Helimagnetic Research Articles

Spin-induced strongly correlated magnetodielectricity, magnetostriction effect, and spin-phonon coupling in helical magnet Fe3(PO4)O3

Helical magnets are extremely promising, as they have fascinating magnetic, electric, and phononic properties. Here, we report a spectrum of simultaneously occurring and highly entangled intriguing phenomena induced by helical spin ordering in a noncentrosymmetric and spin-frustrated system ${\mathrm{Fe}}_{3}(\mathrm{P}{\mathrm{O}}_{4}){\mathrm{O}}_{3}$. These phenomena include magnetodielectric effect in the form of a frequency-independent pronounced dielectric peak, clear magnetostriction effect manifested as a dramatic downturn in the thermal variation of lattice parameters, and strong spin-phonon coupling (which displays a unique anomalous hardening and softening of various phonon modes) at temperatures as high as ${T}_{N}=163\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The observed dielectric peak is seemingly associated with a structural distortion via the strong magnetostriction effect.

Chirality-dependent spin-transfer torque and current-induced spin rotation in helimagnets

We have built a theory of the spin-transfer torque (STT) effect in a conductive chiral helical magnet (CHM). It is shown that the STT effect induced by a spin current flowing through CHM leads to the rotation of the CHM magnetization spiral around its axis. The frequency of such rotation of the CHM magnetization is found. The former is expressed in terms of the parameters of the quantum-exchange Hamiltonian that specifies helical magnetic ordering in a conductive crystal. We have showcased that both the direction of rotation of the CHM magnetization and the direction of changes in the shape of the magnetic spiral are determined by the electron flow direction and the chirality of the magnet. The theory developed accounts for the generation of an electromagnetic field when rotating the magnetic spiral in the CHM subjected to an electric current flowing through the helimagnet.

Origin of circular dichroism in resonant elastic x-ray scattering from magnetic and polar chiral structures

Circular dichroism in resonant elastic x-ray scattering (CD-REXS) has recently been observed in chiral structures composed of multiferroic materials as well as magnetic moments or electric polarization vectors. In order to comprehensively understand the experimental results of these previous studies, we present here in detail the analytical formulation of CD-REXS for one-dimensional helices composed of magnetic moments and electric polarization vectors, respectively. In particular, by comparing CD-REXS for a proper-screw-shaped Bloch-type helix and cycloid-shaped N\'eel-type helix, we found that CD-REXS for both magnetic and polar helices can discriminate between both types of helices. We also found that the x-ray polarization factor depending on the scattering geometry is a significant factor in determining the characteristics of CD-REXS for chiral structures. The results obtained from the detailed formulas can be intuitively understood using the concept of mirror reflection. In particular, in this way it can be understood that Bloch- and N\'eel-type helices correspond to truly chiral and achiral structures, respectively, and why CD-REXS is able to distinguish between these two types of helices.

Curvilinear manipulation of polarized spin waves

Polarization, the precession direction with respect to the background magnetization, is an intrinsic degree of freedom of spin wave. Introducing symmetry-breaking mechanisms lies in the heart of lifting the degeneracy between polarized spin-wave modes and is essential in constructing polarization-based magnonic devices. Here we show that polarized spin waves can be naturally harnessed in a curved antiferromagnetic wire via tuning its curvature and torsion. Specifically, we investigate evolution of polarized spin wave in a spin-wave rotator and a spin-wave interferometer based on magnetic circular helices and correlate these curvilinear effects to the Berry phase accumulated along the wire.

Disentangling Magnetic and Grain Contrast in Polycrystalline FeGe Thin Films Using Four-Dimensional Lorentz Scanning Transmission Electron Microscopy

