Chiral Spin Texture Research Articles

Two-dimensional chiral perovskites with large spin Hall angle and collinear spin Hall conductivity.

Two-dimensional hybrid organic-inorganic perovskites with chiral spin texture are emergent spin-optoelectronic materials. Despite the wealth of chiro-optical studies on these materials, their charge-to-spin conversion efficiency is unknown. We demonstrate highly efficient electrically driven charge-to-spin conversion in enantiopure chiral perovskites (R/S-MB)2(MA)3Pb4I13 (〈n〉 = 4), where MB is 2-methylbutylamine, MA is methylamine, Pb is lead, and I is iodine. Using scanning photovoltage microscopy, we measured a spin Hall angle θsh of 5% and a spin lifetime of ~75 picoseconds at room temperature in 〈n〉 = 4 chiral perovskites, which is much larger than its racemic counterpart as well as the lower 〈n〉 homologs. In addition to current-induced transverse spin current, the presence of a coexisting out-of-plane spin current confirms that both conventional and collinear spin Hall conductivities exist in these low-dimensional crystals.

Unusual Dzyaloshinskii-Moriya Interaction in Graphene/Fe3GeTe2 Van der Waals Heterostructure.

Dzyaloshinskii-Moriya interaction (DMI) is shown to induce a topologically protected chiral spin texture in magnetic/nonmagnetic heterostructures. In the context of van der Waals spintronic devices, graphene emerges as an excellent candidate material. However, due to its negligible spin-orbit interaction, inducing DMI to stabilize topological spins when coupled to 3d-ferromagnets remains challenging. Here, it is demonstrated that, despite these challenges, a sizeable Rashba-type spin splitting followed by significant DMI is induced in graphene/Fe3GeTe2. This is made possible due to an interfacial electric field driven by charge asymmetry together with the broken inversion symmetry of the heterostructure. These findings reveal that the enhanced DMI energy parameter, resulting from a large effective electron mass in Fe3GeTe2, remarkably contributes to stabilizing non-collinear spins below the Curie temperature, overcoming the magnetic anisotropy energy. These results are supported by the topological Hall effect, which coexists with the non-trivial breakdown of Fermi liquid behavior, confirming the interplay between spins and non-trivial topology. This work paves the way toward the design and control of interface-driven skyrmion-based devices.

Chiral spin textures creation and dynamics in a rectangular nanostructure

Abstract Controlled creation of stable chiral spin textures is required to use them as an energy-efficient information carrier in spintronics. Here we have studied the stable creation of isolated chiral spin texture (skyrmion and antiskyrmion) and its pair through the magnetization reversal of a rectangular nanostructure using spin-polarized currents. An isolated spin texture is created through a negative current pulse. Dynamics of the stable spin texture are explored under external magnetic fields, and the resonant frequencies are calculated. A stable skyrmion pair is created using an asymmetric current pulse, and their interaction is studied using the Thiele equation. The stability of isolated or paired spin texture depends on the Dzyaloshinskii–Moriya interaction strength, spin-polarized current density, and pulse duration. In addition, the stability of the skyrmion pair depends on their initial separation, and a threshold for the separation between skyrmions of 78 nm is observed.

Doping-dependent charge- and spin-density wave orderings in a monolayer of Pb adatoms on Si(111)

In this work we computed the phase diagram as a function of temperature and doping for a system of lead adatoms allocated periodically on a silicon (111) surface. This Si(111):Pb material is characterized by a strong and long-ranged Coulomb interaction, a relatively large value of the spin-orbit coupling, and a structural phase transition that occurs at low temperature. In order to describe the collective electronic behavior in the system, we perform many-body calculations consistently taking all these important features into account. We find that charge- and spin-density wave orderings coexist with each other in several regions of the phase diagram. This result is in agreement with the recent experimental observation of a chiral spin texture in the charge density wave phase in this material. We also find that the geometries of the charge and spin textures strongly depend on the doping level. The formation of such a rich phase diagram in the Si(111):Pb material can be explained by a combined effect of the lattice distortion and electronic correlations.

