Formation Of Magnetic Structure Research Articles

Role of internal stresses in the formation of magnetic structure and magnetic properties of iron-based glass coated microwires

Based on the experimental data of the magnetic hysteresis and magneto-optical images of the magnetic structure during magnetization process, in this work, we have carried out estimation of the value of radial mechanical stresses in the surface region of the metal core of microwire. The composition of the sample microwire was Fe73.9B13.2Si10.9C2 with a positive magnetostriction constant. The magneto-optical images of the domain structure during magnetization and the hysteresis loops have been used to define the magnetic characteristics of the sample (the saturation magnetization Ms and the saturation field Hs). We have determined that there is a relationship between the orientation of the magnetic moments of the ring-shaped surface domains and the internal cylindrical domains. On the basis of the experimental data, we have estimated the value of radial mechanical stress of the microwire in the surface region of the metal core where the ring-shaped domains are located.

Structure and magnetic properties of BaFe11.9In0.1O19 hexaferrite in a wide temperature range

The investigations of the crystal and magnetic structure by powder neutron diffractometry as well as the magnetic properties by vibration sample magnetometry for the BaFe11.9In0.1O19 polycrystalline sample have been performed in a wide temperature range from 10 K up to 730 K and in the magnetic fields up to 14 T. The atomic coordinates and lattice parameters have been Rietveld refined. The Invar effect has been observed in low temperature range below 150 K. The temperature evaluation of the main FeO bond lengths and FeO Fe bond angles are constructed. The increase of microstress value with decreasing temperature has been defined from Rietveld refinement. From the microscopic diffraction measurement the magnetic moments at different atomic position and total magnetic moment per iron ion have been defined at different temperatures. The nature of the intrasublattice and intersublattice Fe3+O2−Fe3+ indirect exchange interactions is investigated at the different temperatures. From the macroscopic magnetization measurement the Curie temperature and the ordered magnetic moment per nominal iron ion are obtained. It is established that the ferrimagnet-paramagnet phase transition is a standard second-order one. The magnetic structure model is proposed. The most likely reasons and the mechanism of magnetic structure formation are discussed.

Numerical simulations of the structure of ferromagnetic fluids based on dissipative particle dynamics method

An algorithm based on dissipative particle dynamics (DPD) method is employed to simulate the structure of ferromagnetic fluids at thermodynamic equilibrium state. Two cases are considered. First, the effects of the magnetic particle–particle interaction strength on the structure of magnetic fluids are studied for the fixed-magnetic particle area fraction using the above DPD-based method. The obtained aggregate structures of magnetic particles agree well qualitatively with the corresponding simulation and experimental results in the literature. The radial distribution functions (RDFs) characterising quantitatively the internal structure of magnetic fluids are also calculated and analysed. As a result, the magnetic particle–particle interaction strength plays a crucial role in the formation of magnetic particle chain structure. Second, the influences of the magnetic particle area fraction φ on the structure of magnetic fluids are investigated for the fixed-magnetic particle–particle interaction strength using the above method. The simulated results are in qualitatively good agreement with simulation and experimental results in the literature. The influence of the size of periodic boundary on the height of the first peaks of RDFs is discussed and for the present system, the influence can be neglected. All the simulations and calculations show the employed DPD-based method is very effective. In a word, the present DPD-based method is indeed a powerful tool for simulating the structure of magnetic fluids.

Crystal structure and magnetic properties of the BaFe12−xInxO19 (x=0.1–1.2) solid solutions

The investigations of the crystal and magnetic structure by powder neutron diffractometry as well as the magnetic properties by vibration sample magnetometry for the BaFe12−xInxO19 (x=0.1–1.2) solid solutions have been performed at different temperatures and magnetic fields. The atomic coordinates and lattice parameters have been Rietveld refined. The Invar effect has been observed in low temperature range (from 150K to 10K). It was explained by the thermal oscillation anharmonicity of atoms. The increase of microstress value with decreasing temperature has been defined from Rietveld refinement. The Curie temperature and change of total magnetic moment per formula unit have been defined for all the compositions of the barium hexaferrites BaFe12−xInxO19 (x=0.1–1.2) solid solutions. The magnetic structure model is proposed. The most likely reasons and the mechanism of magnetic structure formation are discussed.