The study of nanoscale chiral magnetic order in polycrystalline materials with a strong Dzyaloshinskii-Moriya interaction is interesting for the observation of magnetic phenomena at grain boundaries and interfaces. This is especially true for polycrystalline materials, which can be grown using scalable techniques, the scalability of which is promising for future device applications. One such material is sputter-deposited B20 $\mathrm{Fe}\mathrm{Ge}$ on $\mathrm{Si}$, which is actively investigated as the basis for low-power high-density magnetic memory technology in a scalable material platform. Although conventional Lorentz electron microscopy provides the requisite spatial resolution to probe chiral magnetic textures in single-crystal $\mathrm{Fe}\mathrm{Ge}$, probing the magnetism of sputtered B20 $\mathrm{Fe}\mathrm{Ge}$ is more challenging because the submicron crystal grains add confounding contrast. This is a more general problem for polycrystalline magnetic devices, where scattering from grain boundaries tends to hide comparably weaker signals from magnetism. We address the challenge of disentangling magnetic and grain contrast by applying four-dimensional Lorentz scanning transmission electron microscopy using an electron-microscope pixel-array detector. Supported by analytical and numerical models, we find that the most important parameter for imaging magnetic materials with polycrystalline grains is the ability for the detector to sustain large electron doses, where having a high-dynamic-range detector becomes extremely important. Despite the small grain size in sputtered B20 $\mathrm{Fe}\mathrm{Ge}$ on $\mathrm{Si}$, using this approach, we are still able to observe helicity switching of skyrmions and magnetic helices across two adjacent grains, as they thread through neighboring grains. We reproduce this effect using micromagnetic simulations by assuming that the grains have distinct orientation and magnetic chirality and find that magnetic helicity couples to crystal chirality. Our methodology for imaging magnetic textures is applicable to other thin-film magnets used for spintronics and memory applications, where an understanding of how magnetic order is accommodated in polycrystalline materials is important.

Magnetic and anisotropic magnetocaloric effects of HoCoSi fast quenching ribbons

The performance of magnetocaloric effect materials is one of the key factors restricting the development of magnetic refrigeration technology. Materials with anisotropic magnetocaloric effect can be used in the rotary magnetic refrigeration technology, which is beneficial to the simplification of refrigeration devices. In this work, the magnetic properties, magnetocaloric effects, and magnetic anisotropies of rapidly quenched HoCoSi compounds are investigated. At low temperatures below <i>T</i><sub>t</sub> = 5.7 K, the HoCoSi ferromagnetism and helical magnetism coexist. With the increase of temperature, the HoCoSi undergoes a second-order phase transition from ferromagnetic (FM) to paramagnetic (PM) phase at <i>T</i><sub>C</sub> = 13.7 K. Both XRD and SEM show that the HoCoSi has a preferred orientation. In order to obtain a large magnetocaloric effect and to determine the effect of preferred orientation on magnetism and magnetocaloric effect, the isothermal magnetization curves of the 10 m/s–HoCoSi fast quenched belt in the directions of <i>H</i> parallel and perpendicular to texture around the Curie temperature are analyzed. The corresponding magnetic entropy change (–Δ<i>S</i><sub>M</sub>) and magnetic refrigeration capacity (RC) are calculated. Under the magnetic field changing from 0 to 5 T, the value of –Δ<i>S</i><sub>M</sub> is 22 J/(kg·K) in the direction of <i>H</i> parallel to the texture and 12 J/(kg·K) in the direction of <i>H</i> perpendicular to texture , and their corresponding values of RC(RCP)are 360 (393.8) J·kg<sup>–1</sup> and 160 (254.4) J/kg. The value of –Δ<i>S</i><sub>M</sub> reaches 12.5 J/(kg·K)even at <i>μ</i><sub>0</sub><i>H</i> = 0–2 T in the direction of H parallel to the texture. It is obvious that the 10-m/s-HoCoSi fast quenching belt shows a large low-field magnetocaloric effect and obvious magnetic anisotropy, which is expected to be used to realize the magnetic refrigeration technology of rotating samples.

Dynamics of interstitial skyrmions in the presence of temperature gradients

An interstitial skyrmion is the skyrmion state confined in the helical background. While the spin helix naturally provides the one-dimensional channels in helical magnets, the dynamics of an interstitial skyrmion are different from that for free skyrmions in a ferromagnetic or conical background. In this paper, we studied the dynamics of interstitial skyrmions under a temperature gradient numerically and analytically. We show that the interstitial skyrmion moves from the hot area to the cold area in the presence of temperature gradients, in contrast to skyrmions in the ferromagnetic background, where they move towards the hot region. Because the interstitial skyrmion is confined in the one-dimensional channel, it moves in the same direction as the magnon flow induced by the temperature gradient, where the $\ensuremath{\beta}$-type thermomagnonic torque plays an essential role.

