Nontrivial Spin Textures Research Articles

Polaron induced local spin texture and anomalous Hall effect in the quadrilateral prism-shaped nanotube with Rashba and Dresselhaus spin–orbit coupling

We theoretically study the spin-texture dynamics and the transverse asymmetric charge deflection induced by the polaron in a quadrilateral prism-shaped nanotube with the Rashba and Dresselhaus spin–orbit coupling (SOC). We reveal the polaron gives rise to the nontrivial local spin textures in the nanotube within the cross section plane. The spins demonstrate oscillations and the oscillating patterns are dependent on the SOC type. For the nanotube containing a segment of the ferromagnetic domain, the sizable asymmetric charge deflections could additionally take place, namely, the anomalous Hall effect. The amount of the deflected charges is determined by the strength and orientations of the ferromagnetic magnetization as well as the SOC type. The work provides a valuable insight of the coherent transport of polaron through a quasi-one-dimensional nanotube with Rashba and Dresselhaus SOC and open avenues for the potential device applications.

Noncollinear twisted RKKY interaction on the optically driven SnTe(001) surface

The nontrivial spin texture on the (001) surface of topological crystalline insulator SnTe hosts exotic scientific importance and spintronic applications. Here, we study the effects of weak Floquet optical driving on the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between two magnetic impurities on a doped SnTe(001) surface. Due to peculiar spin-orbit hybridization, we find a noncollinear twisted RKKY interaction with XYZ-Heisenberg, symmetric in-plane, and asymmetric Dzyaloshinskii-Moriya (DM) terms. We see that contributions from the $z$ $(x)$ component of the XYZ-Heisenberg (DM) interaction are dominant for most parameters. The interactions, including DM terms that are responsible for interesting spin textures, require doping in most cases. We propose to modify the interactions in situ via optical control of band structure, and thereby doping. A notable aspect of this control protocol is breaking of electron-hole symmetry, which stems from the DM interaction.

Revealing the band structure of ZrTe5 using multicarrier transport

The layered material ZrTe$_5$ appears to exhibit several exotic behaviors which resulted in significant interest recently, although the exact properties are still highly debated. Among these we find a Dirac/Weyl semimetallic behavior, nontrivial spin textures revealed by low temperature transport, and a potential weak or strong topological phase. The anomalous behavior of resistivity has been recently elucidated as originating from band shifting in the electronic structure. Our work examines magnetotransport behavior in ZrTe$_5$ samples in the context of multicarrier transport. The results, in conjunction with ab-initio band structure calculations, indicate that many of the transport features of ZrTe$_5$ across the majority of the temperature range can be adequately explained by the semiclassical multicarrier transport model originating from a complex Fermi surface.

Rashba spin-orbit coupling induced modulation of magnetic anisotropy in canted antiferromagnetic heterostructures

The combined effect of charge transfer, broken symmetries, and spin-orbit coupling (SOC) yields interfacial magnetism and transport in spatially asymmetric quantum oxide heterostructures. The polar nature of such interfaces activates the Rashba effect which provides a platform to modify the electronic band structure, whereas the presence of competing magnetic anisotropies is essential for deriving the topologically nontrivial spin textures. Entanglement of these two effects wherein the Rashba physics can be utilized to manipulate magnetic anisotropy (MA) is contemplated as a pathway to alternative electronic states. Here, using the $\mathrm{CaMn}{\mathrm{O}}_{3}/\mathrm{CaIr}{\mathrm{O}}_{3}$ heterostructure as a template, we demonstrate that Rashba SOC modulates the MA and controls the sign and magnitude of anomalous Hall conductivity consequent to the modification in the electronic band structure via the reconstruction of Berry curvature. This approach to tune and control the multipronged magnetotransport phenomena via Rashba SOC (without external bias voltage) is potentially relevant for spin-orbitronics functionalities.

