Helical Magnetic Structures Research Articles

Review of Helical Magnetic Structures in Magnetic Microwires

We provide an overview of the helical magnetic structures in magnetic microwires. Having analyzed the experimental data describing the magnetic behavior of magnetic microwires since the 1990s, we found indirect evidence of the existence of various types of helical magnetic structures. Purposeful research has allowed us to discover the spiral magnetic structure as one of the most unusual helical structures. A comparison of the spiral structure with another type of helical structure—elliptical—was carried out. In the analysis, emphasis was placed on the length of the domain wall as one of the most important parameters. The difference in the dynamic properties of the spiral and elliptical domain walls has been demonstrated.

Helical versus collinear antiferromagnetic order tuned by magnetic anisotropy in polar and chiral (Ni,Mn)3TeO6

Polar and chiral ${\mathrm{Ni}}_{3}{\mathrm{TeO}}_{6}$ was found to exhibit a colossal magnetoelectric (ME) effect associated with collinear antiferromagnetic order. We have investigated the evolution of its magnetic state with the substitution of the ${\mathrm{Ni}}^{2+}$ spins by isotropic ${\mathrm{Mn}}^{2+}$ spins, namely ${(\mathrm{Ni},\mathrm{Mn})}_{3}{\mathrm{TeO}}_{6}$ (NMTO). The ground state of NMTO maintains an Ising nature, but a new phase with XY-type magnetic anisotropy is discovered at an intermediate temperature range. Neutron powder diffraction experiments reveal that (1) Mn ions tend to occupy a specific transition-metal site, consistent with the first-principle calculation, and (2) the intermediate phase is an incommensurate (IC) helical magnetic state propagating along the $c$ axis with ${\stackrel{P\vec}{q}}_{\text{IC}}=(0,\phantom{\rule{0.16em}{0ex}}0,\phantom{\rule{0.16em}{0ex}}1.5\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta})$. We also found that the balance between these two magnetic states can be readily manipulated with magnetic fields, which results in significant ME effects. This selective site occupancy of NMTO allows for a unique competition between collinear and helical magnetic structures with minimal chemical disorder. Thus, NMTO serves as a model system to study site-specific chemical control of noncollinear magnetism and ME coupling.

Generation of astrophysics-relevant helical magnetic structures in laser-produced plasma

Helical magnetic fields play an important role in star formation and stellar wind shaping. In recent years, there have been many experiments using pure poloidal or toroidal fields to confine hot plasma species that have successfully reproduced jets and instability patterns similar to those in the universe, but the role of helical magnetic structures is rarely explored due to the complexity of their generation. However, certain processes like cloud angular momentum extraction will only manifest in helical field configurations. In this article, we propose and numerically illustrate a method that can generate helical magnetic structures inside the plasma by merging a self-generated toroidal field and an externally applied poloidal field. The resulting helical field is dynamically important regarding pressure, adjustable in strength and pitch angle, and consistent with the actual astrophysical outflow in terms of boundary conditions. This method shows how to integrate a helical field structure into laser-plasma experiments, and it provides a framework for future studies on stellar outflow interaction with their embedded helical fields.

Helical magnetic structures in magnetostrictive amorphous microwires

Surface magnetic structures have been studied in amorphous glass-covered microwires using the magneto-optical technique. Different types of helical structure have been recognized depending on the external conditions. The simulation of the magnetic structure in magnetic cylinder demonstrates dependence of the type of the structure on the value of the magnetostriction coefficient. The performed comparative analysis of experiment and results of the simulation permitted to establish the direct relation between the value of the magnetostriction coefficient and the type of the helical structure.

Helical magnetic structure and exchange bias across the compensation temperature of Gd/Co multilayers

Antiferromagnetic coupling between rare-earth (RE) and transition metals (TM) gives rise to various magnetic ground states in RE/TM heterostructures. Interface structure and morphology tend to play important roles in defining the magnetic properties of heterostructures of these materials. Using spin-dependent polarized neutron reflectivity (PNR), we found a distinct magnetic structure for RE/TM (Gd/Co) multilayers having different interface morphologies. Two Gd/Co multilayers, each consisting of eight bilayers of Gd and Co, were grown at different argon pressures to accomplish variation in the interface morphology. The Gd/Co multilayer, which is grown at a lower argon pressure, exhibits lower intermixing/interdiffusion at the interfaces and shows asymmetric magnetic hysteresis loops below the compensation temperature (Tcomp ∼ 140 K) and antisymmetric magnetoresistance at Tcomp. The other multilayer, grown at two different argon pressures for the top and bottom four bilayers, shows a double hysteresis loop with an exchange-bias-like shift to the field axis at temperatures above Tcomp (∼150 K). From PNR measurements across Tcomp, these behaviors were attributed to interface dependent magnetic helical structures. The PNR results indicate the formation of a twisted helical magnetic structure with planar 2π domain walls and the evolution of magnetic inhomogeneities below Tcomp. The interface dependent twisted helical magnetic structures and the development of highly correlated magnetic inhomogeneities may find applications in all-spin-based technologies.

