Magnetic Axis Research Articles

Joule magnetostriction and volume magnetostriction of Iron Gallium thin films

The main aim of this work is to evaluate magnetostriction performances, including Joule magnetostriction and volume magnetostriction, of FeGa thin films synthesized by electrodeposition and ion-beam sputtering. This research demonstrated that the electrodeposited Fe(100-x)Gax (x: 19 ∼ 26) films had a tensile stress, and the ion-beam sputtered Fe81Ga19 film had a compressive stress. In both films, the easy axis of magnetization was in the in-plane direction. Both films exhibited nearly (110)-oriented polycrystalline structures. For the ion-beam sputtered Fe81Ga19 film, the observed volume magnetostriction was 39 ppm, approximately one-third of the Joule magnetostriction of 116 ppm. For electrodeposited Fe81Ga19, the Joule and volume magnetostrictions were 64 ppm and 46 ppm, respectively. An important conclusion of this research was that the FeGa films formed by non-equilibrium growth processes displayed a large volume magnetostriction.

Aluminum substitution in low damping epitaxial lithium ferrite films

Ferromagnetic insulators with ultra-low damping and tunable magnetic properties are of great interest for the study of spin wave-based phenomena as well as for applications in spin wave spintronics. We have developed epitaxial spinel structure ferrite thin films of Li0.5(AlxFe2.5−x)O4 (LAFO) with ultra-low magnetic damping. The degree of Al substitution tunes the saturation magnetization and in-plane magnetic anisotropy while maintaining excellent epitaxy and crystalline quality as well as low magnetic damping. All LAFO films, deposited on (001)-oriented MgAl2O4 (MAO), are under epitaxial compressive strain, giving rise to an easy magnetic axis in-plane and a hard out-of-plane axis. Element-specific x-ray magnetic circular dichroism measurements confirm that the magnetism originates from the majority of Fe3+ cations occupying the octahedral sites, which are displaced by increased Al3+ substitution. Broadband ferromagnetic resonance reveals the lowest Gilbert damping parameters of 5×10−4 for films 10–15 nm thick. Our films demonstrate the robustness of the low loss dynamic properties of Li0.5(AlxFe2.5−x)O4 that are promising for spin wave applications.

Magnetic properties of CoNi nanowires and nanotubes: Insights from micromagnetic simulations and composition-dependent equations

We conducted micromagnetic simulations of CoNi nanowires and nanotubes to study their magnetic properties as a function of their composition. We found that the hysteresis curves for isolated CoNi nanowires exhibit a square trend with an easy axis of magnetization along the nanowire axis. The coercivity and normalized remanence show a U-shaped behavior, with maximum values at Ni and Co and minimum values for the Co40Ni60 alloy. The hysteresis curves for CoNi nanotubes maintain the square trend of nanowires, but with a clear decrease in coercivity and an asymmetric V-shaped behavior for coercivity and normalized remanence. Our research offers valuable and comprehensive insights into the magnetic properties of CoNi nanowires and nanotubes. It underscores the significant influence of composition on magnetic properties, which holds great relevance for potential applications. Our approach, involving the prediction of static magnetic properties through micromagnetic simulations and tailored equations designed for nanostructures, rather than relying on bulk parameters, aligns more closely with the nature of these physical systems. This alignment is validated by the strong agreement with previously reported experimental results. Our work lays a solid foundation for the deliberate design of CoNi nanowires and nanotubes with specific magnetic properties tailored for articular applications. We anticipate that our study will facilitate the development of innovative magnetic nanostructures with controllable properties and enhanced performance.

Investigation of the electronic, magnetic, exchange interactions, and magnetocaloric properties in orthotitanate oxides RTiO3 (R = Er, Ho)

Strong correlated materials are emerging materials in low magnetocaloric cooling applications. The development of these fields necessitates materials with appealing magnetic, and magnetocaloric features. In this context, Density Functional Theory (DFT) combined with Monte Carlo simulations (MCs) are performed to comprehensively examine various physical, and magnetic features of the orthorhombic perovskite-type RTiO3. A metallic behavior of (Ho, Er)TiO3 has been found via the density of state using GGA+U approach. X-ray magnetic circular dichroism (XMCD) was carried out to identify the magnetic contributions of Ho, Er, and Ti atoms in HoTiO3, and ErTiO3 compounds. The spin-orbit coupling was added in our calculations to study and evaluate the magnetocrystalline anisotropies of RTiO3 with (R= Ho, Er). We noticed an easy magnetization axis along the b-direction for HoTiO3, and along the c-direction for ErTiO3. Low magnetocrystalline anisotropy energies were found compared to other perovskite oxides such as RVO3, and RMnO3 indicating that a weak rotating magnetocaloric effect (RMCE) is expected in (Ho, Er)TiO3. Moreover, the exchange coupling interactions were calculated using Ising model, the (Ti-Ti) interaction coupling is ferromagnetic, and larger than the other exchange interactions in (Ho, Er)TiO3. Furthermore, these exchange coupling interactions were utilized within the Heisenberg model for MCs, allowing us to understand the driving mechanisms underlying magnetocaloric and magnetic behaviors in (Ho, Er)TiO3.