SPONTANEOUS FORMATION OF SURFACE MAGNETIC STRUCTURE FROM LARGE-SCALE DYNAMO IN STRONGLY STRATIFIED CONVECTION

ABSTRACT We report the first successful simulation of spontaneous formation of surface magnetic structures from a large-scale dynamo by strongly stratified thermal convection in Cartesian geometry. The large-scale dynamo observed in our strongly stratified model has physical properties similar to those in earlier weakly stratified convective dynamo simulations, indicating that the α 2-type mechanism is responsible for the dynamo. In addition to the large-scale dynamo, we find that large-scale structures of the vertical magnetic field are spontaneously formed in the convection zone (CZ) surface only in cases with a strongly stratified atmosphere. The organization of the vertical magnetic field proceeds in the upper CZ within tens of convective turnover time and band-like bipolar structures recurrently appear in the dynamo-saturated stage. We consider several candidates to be possibly be the origin of the surface magnetic structure formation, and then suggest the existence of an as-yet-unknown mechanism for the self-organization of the large-scale magnetic structure, which should be inherent in the strongly stratified convective atmosphere.

Bipolar region formation in stratified two-layer turbulence

This work presents an extensive study of the previously discovered formation of bipolar flux concentrations in a two-layer model. We interpret the formation process in terms of negative effective magnetic pressure instability (NEMPI), which is a possible mechanism to explain the origin of sunspots. In our simulations, we use a Cartesian domain of isothermal stratified gas that is divided into two layers. In the lower layer, turbulence is forced with transverse nonhelical random waves, whereas in the upper layer no flow is induced. A weak uniform magnetic field is imposed in the entire domain at all times. In this study we vary the stratification by changing the gravitational acceleration, magnetic Reynolds number, strength of the imposed magnetic field, and size of the domain to investigate their influence on the formation process. Bipolar magnetic structure formation takes place over a large range of parameters. The magnetic structures become more intense for higher stratification until the density contrast becomes around $100$ across the turbulent layer. For the Reynolds numbers considered, magnetic flux concentrations are generated at magnetic Prandtl number between 0.1 and 1. The magnetic field in bipolar regions increases with higher imposed field strength until the field becomes comparable to the equipartition field strength of the turbulence. A larger horizontal extent enables the flux concentrations to become stronger and more coherent. The size of the bipolar structures turns out to be independent of the domain size. In the case of bipolar region formation, we find an exponential growth of the large-scale magnetic field, which is indicative of a hydromagnetic instability. Additionally, the flux concentrations are correlated with strong large-scale downward and converging flows. These findings imply that NEMPI is responsible for magnetic flux concentrations.

Fast magnetic-field annihilation in the relativistic collisionless regime driven by two ultrashort high-intensity laser pulses.

The magnetic quadrupole structure formation during the interaction of two ultrashort high power laser pulses with a collisionless plasma is demonstrated with 2.5-dimensional particle-in-cell simulations. The subsequent expansion of the quadrupole is accompanied by magnetic-field annihilation in the ultrarelativistic regime, when the magnetic field cannot be sustained by the plasma current. This results in a dominant contribution of the displacement current exciting a strong large scale electric field. This field leads to the conversion of magnetic energy into kinetic energy of accelerated electrons inside the thin current sheet.

Fast magnetic energy dissipation in relativistic plasma induced by high order laser modes

Fast magnetic field annihilation in a collisionless plasma is induced by using TEM(1,0) laser pulse. The magnetic quadrupole structure formation, expansion and annihilation stages are demonstrated with 2.5-dimensional particle-in-cell simulations. The magnetic field energy is converted to the electric field and accelerate the particles inside the annihilation plane. A bunch of high energy electrons moving backwards is detected in the current sheet. The strong displacement current is the dominant contribution which induces the longitudinal inductive electric field.

Evolution of structure and physical properties in Al-substituted Ba-hexaferrites**Project supported by the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST “MISiS” (Grant No. K4-2015-040). L. Panina acknowledges support under the Russian Federation State contract for organizing a scientific work.

The investigations of the crystal and magnetic structures of the BaFe12−xAlxO19 (x = 0.1–1.2) solid solutions have been performed with powder neutron diffractometry. Magnetic properties of the BaFe12−xAlxO19 (x = 0.1–1.2) solid solutions have been measured by vibration sample magnetometry at different temperatures under different magnetic fields. The atomic coordinates and lattice parameters have been Rietveld refined. The invar effect is observed in low temperature range (from 4.2 K to 150 K). It is explained by the thermal oscillation anharmonicity of atoms. The increase of microstress with decreasing temperature is found from Rietveld refinement. The Curie temperature and the change of total magnetic moment per formula unit are found for all compositions of the BaFe12−xAlxO19 (x = 0.1–1.2) solid solutions. The magnetic structure model is proposed. The most likely reasons and the mechanism of magnetic structure formation are discussed.