Field dependent helical magnetic structure in a compensated Gd/Co multilayer

Rare earth –transition metal heterostructures are known to exhibit a number of magnetic structures at different temperatures and magnetic fields due to the strong antiferromagnetic interaction at the interfaces. Using spin-dependent specular and off-specular polarized neutron reflectivity (PNR), we observed the field-dependent twisted helical magnetic structure with a planar 2π domain wall (DW) in a Gd/Co multilayer. Macroscopic magnetization measurements revealed the field-dependent compensation temperature, which increases with an increase in the applied field. Specular PNR at 5 K, after field cooling the multilayer in an applied field of 100 Oe, reveals the formation of planar 2π DWs and antiferromagnetic coupling at the interfaces. Off-specular PNR data from multilayer at 5 K also revealed the development of magnetic inhomogeneities for the central part (thickness ~ 25 Å) of each Gd layer in the multilayer. In contrast, a similar magnetic property of the multilayer was observed at a higher temperature (125 K) on applying a field of 500 Oe. The field-dependent tunability of twisted helical magnetic structures with planar 2π DWs and the development of highly correlated magnetic inhomogeneities may be key for application in spin-based technology.

Magnetized interstellar molecular clouds – II. The large-scale structure and dynamics of filamentary molecular clouds

Abstract We perform ideal magnetohydrodynamics high-resolution adaptive mesh refinement simulations with driven turbulence and self-gravity and find that long filamentary molecular clouds are formed at the converging locations of large-scale turbulence flows and the filaments are bounded by gravity. The magnetic field helps shape and reinforce the long filamentary structures. The main filamentary cloud has a length of ∼4.4 pc. Instead of a monolithic cylindrical structure, the main cloud is shown to be a collection of fibre/web-like substructures similar to filamentary clouds such as L1495. Unless the line-of-sight is close to the mean field direction, the large-scale magnetic field and striations in the simulation are found roughly perpendicular to the long axis of the main cloud, similar to L1495. This provides strong support for a large-scale moderately strong magnetic field surrounding L1495. We find that the projection effect from observations can lead to incorrect interpretations of the true three-dimensional physical shape, size, and velocity structure of the clouds. Helical magnetic field structures found around filamentary clouds that are interpreted from Zeeman observations can be explained by a simple bending of the magnetic field that pierces through the cloud. We demonstrate that two dark clouds form a T-shaped configuration that is strikingly similar to the infrared dark cloud SDC13, leading to the interpretation that SDC13 results from a collision of two long filamentary clouds. We show that a moderately strong magnetic field (${{\cal M}_{\rm A}}\sim 1$) is crucial for maintaining a long and slender filamentary cloud for a long period of time ∼0.5 Myr.

Chiral Soliton Lattice Formation in Monoaxial Helimagnet Yb(Ni1−xCux)3Al9

Helical magnetic structures and its responses to external magnetic fields in Yb(Ni$_x$Cu$_{1-x}$)$_3$Al$_9$, with a chiral crystal structure of the space group $R32$, have been investigated by resonant X-ray diffraction. It is shown that the crystal chirality is reflected to the helicity of the magnetic structure by a one to one relationship, indicating that there exists an antisymmetric exchange interaction mediated via the conduction electrons. When a magnetic field is applied perpendicular to the helical axis ($c$ axis), the second harmonic peak of $(0, 0, 2q)$ develops with increasing the field. The third harmonic peak of $(0, 0, 3q)$ has also been observed for the $x$=0.06 sample. This result provides a strong evidence for the formation of a chiral magnetic soliton lattice state, a periodic array of the chiral twist of spins, which has been suggested by the characteristic magnetization curve. The helical ordering of magnetic octupole moments, accompanying with the magnetic dipole order, has also been detected.

Polarized neutron diffraction study in helical magnetic phases of MnP

MnP shows an interesting magnetic phase diagram under pressure up to ∼7.5 GPa, where magnetic long range order disappears and superconductivity appears. Polarized neutron analyses were applied to two different helical magnetic structures (helical-c and helical-b). Helicity was observed in the both structures. The two helical domain ratio becomes more unbalanced in the helical-b structure at a higher pressure, suggesting that the Dzyaloshinskii-Moriya interaction, which is enhanced by a local structural strain, could be related with the helical structures.

