Evolution Of Magnetic Structure Research Articles

Evidence for a nearly orthogonal rotator in GX 301–2 with phase-resolved cyclotron resonant scattering features

Abstract Cyclotron resonant scattering features (CRSFs) are the absorption features in the X-ray spectra of strongly magnetized accretion neutron stars (NSs), which are probably the most reliable probe to the surface magnetic fields of NSs. The high mass X-ray binary GX 301–2 exhibits a very wide, variable and complicated CRSF in the average spectra, which should be two absorption lines based on NuStar and Insight-HXMT observations. With the Insight-HXMT frequent observations, we performed the phase-resolved spectroscopy and confirmed two cyclotron absorption lines in the phase-resolved spectra, with their centroid energy ratio ∼1.6 − 1.7 in the super-critical luminosity case. A major hindrance in understanding those CRSFs is the very poorly constrained magnetic inclination angle, which is also a fundamental property of a NS and key to understanding the emission characteristics of a pulsar. Comparing the phase-resolved CRSF with simulated X-ray spectra, the magnetic inclination angle is found to be ≳ 70○, i.e., nearly orthogonal between the NS’s spin and magnetic axies. The implications of an orthogonal rotator and magnetic structure evolution in the accreting X-ray binary are also discussed.

Effects of Electron Vortices on the Magnetic Structures in the Terrestrial Magnetosheath

AbstractElectron vortices are usually embedded within different magnetic structures in space plasmas. The effects, including the nonideal electric field, energy dissipation and magnetic field, of electron vortices on these magnetic structures are still unclear. Utilizing the unprecedented high‐resolution data from the Magnetospheric Multiscale mission in the terrestrial magnetosheath, we statistically investigate these effects on magnetic structures. Both nonideal electric fields and energy dissipation have no obvious correlations with the scales of electron vortices. However, compared to the scales, stronger correlations are found between the vorticities of electron vortices and nonideal electric fields, and energy dissipation, respectively. Most of electron vortices have positive contributions to magnetic fields of magnetic structures, such as strengthening the decrease (or increase) of Bt for current sheets and magnetic holes (or flux ropes and magnetic peaks). Our results reveal that the electron vortices play an important role in the evolution of magnetic structures.

Non-collinear magnetism driven by a hidden multipolar order in PrO2

Standard microscopic approach to magnetic orders is based on assuming a Heisenberg form for inter-atomic exchange interactions. These interactions are considered as the driving force for the ordering transition with magnetic moments serving as the primary order parameter. Any higher-rank multipoles appearing simultaneously with such magnetic order are typically treated as auxiliary order parameters rather than a principal cause of the transition. In this study, we show that these traditional assumptions are violated in the case of PrO2. Evaluating a full set of Pr-Pr superexchange interactions from a first-principles many-body technique we find that its unusual non-collinear 2k magnetic structure stems from high-rank multipolar interactions, and that the corresponding contribution of the Heisenberg interactions is negligible. The observed magnetic order in PrO2 is thus auxiliary to high-rank “hidden” multipoles. Within this picture we consistently account for previously unexplained experimental observations like the magnitude of exchange splitting and the evolution of magnetic structure in external field. Our findings challenge the standard paradigm of observable magnetic moments being the driving force for magnetic transitions.

Exploring magnetic field properties at the boundary of solar pores: A comparative study based on SDO-HMI observations

Context. The Sun’s magnetic fields play an important role in various solar phenomena. Solar pores are regions of intensified magnetic field strength compared to the surrounding photospheric environment, and their study can help us better understand the properties and behaviour of magnetic fields in the Sun. In this work, we investigate the properties of magnetic fields on the boundaries of solar pores, specifically focusing on the evolution of the vertical magnetic field. Aims. Up to now, there exists only a single study on magnetic field properties at the boundary region of a pore. Therefore, the main goal of this work is to increase the statistics of magnetic properties determining the pore boundary region. To this aim, we study the change of the vertical magnetic field on the boundaries of six solar pores and their time evolution. Methods. We analyse six solar pores using data from the Helioseismic and Magnetic Imager instrument on board the Solar Dynamics Observatory. We apply image processing techniques to extract the relevant features of the solar pores and determine the boundary conditions of the magnetic fields. For each pore, the maximal vertical magnetic field is determined, and the obtained results are compared with the above-mentioned previous study. Results. We find the maximal vertical magnetic field values on the boundaries of the studied solar pores to range from 1400 G to 1600 G, with a standard deviation between 7.8% and 14.8%. These values are lower than those reported in the mentioned preceding study. However, this can be explained by differences in spatial resolution as well as the type of data we used. For all the pores, we find that the magnetic inclination angle lies in a range of 30 ± 7°, which is consistent with the idea that the magnetic field configuration in solar pores is mainly vertical. Conclusions. The vertical magnetic field is an important factor in determining the boundary of solar pores, and it plays a more relevant role than the intensity gradient. The obtained information will be useful for future studies on the formation and evolution of magnetic structures of the Sun. Additionally, this study highlights the importance of high spatial resolution data for the purpose of accurately characterising the magnetic properties of solar pores. Overall, the findings of this work contribute to the understanding of the magnetic field properties of the Sun and will be crucial for improving models of solar dynamics and magnetic flux emergence.