Sunspot magnetic structure and interspot radio source formation

The magnetic structure of a typical sunspot is represented as a set of three interacting magnetic fluxes. The first one, F1, escapes upward from the sunspot umbra and closes through the corona on the sunspot of opposite polarity. The second, F2, forms the sunspot penumbra. It includes Evershed flows emerging from the sunspot, the heavy photospheric plasma of which cannot be pulled up to the corona. For this reason, the F2 flux at the outer edge of the penumbra should close on the photosphere adjacent to the sunspot, i.e., here the F2 field changes sign. The third flux, F3, is the external magnetic field flux and has the same polarity as F1. Thus, magnetic fields of different signs occur near the outer boundary of the sunspot penumbra. Magnetic reconnections will inevitably take place there. Smalland multiscale current sheets in which particles are accelerated are formed. The latter fill short and low magnetic loops that connect sunspots of different polarity in the bipolar group and form interspot radio sources at the tops of these loops.

Fabrication and Characterization of a Microwave Filter Based on a Nanowire-Supported Magnetic Photonic Band Gap Material

This paper presents a novel method for fabrication of a microwave filter based on a magnetic photonic band gap material. To achieve this purpose, the nickel nanowires are prepared using an electrodeposition method in anodic alumina template and, then, they are dispersed and aligned in polydimethylsiloxane matrix. Subsequently, polydimethylsiloxane is cured with and without nanowires and has been cut with a surgical blade in same dimensions, eventually, bonded together for the formation of magnetic photonic band gap structure with plasma bonding technique. The frequency response of the filter has been measured using a vector network analyzer in a frequency range from 1 to 40 GHz. Our results present a good microwave filter response based on these new classes of magnetic photonic band gap structures.

Crystal structure and magnetic properties of the BaFe12−xAlxO19 (x=0.1–1.2) solid solutions

The investigations of the crystal and magnetic structure by powder neutron diffractometry as well as the magnetic properties by vibration sample magnetometry for the BaFe12−xAlxO19 (x=0.1–1.2) solid solutions have been performed at different temperatures and magnetic fields. The atomic coordinates and lattice parameters have been Rietveld refined. The Invar effect has been observed in low temperature range (from 150 to 4.2K). It was explained by the thermal oscillation anharmonicity of atoms. The increase of microstress value with decreasing temperature has been defined from Rietveld refinement. The Curie temperature and change of total magnetic moment per formula unit have been defined for all the compositions of the barium hexaferrites BaFe12−xAlxO19 (x=0.1–1.2) solid solutions. The magnetic structure model is proposed. The most likely reasons and the mechanism of magnetic structure formation are discussed.

Control of an electron beam using strong magnetic field for efficient core heating in fast ignition

To enhance the core heating efficiency in electron-driven fast ignition, we propose fast-electron beam guiding using externally applied longitudinal magnetic fields. Based on particle-in-cell simulations for FIREX-class experiments, we demonstrate sufficient beam guiding performance in collisional dense plasma by kilotesla-class external magnetic fields for the case with moderate mirror ratio (⩽10). Boring of the mirror field was found through the formation of magnetic pipe structure due to the resistive effects, which indicates a possibility of beam guiding in high mirror field for higher laser intensity and/or longer pulse duration.

Effect of heat treatment on the phase composition, structure and magnetic properties of M-type barium hexaferrite

Effect of the heat treatment of carbonate-hydroxide precipitates on the phase composition, structure and magnetic properties of M-type barium hexaferrite has been investigated using the Mőssbauer spectroscopy, X-ray phase analysis and magnetic measurements.The distribution of Fe3+ ions over structural sites of barium hexaferrite with different degree of ferritizations has been defined. The conditions of single-domain behavior of barium hexaferrite nanoparticles in the magnetic field have been discussed.Obtained results explain the formation of magnetic structure during the synthesis of barium hexaferrite. This information could further be used for optimizing conditions for synthesis of nanosized barium hexaferrite with high-level properties.

Evidence for magnetic phase separation in colossal magnetoresistance compound EuB5.99C0.01

EuB5.99C0.01 is a low-carrier density ferromagnet that is believed to be intrinsically inhomogeneous due to fluctuations of carbon content. In accordance with our previous studies, electric transport of EuB5.99C0.01 close above the temperature of the bulk ferromagnetic (FM) ordering is governed by magnetic polarons. Carbon-rich regions are incompatible with FM phase and therefore they act as spacers preventing magnetic polarons to link, to form FM clusters, and eventually to percolate and establish a (homogoneous) bulk FM state in this compound, what consequently causes additional (magneto) resistance increase. Below the temperature of the bulk FM ordering, carbon-rich regions give rise to helimagnetic domains, which are responsible for an additional scattering term in the electrical resistivity. Unfortunately, there has not been provided any direct evidence for magnetic phase separation in EuB5.99C0.01 yet. Here reported results of electrical, heat capacity, Hall resistivity and small-angle neutron scattering studies bring evidence for formation of mixed magnetic structure, and provide consistent support for the previously proposed scenario of the magnetoresistance enhancement in EuB5.99C0.01.