Specific features of magnetic order in a multiferroic compound CuCrO2 determined using NMR and NQR data for 63, 65Cu nuclei

Results of studying the paramagnetic and ordered phases of a CuCrO2 single crystal using nuclear magnetic and nuclear quadrupole resonances on 63,65Cu nuclei are presented. The measurements have been carried out in wide ranges of temperature (T = 4.2–300 K) and magnetic-field strength (Н = 0–94 kOe), with the magnetic fields being directed along a and c axes of the crystal. The components of the electric-field gradient tensor and the magnetic-shift tensor (Ka,c) have been determined. The temperature dependences Ka(H || a) and Kc(H || c) for the paramagnetic phase are described by the Curie–Weiss law and reproduce the behavior of the magnetic susceptibility (χa,c). The hyperfine field on a copper nucleus has been determined, which is equal to hhfa,c= 33 kOe/μB. Below the temperature ТN = 23.6 K, nuclear magnetic resonance and nuclear quadrupole resonance spectra for 63,65Cu nuclei have been recorded typical of helical magnetic structures, which are incommensurable with the lattice period.

Helical magnetic structures in cubic perovskites

One-dimensional helical structures with a finite Hund interaction have been considered in detail. The analytical expressions for the electron spectrum of charge carriers in simplest one-dimensional and three-dimensional helical structures in a simple cubic lattice, which simulates the manganese sublattice in the crystal structure of perovskite, have been derived in the tight-binding approximation. It has been shown that this approach can explain qualitatively the formation of the antiferromagnetic A structure in LaMnO3.

A solar tornado triggered by flares?

Solar tornados are dynamical, conspicuously helical magnetic structures mainly observed as a prominence activity. We investigate and propose a triggering mechanism for the solar tornado observed in a prominence cavity by SDO/AIA on September 25, 2011. High-cadence EUV images from the SDO/AIA and the Ahead spacecraft of STEREO/EUVI are used to correlate three flares in the neighbouring active-region (NOAA 11303), and their EUV waves, with the dynamical developments of the tornado. The timings of the flares and EUV waves observed on-disk in 195\AA\ are analyzed in relation to the tornado activities observed at the limb in 171\AA. Each of the three flares and its related EUV wave occurred within 10 hours of the onset of the tornado. They have an observed causal relationship with the commencement of activity in the prominence where the tornado develops. Tornado-like rotations along the side of the prominence start after the second flare. The prominence cavity expands with acceleration of tornado motion after the third flare. Flares in the neighbouring active region may have affected the cavity prominence system and triggered the solar tornado. A plausible mechanism is that the active-region coronal field contracted by the `Hudson effect' due to the loss of magnetic energy as flares. Subsequently the cavity expanded by its magnetic pressure to fill the surrounding low corona. We suggest that the tornado is the dynamical response of the helical prominence field to the cavity expansion.

Nonlinear collective excitations in helical magnetic structures

Based on the sine-Gordon model for a quasi-one-dimensional ferromagnet without an inversion center, it is shown that the total set of nonlinear modes of a helical structure contains particle-like solitons and dispersing spin waves. New types of solitons in the helical structure are described. It is found that collisions of solitons with each other are accompanied by variations in the coordinates of their centers and in their phases, as well as shifts of the structure. The possibilities of the diagnostics of solitons in the presence of external magnetic field perpendicular to the axis of the magnetic helix are established. The theoretical description of the nonlinear dynamics of spin waves is reduced to the solution of linear integral equations.

Helical equilibria and magnetic structures in the reversed field pinch and analogies to the tokamak and stellarator

The reversed field pinch configuration is characterized by the presence of magnetic structures both in the core and at the edge: in the core, at high plasma current the spontaneous development of a helical structure is accompanied by the appearance of internal electron transport barriers; at the edge strong pressure gradients, identifying an edge transport barrier, are observed too, related to the position of the field reversal surface.The aim of this paper is the experimental characterization of both the internal and edge transport barriers in relation to the magnetic topology, discussing possible analogies and differences with other confinement schemes.

Symmetry constraints on the electrical polarization in multiferroic materials

The symmetry conditions for the development of a macroscopic electrical polarization as a secondary order parameter to a magnetic ordering transition and the constraints on the direction of the polarization vector are determined by a nonconventional application of the theory of irreducible corepresentations. In our approach, which is suitable for both magnetic and structural modulations, antiunitary operators are employed to describe symmetry operations that exchange the propagation vector $\mathbf{k}$ with $\ensuremath{-}\mathbf{k}$, rather than operations combined with time reversal as in classical corepresentation analysis. Unlike the conventional irreducible representations, corepresentations can capture the full symmetry properties of the system even if the propagation vector is in the interior of the Brillouin zone. It is shown that ferroelectricity can develop even for a completely collinear structure, and that helical and cycloidal magnetic structures are not always polar. In some cases, symmetry allows the development of polarization parallel to the magnetic propagation vector. Our analysis also highlights the unique importance of magnetic commensurability, enabling one to derive the different symmetry properties of equivalent commensurate and incommensurate phases even for a completely generic propagation vector.

