Stable Magnetic Structures Research Articles

Effect of substitutional Sb doping on the structural stability, half-metallicity, elastic properties, electronic properties, and magnetism of the Co₂MnSn full Heusler compound

We conducted Density Functional Theory (DFT) calculations using the full potential linearized augmented plane wave (FP-LAPW) method to modify the Fermi level by introducing Sb doping into Co2MnSn.This was done through the Generalized Gradient Approximation (GGA) and modified Becke-Johnson (mBJ-GGA) formalisms to enhance spin polarization and suggest signs of half-metallicity. For both ferromagnetic and paramagnetic phases, lattice optimization was performed to identify the stable magnetic structure. The results designate that the doped alloys are stable in a ferromagnetic phase with L21 structure. The calculated elastic constants suggest that the doped alloys meet the criteria for mechanical stability. The magnetism in these compounds is primarily due to the localized moments on the Mn and Co atoms. According to mBJ-GGA calculations, the total magnetic moment slightly increases with Sb doping. Analysis of the optical constants revealed the optoelectronic properties, confirming semiconducting behavior. Doping elements to achieve half-metallicity is demonstrated to be an effective method for discovering new materials suitable for spintronics applications.

Revisiting High-energy Polarization from Leptonic and Hadronic Blazar Scenarios

X-ray and MeV polarization can be powerful diagnostics for leptonic and hadronic blazar models. Previous predictions are mostly based on a one-zone framework. However, recent IXPE observations of Mrk 421 and 501 strongly favor a multizone framework. Thus, the leptonic and hadronic polarization predictions need to be revisited. Here we identify two generic radiation transfer effects, namely, double depolarization and energy stratification, that can have an impact on the leptonic and hadronic polarization. We show how they are generalized from previously known multizone effects of the primary electron synchrotron radiation. Under our generic multizone model, the leptonic polarization degree is expected to be much lower than the one-zone prediction, unlikely detectable in most cases. The hadronic polarization degree can reach a value as high as the primary electron synchrotron polarization during simultaneous multiwavelength flares, consistent with the one-zone prediction. Therefore, IXPE and future X-ray and MeV polarimeters such as eXTP, COSI, and AMEGO-X, have good chances to detect hadronic polarization during flares. However, the hadronic polarization cannot be well constrained during the quiescent state. Nonetheless, if some blazar jets possess relatively stable large-scale magnetic structures, as suggested by radio observations, a nontrivial polarization degree may show up for the hadronic model after a very long exposure time (≳1 yr).

Large barocaloric and magnetocaloric effects in MnFe0.8+xNi1.2−xSiGe0.5 high-entropy intermetallics

High-entropy intermetallics (HEIs) are a category of materials that exhibit special functional properties. Among the many excellent properties of high-entropy materials, the magnetocaloric performance has a promising future. We design of HEIs of MnFe0.8+xNi1.2−xSiGe0.5 (x = 0, 0.2, 0.3, and 0.4) based on the MnNiSi. The change of Fe content establishes a stable magnetic structure transition from paramagnetic hexagonal phase to ferromagnetic orthorhombic phase, and a large magnetic entropy change of 25.8 and 32.8 J·kg−1 K−1 is obtained in x = 0.2 and 0.3 materials under the field of 3 T. Moreover, the transition temperature of the MnFeNiSiGe0.5 HEI decreases with increasing pressure, and a low-pressure-induced large reversible barocaloric effect is achieved with the transformation entropy change of 43.72 J·kg−1 K−1 by applying hydrostatic pressure of 10 kbar. Our findings demonstrate that MnFe0.8+xNi1.2−xSiGe0.5 HEIs have promising potential as high-performance solid state refrigeration materials.