Magnetic Levitation Type Double Helix Self‐Powered Acceleration Sensor Based on ZnO‐RTV Film

AbstractTraditional accelerometers rely on batteries to provide energy, and the massive use of batteries caused great pressure on the natural environment. In this paper, a self‐powered acceleration sensor is designed. To more easily harvest weak vibration signals and effectively improve the sensitivity, this sensor is built on a magnetic levitation type double helix structure and ZnO‐doped triboelectric film. Experiments indicated that the open‐circuit voltage and short circuit current of the triboelectric nanogenerator (TENG) doped with ZnO are increased by 2.51 times and 2.07 times, respectively. Under shaking table test for simulating machine vibration environment, the sensor's sensitivity reaches 16.31 V (m s−2)−1 and its linearity is 0.993. The sensor power generation is 110 μW and its test range is ≈1–7 m s−2. Also, the sensor can be applied for monitoring human's movement state by installing in a sole. This design provides a new idea for the development of TENG‐based sensors.

Generalized model of MnSi-like spiral magnets

A general, symmetry-allowed model of nearest-neighbour interactions for MnSi-like magnets is presented. A left-handed helical magnet phase occurs within a large parameter space of the model, which is explored via numerical simulation. The relations between microscopic features of the spiral structure and various model parameters, including an external magnetic field, are determined and show good agreement with predictions from free energy considerations. A skyrmion structure is stabilized near the boundary.

Weyl-mediated helical magnetism in NdAlSi.

Emergent relativistic quasiparticles in Weyl semimetals are the source of exotic electronic properties such as surface Fermi arcs, the anomalous Hall effect and negative magnetoresistance, all observed in real materials. Whereas these phenomena highlight the effect of Weyl fermions on the electronic transport properties, less is known about what collective phenomena they may support. Here, we report a Weyl semimetal, NdAlSi, that offers an example. Using neutron diffraction, we found a long-wavelength helical magnetic order in NdAlSi, the periodicity of which is linked to the nesting vector between two topologically non-trivial Fermi pockets, which we characterize using density functional theory and quantum oscillation measurements. We further show the chiral transverse component of the spin structure is promoted by bond-oriented Dzyaloshinskii-Moriya interactions associated with Weyl exchange processes. Our work provides a rare example of Weyl fermions driving collective magnetism.

CEPC Z-pole polarization design studies

In this paper, we give preliminary designs of beam polarization manipulations by inserting three different types of insertions in the Circular Electron–Positron Collider (CEPC) at center-of-mass energies of 91 GeV ([Formula: see text]-pole). With the wigglers in the collider ring, we can obtain 5% transverse polarization in 1.1 h for the precise energy measurement. To overcome depolarization effects as the beam energy rises from 10 GeV to 45.5 GeV in the booster ring, Siberian snakes based on helical magnets are adopted. Finally, for longitudinally polarized beam collisions, a schematic design of spin rotators based on solenoids in the collider ring is studied.

Chiral Helimagnetism and One-Dimensional Magnetic Solitons in a Cr-Intercalated Transition Metal Dichalcogenide.

Chiral magnets endowed with topological spin textures are expected to have promising applications in next-generation magnetic memories. In contrast to the well-studied 2D or 3D magnetic skyrmions, the authors report the discovery of 1D nontrivial magnetic solitons in a transition metal dichalcogenide 2H-TaS2 via precise intercalation of Cr elements. In the synthetic Cr1/3 TaS2 (CTS) single crystal, the coupling of the strong spin-orbit interaction from TaS2 and the chiral arrangement of the magnetic Cr ions evoke a robust Dzyaloshinskii-Moriya interaction. A magnetic helix having a short spatial period of ≈25nm is observed in CTS via Lorentz transmission electron microscopy. In a magnetic field perpendicular to the helical axis, the helical spin structure transforms into a chiral soliton lattice (CSL) with the spin structure evolution being consistent with the chiral sine-Gordon theory, which opens promising perspectives for the application of CSL to fast-speed nonvolatile magnetic memories. This work introduces a new paradigm to soliton physics and provides an effective strategy for seeking novel 2D magnets.