Magnon Interference Tunneling Spectroscopy as a Probe of 2D Magnetism

Probing two-dimensional single-layer quantum magnets remains a significant challenge. In this Letter, we propose exploiting tunneling spectroscopy in the presence of magnetic impurities to obtain information about the magnon dispersion relations in analogy to quasiparticle interference in nonmagnetic materials. We show that this technique can be used to establish the dispersion relations even for frustrated magnets, where the presence of an impurity generally leads to a nontrivial spin texture. Finally, we consider the problem of establishing Chern magnon bands in 2D magnets showing how tunable impurities allow one to probe the nature of the surface states.

Topological phase transition and skyrmions in a Janus MnSbBiSe2Te2 monolayer.

Using first-principles calculations and micro-magnetic simulations, we propose a Janus MnSbBiSe2Te2 (MSBST) monolayer derived from the MnBi2Te4 (MBT) ferromagnet and investigate the influence of biaxial strain on the electronic structures, topological characteristics and spin textures. Different from pristine MBT with an out-of-plane easy axis, the anisotropy of MSBST prefers an in-plane direction. Intriguingly, switching the easy axis direction of MSBST will significantly modify the band structure. Topological phase transition can be achieved by applying a compressive strain, making MSBST become a topological insulator with . Moreover, due to the inherent inversion asymmetry of Janus MSBST, a large Dzyaloshinskii-Moriya interaction (DMI) is induced for generating and stabilizing skyrmions. By micro-magnetic simulations, the results of spin textures show that the skyrmions phase can be achieved in MSBST with an external magnetic field of 0.8 T. Our findings provide guidelines for the development and application of spintronic devices with nontrivial topological properties and a large DMI.

Tuning nano-skyrmions and nano-skyrmioniums in Janus magnets.

We study non-trivial spin textures, nanoscale magnetic skyrmions and skyrmioniums, in two-dimensional (2D) Janus magnets, such as MnSTe and MnSeTe, based on the micromagnetism approach and Landau-Lifshitz-Gilbert (LLG) equation. It is found that the Janus magnetic structures can host stable Néel nano-skyrmions with sub-10 nm diameters, and skyrmioniums with zero topological charge. The skyrmion size can be squeezed by external magnetic fields, and even the topological charge can be changed. The diameters of the skyrmioniums are about twice the size of the skyrmions. Moreover, the switching of the topological charge Q = ±1 can be realized by changing the direction of the external magnetic fields. Our results clearly show that magnetic skyrmions in Janus magnets can be used to construct new types of efficient spintronic nanodevices.

Manipulating topological Hall-like signatures by interface engineering in epitaxial ruthenate/manganite heterostructures.

Topologically protected non-trivial spin textures (e.g. skyrmions) give rise to a novel phenomenon called the topological Hall effect (THE) and have promising implications in future energy-efficient nanoelectronic and spintronic devices. Here, we have studied the Hall effect in SrRuO3/La0.42Ca0.58MnO3 (SRO/LCMO) bilayers. Our investigation suggests that pure SRO has hard and soft magnetic characteristics but the anomalous Hall effect (AHE) in SRO is governed by the high coercivity phase. We have shown that the proximity effect of a soft magnetic LCMO on SRO plays a critical role in interfacial magnetic coupling and transport properties in SRO. Upon reducing the SRO thickness in the bilayer, the proximity effect becomes the dominant feature, enhancing the magnitude and temperature range of THE-like signatures. The THE-like features in bilayers can be explained by a diffusive Berry phase transition model in the presence of an emergent magnetic state due to interface coupling. This work provides an alternative understanding of THE-like signatures and their manipulation in SRO-based heterostructures, bilayers and superlattices.