Numerical calculation for instability criterion of tearing modes influenced by energetic ions

The instability criterion of tearing modes, including the effects of circulating energetic ions (CEIs), is numerically solved through an iterative method and fast Fourier transform algorithm. The influence of beta (the ratio of pressure to the magnetic pressure) profile of CEIs and magnetic shear on instability criterion of the tearing modes are investigated systematically. CEIs impact the tearing modes through altering the total perturbed parallel current density, and the gradient of their beta profile at the rational surface (the location of tearing mode) plays an important role. The effect of CEIs on tearing modes depends on the deposition position and the toroidal circulating direction of CEIs. Specifically, the on-axis peaked co-CEIs have a stabilizing effect on tearing modes. For off-axis peaked co-CEIs, in order to stabilize the tearing modes, the deposition position of co-CEIs should be located between the magnetic axis and rational surface, and there is an optimal peak width and deposition location where the stabilizing effect is strongest. For counter-CEIs, they would destabilize the tearing modes when the deposition position is located between the magnetic axis and rational surface. However, if the deposition position is located near the outside rational surface, counter-CEIs have a stabilizing effect on tearing modes. Furthermore, there exists a critical deposition position for CEIs. Whether CEIs plays a stable role or an unstable role depends on the deposition position within or outside the critical deposition position. The critical deposition position is related to the orbital width of CEIs. Finally, the magnetic shear can stabilize the tearing modes, but the effect of co-CEIs on tearing modes is weakened as the magnetic shear increases.

Experiments on excitation of Alfvén eigenmodes by alpha-particles with bump-on-tail distribution in JET DTE2 plasmas

Dedicated experiments were performed in JET DTE2 plasmas for obtaining an α-particle bump-on-tail (BOT) distribution aiming at exciting Alfvén eigenmodes (AEs). Neutral beam injection-only heating with modulated power was used so that fusion-born α-particles were the only ions present in the MeV energy range in these DT plasmas. The beam power modulation on a time scale shorter than the α-particle slowing down time was chosen for modulating the α-particle source and thus sustaining a BOT in the α-particle distribution. High-frequency modes in the toroidicity-induced Alfven eigenmode (TAE) frequency range and multiple short-lived modes in a wider frequency range have been detected in these DT discharges with interferometry, soft x-ray cameras, and reflectometry. The modes observed were localised close to the magnetic axis, and were not seen in the Mirnov coils. Analysis with the TRANSP and Fokker-Planck FIDIT codes confirms that α-particle distributions with BOT in energy were achieved during some time intervals in these discharges though no clear correlation was found between the times of the high-frequency mode excitation and the BOT time intervals. The combined magneto-hydrodynamic (MHD) and kinetic modelling studies show that the high-frequency mode in the TAE frequency range is best fitted with a TAE of toroidal mode number n= 9. This mode is driven mostly by the on-axis beam ions while the smaller drive due to the pressure gradient of α-particles allows overcoming the marginal stability and exciting the mode (Oliver et al 2023 Nucl. Fusion submitted). The observed multiple short-lived modes in a wider frequency range are identified as the on-axis kinetic AEs predicted in Rosenbluth and Rutherford (1975 Phys. Rev. Lett. 34 1428).

Electric Power Flow Control in Double Three-Phase Machines without Shift between Magnetic Axes

This paper examines the control strategy adopted for six-phase machines that have been designed as double three-phase machines without a displacement between the magnetic axes of the two sets of three-phase windings (6PH-ZS). The approach is based on vector control principles, enabling the independent control of direct- and quadrature-current components in both three-phase windings. The advantages of such an approach stem from the flexibility of the current distribution between windings, which aims to achieve the required flux and torque. The proposed control strategy is considered for both motor and generator operating regimes. However, an additional quasi-transformer regime is also addressed. While the classic operation regime requires the machine to produce either driving or generating/braking torque, the quasi-transformer regime enables the control of electric power flow between two three-phase windings. A theoretical analysis of the machine model and control algorithm for the flow of electric power through machines is presented for the three aforementioned operating regimes. Verification of the proposed control algorithm is performed through extensive experimental verification.