Direct observation of Néel-type skyrmions and domain walls in a ferrimagnetic DyCo3 thin film

Isolated magnetic skyrmions are stable, topologically protected spin textures that are at the forefront of research interests today due to their potential applications in information technology. A distinct class of skyrmion hosts are rare earth - transition metal (RE-TM) ferrimagnetic materials. To date, the nature and the control of basic traits of skyrmions in these materials are not fully understood. We show that for an archetypal ferrimagnetic material DyCo3 that exhibits a strong perpendicular anisotropy, the ferrimagnetic skyrmion size can be tuned by an external magnetic field. Moreover, by taking advantage of the high spatial resolution of scanning transmission X-ray microscopy (STXM) and utilizing a large x-ray magnetic linear dichroism (XMLD) contrast that occurs naturally at the RE resonant edges, we resolve the nature of the magnetic domain walls of ferrimagnetic skyrmions. We demonstrate that through this method one can easily discriminate between Bloch and Néel type domain walls for each individual skyrmion. For all isolated ferrimagnetic skyrmions, we observe that the domain walls are of Néel-type. This key information is corroborated with results of micromagnetic simulations and allows us to conclude on the nature of the Dzyaloshinskii-Moriya interaction (DMI) which concurs to the stabilisation of skyrmions in this ferrimagnetic system. Establishing that an intrinsic DMI occurs in RE-TM materials will also be beneficial towards a deeper understanding of chiral spin texture control in ferrimagnetic materials.

Giant tunability of microwave responses for current-driven skyrmions in a tapered nanostructure with notches

The investigation of the gigahertz dynamics of a skyrmion—a topologically protected chiral spin texture, offers several applications in high-frequency magnonic devices. In this work, we have investigated the motion and microwave resonant dynamics of skyrmions in an engineered nanostructure using micromagnetic simulations. The structure is a tapered ferromagnetic/heavy metal bilayer with notches to stabilize the skyrmion for dynamic studies. The skyrmion is moved along the nanostructure via current-induced spin–orbit torques. Multiple stable skyrmion states were demonstrated in the structure, where a remarkable tunability of 6 GHz is observed between the initial and the final state. This large shift in the resonant frequency has been attributed to the variation in the size of the skyrmion at various locations in the engineered nanostructure. The dependency of the skyrmion velocity and their microwave responses on the DzyalosnDzyaloshinskii–Moriya interaction strength and the perpendicular magnetic anisotropy is described. The calculation of the skyrmion Hall angle shed light on the motion and stability of the skyrmion in the structure.

Chiral Interlayer Exchange Coupling for Asymmetric Domain Wall Propagation in Field-Free Magnetization Switching

The discovery of chiral spin texture has unveiled many unusual yet extraordinary physical phenomena, such as the Néel type domain walls and magnetic skyrmions. A recent theoretical study suggests that a chiral exchange interaction is not limited to a single ferromagnetic layer; instead, three-dimensional spin textures can arise from an interlayer Dzyaloshinskii-Moriya interaction. However, the influence of chiral interlayer exchange coupling on the electrical manipulation of magnetization has rarely been addressed. Here, the coexistence of both symmetric and chiral interlayer exchange coupling between two orthogonally magnetized CoFeB layers in PtMn/CoFeB/W/CoFeB/MgO is demonstrated. Images from polar magneto-optical Kerr effect microscopy indicate that the two types of coupling act concurrently to induce asymmetric domain wall propagation, where the velocities of domain walls with opposite chiralities are substantially different. Based on this microscopic mechanism, field-free switching of the perpendicularly magnetized CoFeB is achieved with a wide range of W thicknesses of 0.6-4.5 nm. This work enriches the understanding of interlayer exchange coupling for spintronic applications.

Spontaneous Formation of Exceptional Points at the Onset of Magnetism.