Structural and electronic properties of Ba[formula omitted]TiMnO[formula omitted] from first principles

In this study, we present ab-initio calculations of the structural, spin arrangement properties, and electronic properties of the Ba2TiMnO6 double perovskite. Our calculations employ the Full-Potential Linear Augmented Plane-Wave method (FP-LAPW) method and both Local Spin Density (LSDA) and Generalized Gradient (GGA) approximations. We additionally take into account the relativistic effects of spin–orbit coupling and the Hubbard term corrections U and J. According to our findings, the most stable magnetic structure is antiferromagnetic, with collinear Mn spins aligned along the [001] direction and an ordered moment μord of roughly 2.56 μB. It turns out that μord drops with increasing the Hubbard parameter, down to 2.28 μB at U = 4 and J = 1 eV. The careful analysis of the density of states (DOS) and the electronic band structure (EBS) reveals that Ba2TiMnO6 is a direct band gap semiconductor with a semiconducting energy gap of approximately Eg = 0.98 eV. The effects of U-J parameters were examined for Eg. We found that in opposite to μord, Eg shows an upward trend with U, reaching a value of 1.33 eV at U = 5 eV, J = 1 eV. We compared our findings to those found in the literature.

Study on the magnetic viscosity of multi-step magnetized heterogeneous alloys

The multi-step magnetization behavior caused by a variety of microstructures is common in magnetic materials. The study of the effect of heterogeneous structure magnetization behavior on magnetic viscosity provides extensive guidance for the long-term stability of magnetic materials. The microstructure of Fe–Co–Ni–Al–Ti–Cu alloy was regulated by heat treatment process, and homogeneous structure (S810) and heterogeneous structure (S830) samples were prepared. In the S830 sample, there are three steps of magnetization reversal due to the presence of α1′ phases (split α1 phases). Part of the α1′ phases are reversed under a lower magnetic field, and the other part has strong interaction with the α1 phases forming a closed stable magnetic domain structure, which enhances the switch field of α1 phases. Both first-order reversal curves (FORC) analysis and electron holography have confirmed this phenomenon. The magnetic viscosity coefficient of S830 is higher, but its magnetic field dependence is better. The quantitative calculation of energy barriers shows that the energy barriers of α1′ phases are lower, which leads to the instability of S830 sample under low magnetic fields. The superposition of three magnetization reversal steps makes the variation of magnetic viscosity coefficient gentler.

Crystal structure and magnetic properties of EuZrO3 solid solutions

The stable magnetic structure of EuZrO3 solid solution is tuned with the change of lattice volume. In particular, the ferromagnetic state is stabilized by the increase in lattice volume, which experimentally verifies the theoretical prediction.

Tailoring the magnetic exchange interaction in MnBi2Te4 superlattices via the intercalation of ferromagnetic layers

The intrinsic magnetic topological insulator MnBi2Te4 (MBT) provides a platform for the creation of exotic quantum phenomena. Novel properties can be created by modification of the MnBi2Te4 framework, but the design of stable magnetic structures remains challenging. Here we report ferromagnet-intercalated MnBi2Te4 superlattices with tunable magnetic exchange interactions. Using molecular beam epitaxy, we intercalate ferromagnetic MnTe layers into MnBi2Te4 to create [(MBT)(MnTe)m]N superlattices and examine their magnetic interaction properties using polarized neutron reflectometry and magnetoresistance measurements. Incorporation of the ferromagnetic spacer tunes the antiferromagnetic interlayer coupling of the MnBi2Te4 layers through the exchange-spring effect at MnBi2Te4/MnTe hetero-interfaces. The MnTe thickness can be used to modulate the relative strengths of the ferromagnetic and antiferromagnetic order, and the superlattice periodicity can tailor the spin configurations of the synthesized multilayers.

Magnetic domain structure of the ferrimagnetic (001) NiCo2O4 film with perpendicular magnetic anisotropy

A 65-nm thick epitaxial (001) NiCo2O4 film that has metallic and ferrimagnetic properties at room temperature was grown on a (001) MgAl2O4 substrate. Evolutions in magnetic domains of the ferrimagnetic (001) NiCo2O4 film with perpendicular magnetic anisotropy were observed using polar magneto-optical Kerr effect microscopy with changes in magnetic field, temperature, or time. The magnetic domain structure in the magnetic field exhibited noticeable changes around a coercive field; whereas, its change in temperature or time at a fixed magnetic field was indistinct. This suggests that the ferrimagnetic (001) NiCo2O4 film has a relatively stable magnetic domain structure in comparison with Co alloy films with perpendicular magnetic anisotropy and fast domain wall motion. Conclusively, the magnetic domain variation in the NiCo2O4 film signified that the magnetization process at an early stage was mainly caused by domain rotation behavior rather than domain wall motion.