Formation of cube-on-face texture {0 0 1}〈uv0〉 in a high saturation soft magnetic iron-cobalt-vanadium alloy by controlled phase-transformation γ → α

In this investigation, the soft magnetic properties of a high saturation iron-cobalt-vanadium alloy with 17 wt% Co and 1.5 wt% V have been greatly enhanced by a special heat treatment in the γ-region, which effectively leads to the formation of a magnetically favorable strong cube-on-face texture during cooling. Based on EBSD analyses, a high degree of cube-on-face texture {001}〈uv0〉 of up to 80 % was obtained. The underlying mechanism is presumed to be a preferred surface nucleation of {001}-grains during cooling through the two-phase-region α + γ. The annealing of all samples was performed in ceramic powder. Therefore, mechanical stress is not considered the primary cause for this preferred nucleation, which should simplify the upscaling to an industrial process. As the 〈100〉 direction is the magnetic easy axis in this material system, this preferred crystal orientation goes along with very good soft magnetic properties such as a high maximum permeability µmax of 16,000, an increased induction B(160 A/cm) of 2.14 T and a reduced coercivity Hc of about 30 A/m. The core loss is reduced for frequencies up to 1 kHz. This favorable combination of properties, along with the reduced Co content, opens up new possibilities for the use of this material in electrical machines with both a high efficiency and a high power density.

Electric field-controlled deterministic magnetization reversal in nanomagnet Fe3Si/PMN-PT multiferroic heterostructures

Pure electric field-controlled 180° magnetization switching plays a vital role in low-power magnetoelectric memory devices. Using micromagnetic simulation, we engineered a square-shaped epitaxial Fe3Si nanomagnet on a PMN-PT piezoelectric substrate to make the magnetic easy axis slightly deviate 18° from the piezostrain axis, aiming to break the symmetry of the magnetization distribution and achieve deterministic magnetization reversal paths. Under the coaction of a magnetic field and an electric field, the simulated magnetic hysteresis loops and magnetic domain patterns reveal a fourfold to twofold magnetic anisotropy transition and magnetization reversal paths. Stimulated by pure electric field-induced piezoelectric strain, deterministic 180° magnetization reversals are accomplished by the two successive clockwise 90° switching process. The results help to comprehend electrically regulated deterministic magnetization reversal and pave an avenue for designing multistate spintronics devices.

Coupling finite elements of class C1 on composite curved meshes for second order elliptic problems

SummaryFinite elements of class are suitable for the computation of magnetohydrodynamics instabilities in tokamak plasmas. In addition, isoparametric approximations allow for a precise alignment of the mesh with the magnetic field line. Mesh alignment is crucial to achieve axisymmetric equilibria accurately. It is also helpful to deal with the anisotropy nature of magnetized plasma flows. In this numerical framework, several practical simulations are now available. They help to understand better the operation of existing devices and predict the optimal strategies for using the international ITER tokamak under construction. However, a mesh‐aligned isoparametric representation suffers from the presence of critical points of the magnetic field (magnetic axis, X‐point). We here explore a strategy that combines aligned mesh out of the critical points with non‐aligned unstructured mesh in a region containing these points. By this strategy, we can avoid highly stretched elements and the numerical difficulties that come with them. The mesh‐aligned interpolation uses bi‐cubic Hemite‐Bézier polynomials on a structured mesh of curved quadrangular elements. On the other hand, we assume reduced cubic Hsieh‐Clough‐Tocher finite elements on an unstructured triangular mesh. Both meshes overlap, and the resulting formulation is a coupled discrete problem solved iteratively by a suitable one‐level Schwarz algorithm. In this paper, we will focus on the Poisson problem on a two‐dimensional bounded regular domain. This elliptic equation is a simplified version of the axisymmetric tokamak equilibrium one at the asymptotic limit of infinite major radius (large aspect ratio).