We reveal how symmetry-protected nodal points in topological semimetals may be promoted to pairs of generically stable exceptional points (EPs) by symmetry-breaking fluctuations at the onset of long-range order. This intriguing interplay between non-Hermitian (NH) topology and spontaneous symmetry breaking is exemplified by a magnetic NH Weyl phase spontaneously emerging at the surface of a strongly correlated three-dimensional topological insulator, when entering the ferromagnetic regime from a high-temperature paramagnetic phase. Here, electronic excitations with opposite spin acquire significantly different lifetimes, thus giving rise to an anti-Hermitian structure in spin that is incompatible with the chiral spin texture of the nodal surface states, and hence facilitate the spontaneous formation of EPs. We present numerical evidence of this phenomenon by solving a microscopic multiband Hubbard model nonperturbatively in the framework of dynamical mean-field theory.

Strain-enabled control of chiral magnetic structures in MnSeTe monolayer

Abstract Chiral magnetic states are promising for the future spintronic applications. Recent progress of chiral spin textures in two-dimensional magnets, such as chiral domain walls (DW), skyrmions, and bimerons, have been drawing extensive attention. Here, via first-principles calculations, we show that biaxial strain can effectively manipulate the magnetic parameters of the Janus MnSeTe monolayer. Interestingly, we find that both the magnitude and the sign of the magnetic constants of the Heisenberg exchange coupling, Dzyaloshinskii-Moriya interaction (DMI) and magnetocrystalline anisotropy (MCA) can be tuned by strain. Moreover, using micromagnetic simulations, we obtain the distinct phase diagram of chiral spin texture under different strain. Especially, we demonstrate that abundant chiral magnetic structures including ferromagnetic skyrmion, skyrmionium, bimeron, and antiferromagnetic spin spiral can be induced in the MnSeTe monolayer. We have also discussed the effect of temperature on these magnetic structures. These findings highlight the Janus MnSeTe monolayer as a good candidate for spintronic nanodevices.

Strain-Enabled Control of Chiral Magnetic Structures in MnSeTe Monolayer

Chiral magnetic states are promising for future spintronic applications. Recent progress of chiral spin textures in two-dimensional magnets, such as chiral domain walls, skyrmions, and bimerons, have been drawing extensive attention. Here, via first-principles calculations, we show that biaxial strain can effectively manipulate the magnetic parameters of the Janus MnSeTe monolayer. Interestingly, we find that both the magnitude and the sign of the magnetic constants of the Heisenberg exchange coupling, Dzyaloshinskii–Moriya interaction and magnetocrystalline anisotropy can be tuned by strain. Moreover, using micromagnetic simulations, we obtain the distinct phase diagram of chiral spin texture under different strains. Especially, we demonstrate that abundant chiral magnetic structures including ferromagnetic skyrmion, skyrmionium, bimeron, and antiferromagnetic spin spiral can be induced in the MnSeTe monolayer. We also discuss the effect of temperature on these magnetic structures. The findings highlight the Janus MnSeTe monolayer as a good candidate for spintronic nanodevices.

Theoretical Investigation of Skyrmion Dynamics in Pt/Co/MgO Nanodots.

In this article, we present a numerical study on stabilization and eigenmodes of the so-called skyrmion chiral spin texture in nanometric dots. The first aim of this study is to identify the appropriate multilayer in a set of Pt/Co/MgO structures with different Co thicknesses that have been previously experimentally characterized. Stabilization occurs if the energy favoring skyrmions is greater than the geometric mean of the exchange and anisotropy energies. Both the energy favoring skyrmions and the anisotropy contribution depend on the Co thickness. The appropriate multilayer is obtained for a specific Co thickness. MuMax simulations are used to calculate the precise static magnetization configuration for the experimental parameters, allowing us select the appropriate structure. Moreover, in view of experimental study of skyrmion dynamics by means of Brillouin light scattering, the eigenfrequency, eigenmode profile, and spectral density are calculated for different dot sizes. Finally, the optimal dot size that allows for a feasible experiment is obtained.

Noncollinear Spin Current for Switching of Chiral Magnetic Textures.