Dramatic Tuning of the Topological Hall Effect in AxRhO2 (A = K, Rb, and Cs) Crystals by Electron Concentration or Cation

AbstractTopological Hall effect, being an unconventional anomalous Hall effect, is originated from the real‐space Berry curvature caused by the nontrivial topological spin textures in materials. Manipulations of nontrivial magnetic structure and related topological Hall effect are very important for the study of the chiral magnet. Herein, it is experimentally observed that the significant topological Hall conductivity σxy in antiferromagnetic K0.5RhO2 can reach 3.5% of ν = 1 quantum conductivity below 20 K. Furthermore, by adjusting the concentration of K‐cation different from 0.5 in KxRhO2 or substituting the K cations by Rb or Cs to form Rb0.5RhO2 or Cs0.5RhO2, it is observed that the topological Hall effect is much weakened or even disappeared. This evolution, verified by the theoretical calculations, is attributed to the unstable ground state of the non‐coplanar spin structure in KxRhO2 (x = 0.4 and 0.6) and Cs0.5RhO2. The significantly tunable topological Hall effect in AxRhO2 makes it prospective on logical/sensor devices of spintronics.

Anisotropic linear and nonlinear charge-spin conversion in topological semimetal SrIrO3

Over the past decade, utilizing spin currents in the linear response of electric field to manipulate magnetization states via spin-orbit torques (SOTs) is one of the core concepts for realizing a multitude of spintronic devices. Besides the linear regime, recently, nonlinear charge-spin conversion under the square of electric field has been recognized in a wide variety of materials with nontrivial spin textures, opening an emerging field of nonlinear spintronics. Here, we report the investigation of both linear and nonlinear charge-spin conversion in one single topological semimetal ${\mathrm{SrIrO}}_{3}(110)$ thin film that hosts strong spin-orbit coupling and nontrivial spin textures in the momentum space. In the nonlinear regime, the observation of crystalline direction dependent response indicates the presence of anisotropic surface states induced spin-momentum locking near the Fermi level. Such anisotropic spin textures also give rise to spin currents in the linear response regime, which mainly contributes to the fieldlike SOT component. Our work demonstrates the power of combination of linear and nonlinear approaches in understanding and utilizing charge-spin conversion in topological materials.

Topological Nernst effect emerging from real-space gauge field and thermal fluctuations in a magnetic skyrmion lattice

Recent studies on quantum transport in metals have revealed that a gauge field acting on the electron wavefunction yields a peculiar Hall or Nernst effect. When topologically nontrivial spin textures are present, a gauge field appears in real space and affects the electron transport. However, the understanding of the Nernst effect emerging from a real-space gauge field (topological Nernst effect) remains qualitative, and moreover, the influence of thermal fluctuations has been elusive. Here, we report a pronounced temperature-dependent topological Nernst effect in the metastable skyrmion lattice in MnSi. Our density functional theory, assuming a temperature-independent gauge field, is successful in an order-of-magnitude estimate of the Nernst signal, whereas the experimental values decrease more significantly with increasing temperature. A similar tendency is observed for the topological Hall effect, thus indicating that pronounced suppression of the real-space gauge field is crucial for the quantitative understanding of the quantum transport induced by topological spin textures at finite temperatures.

Multiferroicity in a Two-Dimensional Non-van der Waals Crystal of AgCr2X4 (X = S or Se).

Two-dimensional (2D) intrinsic multiferroics have long been pursued not only for their potential technological applications but also as model systems for studying emergent quantum phenomena and coupling mechanisms between various order parameters in low-dimensional space. However, the realization of 2D multiferroics is still a challenge. In this paper, we reveal that 2D AgCr2X4 (X = S or Se) crystals, which have been synthesized from the non-van der Waals (non-vdW) AgCrX2 bulk phase, are type I half-metallic/metallic multiferroics in which ferroelectricity and ferromagnetism coexist. The off-centering displacement of the Ag ion introduces out-of-plane polarization, and the magnetism originates from the interactions between Cr atoms. Remarkably, AgCr2Se4 shows topologically nontrivial spin textures, such as Meron pairs and Néel-type skyrmions, under suitable temperatures and magnetic fields. Our findings demonstrate that 2D multiferroics can be achieved from non-vdW materials and in turn open a new avenue for 2D multiferroics.

