Helimagnetic Research Articles

Magnetic-field-controllable elasticity of helical spring magnets composed of magnetic-particle-polymer composites

Abstract We experimentally demonstrated the ability to control the spring constant of helical mechanical springs using a magnetic field. These springs were composed of magnetic nanoparticles embedded within an elastic polymer matrix. The composite material, in its gel form, was injected into a 3D-printed mold featuring a helical-spring-shaped cavity. An external magnetic field applied perpendicular to the coil axis of the spring allows the aligmment of the magnetic nanoparticle assemblies (chain axis) in the field direction. This alignment process determines the preferred magnetization orientation of the particle assembly chain, thereby balancing the magnetic force between the magnetic anisotropy field and the Zeeman field under a given external field. When the spring is subjected to compression or stretching loads under an externally applied magnetic field, these two magnetic fields modify the effective spring constant of the magnetic helical spring by ~ 31 %, incresing it from 8.7 N/m (under no field) to 11.5 N/m at 300 mT. Analytical modeling using a simplified rod geometry aptly explains the experimental results, demonstrating that the spring constant linearly increases with the field strength up to 300 mT. Such composite magnetic helical springs could be utilized as active vibration absorbers or isolators due to their field-controllable elasticity.

Spontaneous reversal of spin chirality and competing phases in the topological magnet EuAl4

AbstractMaterials exhibiting a spontaneous reversal of spin chirality have the potential to drive the widespread adoption of chiral magnets in spintronic devices. Unlike the majority of chiral magnets that require the application of an external field to reverse the spin chirality, we observe the spin chirality to spontaneously reverse in the topological magnet EuAl4. Using resonant elastic x-ray scattering we demonstrate that all four magnetic phases in EuAl4 are single-k, where the first two magnetic phases are characterized by spin density wave order and the last two by helical spin order. A single spin chirality was stabilised across the 1mm2 sample, and the reversal of spin chirality occurred whilst maintaining a helical magnetic structure. At the onset of the helical magnetism, the crystal symmetry lowers to a chiral monoclinic space group, explaining the asymmetry in the chiral spin order, and establishing a mechanism by which the spin chirality could reverse via magnetostructural coupling.

Magnetostriction related to skyrmion-lattice formation in chiral magnet FeGe

The B20 type chiral magnet FeGe exhibits the formation of skyrmion-lattice (SkL) phases in the vicinity of the magnetic ordering temperature. The SkL is a magnetic superlattice composed of vortex-type topological spin objects, and it has experimentally been known that its formation requires the existence of an intermediate (IM) phase between SkL and the paramagnetic (PM) phases. We take interest in how the crystal lattice experiences the formation of these topological spin texture. In this study, we observed the so-called spin–orbit coupling induced magnetostriction related to these topological spin texture formation, in addition to the ac magnetization anomalies. The temperature and magnetic field dependences of the lattice parameter reflected the transformation of phases, such as helimagnetic (HM), SkL, IM, conical (CM), and PM phases. In the PM region, a phase characterized as gaseous skyrmions was detected similarly to the case of the same B20 type MnSi. Furthermore, the HM, CM, and IM phases were also divided into two regions. Thus, the precise phase diagram near Tc was reconstructed from the prospect of the magnetostriction such that we demonstrated that the stabilization of skyrmions needs a finite magnetic field.

Observation of Circular Dichroism Induced by Electronic Chirality.

Chiral phases of matter, characterized by a definite handedness, abound in nature, ranging from the crystal structure of quartz to spiraling spin states in helical magnets. In 1T-TiSe_{2} a source of chirality has been proposed that stands apart from these classical examples as it arises from combined electronic charge and quantum orbital fluctuations. This may allow its chirality to be accessed and manipulated without imposing either structural or magnetic handedness. However, direct bulk evidence that broken inversion symmetry and chirality are intrinsic to TiSe_{2} remains elusive. Here, employing resonant elastic x-ray scattering technique, we reveal the presence of circular dichroism, i.e., polarization dependence of the resonant diffraction intensity, up to ∼40% at forbidden Bragg peaks that emerge at the charge and orbital ordering transition. The dichroism varies dramatically with incident energy and azimuthal angle. Comparison to calculated scattering intensities traces its origin to bulk chiral electronic order in TiSe_{2} and establishes resonant elastic x-ray scattering as a sensitive probe to electronic chirality.