The temperature–strain phase diagrams of ferromagnetic thin films under different magnetic fields

The topological magnetic structures in ferromagnetic thin films, such as magnetic skyrmions, are considered as the potential information carriers for future spintronics memory and logic devices due to their topological stability and controllability. In the application, ferromagnetic thin films often experience different temperatures, strains and magnetic fields. To understand the stability of topological magnetic structures in ferromagnetic thin films under different external conditions is not only of scientific significance but also of practical importance. In this work, a temperature-dependent real space phase field model is proposed to investigate the stable topological magnetic structures in ferromagnetic thin films under different magnetic fields, temperatures and strains. The skyrmions phase, helical phase and ferromagnetic phase are predicted in the ferromagnetic thin films with different magnetic fields, temperatures and strains. The strain is applied in the plane of the films, whereas the magnetic field is applied perpendicular to the plane of the thin films. The temperature–strain phase diagrams of ferromagnetic thin films are constructed under different magnetic fields. It is found that a tensile biaxial strain enhances the stability of skyrmions while skyrmions gradually become unstable when the biaxial strain changes from tensile to compressive. For the uniaxial strain, however, skyrmions can be stabilized under both tensile and compressive strains, which indicates the uniaxial strain is more preferable than biaxial strain for the stability of skyrmions.

MagGene: A genetic evolution program for magnetic structure prediction

We have developed a software MagGene to predict magnetic structures by using genetic algorithm. Starting from an atom structure, MagGene repeatedly generates new magnetic structures and calls first-principles calculation engine to get the most stable magnetic structure. This software is applicable to both collinear and noncollinear systems. It is particularly convenient for predicting the magnetic structures of atomic systems with strong spin–orbit couplings and/or strong spin frustrations. Program summaryProgram Title:MagGeneCPC Library link to program files:https://doi.org/10.17632/m83gcp5z48.1Licensing provisions: MITProgramming language: Fortran 90Nature of problem: In complex magnetic systems, such as systems with strong spin–orbit couplings and/or strong spin-frustrations, the associated magnetic structures could also be quite complex. The traditional methods, such as theoretical analysis based on crystal symmetries and simulated annealing based on a special model Hamiltonian, are inefficient. Then the electronic structures, spin wave dispersions, and other properties based on correct magnetic structures could not be obtained accurately. Therefore, an efficient method to predict magnetic structures is required.Solution method: The magnetic structures can be predicted efficiently by using genetic algorithm.Additional comments including unusual features: The magnetic structures can be predicted for both collinear and noncollinear atomic systems by using genetic evolution algorithm. It is flexible to use in variety of systems. The total magnetic moment can be fixed.

Computational screening of Fe-Ta hard magnetic phases

In this paper, we perform a systematic calculation of the Fe-Ta phase diagram to discover hard magnetic phases. By using structure prediction methods based on evolutionary algorithms, we identify two energetically stable magnetic structures: a tetragonal ${\mathrm{Fe}}_{3}\mathrm{Ta}$ (space group 122) and a cubic ${\mathrm{Fe}}_{5}\mathrm{Ta}$ (space group 216) binary phase. The tetragonal structure is estimated to have both high saturation magnetization (${\ensuremath{\mu}}_{0}{\mathrm{M}}_{s}=1.14$ T) and magnetocrystalline anisotropy (${\mathrm{K}}_{1}=2.17 \mathrm{MJ}/{\mathrm{m}}^{3}$) suitable for permanent magnet applications. The high-throughput screening of magnetocrystalline anisotropy also reveals two low-energy metastable hard magnetic phases: ${\mathrm{Fe}}_{5}{\mathrm{Ta}}_{2}$ (space group 156) and ${\mathrm{Fe}}_{6}\mathrm{Ta}$ (space group 194), that may exhibit intrinsic magnetic properties comparable to ${\mathrm{SmCo}}_{5}$ and ${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$, respectively.