Zero thermal expansion in non-stoichiometric and single-phase (Hf,Nb)Fe2.5 alloy

Zero thermal expansion (ZTE) alloys are dimensionally stable under the influence of heat, which is critical in high-precision components but remains rare among single phase materials. This work reports a single phase and nearly isotropic ZTE alloy (Hf0.8Nb0.2)Fe2.5 over a wide temperature range of 5–460 K (αl≈1.07 × 10−6 K−1) via non-stoichiometric regulation. The introduction of excess Fe breaks the dual-phase nature of parent alloy, extends the magnetic ordering temperature, and induces a spin reorientation. The evolution of magnetic structure versus temperature gives rise to the ZTE behavior. The design concept of changing the internal magnetic interaction through non-stoichiometric regulation provides support for the discovery of novel ZTE materials.

On the magnetic-structure origin of giant magnetostrictive effect in MnCoSi-based metallic helimagnets

Giant magnetostrictive effect, the huge volume and/or shape change of a material under an external magnetic field, is recently discovered in helimagnets with large magnetoelastic interactions. Here we revisit this effect in MnCoSi, a helical metamagnet with a unique Lifshitz tricritical point. By introducing minimal amount of Ni, the modified competing exchange interaction simultaneously lowers the tricritical temperature and even critical field of metamagnetism, leading to a giant, reversible, low-field magnetostrictive effect at ambient temperature. When applying a cyclic low field of 0.5 T, a giant reversible magnetostriction of 550 ppm can be achieved and maintained at 300 K. Interestingly, by advanced 3D-vector magnetic field measurement, a strong relation is found between magnetocrystalline anisotropy and the critical field of magnetostrictive effect. In-situ neutron powder diffraction results further reveal that the incommensurate helical antiferromagnetic structure with moment lying in ab plane transits into a ferromagnetic structure along b axis with increasing magnetic fields. During this metamagnetic transition, dramatical spin rotation triggers sharp anisotropic lattice change via strong magnetoelastic coupling, resulting in giant magnetostrictive effect. The saturated magnetostriction value is closely related with the amplitude of spin rotations. These findings not only design of a potential MnCoSi-based magnetostrictive material but also uncover the specific evolution of magnetic structures in the metamagnetic transition and the possible mechanism behind the macroscopic size change.

Magnetic field reversal of electric polarization and pressure-temperature-magnetic field magnetoelectric phase diagram of the hexaferrite Ba0.4Sr1.6Mg2Fe12O22

Pressure, as an independent thermodynamic parameter, is an effective tool to obtain novel material system and exotic physical phenomena not accessible at ambient conditions, because it profoundly modifies the charge, orbital and spin state by reducing the interatomic distance in crystal structure. However, the studies of magnetoelectricity and multiferroicity are rarely extended to high pressure dimension due to properties measured inside the high pressure vessel being a challenge. Here we reported the temperature-magnetic field-pressure magnetoelectric (ME) phase diagram of Y type hexaferrite Ba0.4Sr1.6Mg2Fe12O22 derived from static pyroelectric current measurement and dynamic magnetodielectric in diamond anvil cell and piston cylinder cell. We found that a new spin-driven ferroelectric phase emerged at P = 0.7 GPa and sequentially ME effect disappeared around P = 4.3 GPa. The external pressure may enhance easy plane anisotropy to destabilize the longitudinal conical magnetic structure with the suppression of ME coefficient. These results offer essential clues for the correlation between ME effect and magnetic structure evolution under high pressure.