Nonreciprocity of spin waves by chiral fluctuations induced in the conical state

Nonreciprocity in chiral magnets is promising for applications in chiral spintronics. Spin-wave nonreciprocity happens when applying an external magnetic field to a chiral magnet while a stream of particles parallel to the field flows through it. Here, we found through micromagnetic simulations that, in the absence of lattice chirality, the excitation spectra of spin waves in the conical magnetic state depend on the relative orientations of the spin-polarized current and the external magnetic field applied along the magnetic helix axis. When both are in the same direction, the spin component of the magnet along the helical axis increases with time while the chiral index decreases. Statistical analysis shows that the spin fluctuations are anisotropic, where the fluctuations along the helical axis have chiral characters as multi-modal, high-frequency, and low-intensity. We give an intuitive interpretation of these observations from the point of view of symmetry breaking and explore the factors affecting the nonreciprocity of the spin-wave spectrum, guiding experimental observations.

Magnon-mediated topological superconductivity in a quantum wire

Many emergent phases of matter stem from the intertwined dynamics of quasiparticles. Here we show that a topological superconducting phase emerges as the result of interactions between electrons and magnons in a quantum wire and a helical magnet. The magnon-mediated interaction favors triplet superconductivity over a large magnetic phase space region, and stabilizes topological superconductivity over an extended region of chemical potentials. The superconducting gap depends exponentially on the spin-electron coupling, allowing it to be enhanced through material engineering techniques. Published by the American Physical Society 2024

Lattice dynamics and enhanced electron–phonon coupling of itinerant magnet Fe1-xCoxSi (x ≥ 0.3): A 57Fe Mössbauer spectroscopy study

The magnetic phase transition in itinerant magnets involves many fundamental physical problems. Here we report a systematic investigation on the 57Fe Mössbauer spectroscopy of B20-type Fe1-xCoxSi (0.3 ≤ x ≤ 0.8) compounds in combination with macroscopic magnetism and resistance measurements. We observed non-Fermi liquid behavior at low temperatures in the region of phase transition (close to x = 0.7), as well as significant phonon softening. In addition, with the increase of Co content, the effect of electron–phonon coupling on the hyperfine field is gradually comparable to that of spin fluctuations. Our observations indicate that electron doping promotes electron–electron correlations and facilitates electron–phonon interactions at the cost of suppression of magnetic ordering and magnetic fluctuations, suggesting that interactions between phonons and other quasiparticles play a crucial role in the magnetic phase transitions of itinerant helical magnets.

The influence of antiferromagnetic spin cantings on the magnetic helix pitch in cubic helimagnets

In cubic helimagnets MnSi and Cu2OSeO3 with their nearly isotropic magnetic properties, the magnetic structure undergoes helical deformation, which is almost completely determined by the helicoid wavenumber k=D/J , where magnetization field stiffness J is associated with isotropic spin exchange, and D is a pseudoscalar value characterizing the antisymmetric Dzyaloshinskii–Moriya (DM) interaction. Another magnetic feature of these crystals, also caused by the DM interactions, are antiferromagnetic spin cantings, similar to the ferromagnetic cantings responsible for the phenomenon of weak ferromagnetism. Here we show that cantings can strongly influence the helical order through the value of the parameter D . Changing the cantings in a strong magnetic field is predicted to affect the magnon spectrum of the crystals.

Helical Undulators of Magnetized Helices and Ring Sectors

The periodic system of a spiral array of magnetized ring sectors (quasi-helix) creates a helical field, which is close in structure and magnitude to the field in the system of helical magnets. Such a system of a relatively small number of readily accessible magnets can be easier to manufacture and assemble than a system containing mag-netized helices made from single pieces. In this paper, we study the dependence of the helical field on the number of sectors per undulator period. Short prototypes consisting of longitudinally and radially magnetized sectors were experimentally studied. The maximum value of the helical field on the axis of a quasi-helix of longitudinally and radially magnetized ring NdFeB sectors with a period of 2 cm and a relatively large inner diameter of 8 mm is 0.28 T and 0.34 T, respectively. This corresponds to a field of 0.74 T in the case of four alternately longitudinally and radially magnetized Halbach-like quasi-helices of pre-magnetized sectors and a hybrid system assembled from longitudinally pre-magnetized sectors and preliminarily non-magnetized steel sectors. Such undulators can provide a high oscillatory electron velocity and seem promising for increasing the efficiency of FELs and inverse FELs in various frequency ranges.