Magnetic triangular bubble lattices in bismuth-doped yttrium iron garnet

Magnetic bubbles have again become a subject of significant attention following the experimental observation of topologically nontrivial magnetic skyrmions. In recent work, tailoring the shape of the bubbles is considered a key factor for their dynamics in spintronic devices. In addition to the reported circular, elliptical, and square bubbles, here we observe triangular bubble domains in bismuth-doped yttrium iron garnet (Bi-YIG) using Kerr microscopy. The bubble domains evolve from discrete circular to latticed triangular and hexagonal shapes. Further, the orientation of the triangular bubbles in the hexagonal lattices can be flipped by decreasing the magnetic field. The sixfold in-plane magnetic anisotropy of Bi-YIG(111) crystal, which is presumably the mechanism underlying the triangular shape of the bubbles, is measured as 1179 erg/cm3. The study of the morphologies of topologically trivial bubbles in YIG offers insight into nontrivial spin textures, which is appealing for future spintronic applications.

Intrinsic Ferroelectric Quantum Spin Hall Insulator in Monolayer Na3Bi with Surface Trimerization.

Two-dimensional (2D) ferroelectric quantum spin Hall (FEQSH) insulator, which features coexisting ferroelectric and topologically insulating orders in two-dimension, is generally considered available only in engineered 2D systems. This is detrimental to the synthesis and application of next generation nonvolatile functional candidates. Therefore, exploring the intrinsic 2D FEQSH insulator is crucial. Here, by means of first-principles, we report a long-thought intrinsic 2D FEQSH insulator in monolayer Na3Bi with surface trimerization. The material harbors merits including large ferroelectric polarization, sizable nontrivial band gap, and low switching barrier, which are particularly beneficial for the detection and observation of ferroelectric topologically insulating states. Also, it is capable of nonvolatile switching of nontrivial spin textures via inherent ferroelectricity. The fantastic combination of excellent ferroelectric and topological phases in intrinsic the Na3Bi monolayer serves as an alluring platform for accelerating both scientific discoveries and innovative applications.

Spontaneous Topological States and Their Mutual Transformations in a Rare-Earth Ferrimagnet.

Nontrivial chiral spin textures with nanometric sizes and novel characteristics (e.g., magnetic skyrmions) are promising for encoding information bits in future energy-efficient and high-density spintronic devices. Because of antiferromagnetic exchange coupling, skyrmions in ferrimagnetic materials exhibit many advantages in terms of size and efficient manipulation, which allow them to overcome the limitations of ferromagnetic skyrmions. Despite recent progress, ferrimagnetic skyrmions have been observed only in few films in the presence of external fields, while those in ferrimagnetic bulks remain elusive. This study reports on spontaneously generated zero-field ground-state magnetic skyrmions and their subsequent transformation into traditional magnetic bubbles via intermediate states of (bi-)target bubbles during a magnetic anisotropy change in the rare-earth ferrimagnetic crystal DyFe11 Ti. Spontaneous reversible topological transformation driven by a temperature-induced spin reorientation transition is directly distinguished using Lorentz transmission electron microscopy. The spontaneous generation of magnetic skyrmions and successive topological transformations in ferrimagnetic DyFe11 Ti are expected to advance the design of topological spin textures with versatile properties and potential applications in rare-earth magnets.

Unified theory of the anomalous and topological Hall effects with phase-space Berry curvatures.

Spontaneously broken time-reversal symmetry in magnetic materials leads to a Hall response, with a nonzero voltage transverse to an applied current, even in the absence of external magnetic fields. It is common to analyze the Hall resistivity of chiral magnets as the sum of two terms: an anomalous Hall effect arising from spin-orbit coupling and a topological Hall signal coming from skyrmions, which are topologically nontrivial spin textures. The theoretical justification for such a decomposition has long remained an open problem. Using a controlled semiclassical approach that includes all phase-space Berry curvatures, we show that the solution of the Boltzmann equation leads to a Hall resistivity that is just the sum of an anomalous term arising from momentum-space curvature and a topological term related to the real-space curvature. We also present numerically exact results from a Kubo formalism that complement the semiclassical approach.