Successive electric polarization transitions induced by high magnetic field in the single-crystal antiferromagnet Co2Mo3O8

The polar antiferromagnet ${\mathrm{Co}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$ was recently reported as a new multiferroic material exhibiting remarkable second-order magnetoelectric (ME) coupling effect while no other metamagnetic transitions occurred at low magnetic field. Herein, we have investigated the ME phenomena under high magnetic field (up to 60 T) in ${\mathrm{Co}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$ single crystals and observed unique ME response associated with the changes in the hidden magnetic moment on the honeycomb lattice. Two spin-flop transitions are unambiguously defined at ${H}_{c1}\ensuremath{\sim}27\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ and ${H}_{c2}\ensuremath{\sim}31\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ under $H$ along the $c$ axis at 1.7 K, accompanied by two successive colossal changes of electric polarization. The results on the anisotropy of magnetoelectricity as well as the angular-dependent polarization are well consistent with the ME tensor prediction, providing a better approach to understand the evolution of magnetic structures under high magnetic field. Therefore, the hidden magnetic transitions and distinctive magnetoelectricity provide a unique platform on which the ME coupling mechanism in the presence of rich magnetic phase transitions can be explored in this 238 family.

Ultrafine-grained microstructure and controllable magnetic domains in Fe-based nanocrystalline alloys with excellent magnetic properties induced by hot isostatic pressing

The coarsening and inhomogeneous distribution of nano-grains caused by traditional heat treatment processes are the biggest obstacles for obtaining ultrahigh permeability and excellent comprehensive soft magnetic properties in Fe-based nanocrystalline alloys. Here, we propose a new strategy to control grain growth and magnetic anisotropy, by introducing isotropic compressive stress field in the annealing process to optimize the magnetic properties. Compared with conventional isothermal annealing, the coupling of stress-temperature field generated by hot isostatic pressing (HIP) induced the microstructure, magnetic domain evolution and magnetic properties in Si-rich Fe73.5Si15.5B7Nb3Cu1 amorphous ribbons have been systematically investigated. We demonstrate that HIP treatment promotes more Si atoms to dissolve in Fe unit cell to form Fe(Si) solid solution causing an increase in interplanar spacing. Also, the high-density Cu clusters precipitate in the amorphous matrix upon HIP serving as nucleation sites for α-Fe(Si) grains, which facilitate a high volume fraction, uniform and ultrafine-grained microstructure. Moreover, the existence of stress field introduces induced anisotropy effectively controls the magnetization mechanism and magnetic structure evolution. Furthermore, the Fe73.5Si15.5B7Nb3Cu1 nanocrystalline alloy treated by HIP exhibit excellent comprehensive magnetic properties, such as high Ms, high μe, low Pcv, low AC Hc and high Br/Bm.

Formations of Narrow Stripes and Vortex–Antivortex Pairs in a Quasi-Two-Dimensional Ferromagnet K2CuF4

Using cryogenic Lorentz transmission electron microscopy, we investigate the temperature evolution of magnetic structures in single crystal samples of K2CuF4 with a magnetic phase transition at 6.1...

Temperature Evolution of Magnetic Structure in Fe-Co-Si-B-Mo-P Metallic Glass by Mössbauer Spectrometry

Temperature Evolution of Magnetic Structure in Fe-Co-Si-B-Mo-P Metallic Glass by Mössbauer Spectrometry

Near room temperature spin reorientation and temperature evolution of magnetic structure in Mn substituted HoFeO3

Magnetic structure and spin reorientation transition of 45% Mn substituted HoFeO3 have been investigated using bulk magnetization, neutron powder diffraction and specific heat capacity techniques. These studies confirm the presence of antiferromagnetic ordering of Fe/Mn sublattice below 336 K. The spin reorientation (SR) transition (TSR ~ 290 K) where the magnetic structure changes from Γ4(Ax, Fy, Gz) to Γ1(Gx, Cy, Az) is confirmed by Rietveld analysis of neutron diffraction patterns taken at various temperatures. Though the spin reorientation (SR) transition is happening at 290 K, the system is found to be in mixed Γ4(Ax, Fy, Gz) + Γ1(Gx, Cy, Az) phase from room temperature to 275 K. Below 275 K, the sample exhibits a pure Γ1(Gx, Cy, Az) structure in which the weak ferromagnetic component is absent. Bulk magnetization and specific heat capacity measurements also confirm the Néel temperature and spin reorientation transitions and support the neutron diffraction analysis.