Continuum Enhancements, Line Profiles, and Magnetic Field Evolution during Consecutive Flares

Abstract During solar flares, magnetic energy can be converted into electromagnetic radiation from radio waves to γ-rays. Enhancements in the continuum at visible wavelengths, as well as continuum enhancements in the FUV and NUV passbands, give rise to white-light flares. In addition, the strong energy release in these events can lead to the rearrangement of the magnetic field at the photospheric level, causing morphological changes in large and stable magnetic structures like sunspots. In this context, we describe observations acquired by satellite instruments (Interface Region Imaging Spectrograph (IRIS), Solar Dynamics Observatory/Helioseismic and Magnetic Imager, Hinode/Solar Optical Telescope) and ground-based telescopes (Rapid Oscillations in the Solar Atmosphere (ROSA)/Dunn Solar Telescope) during two consecutive C7.0 and X1.6 flares that occurred in active region NOAA 12205 on 2014 November 7. The flare was accompanied by an eruption. The results of the analysis show the presence of continuum enhancements during the evolution of the events, observed both in ROSA images and in IRIS spectra. In the latter, a prominent blueshifted component is observed at the onset of the eruption. We investigate the role played by the evolution of the δ sunspots of the active region in the flare triggering, and finally we discuss the changes in the penumbrae surrounding these sunspots as a further consequence of these flares.

Topology analysis for anomalous Hall effect in the noncollinear antiferromagnetic states of Mn3AN (A=Ni, Cu, Zn, Ga, Ge, Pd, In, Sn, Ir, Pt)

We investigate topological features of electronic structures which produce large anomalous Hall effect in the non-collinear antiferromagnetic metallic states of anti-perovskite manganese nitrides by first-principles calculations. We first predict the stable magnetic structures of these compounds to be non-collinear antiferromagnetic structures characterized by either $T_{1g}$ or $T_{2g}$ irreducible representation by evaluating the total energy for all of the magnetic structures classified according to the symmetry and multipole moments. The topology analysis is next performed for the Wannier tight-binding models obtained from the first-principles band structures. Our results reveal the small Berry curvature induced through the coupling between occupied and unoccupied states with the spin-orbit coupling, which is widely spread around the Fermi surface in the Brillouin zone, dominantly contributes after the $k$-space integration to the anomalous Hall conductivity, while the local divergent Berry curvature around Weyl points has a rather small contribution to the anomalous Hall conductivity.

Long quasi-periodic oscillations of the faculae and pores

Aims. The main goal of this work is to analyze the structural and temporal evolution of small-scale magnetic structures (SSMSs) observed in the solar atmosphere, such as solitary faculae and pores, and reveal long quasi-periodic oscillations of these structures. Methods. The statistical method of regression analysis and the wavelet transform were used to obtain the periods of oscillations and dependences between the parameters of magnetic structures and periods of oscillations. Results. Long-period oscillations with periods in the interval of 18−260 min are found for the structurally stable phase of SSMSs at the level of the solar photosphere. These long-period oscillations were interpreted as natural oscillations of the structurally stable long-lived magnetic structures around their equilibrium position. These oscillations, which are of similar nature, are observed in the chromospheric bright formations associated with photospheric SSMSs. Dependences between the magnetic field and the continuum intensity of the facula elements were found. It is shown that the continuum intensity of a SSMS decreases when its magnetic field increases.

Comparative study of magnetic and electronic properties of room‐temperature polar magnets ScFeO3 and InFeO3

We performed calculations based on collinear and non‐collinear spin density functional theory to investigate magnetic and electronic properties of new room‐temperature polar magnetic compounds AFeO3 (A = Sc, In). Exchange and correlation effects were simulated by standard local spin density approximation and the generalized gradient approximation. We determined that the canted G‐type antiferromagnetic (G‐AFM) structure of the Fe spin moments is the most stable magnetic structure for both compounds, in accordance with experimental facts. The calculated Fe spin magnetic moments and resulting weak ferromagnetic components perpendicular to the c‐axis also agree well with the experiment. The magnitude of the Fe spin moment does not depend on the fact if the G‐AFM structure is collinear or canted. The energy difference between these two structures is found to be much larger for InFeO3, so as the exchange interaction constant Jnn between nearest‐neighbor magnetic Fe ions. The ScFeO3 and InFeO3 electronic structures, which reproduce well their magnetic properties are presented and discussed. By comparing them with the band structures of some typical multiferroics, we conclude that different electronic mechanisms should be responsible for their ferroelectric distortion. In ScFeO3, the cause of this distortion should be attributed to the hybridization between almost empty Sc 3d and occupied O 2p states along the hexagonal c‐axis. For InFeO3, however, the stereochemical activity of the In 5s localized electrons should be the principal driving force for ferroelectric distortion.