Простейшая аппроксимация интегральных коэффициентов в уравнениях корреляционной магнитодинамики для ферромагнетиков

The equations of correlational magnetodynamics (CMD) describe a magnet in the continuum approximation. The main problem in constructing CMD is the calculation of integral coefficients, in particular, the coefficient describing the production of short-range order, depending on the three-particle distribution functions and the structure of the crystal lattice. The work provides the simplest approximations for the integral coefficients of CMD based on the value of pair correlations at the phase transition point. To ensure an equilibrium solution, the coefficients are additionally determined in the upper part of the phase plane according to the assumption of a helical magnetization structure. The resulting approximation provides qualitative agreement with the simulation results within the framework of the original atomistic model of the magnet, and at the same time it turns out to be simple enough for further analysis.

Magnetic Helix in a Multilayer Ferromagnetic Nanoparticle and Its Rotation Induced by an Electric Current

The dynamics of the magnetization induced by an electric current flowing in a multilayer nanoparticle is studied theoretically. A region of the parameters where the coherent rotation of a magnetic helix, which is formed in this system due to the magnetostatic interaction of ferromagnetic layers, has been determined analytically. Estimates indicate that the predicted nonlinear oscillation mode of the magnetization can be observed experimentally.

Light irradiation controlled spin selectivity in a magnetic helix

Light irradiation controlled spin selectivity in a magnetic helix

Magnetic Helix in a Multilayer Ferromagnetic Nanoparticle and Its Rotation Induced by an Electric Current

Magnetic Helix in a Multilayer Ferromagnetic Nanoparticle and Its Rotation Induced by an Electric Current

Intergranular Spin Dependent Tunneling Dominated Magnetoresistance in Helimagnetic Manganese Phosphide Thin Films.

Helical magnets are emerging as a novel class of materials for spintronics and sensor applications; however, research on their charge- and spin-transport properties in a thin film form is less explored. Herein, we report the temperature and magnetic field-dependent charge transport properties of a highly crystalline MnP nanorod thin film over a wide temperature range (2 K < T < 350 K). The MnP nanorod films of ~100 nm thickness were grown on Si substrates at 500 °C using molecular beam epitaxy. The temperature-dependent resistivity ρ(T) data exhibit a metallic behavior (dρ/dT > 0) over the entire measured temperature range. However, large negative magnetoresistance (Δρ/ρ) of up to 12% is observed below ~50 K at which the system enters a stable helical (screw) magnetic state. In this temperature regime, the Δρ(H)/ρ(0) dependence also shows a magnetic field-manipulated CONE + FAN phase coexistence. The observed magnetoresistance is dominantly governed by the intergranular spin dependent tunneling mechanism. These findings pinpoint a correlation between the transport and magnetism in this helimagnetic system.

Stripe helical magnetism and two regimes of anomalous Hall effect in NdAlGe

We report the magnetic and electronic transport properties of the inversion and time-reversal symmetry breaking Weyl semimetal NdAlGe. This material is analogous to NdAlSi, whose helical magnetism presents a rare example of a Weyl-mediated collective phenomenon, but with a larger spin-orbit coupling. Our neutron diffraction experiments revealed that NdAlGe, similar to NdAlSi, supports an incommensurate Ising spin density wave ($T_{\text{inc}}=6.8$ K) with a small helical spin canting of 3$^\circ$ and a long-wavelength of $\sim$ 35 nm, which transitions to a commensurate ferrimagnetic state below $T_{\text{com}}=5.1$ K. Using small-angle neutron scattering, we showed that the zero-field cooled ferrimagnetic domains form stripes in real space with characteristic length scales of 18 nm and 72 nm parallel and perpendicular to the [110] direction, respectively. Interestingly, for the transport properties, NdAlSi does not exhibit an anomalous Hall effect (AHE) that is commonly observed in magnetic Weyl semimetals. In contrast to NdAlSi, we identify two different AHE regimes in NdAlGe that are respectively governed by intrinsic Berry curvature and extrinsic disorders/spin fluctuations. Our study suggests that Weyl-mediated magnetism prevails in this group of noncentrosymmetric magnetic Weyl semimetals NdAl$X$, but transport properties including AHE are affected by material-specific extrinsic effects such as disorders, despite the presence of prominent Berry curvature.