Current-induced H-shaped-skyrmion creation and their dynamics in the helical phase

Inevitable for the basic principles of skyrmion racetrack-like applications is not only their confined motion along one-dimensional channels but also their controlled creation and annihilation. Helical magnets have been suggested to naturally confine the motion of skyrmions along the tracks formed by the helices, which also allow for high-speed skyrmion motion. We propose a protocol to create topological magnetic structures in a helical background. We furthermore analyse the stability and current-driven motion of the skyrmions in a helical background with in-plane uniaxial anisotropy fixing the orientation of the helices.

Archimedean screw in driven chiral magnets

In chiral magnets a magnetic helix forms where the magnetization winds around a propagation vector {q}q. We show theoretically that a magnetic field B_\bot(t) \bot qB⊥(t)⊥q, which is spatially homogeneous but oscillating in time, induces a net rotation of the texture around {q}q. This rotation is reminiscent of the motion of an Archimedean screw and is equivalent to a translation with velocity v_{\text{screw}}vscrew parallel to q. Due to the coupling to a Goldstone mode, this non-linear effect arises for arbitrarily weak B_\bot(t)B⊥(t) with v_{\text{screw}} \propto |{ B_\perp}|^2vscrew∝|B⊥|2 as long as pinning by disorder is absent. The effect is resonantly enhanced when internal modes of the helix are excited and the sign of v_{\text{screw}}vscrew can be controlled either by changing the frequency or the polarization of B_\bot(t)B⊥(t). The Archimedean screw can be used to transport spin and charge and thus the screwing motion is predicted to induce a voltage parallel to q. Using a combination of numerics and Floquet spin wave theory, we show that the helix becomes unstable upon increasing B_\botB⊥, forming a `time quasicrystal’ which oscillates in space and time for moderately strong drive.

Nonreciprocity of spin waves in magnetic nanotubes with helical equilibrium magnetization

Spin waves (SWs) in magnetic nanotubes have shown interesting nonreciprocal properties in their dispersion relation, group velocity, frequency linewidth, and attenuation lengths. The reported chiral effects are similar to those induced by the Dzyaloshinskii–Moriya interaction but originating from the dipole–dipole interaction. Here, we show that the isotropic-exchange interaction can also induce chiral effects in the SW transport; the so-called Berry phase of SWs. We demonstrate that with the application of magnetic fields, the nonreciprocity of the different SW modes can be tuned between the fully dipolar governed and the fully exchange governed cases, as they are directly related to the underlying equilibrium state. In the helical state, due to the combined action of the two effects, every single sign combination of the azimuthal and axial wave vectors leads to different dispersions, allowing for a very sophisticated tuning of the SW transport. A disentanglement of the dipole–dipole and exchange contributions so far was not reported for the SW transport in nanotubes. Furthermore, we propose a device based on coplanar waveguides that would allow to selectively measure the exchange or dipole induced SW nonreciprocities. In the context of magnonic applications, our results might encourage further developments in the emerging field of 3D magnonic devices using curved magnetic membranes.

Reentrant superconductivity in proximity to a topological insulator

Superconducting hybrid structures with topological order and induced magnetization offer a promising way to realize fault-tolerant quantum computation. However, the effect of the interplay between magnetization and the property of the topological insulator surface, otherwise known as spin-momentum locking on the superconducting proximity effect, still remains to be investigated. We relied on the quasiclassical self-consistent approach to consider the superconducting transition temperature in the two-dimensional superconductor/topological insulator (S/TI) junction with an in-plane helical magnetization on the TI surface. It has emerged that the presence of the helical magnetization leads to the nonmonotonic dependence of the critical temperature on the TI thickness for both cases when the magnetization evolves along or perpendicular to the interface. The results obtained can be helpful for designing novel superconducting nanodevices and better understanding the nature of superconductivity in S/TI systems with nonuniform magnetization.