The subtlety of resolving orbital angular momenta in calculating Hubbard U parameters in the density functional tight-binding theory and its delicacy is illustrated by the calculated magnetic properties of carbon clusters

We study magnetic properties of carbon clusters Cn by combining the spin-polarized parametrized density functional tight-binding (SDFTB) theory with an unbiased modified basin hopping (MBH) optimization algorithm. With an intent to develop a physically self-consistent technique, we deliberately treat valence electronic charges, spin charges and ionic charges on equal footing. Within the density functional tight-binding (DFTB) theory, we examine the effect of using the orbital angular-momentum unresolved and resolved schemes in calculating the on-site Coulombic energy which, we judge, will have subtle but significant influence on the Cn’s magnetism, their topologies and the change with size n their conformational structures. As a concrete means to substantiate our conjecture, we apply the SDFTB/MBH method to Cns and determine their stable magnetic structures within the angular-momentum resolved scheme. Our calculations show that the lowest energy Cn changes from a linear-chain shape for n = 3–9, turns over to a singlet monocyclic ring for n = 10–18, becomes a polycyclic ring for n = 19–25 and finally assumes a cage-like geometry at n = 26. Except for n = 4, 6 and 8, all other Cns are found unmagnetized. Accordingly, the newly discovered n that marks the first occurrence of a bi- to tridimensional transition occurs at n = 26, and this size is in contrast to n = 24 which was predicted by similar calculations using the unresolved scheme. Our calculations reveal furthermore two different features. The first one is that the predicted optimized geometries for all of the Cns, except for C24, are structurally the same as the size n = 3–23 of Cn calculated by the DFTB/MBH employing the unresolved orbital angular-momentum scheme. As a result, the present calculations which employ the resolved angular-momentum scheme thus showed that the latter affects only the larger size Cn starting at C24; this finding redefines therefore the turnover transition point of Cn from a bidimensional planar at n = 25 to a tridimensional cage-like at n = 26. The second feature is that the SDFTB/MBH method yields only triplet C4, C6 and C8, whereas in our previous work employing DFTB/MBH, not only C4 and C6, all of C13, C15, C17, C19, C22 and C23 were predicted to carry a magnetic moment of 2 μB. These differences in the magnetism obtained are attributed to the combined use of both the SDFTB/MBH procedure and the orbital angular-momentum resolved scheme.

Suppression of the dayside magnetopause surface modes

Magnetopause surface eigenmodes were suggested as a potential source of dayside high-latitude broadband pulsations in the Pc5-6 band (frequency about 1–2 mHz). However, the search for a ground signature of these modes has not provided encouraging results. The comparison of multi-instrument data from Svalbard with the latitudinal structure of Pc5-6 pulsations, recorded by magnetometers covering near-cusp latitudes, has shown that often the latitudinal maximum of pulsation power occurs about 2–3° deeper in the magnetosphere than the dayside open-closed field line boundary (OCB). The OCB proxy was determined from SuperDARN radar data as the equatorward boundary of enhanced width of a return radio signal. The OCB-ULF correspondence is further examined by comparing the latitudinal profile of the near-noon pulsation power with the equatorward edge of the auroral red emission from the meridian scanning photometer. In most analyzed events, the “epicenter” of Pc5-6 power is at 1–2° lower latitude than the optical OCB proxy. Therefore, the dayside Pc5-6 pulsations cannot be associated with the ground image of the magnetopause surface modes or with oscillations of the last field line. A lack of ground response to these modes beneath the ionospheric projection of OCB seems puzzling. As a possible explanation, we suggest that a high variability of the outer magnetosphere near the magnetopause region may suppress the excitation efficiency. To quantify this hypothesis, we consider a driven field line resonator terminated by conjugate ionospheres with stochastic fluctuations of its eigenfrequency. A solution of this problem predicts a substantial deterioration of resonant properties of MHD resonator even under a relatively low level of back-ground fluctuations. This effect may explain why there is no ground response to magnetopause surface modes or oscillations of the last field line at the OCB latitude, but it can be seen at somewhat lower latitudes with more regular and stable magnetic and plasma structure.