Thermal Decomposition Fabrication, Growth Mechanism, and Characterization of Rod-Like γ-Fe2O3

A facile method of synthesizing polycrystalline rod-like γ-Fe2O3 from inexpensive, nontoxic precursors FeC2O4 via a thermal decomposition reaction of 300–500°C for 1 h has been developed. A ligand directed growth mechanism was proposed for the magnetic anisotropic structure formation and the aspect heat treatment temperatures change. The morphology and structure of the synthesized rod-like γ-Fe2O3 were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM). The results show that these rod-like γ-Fe2O3 presents cubic structure and typical diameters of 1.0–2.0 μm with a self-assembly of nanoparticles and average diameter of about 36 nm. The as-prepared rod-like γ-Fe2O3 have a high surface area of about 102.03 m2/g. Compared with previous methods; this present method shows an easy processing and can be applied on the large-scale produce of rod-like γ-Fe2O3.

LARGE-SCALE MAGNETIC STRUCTURE FORMATION IN THREE-DIMENSIONAL MAGNETOHYDRODYNAMIC TURBULENCE

The inverse cascade of magnetic helicity in 3D-MHD turbulence is believed to be one of the processes responsible for large scale magnetic structure formation in astrophysical systems. In this work we present an exhaustive set of high resolution direct numerical simulations (DNS) of both forced and decaying 3D-MHD turbulence, to understand this structure formation process. It is first shown that an inverse cascade of magnetic helicity in small-scale driven turbulence does not necessarily generate coherent large-scale magnetic structures. The observed large-scale magnetic field, in this case, is severely perturbed by magnetic fluctuations generated by the small-scale forcing. In the decaying case, coherent large-scale structure form similar to those observed astronomically. Based on the numerical results the formation of large-scale magnetic structures in some astrophysical systems, is suggested to be the consequence of an initial forcing which imparts the necessary turbulent energy into the system, which, after the forcing shuts off, decays to form the large-scale structures. This idea is supported by representative examples e.g. cluster of galaxies.

BIPOLAR MAGNETIC STRUCTURES DRIVEN BY STRATIFIED TURBULENCE WITH A CORONAL ENVELOPE

We report the spontaneous formation of bipolar magnetic structures in direct numerical simulations of stratified forced turbulence with an outer coronal envelope. The turbulence is forced with transverse random waves only in the lower (turbulent) part of the domain. Our initial magnetic field is either uniform in the entire domain or confined to the turbulent layer. After about 1--2 turbulent diffusion times, a bipolar magnetic region of vertical field develops with two coherent circular structures that live during one turbulent diffusion time, and then decay during 0.5 turbulent diffusion times. The resulting magnetic field strengths inside the bipolar region are comparable to the equipartition value with respect to the turbulent kinetic energy. The bipolar magnetic region forms a loop-like structure in the upper coronal layer. We associate the magnetic structure formation with the negative effective magnetic pressure instability in the two-layer model.

Specific features of magnetic structure formation in orbitally degenerate BiMnO3 manganite

The orbital structure and magnetic ordering of the Jahn-Teller multiferroic BiMnO3 manganite have been theoretically studied. It is shown that the orbital structure depends not only on the nearest-neighbor oxygen environment of manganese ions, but also on their next-to-nearest neighbors. The orbital structure significantly influences the magnetic order that forms as a result of competition between ferromagnetic and antiferromagnetic exchange interactions.

Ab initio study on magnetic anisotropy change of SrCoxTixFe12−2xO19

The magnetic anisotropy change of Co-Ti substituted Sr-M hexaferrite has been investigated by using general gradient approximation (GGA) with on-site coulomb repulsion added (GGA + U) and force theorem method. The calculated dependence of magnetic anisotropy energy on angles of SrFe12O19 fits very well with theoretical description, and anisotropy constant K1 agrees well with experimental value. The impact of occupancy of Co2+ and Ti4+ on magnetic anisotropy in SrCo2Ti2Fe8O19 has been studied. The results show that Co2+ in 4f2 site and Ti4+ in 12 k site both contribute to the reduction of uniaxial magnetic anisotropy, and as Co2+ and Ti4+ in their first preference of substitution site, magnetic anisotropy of SrCo2Ti2Fe8O19 is weak, which would facilitate the formation of conical magnetic structure to induce electric polarization. As substitution rate x increases from 0 to 3, the magnetic anisotropy of SrCoxTixFe12−2xO19 is found to change from uniaxial to planar.