Multiple quantum phase transitions of different nature in the topological kagome magnet Co3Sn2\u2212xInxS2

The exploration of topological electronic phases that result from strong electronic correlations is a frontier in condensed matter physics. One class of systems that is currently emerging as a platform for such studies are so-called kagome magnets based on transition metals. Using muon spin-rotation, we explore magnetic correlations in the kagome magnet Co3Sn2−xInxS2 as a function of In-doping, providing putative evidence for an intriguing incommensurate helimagnetic (HM) state. Our results show that, while the undoped sample exhibits an out-of-plane ferromagnetic (FM) ground state, at 5% of In-doping the system enters a state in which FM and in-plane antiferromagnetic (AFM) phases coexist. At higher doping, a HM state emerges and becomes dominant at the critical doping level of only xcr,1 ≃ 0.3. This indicates a zero temperature first order quantum phase transition from the FM, through a mixed state, to a helical phase at xcr,1. In addition, at xcr,2 ≃ 1, a zero temperature second order phase transition from helical to paramagnetic phase is observed, evidencing a HM quantum critical point (QCP) in the phase diagram of the topological magnet Co3Sn2−xInxS2. The observed diversity of interactions in the magnetic kagome lattice drives non-monotonous variations of the topological Hall response of this system.

Triple helix versus skyrmion lattice in two-dimensional noncentrosymmetric magnets

It is commonly assumed that a lattice of skyrmions, emerging in two-dimensional non-centrosymmetric magnets in external magnetic fields, can be represented as a sum of three magnetic helices. In order to test this assumption we compare two approaches to a description of regular skyrmion structure. We construct (i) a lattice of Belavin-Polyakov-like skyrmions within the stereographic projection method, and (ii) a deformed triple helix defined with the use of elliptic functions. The estimates for the energy density and magnetic profiles show that these two ansatzes are nearly identical at zero temperature for intermediate magnetic fields. However at higher magnetic fields, near the transition to topologically trivial uniform phase, the stereographic projection method is preferable, particularly, for the description of disordered skyrmion liquid phase. We suggest to explore the intensities of the secondary Bragg peaks to obtain the additional information about the magnetic profile of individual skyrmions. We estimate these intensities to be several percents of the main Bragg peak at high magnetic fields.

Direct Observation of Fe‐Ge Ordering in Fe5−xGeTe2 Crystals and Resultant Helimagnetism

AbstractMicroscopic structures and magnetic properties are investigated for Fe5−xGeTe2 single crystal, recently discovered as a promising van der Waals (vdW) ferromagnet. An Fe atom (Fe(1)) located in the outermost Fe5Ge sublayer has two possible split‐sites which are either above or below the Ge atom. Scanning tunneling microscopy shows √3 × √3 superstructures which are attributed to the ordering of Fe(1) layer. The √3 × √3 superstructures have two different phases due to the symmetry of Fe(1) ordering. Intriguingly, the observed √3 × √3 ordering breaks the inversion symmetry of crystal, resulting in substantial antisymmetric exchange interaction. The temperature dependence of magnetization reveals a sharp magnetic anomaly suggesting helical magnetism of the Fe5−xGeTe2 due to its non‐centrosymmetricity. Analytical study also supports that the observed ordering can give rise to the helimagnetism. The work will provide essential information to understand the complex magnetic properties and the origin of the new vdW ferromagnet, Fe5−xGeTe2 for future topology‐based spin devices.

Conformational effect on spin filtration through a multi-terminal magnetic helix

We investigate spin dependent electron transport through a magnetic helix (MH) which is coupled to one input and two output leads. Suitably adjusting the physical parameters and lead positions we can achieve high degree of spin polarizations with opposite phase at the output terminals. We consider both long-range and short-range hopping magnetic helices and critically examine the interplay between these hopping integrals on spin polarization. From our results we find that the long-range hopping MH is much superior than the short-range one. Simulating the quantum system within a tight-binding framework we compute spin dependent transmission probabilities and spin polarization co-efficient using the well known Green’s function formalism. Our proposition can provide a deeper impact in realizing efficient spin based devices in near future.