Tuning of topological properties in the strongly correlated antiferromagnet Mn3Sn via Fe doping

Magnetic topological materials, in which strong correlations between magnetic and electronic properties of matter, give rise to various exotic phenomena such as anomalous Hall effect (AHE), topological Hall effect (THE), and skyrmion lattice. Here, we report on the electronic, magnetic, and topological properties of Mn$_{3-\it{x}}$Fe$_{\it{x}}$Sn single crystals ($\it{x}$=0, 0.25, and 0.35). Low temperature magnetic properties have been significantly changed with Fe doping. Most importantly, we observe that large uniaxial magnetocrystalline anisotropy that is induced by the Fe doping in combination with competing magnetic interactions at low temperature produce nontrivial spin-texture, leading to large topological Hall effect in the doped systems at low temperatures. Our studies further show that the topological properties of Mn$_{3-\it{x}}$Fe$_{\it{x}}$Sn are very sensitive to the Fe doping.

Canonical Spin Glass and the Anomalous Hall Effect in a Centrosymmetric Ferrimagnet.

Centrosymmetric ferro/ferrimagnets provide an ideal arena for fundamental research due to their fascinating magnetic and structural characters. In this work, the Co0.8MnSn compound with a single hexagonal phase was successfully synthesized, and the magnetic phase transition and magnetic and electronic properties have been systematically investigated. Interestingly, Arrott plots and normalized magnetic entropy changes derived from the isothermal magnetizing curves may imply the first-order nature of the magnetic ordering transition around TC ∼ 121 K. The AC susceptibility analysis and detailed nonequilibrium dynamical studies (including magnetic aging, rejuvenation, and memory effects) reveal the canonical spin-glass state of Co0.8MnSn at lower temperature. Further, negative magnetoresistance and the anomalous Hall effect dominated by a commonly intrinsic term are obtained. Moreover, the field-dependent AC susceptibility data indicated that complicated and nontrivial magnetic spin textures should exist in the compound. These studies may open up further research opportunities in exploring emergent physical phenomena and potential applications in centrosymmetric magnets.

Clockless skyrmion logic gate based on voltage-controlled skyrmion propagation

Magnetic skyrmions are nanoscale topologically nontrivial spin texture that offer great promise as information carriers for the next-generation spintronic computing schemes. However, the current skyrmion-based logic gates require precise control of skyrmion collisions through clocked synchronizers, which leads to the complexity of logic implementation. To address this challenge, we propose a clockless skyrmion logic gate that can be implemented into large-scale computing networks without skyrmions synchronization. The clockless operations are achieved in a cross-shaped skyrmion track based on the skyrmion gating method with the aid of the voltage-controlled magnetic anisotropy gate. A complete set of Boolean operations and a cascaded full adder can be realized through the different combinations of inputs and interconnections of these logic gates. This computing paradigm paves the way for the design of a highly efficient and robust computing architecture using fully skyrmion-based logic devices.

Two-channel anomalous Hall effect originating from the intermixing in Mn2CoAl/Pd thin films

The anomalous Hall effect (AHE) is an electronic transport phenomenon with rich physics. In thin magnetic films and multilayers, an unusual peak observed in AHE is often identified as the topological Hall effect (THE), induced by topologically nontrivial spin textures. However, it is a challenge to determine whether this unusual peak truly originates from such spin textures or is an artifact due to inhomogeneities. In this work, we systematically study the Pd thickness and temperature dependence of the Hall effect in ${\mathrm{Mn}}_{2}\mathrm{CoAl}$/Pd thin films. We observe an unusual peak in the Pd thickness dependence and rule out that the origin of the peak is from the topologically nontrivial spin textures by directly imaging the magnetic domain structures. We also build a model for the AHE based on the sum of contributions from ${\mathrm{Mn}}_{2}\mathrm{CoAl}$ and an intermixed CoPd alloy. These materials have opposite signs of the AHE and different coercive fields, which successfully explains the thickness and temperature behaviors of the unusual peak. Our results show that the interface mixing layer can play an important role in the interpretation of the AHE in ferromagnet/heavy metal systems.