Simultaneous tuning of magnetocrystalline anisotropy and spin reorientation transition via Cu substitution in Mn-Ni-Ga magnets for nanoscale biskyrmion formation

Skyrmions with multiple helicity or topology in centrosymmetric crystals are intriguing magnetic-domain objects due to their diverse dynamics under external stimuli. Here we illustrate how the two key gradients of magnetocrystalline anisotropy (MCA) and spin reorientation transition (SRT) affect the skyrmion formation and topology by Cu substitution in the biskyrmion-host MnNiGa alloy. The MCA and SRT are simultaneously tuned in a large scope, while the original high Curie temperature (${T}_{C}$) is retained. Detailed neutron-scattering studies revealed the construction of a noncollinear canted magnetic structure below the SRT temperature (${T}_{\mathrm{SR}}$), which effectively correlates the SRT with the evolution of the MCA, as well as the exchange interaction. The Cu substitution raises the ${T}_{\mathrm{SR}}$ to merge with the ${T}_{\mathrm{C}}$, and meanwhile, reduces the $c$-axis anisotropy. Lorentz transmission electron microscopy revealed the formation of stacked biskyrmions from above room temperature to lower temperatures in $\mathrm{MnN}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{u}}_{x}\mathrm{Ga}\phantom{\rule{0.28em}{0ex}}(x=0--0.3)$ in the presence of proper MCA. Micromagnetic simulations further confirmed the great effect of uniaxial anisotropy on the stabilization of biskyrmions. Our work has helped clarify the evolution of magnetic structures and their correlation to the SRT, providing an account of the effect of MCA and exchange interaction on the biskyrmion formation.

From Nano to Micro: Evolution of Magnetic Domain Structures in Multidomain Magnetite

AbstractReliability of magnetic recordings of the ancient magnetic field is strongly dependent on the magnetic mineralogy of natural samples. Theoretical estimates of long‐term stability of remanence were restricted to single‐domain (SD) states, but micromagnetic models have recently demonstrated that the so‐called single‐vortex (SV) domain structure can have even higher stability that SD grains. In larger grains ( 10 μm in magnetite) the multidomain (MD) state dominates, so that large uniform magnetic domains are separated by narrow domain walls. In this paper we use a parallelized micromagnetic finite element model to provide resolutions of many millions of elements allowing us, for the first time, to examine the evolution of magnetic structure from a uniform state, through the SV state up to the development of the domain walls indicative of MD states. For a cuboctahedral grain of magnetite, we identify clear domain walls in grains as small as ∼3 μm with domain wall widths equal to that expected in large MD grains; we therefore put the SV to MD transition at ∼3 μm for magnetite and expect well‐defined, and stable, SV structures to be present until at least ∼1 μm when reducing the grain size. Reducing the size further shows critical dependence on the history of domain structures, particularly with SV states that transition through a so‐called “unstable zone” leading to the recently observed hard‐aligned SV states that proceed to unwind to SD yet remain hard aligned.

Signatures of Magnetic Flux Ropes in the Low Solar Atmosphere Observed in High Resolution

Magnetic flux ropes (MFRs) are important physical features closely related to solar eruptive activities with potential space weather consequences. Studying MFRs in the low solar atmosphere can shed light on their origin and subsequent magnetic structural evolution. In recent years, observations of solar photosphere and chromosphere reached a spatial resolution of 0.1 to 0.2 arcsec with the operation of meter class ground-based telescopes, such as the 1.6m Goode Solar Telescope at Big Bear Solar Observatory and the 1m New Vacuum Solar Telescope at Yunnan Observatory. The obtained chromospheric Halpha filtergrams with the highest resolution thus far have revealed detailed properties of MFRs before and during eruptions, and the observed pre-eruption structures of MFRs are well consistent with those demonstrated by nonlinear force-free field extrapolations. There are also evidences that MFRs may exist in the photosphere. The magnetic channel structure, with multiple polarity inversions and only discernible in high-resolution magnetograph observations, may be a signature of photospheric MFRs. These MFRs are likely formed below the surface due to motions in the convection zone and appear in the photosphere through flux emergence. Triggering of some solar eruptions is associated with an enhancing twist in the low-atmospheric MFRs.