Magnetic phase diagram of Cr1/3NbS2: SANS study

The magnetic structure of the Cr1/3NbS2 compound was studied with help of small-angle neutron scattering under an applied magnetic field in a wide temperature range below TN ​= ​130 К. At zero magnetic field the magnetic moments of Cr3+ ions build the helix structure with the wave vector k oriented along the c-axis of the chiral structure of P6322 space group. The detected diffraction ring is interpreted as scattering on the magnetic helices of randomly oriented crystallites within powder sample. The intensity of the diffraction peak decreases with temperature and dissolves at T ​= ​115 К. The magnetic field differently affects the crystallites with helix wave vector k along the field and those with k perpendicular to the field. The magnetic field, when applied, leads to appearance of the chiral soliton lattice in the grains with k oriented perpendicularly to the field. The period of the chiral soliton lattice increases rapidly as H approaches HC1 ​= ​0.25 ​T. The transition from chiral soliton lattice to the field-induced ferromagnet, however, is not homogeneous but is accompanied by strong ferromagnetic fluctuations in the wide field range around HC1. These fluctuations are well visible in the experiment as scattering centered at Q ​= ​0. In the grains with k parallel to the field, the chiral soliton lattice undergoes a series of transitions from helimagnet to the conical state, and then to the field-induced ferromagnet at HC2 ​= ​1.25 ​T. The period of the conical helix increases slightly (for 5%) with the field, while the transition to the field–induced ferromagnetic state is again accompanied by strong ferromagnetic fluctuations. The magnetic field–temperature phase diagrams for H perpendicular to the c-axis and for H parallel to the c-axis are plotted on the basis of the data obtained.

Remote magnetic-field-actuated helical spring magnets: Micromagnetic simulation and analytical modeling

Remote magnetic-field-actuated helical spring magnets: Micromagnetic simulation and analytical modeling

Magnetic Soft Helical Manipulators with Local Dipole Interactions for Flexibility and Forces.

Magnetic continuum manipulators (MCMs) are a class of continuum robots that can be actuated without direct contact by an external magnetic field. MCMs operating in confined workspaces, such as those targeting medical applications, require flexible magnetic structures that contain combinations of magnetic components and polymers to navigate long and tortuous paths. In cylindrical MCM designs, a significant trade-off exists between magnetic moment and bending flexibility as the ratio between length and diameter decreases. In this study, we propose a new MCM design framework that enables increasing diameter without compromising on flexibility and magnetic moment. Magnetic soft composite helices constitute bending regions of the MCM and are separated by permanent ring magnets. Local dipole interactions between the permanent magnets can reduce bending stiffness, depending on their size and spacing. For the particular segment geometry presented herein, the local dipole interactions result in a 31% increase in angular deflection of composite helices inside an external magnetic field, compared to helices without local interactions. In addition, we demonstrate fabrication, maneuverability, and example applications of a multisegment MCM in a phantom of the abdominal aorta, such as passing contrast dye and guidewires.

Incommensurate magnetic states induced by ordering competition in

Quantum criticality near a certain magnetic phase transition beneath the superconducting (SC) dome of is attentively studied by virtue of a phenomenological theory in conjunction with the renormalization group approach. We report that ordering competition between magnetic and SC fluctuations is capable of coaxing incommensurate (IC) magnetic states to experience distinct fates depending upon their spin configurations. The C 2-symmetry IC magnetic stripe with a perpendicular magnetic helix dominates over other C 2-symmetry magnetic competitors and hints at a potential candidate for the unknown C 2-symmetry magnetic state. Of the C 4-symmetry IC magnetic phases, the IC charge spin density wave is substantiated to be superior, shedding light on the significant intertwining of charge and spin degrees of freedom. Meanwhile, ferocious fluctuations render a sharp fall in superfluid density alongside a dip in critical temperature as well as intriguing behavior of the London penetration depth.

Spintronics in double stranded magnetic helix: role of non-uniform disorder

The spin dependent transport phenomena are investigated in a double stranded (ds) magnetic helix (MH) structure. Two different helical systems, short-range hopping helix and long range hopping (LRH) helix, are taken into account. We explore the role of these two kinds of geometries on spin dependent transport phenomena. Using Green’s function formalism within a tight-binding framework we compute transport quantities which include spin dependent transmission probabilities, junction currents and spin polarization (SP) coefficient. High degree of SP is obtained for the LRH MH. The SP can be tuned by changing the inter-strand hopping and the direction of magnetic moments at different lattice sites. We find atypical features when we include impurities in one strand of the MH, keeping the other strand free. Unlike uniform disordered systems, SP gets increased with impurity strength beyond a critical value. The effect of temperature on SP and experimental possibilities of our proposed quantum system are also discussed, to make the present communication a self-contained one. Our analysis may provide a new route to explore interesting spintronic properties using similar kind of fascinating helical geometries, possessing higher order electron hopping and subjected to non-uniform disorder.