Подавление поверхностных мод на дневной магнитопаузе

Magnetopause surface eigenmodes were suggested as a potential source of dayside high-latitude broadband pulsations in the Pc5-6 band (frequency about 1–2 mHz). However, the search for a ground signature of these modes has not provided encouraging results. The comparison of multi-instrument data from Svalbard with the latitudinal structure of Pc5-6 pulsations, recorded by magnetometers covering near-cusp latitudes, has shown that often the latitudinal maximum of pulsation power occurs about 2–3° deeper in the magnetosphere than the dayside open-closed field line boundary (OCB). The OCB proxy was determined from SuperDARN radar data as the equatorward boundary of enhanced width of a return radio signal. The OCB-ULF correspondence is further examined by comparing the latitudinal profile of the near-noon pulsation power with the equatorward edge of the auroral red emission from the meridian scanning photometer. In most analyzed events, the “epicenter” of Pc5-6 power is at 1–2° lower latitude than the optical OCB proxy. Therefore, the dayside Pc5-6 pulsations cannot be associated with the ground image of the magnetopause surface modes or with oscillations of the last field line. A lack of ground response to these modes beneath the ionospheric projection of OCB seems puzzling. As a possible explanation, we suggest that a high variability of the outer magnetosphere near the magnetopause region may suppress the excitation efficiency. To quantify this hypothesis, we consider a driven field line resonator terminated by conjugate ionospheres with stochastic fluctuations of its eigenfrequency. A solution of this problem predicts a substantial deterioration of resonant properties of MHD resonator even under a relatively low level of background fluctuations. This effect may explain why there is no ground response to magnetopause surface modes or oscillations of the last field line at the OCB latitude, but it can be seen at somewhat lower latitudes with more regular and stable magnetic and plasma structure.

First-principles study on the magnetic properties of ordered Nd6(Fe,Ga)14 alloys

We studied the stable magnetic structure of ordered Nd6Fe14-xGax (x = 0, 1) alloys, which appears in the grain-boundary phase of Nd-Fe-B permanent magnets, using first-principles techniques. Slight Ga doping (x = 1) was shown to contribute to the stabilization of an anti-ferromagnetic (AF) state, whereas the non-doped case (x = 0) was revealed to favor the ferromagnetic state rather than the AF state with a slight energy difference.

Conditions for magnetic and electronic properties of ultrathin Ni–Fe hydroxide nanosheets as catalysts: a DFT+U study

The effects of cation ordering and surface compensating anions on the magnetic structure and catalytic properties of unilamellar Ni–Fe hydroxide nanosheets are studied by using the density functional theory (DFT) plus U method. Fe-segregation in the nanosheets yields magnetic domains with different spin alignments, while the surface compensating anions affect the local moments and valence states of the Fe atoms. The two conditions do not radically change the super-exchange nature of interactions between the paramagnetic metal centers, but facilitate the formation of various magnetic superlattices in the nanosheets. The calculated free energy change of the intermediates shows that the most stable magnetic structure of Ni–Fe hydroxide nanosheets exhibits superior catalytic activity towards oxygen reduction/evolution reactions, which is indicative of magnetic catalyst. This is due to the cycle transition between Fe2+ and Fe3+ ions in the reactions, which determines the sequence of cleavage of the O–H bond and the release of the OH group, controlling the rate-limiting steps of the reaction. The relationship of magnetism and catalytic activity of Ni–Fe hydroxide nanosheets is established by the valence state change of the Fe ions, which will be helpful to open the way for the design of hydroxide/layered double hydroxides (LDHs)-based magnetic catalysts.