Low-temperature magnetic ordering in Co core/CoO shell nanoparticles on the copper surface

Cobalt/cobalt oxide nanoparticles electrochemically deposited onto a copper surface revealed an unusual behavior of the coercivity and exchange bias at liquid helium temperature [V.G. Bayev et al., Appl. Surf. Sci. 440 (2018) 1252–1260], pointing out the necessity of careful consideration of magnetic properties at these temperatures. Here, it is shown that the approach based on the random anisotropy model and analysis of law of the approach to magnetic saturation adopted for the two-dimensional (2D) magnetic system allows obtaining the correlation function of the magnetic anisotropy axes in real space, as well as the exchange, random and coherent anisotropy magnetic fields. The long-range correlation in the magnetic anisotropy axes and the oscillating character of the correlation function is the characteristic feature, suggesting the important role of the interplay between the exchange bias and random anisotropy on the one hand and indirect exchange coupling via the conducting electrons of copper and magnetic dipole interaction, on the other.

Design of an optimized load-resilient conjugate T for the ICRH system in the LHD using a novel hybrid circuit/3DLHDAP code and experimental results

It is crucial to correctly predict the S-matrix with plasma and set the optimal impedance matching device in the ion cyclotron resonance heating (ICRH) antenna system design. In this paper, a hybrid circuit/3DLHDAP code to verify the S-matrix measurements in the presence of plasma and optimize the load-resilient conjugate-T circuit for Large Helical Device (LHD) ICRH antennas has been developed and benchmarked. The variation of S-matrices for handshake form (HAS) and field-aligned-impedance-transforming (FAIT) antenna systems with density, magnetic field and coupling distance during heating obtained by the code’s simulations agrees with that of with the LHD ICRH experiments. The mutual coupling of toroidally aligned HAS antennas is larger than that of poloidally aligned FAIT antennas over a wide range of densities. When the density and coupling distance increase, under a magnetic field on the magnetic axis of 2.75 T and 1.0 T, within a certain density change range, at the minimum voltage position with vacuum injection, the change rule of the antennas’ Sa_minV_ 11 and Sa_minV_ 22 with density is opposite to that with coupling distance, which means that under certain conditions, adjusting the coupling distance may make up for the S-parameters changes caused by plasma density variation, keeping the minimum voltage position fixed, and may make impedance matching easier to achieve during long-pulse operation. Based on obtaining the S parameters, conjugate-T circuits for the HAS and FAIT antennas are designed with the hybrid circuit/3DLHDAP code, which can keep the reflection coefficients low without controlling impedance matching device over a wide range of plasma parameters region. The related results in this paper may provide some guidance for the high-power long-pulse operation of the ICRH antenna system on the fusion device.

Magnetized rotating isothermal winds

ABSTRACT We consider the general problem of a Parker-type non-relativistic isothermal wind from a rotating and magnetic star. Using the magnetohydrodynamics code athena++, we construct an array of simulations in the stellar rotation rate Ω* and the isothermal sound speed cT, and calculate the mass, angular momentum, and energy loss rates across this parameter space. We also briefly consider the 3D case, with misaligned magnetic and rotation axes. We discuss applications of our results to the spin-down of normal stars, highly irradiated exoplanets, and to nascent highly magnetic and rapidly rotating neutron stars born in massive star core-collapse.

Prediction of novel 2D metallic ferroelastic ferromagnet: Monoclinic CrTe2

By combining the USPEX evolutionary algorithm and density functional theory at the HSE06 level, we identified a new CrTe2 monolayer crystallizing in monoclinic phase with C2h symmetry, which is different from the well-known 1T-CrTe2 monolayer. The monoclinic phase is energetically favored over that in 1T phase, is thermodynamically, dynamically, and mechanically stable. The monoclinic phase is an intrinsically ferromagnetic metal with a relatively high Curie temperature of 215 K showing an out-of-plane easy axis with 25° off the z-axis. In particular, the predicted Curie temperature and out-of-plane ferromagnetic order are in agreement with the experimental evidence. In addition, a uniaxial strain can induce a ferroelastic switching together with the magnetic easy axis, resulting in a strong intrinsic magnetoelasticity. Our results open up a new possibility for the realization of novel two-dimensional ferromagnets and provide a platform for understanding the underlying physical mechanism.