Chiral scattering in complex magnets

General properties of the chiral scattering of polarized neutrons are considered for two possible axial vector interactions: Zeeman energy and non-alternating Dzyaloshinskii–Moriya interaction. Behavior in magnetic field of helical magnetic structures is discussed for MnSi and magneto-electric materials. The dynamical chiral fluctuations in magnetic field are considered briefly. The chiral fluctuations in materials with the Dzyaloshinskii–Moriya interaction are discussed and an assumption is made that above the transition temperature they have to be incommensurate.

Three-dimensional dynamics of relativistic flows in pair plasmas with force-free magnetic configuration

Dynamics of the relativistic flow in pair plasmas with force-free magnetic configuration is investigated by using a three-dimensional fully relativistic electromagnetic particle-in-cell code. This study is an extension of the work by Haruki and Sakai [Phys. Plasmas 8, 1538 (2001)] that was done in a two-dimensional force-free magnetic configuration. They found that during the early stage of the interaction there occurs the streaming instability, which induces the electromagnetic perturbations associated with generation of quasi-static magnetic field. In the nonlinear stage the force-free magnetic field becomes unstable against the firehose instability and then magnetic islands are formed through magnetic reconnection. The dissipated magnetic field energy is converted to the plasma heating as well as the high-energy particle production. It is found that the three-dimensional configuration could result in completely different dynamics, except for the initial phase where the streaming instability develops. It is also found that the dynamical interaction between the force-free magnetic configuration and the relativistic plasma flows develops sequentially through four different physical processes: (I) The phase of streaming instability, (II) the phase of magnetic reconnection triggered by the first streaming instability, (III) the phase of Alfvén wave excitation through the magnetic reconnection process, and (IV) the phase of dissipation of the Alfvén waves through the magnetic reconnection. It is shown that three-dimensional Alfvén waves with helical magnetic structures can be excited through complicated three-dimensional tearing instability triggered from the streaming instability. During these dynamical processes the pair plasma can be heated through the magnetic reconnection and also the high-energy particles are generated. The interaction process between the force-free collisionless plasmas and the relativistic plasma flows may play an important role for the effective magnetic field energy dissipation, formation of filament structures, and high-energy particle production in astrophysical plasmas.

Helical Surface Structures

Over the past few years there has been growing interest in helical magnetic field structures seen at the solar surface, in coronal mass ejections, as well as in the solar wind. Although there is a great deal of randomness in the data, on average the extended structures are mostly left-handed on the northern hemisphere and right-handed on the southern. Surface field structures are also classified as dextral (= right bearing) and sinistral (= left bearing) occurring preferentially in the northern and southern hemispheres respectively. Of particular interest here is a quantitative measurement of the associated emergence rates of helical structures, which translate to magnetic helicity fluxes. In this review, we give a brief survey of what has been found so far and what is expected based on models. Particular emphasis is put on the scale dependence of the associated fields and an attempt is made to estimate the helicity flux of the mean field vs. fluctuating field.

The Structure and Evolution of a Sigmoidal Active Region

Solar coronal sigmoidal active regions have been shown to be precursors to some coronal mass ejections. Sigmoids, or S-shaped structures, may be indicators of twisted or helical magnetic structures, having an increased likelihood of eruption. We present here an analysis of a sigmoidal region's three-dimensional structure and how it evolves in relation to its eruptive dynamics. We use data taken during a recent study of a sigmoidal active region passing across the solar disk (an element of the third Whole Sun Month campaign). While S-shaped structures are generally observed in soft X-ray (SXR) emission, the observations that we present demonstrate their visibility at a range of wavelengths including those showing an associated sigmoidal filament. We examine the relationship between the S-shaped structures seen in SXR and those seen in cooler lines in order to probe the sigmoidal region's three-dimensional density and temperature structure. We also consider magnetic field observations and extrapolations in relation to these coronal structures. We present an interpretation of the disk passage of the sigmoidal region, in terms of a twisted magnetic flux rope that emerges into and equilibrates with overlying coronal magnetic field structures, which explains many of the key observed aspects of the region's structure and evolution. In particular, the evolving flux rope interpretation provides insight into why and how the region moves between active and quiescent phases, how the region's sigmoidicity is maintained during its evolution, and under what circumstances sigmoidal structures are apparent at a range of wavelengths.

Helical magnetic structures above the Curie point

The behavior of a ferromagnet with the absence of mirror symmetry in the region above the Curie point has been studied. It is shown that the presence of a pseudoscalar of mrot m type in the system leads to the transition point smearing and to the generation of a new large-scale magnetic structure above the Curie point. The obtained result may be of importance in the study of the maintenance of the dipole component of magnetic fields of the Earth.