We propose a concept of noncollinear spin current, whose spin polarization varies in space even in nonmagnetic crystals. While it is commonly assumed that the spin polarization of the spin Hall current is uniform, asymmetric local crystal potential generally allows the spin polarization to be noncollinear in space. Based on microscopic considerations, we demonstrate that such noncollinear spin Hall currents can be observed, for example, in layered Kagome Mn_{3}X (X=Ge, Sn) compounds. Moreover, by referring to atomistic spin dynamics simulations we show that noncollinear spin currents can be used to switch the chiral spin texture of Mn_{3}X in a deterministic way even in the absence of an external magnetic field. Our theoretical prediction can be readily tested in experiments, which will open a novel route toward electric control of complex spin structures in noncollinear antiferromagnets.

Nonequilibrium dynamical behavior in noncoplanar magnets with chiral spin texture

We observe nonequilibrium dynamical magnetic behavior in the magnetically ordered phase of a Tsai-type Tb-Au-Si quasicrystal approximant system. The magnetic texture in the ordered phase is found to exhibit scalar spin chirality (SSC) order, inferring that SSC is the order parameter of the present magnetic system. We further find that the introduction of ``pseudo-Tsai'' clusters, associated with additional Tb atoms in the structure, induces spin-glass dynamics. We discuss the observed dynamical magnetic behavior in the Tb-Au-Si systems, considering the effect of the pseudo-Tsai clusters on the magnetic configuration and local spin chirality.

Generation of nonreciprocity in gapless spin waves by chirality injection

In chiral magnets with intrinsic inversion symmetry breaking, it has been known that two spin waves moving in opposite directions can propagate at different velocities, exhibiting a phenomenon called magnetochiral nonreciprocity which allows for realizations of certain spin logic devices such as a spin-wave diode. Here, we theoretically demonstrate that the spin-wave nonreciprocity can occur without intrinsic bulk chirality in certain magnets including easy-cone ferromagnets and easy-cone antiferromagnetic. Specifically, we show that nonlocal injection of a spin current from proximate normal metals to easy-cone magnets engenders a non-equilibrium chiral spin texture, on top of which spin waves exhibit nonreciprocity proportional to the injected spin current. One notable feature of the discovered nonreciprocal spin waves is its gapless nature, which can lead to a large thermal rectification effect at sufficiently low temperatures. We envision that nonlocal electric injection of chirality into otherwise nonchiral magnets may serve as a versatile route to realize electrically controllable magnetochiral phenomena in a wide class of materials.

Soft-Magnetic Skyrmions Induced by Surface-State Coupling in an Intrinsic Ferromagnetic Topological Insulator Sandwich Structure.

A magnetic skyrmion induced on a ferromagnetic topological insulator (TI) is a real-space manifestation of the chiral spin texture in the momentum space and can be a carrier for information processing by manipulating it in tailored structures. Here, a sandwich structure containing two layers of a self-assembled ferromagnetic septuple-layer TI, Mn(Bi1-xSbx)2Te4 (MnBST), separated by quintuple layers of TI, (Bi1-xSbx)2Te3 (BST), is fabricated and skyrmions are observed through the topological Hall effect in an intrinsic magnetic topological insulator for the first time. The thickness of BST spacer layer is crucial in controlling the coupling between the gapped topological surface states in the two MnBST layers to stabilize the skyrmion formation. The homogeneous, highly ordered arrangement of the Mn atoms in the septuple-layer MnBST leads to a strong exchange interaction therein, which makes the skyrmions "soft magnetic". This would open an avenue toward a topologically robust rewritable magnetic memory.

Unveiling the Emergent Traits of Chiral Spin Textures in Magnetic Multilayers.

Magnetic skyrmions are topologically wound nanoscale textures of spins whose ambient stability and electrical manipulation in multilayer films have led to an explosion of research activities. While past efforts focused predominantly on isolated skyrmions, recently ensembles of chiral spin textures, consisting of skyrmions and magnetic stripes, are shown to possess rich interactions with potential for device applications. However, several fundamental aspects of chiral spin texture phenomenology remain to be elucidated, including their domain wall (DW) structure, thermodynamic stability, and morphological transitions. Here the evolution of these textural characteristics are unveiled on a tunable multilayer platform—wherein chiral interactions governing spin texture energetics can be widely varied—using a combination of full‐field electron and soft X‐ray microscopies with numerical simulations. With increasing chiral interactions, the emergence of Néel helicity, followed by a marked reduction in domain compressibility, and finally a transformation in the skyrmion formation mechanism are demonstrated. Together with an analytical model, these experiments establish a comprehensive microscopic framework for investigating and tailoring chiral spin texture character in multilayer films.

