFM) preparation. UMC needs to be pre-oxidized to eliminate FeO which easily generates nonmagnetic phases in PFM. The magnetic powder and PFM products prepared with UMC and IOR had close magnetic properties, but the calcination temperature was lower and calcination time was shorter with UMC. Si and Al impurities in UMC could not only form eutectic compounds but also dissolve into the ferrite grain by substituting Fe. Those eutectic compounds benefited uniformity of grains by inhibiting grain overgrowth but decreased the purity of PFM because they were non-magnetic substances. Mn impurity in IOR entered into the ferrite grain by replacing Fe instead of participating in the formation of eutectic mixtures, while Cl impurity was evaporated during calcination, causing equipment erosion and environment pollution. UMC has the advantages of low raw material cost, good product property and non-pollution, and thus it has a broad application prospect in PFM preparation.

Determination of the Boundaries of the Non-Magnetic Phase of the Dual-Phase Magnetic Material of the Starter-Generator Rotor

Dual-phase magnetic material - a magnetic material in which, by processing, localized non-magnetic areas can be obtained to control the direction of the magnetic flux. Such properties can significantly improve the output characteristics of a number of topologies of electrical machines, as well as neutralize a number of the main disadvantages inherent in traditional designs of electrical machines. This paper presents a numerical study of the strength characteristics of the rotor of an electric starter-generator with permanent magnets with a dual-phase magnetic material. The optimal boundaries of racks for processing in the non-magnetic phase are determined.

Experimental study on the melting performance of magnetic NEPCMs embedded in metal foam subjected to a non-uniform magnetic field

This experimental study aims to evaluate the melting performance of non-magnetic and magnetic nano-enhanced phase change materials (NEPCMs) composited with metal foam inside a cavity heated from the left in a non-uniform magnetic field via thermochromic liquid crystal (TLC) visualization. The evolutions of solid-liquid interface and mushy zone during melting are captured by TLC technique and temperature distributions are recorded by ten thermocouples. The effects of particles concentrations of NEPCM (0 wt%, 1 wt%, 3 wt%, 5 wt%), filling arrangements of metal foam (filling height, porosity gradient direction) and directions of magnetic field (left, right) are studied with emphasis on the heat transfer and energy storage characteristics including liquid fraction, energy stored and Nusselt number. The results show that comparing to pure PCM, the 3 wt% Al 2 O 3 NEPCM maximizes the enhancement of melting performance for decreasing complete melting time as 29.32% and increasing energy storage as 0.26%. For partial porous filling, as the filling height increases, the melting rate firstly increases then decreases, but energy stored shows the opposite trend. The filling height of 3/4h is considered as the optimal filling height with 43.51% decreasing of complete melting time and only 6.81% decreasing of energy stored. For gradient porosity filling, positive gradient has faster melting rate (62.16% amplification), but negative gradient has less energy loss (only 6.30% reduction). Under a non-uniform magnetic field, the convection of magnetic NEPCM is greatly suppressed by the Kelvin force, with 6.76% and 19.59% increasing of complete melting time and 0.07% reduction and 2.45% growth of energy stored for left and right magnet cases, respectively. • Conduct experiments to explore the effect of magnetic field on NEPCM melting performance. • Use TLC technique to capture evolution of melting front and mushy zone. • Gradient porosities of full filling can improve the melting performance but decrease energy storage. • Kelvin force suppresses the convection of magnetic NEPCM and reduces the width of mushy zone.

Pressure effect on the structural, electronic, and magnetic properties of the battery cathode material LiMn2O4: An ab-initio study

LiMn2O4 is a battery cathode material with desirable properties such as low cost, low toxicity, high natural abundance of Mn, and environmental compatibility. By means of first-principles calculations, we study the structural, magnetic, and electronic properties of LiMn2O4 under ambient conditions and high hydrostatic pressures (until 20 GPa). We obtain two oxidation states for Mn, even using a cubic structure, which differ in all analyzed properties: structural, electronic, and magnetic. At P > 0, such properties were found to display a standard behavior decreasing smoothly and linearly with pressure. Furthermore, the enthalpy of cubic and orthorhombic structures under low and high-pressure conditions were examined, showing that no cubic to orthorhombic phase transition exists in all the investigated pressure range, nor is a magnetic cubic to a non-magnetic cubic phase transition possible.

Formation and Behaviour of Active Droplets and Bubbles in a Magnetic Fluid in an Inhomogeneous Magnetic Field

This work proposes a new technique for creating active bubbles and droplets with a non-magnetic core and a coating formed by a magnetic fluid. The procedure consists of the injection of a non-magnetic phase into a magnetic one that is supported by the presence of an inhomogeneous magnetic field from the source, which combines an annular magnet and an electromagnet. We explored various modes leading to different active bubbles and drops as well as the influence of the magnetic field on the size, velocity, and acceleration of the formed active droplets. It is shown that active bubbles change their trajectory under the action of a constant magnetic field and also disintegrate under the action of a pulsed one. This provides a new mechanism for controlling the absorption of droplets and bubbles using a magnetic field. Therefore, these results can be applied to create droplet-based microfluidics systems, in which an inhomogeneous magnetic field can be used for focusing droplet and bubble flows in a magnetic fluid.