Magnetohydrodynamic Simulations for Solar Active Regions using Time-series Data of Surface Plasma Flow and Electric Field Inferred from Helioseismic Magnetic Imager Vector Magnetic Field Measurements

Abstract Temporal evolution of magnetic structures of the solar active region (AR) NOAA AR 11158, is simulated with our magnetohydrodynamic (MHD) simulation models using time-dependent solar-surface electric field or plasma flow data. Using the Solar Dynamics Observatory/Helioseismic Magnetic Imager vector magnetogram data, the solar-surface boundary electric field maps are derived with our recently developed algorithm to reproduce the temporal evolution of solar-surface vector magnetic field as observed. The plasma motion velocity maps are calculated through the Differential Affine Velocity Estimator for Vector Magnetograms. In both data-driven models, the simulated evolutionary magnetic field structures at strong-field low-beta regions appear near force-free state, as the current helicity density ( ) are roughly constant along each field line. Although the magnetic energy simulated with the newly developed plasma-velocity-driven model is about 10% of that by the electric-field driven model, the plasma-velocity-driven model can maintain the frozen-in condition, and evolution of current and free energy generated by the solar-surface plasma motions can be spatially and temporally traced. The present MHD simulation models for AR system can be a step toward better, more realistic data-driven evolutionary modeling, in particular, establishing boundary treatments for introducing the time-dependent observation data in a physically and mathematically consistent manner.

Temperature controlled Fe/Au/FeRh spin valves

We demonstrate that FeRh layers can be implemented to the epitaxial Fe/Au/FeRh spin valve structures grown on MgO(001). Owing to the AFM-FM phase transition in the FeRh film, the magnetic structure of our Fe/Au/FeRh system can be temperature controlled. The indirect exchange coupling between Fe and FeRh films mediated by the Au spacer enables to control the relative orientation of the Fe and FeRh magnetizations by the Au spacer thickness between ferromagnetic and non-collinear with nearly orthogonal magnetizations. Moreover, the evolution of magnetic structure of the Fe/Au/FeRh system along with the AFM-FM transition is accompanied by the reversible in-plane rotation of the top Fe-layer magnetization.

The regulation of magnetoelectric coupling and magnetic exchange bias effects in La doped Bi5Ti3FeO15 multiferroic films

La doped Bi5Ti3FeO15 (BTFO) multiferroic films with various doping concentration were synthesized by chemical solution deposition method. The structure, ferroelectric, magnetic properties, and magnetoelectric coupling effect were systemically investigated. Room temperature magnetoelectric effect was obtained. And the magnetoelectric coupling and magnetic exchange bias effects are regulated by the doping concentration of La. Such regulation originates from the evolution of magnetic structure derived from doping rare earth La. The coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) orders can be controlled by the doping concentration of La. Thus, the superexchange interaction between FM and AFM orders was enhanced strongly, which contributes to the strong magnetoelectric coupling effect. This work provides an avenue on regulating the magnetic structure and magnetoelectric coupling effects.

Temperature evolution of magnetic structure of HoFeO3 by single crystal neutron diffraction

We have investigated the temperature evolution of the magnetic structures of HoFeO3 by single crystal neutron diffraction. The three different magnetic structures werevfound as a function of temperature for HoFeO3. In all three phases the fundamental coupling between the Fe sub-lattices remains the same and only their orientation and the degree of canting away from the ideal axial direction varies. The magnetic polarisation of the Ho sub-lattices in these two higher temperature regions, in which the major components of the Fe moment lie along x and y, is very small. The canting of the moments from the axial directions is attributed to the antisymmetric interactions allowed by the crystal symmetry. In the low temperature phase two further structural transitions are apparent in which the spontaneous magnetisation changes sign with respect to the underlying antiferromagnetic configuration. In this temperature range the antisymmetric exchange energy varies rapidly as the the Ho sub-lattices begin to order. So long as the ordered Ho moments are small the antisymmetric exchange is due only to Fe-Fe interactions, but as the degree of Ho order increases the Fe-Ho interactions take over whilst at the lowest temperatures, when the Ho moments approach saturation the Ho-Ho interactions dominate. The reversals of the spontaneous magnetisation found in this study suggest that in HoFeO3 the sums of the Fe-Fe and Ho-Ho antisymmetric interactions have the same sign as one another, but that of the Ho-Fe terms is opposite.

Effect of anisotropy on the evolution of magnetic configuration in a helimagnet nanodisk

We study the effect of magnetic anisotropy on the field-driven evolution of magnetic structure in a nanodisk with existence of Dzyaloshinskii-Moriya interaction by using micromagnetic simulations. We obtain the phase diagram of magnetic state in the magnetic field-magnetic anisotropy constant (Hz-K) plane. It is found that target skyrmion can stably exist at zero external magnetic field when the magnetic anisotropy constant changing in a large range. The evolution of magnetic state with the perpendicular magnetic field strongly depends on the magnetic anisotropy constant.