Emergence of the topological Hall effect in a tetragonal compensated ferrimagnet Mn2.3Pd0.7Ga

Topological spin textures such as magnetic skyrmions have attracted considerable interest due to their potential application in spintronic devices. However, there still remain several challenges to overcome before their practical application, for instance, achieving high scalability and thermal stability. Recent experiments have proposed a new class of skyrmion materials in the Heusler family, Mn1.4Pt0.9Pd0.1Sn and Mn2Rh0.95Ir0.05Sn, which possess noncollinear magnetic structures. Motivated by these experimental results, we suggest another Heusler compound hosted by Mn3Ga to overcome the above limitations. We fabricate Mn3-xPdxGa thin films, focusing on the magnetic compensation point. In Mn2.3Pd0.7Ga, we find a spin-reorientation transition around TSR = 320 K. Below the TSR, we observe the topological Hall effect and a positive magnetic entropy change, which are the hallmarks of a chiral noncollinear spin texture. By integrating all the data, we determine the magnetic phase diagram, displaying a wide chiral noncollinear spin phase even at room temperature. We believe that this compensated ferrimagnet shows promise for opening a new avenue toward chiral spin-based, high-density, and low-power devices.

Spin‐Torque Memristors: Spin‐Torque Memristors Based on Perpendicular Magnetic Tunnel Junctions for Neuromorphic Computing (Adv. Sci. 10/2021)

In article number 2004645, Weisheng Zhao and co-workers develop a nano-scale memristor device based on magnetic tunneling junction. A kind of chiral vortex spin texture plays a key role in forming the multi-level magnetoresistance. Thanks to its advantages such as long endurance and high speed, this spintronic device may take neuromorphic computing one step further.

Topological Hall effect in magnetic topological insulator films

Geometric Berry phase can be induced either by spin–orbit coupling, giving rise to the anomalous Hall effect in ferromagnetic materials, or by chiral spin texture, such as skyrmions, leading to the topological Hall effect. Recent experiments have revealed that both phenomena can occur in topological insulator films with magnetic doping, thus providing us with an intriguing platform to study the interplay between these two phenomena. In this work, we report on a numerical simulation of the anomalous Hall and topological Hall effects in a four-band model that can properly describe the quantum well states in the magnetic topological insulator films by combining Landauer–Büttiker formula and the iterative Green’s function method. Our numerical results suggest that spin–orbit coupling in this model plays a different role in the quantum transport in the clean and disordered limits. In the clean limit, spin–orbit coupling mainly influences the longitudinal transport but does not have much effect on topological Hall conductance. In the disordered limit, the longitudinal transport is determined by disorder scattering and spin–orbit coupling is found to affect strongly the topological Hall conductance. This sharp contrast unveils a dramatic interplay between spin–orbit coupling and disorder effect in topological Hall effect in magnetic topological insulator systems.

Characterizing the Magnetic Interfacial Coupling of the Fe/FeGe Heterostructure by Ferromagnetic Resonance

We characterize the magnetic interfacial coupling of Fe/FeGe heterostructure and its influence on the magnetic damping via ferromagnetic resonance at the temperature range of 200-300 K. When the temperature below the critical temperature of FeGe, the interfacial coupling rises. The strength of magnetic interfacial coupling is determined as the function of temperature and reaches up 0.194 erg/cm2 at 200 K. Meanwhile, the Gilbert damping of Fe layer is enhanced from 0.035 of 300 K to 0.050 of 200 K. The enhancement is linearly proportional to the strength of magnetic interfacial coupling. We attribute the enhancement to the interfacial coupling which transfers spin angular momentum from Fe to FeGe via the exchange interaction. Our results reveal that the interfacial coupling is an effective approach to inject spin current into the chiral spin texture.