Nanosized antiferromagnetic domains in NiO/Ag(001) films probed by polarised X-ray and unpolarised low-energy electrons

Surface antiferromagnetism of NiO film, epitaxially deposited on Ag(001), is investigated using the absorption of linearly polarised X-ray and the diffraction of low-energy unpolarised electrons. Polarisation-dependent X-ray absorption studies reveal a preferential stabilisation of the magnetic domains with the main components of magnetic axes parallel to the interface. Furthermore, the role of interfacial strain-induced magnetocrystalline anisotropy below a critical film thickness (∼ 20 ML) could be demonstrated by the magnetic linear dichroism results. Utilising the exchange scattered electrons from various antiferromagnetic (111) planes, the magnetic twin domains could be probed by a low energy electron microscope operating in the dark-field mode. The linearly polarised X-ray, on the other hand, is sensitive to the magnetic axes orientations of Ni2+ ions and therefore, provides access to the spin domains as well as the twin domains. Comparison of results of these two independent imaging techniques demonstrates that the in-plane component of the magnetic easy axes contained by the complementary T-domains are parallel to each other.

Dynamic susceptibility of ferrogels. Effect of interparticle interaction

We present results of theoretical study of effect of magnetic interparticle interaction on kinetics of remagnetization of magnetic gels with immobilized single-domain particle. Magnetic anisotropy of the particle is supposed very strong. The particles axes of easy magnetization are randomly oriented. Our results demonstrate that effect of interparticle interaction on kinetics of the particles remagnetization is determined by the strength of the applied field: this interaction slows down the composite remagnetization when applied magnetic field is weak and speeds up it when the field is high.

Constructing precisely quasi-isodynamic magnetic fields

We present a novel method for numerically finding quasi-isodynamic stellarator magnetic fields with excellent fast-particle confinement and extremely small neoclassical transport. The method works particularly well in configurations with only one field period. We examine the properties of these newfound quasi-isodynamic configurations, including their transport coefficients, particle confinement and available energy for trapped-electron-instability-driven turbulence, as well as the degree to which they change when a finite pressure profile is added. We finally discuss the differences between the magnetic axes of the optimized solutions and their respective initial conditions, and conclude with the prospects for future quasi-isodynamic optimization.

On the triple pulsar profiles generated by ordinary mode

ABSTRACT A detailed study of the refraction of an ordinary wave in the magnetosphere of radio pulsars was carried out. For this, a consistent theory of the generation of secondary particles was constructed, which essentially takes into account the dependence of the number density and the energy spectrum of secondary particles on the distance from the magnetic axis. This made it possible to determine with high accuracy the refraction of the ordinary O-mode in the central region of the outflowing plasma, which makes it possible to explain the central peak of three-humped mean radio profiles. As shown by detailed numerical calculations, in most cases it is possible to reproduce quite well the observed mean profiles of radio pulsars.

Identification of core ion cyclotron instabilities on HL-2A tokamak

Instabilities in multiplies of ion cyclotron frequency range are identified and termed as core ion cyclotron emission (ICE) in recent HL-2A neutral beam injection heated experiments. Characteristics of the core ICE are presented, including frequency dependence and harmonics features. The detected frequencies are found to agree well with the multiplies of the deuterium cyclotron frequency around the magnetic axis. Additionally, the core ICE exhibits a predominantly compressional property. Observations of distinct spectrum features and individual excitation of each harmonic have demonstrated that the core ICE harmonics are independent multiple modes. Notably, the variation of plasma current is a necessary condition for exciting the 4th harmonic ICE individually. The results suggest that the drive mechanism of core ICE varies between the different frequency ranges.

Field-free spin-orbit switching of perpendicular magnetization enabled by dislocation-induced in-plane symmetry breaking

Current induced spin-orbit torque (SOT) holds great promise for next generation magnetic-memory technology. Field-free SOT switching of perpendicular magnetization requires the breaking of in-plane symmetry, which can be artificially introduced by external magnetic field, exchange coupling or device asymmetry. Recently it has been shown that the exploitation of inherent crystal symmetry offers a simple and potentially efficient route towards field-free switching. However, applying this approach to the benchmark SOT materials such as ferromagnets and heavy metals is challenging. Here, we present a strategy to break the in-plane symmetry of Pt/Co heterostructures by designing the orientation of Burgers vectors of dislocations. We show that the lattice of Pt/Co is tilted by about 1.2° when the Burgers vector has an out-of-plane component. Consequently, a tilted magnetic easy axis is induced and can be tuned from nearly in-plane to out-of-plane, enabling the field-free SOT switching of perpendicular magnetization components at room temperature with a relatively low current density (~1011 A/m2) and excellent stability (> 104 cycles). This strategy is expected to be applicable to engineer a wide range of symmetry-related functionalities for future electronic and magnetic devices.