Effect of sodium addition on structural and magnetic properties of solid state processed SrFe12−xAlxO19 (x ≤ 4)

Aluminum-substituted M-type strontium hexaferrite (SrM-Al, SrFe12−xAlxO19, 0 ≤ x ≤ 4) with uniform microstructure and excellent magnetic properties was synthesized by conventional ceramic processing based on solid state reaction. Coercive field was highly enhanced by heavy doping with Al3+ although magnetization was significantly reduced. However, formation of coarse microstructure and poor development of Hc with low level doping of Al3+ disturbed practical application. To promote phase formation and to suppress growth of coarse grains which were induced by incorporation of Al3+ ions, SrM-Al ceramics was processed with NaOH. Such Na-assisted processing of SrM-Al reduced formation of non-magnetic phases and assisted development of uniform fine grains. In addition, the large residual strain by Al3+ doping was partially relieved by the same approach. As a result, saturation magnetization of SrFe12−xAlxO19 was remarkably enhanced and the BHmax of randomly oriented hard magnetic ceramics was improved to 1.23MGOe by concomitant addition of Na and Al ions.

Intermediate phase induced by dilution in a correlated Dirac Fermi system

Substituting magnetic ions with nonmagnetic ions is a new way to study dilution. Using determinant quantum Monte Carlo calculations, we investigate an interacting Dirac fermion model with the on-site Coulomb repulsion being randomly zero on a fraction $x$ of sites. Based on conductivity, density of states and antiferromagnetic structure factor, our results reveal a novel intermediate insulating phase induced by the competition between dilution and repulsion. With increasing doping level of nonmagnetic ions, this nonmagnetic intermediate phase is found to emerge from the zero-temperature quantum critical point separating a metallic and a Mott insulating phase, whose robustness is proven over a wide range of interactions. Under the premise of strongly correlated materials, we suggest that doping nonmagnetic ions can effectively convert the system back to the paramagnetic metallic phase. This result not only agrees with experiments on the effect of dilution on magnetic order but also provides a possible direction for studies focusing on the metal-insulator transition in honeycomb lattice like materials.

Modification on grain boundary phase distribution in PrNd-Cu alloy diffused hot-deformed Nd-Fe-B magnets

Grain boundary diffusion using PrNd-Cu alloy followed pressure induction were applied to optimize the preferential distribution of the diffused eutectic phases in hot-deformed Nd-Fe-B magnets. A value of high coercivity 2.5 T was achieved while the coercivity of the sample with grain boundary modification was then decreased, and the remanence was significantly increased due to the improved grain orientation and reduced volume fraction of the nonferromagnetic phase. Magnetic domain structures of samples were observed and magnetic isolation by diffused nonmagnetic phases was also obtained. The corresponding magnetic domain structure analysis indicated that coercivity was close correlated with the magnetic isolation of grains by the diffused phases. The coercivity mechanisms of the samples were assumed to be domain wall pinning model by the fitted linear relationship of Hcj1/2 versus T2/3 and the initial magnetization curve. The Lorentz TEM investigation confirmed the domain wall pinning at grain boundaries parallel to c-axis.

Competing spin-orbital singlet states in the 4d4 honeycomb ruthenate Ag3LiRu2O6

When spin-orbit-entangled $d$-electrons reside on a honeycomb lattice, rich quantum states are anticipated to emerge, as exemplified by the $d^5$ Kitaev materials. Distinct yet equally intriguing physics may be realized with a $d$-electron count other than $d^5$. We found that the layered ruthenate Ag$_3$LiRu$_2$O$_6$ with $d^4$ Ru$^{4+}$ ions at ambient pressure forms a honeycomb lattice of spin-orbit-entangled singlets, which is a playground for frustrated excitonic magnetism. Under pressure, the singlet state does not develop the expected excitonic magnetism but experiences two successive transitions to other nonmagnetic phases, first to an intermediate phase with moderate distortion of honeycomb lattice, and eventually to a high-pressure phase with very short Ru-Ru dimer bonds. While the strong dimerization in the high-pressure phase originates from a molecular orbital formation as in the sister compound Li$_2$RuO$_3$, the intermediate phase represents a spin-orbit-coupled $J$-dimer state which is stabilized by the admixture of upper-lying $J_{\rm eff} = 1$-derived states. We argue that the $J$-dimer state is induced by a pseudo-Jahn-Teller effect associated with the low-lying spin-orbital excited states and is unique to spin-orbit-entangled $d^4$ systems. The discovery of competing singlet phases demonstrates rich spin-orbital physics of $d^4$ honeycomb compounds and paves the way for realization of unconventional magnetism.

Mean-Field Study of Antiferromagnetic and Antiferroquadrupolar Orderings in Tetragonal CeCoSi

We investigate the stability of the multipolar orderings in f-electron material CeCoSi based on a self-consistent mean-field calculation for the effective localized model. This material has two ordered phases in the temperature–pressure phase diagram: the antiferromagnetic phase and the nonmagnetic phase, the latter of which has been suggested to be an antiferroquadrupolar phase. Meanwhile, the origin of the antiferroquadrupolar phase has been unclear, since a quadrupole degree of freedom is present only between the ground-state level and highly separated excited-state level under a tetragonal crystalline electric field (CEF), whose energy scale is much larger than the transition temperature. To understand the sequence of the phase transition from the paramagnetic phase, antiferroquadrupolar phase, and antiferromagnetic phase when decreasing the temperature, we examine the important interaction parameters in the effective localized model. We clarify that the 3z2 − r2-type of the antiferroquadrupolar interactions can renormalize the CEF level splitting, which might assist a quadrupolar ordering even in a tetragonal system without orbital degeneracy. Moreover, the stability of the antiferroquadrupolar and antiferromagnetic states in a magnetic field and the behavior of the magnetic/quadrupolar susceptibility are also presented for the information to identify the unknown order parameter in the nonmagnetic ordered phase.