Chiral Spin Research Articles

Manipulating chiral spin transport with ferroelectric polarization.

A magnon is a collective excitation of the spin structure in a magnetic insulator and can transmit spin angular momentum with negligible dissipation. This quantum of a spin wave has always been manipulated through magnetic dipoles (that is, by breaking time-reversal symmetry). Here we report the experimental observation of chiral spin transport in multiferroic BiFeO3 and its control by reversing the ferroelectric polarization (that is, by breaking spatial inversion symmetry). The ferroelectrically controlled magnons show up to 18% modulation at room temperature. The spin torque that the magnons in BiFeO3 carry can be used to efficiently switch the magnetization of adjacent magnets, with a spin-torque efficiency comparable to the spin Hall effect in heavy metals. Utilizing such controllable magnon generation and transmission in BiFeO3, an all-oxide, energy-scalable logic is demonstrated composed of spin-orbit injection, detection and magnetoelectric control. Our observations open a new chapter of multiferroic magnons and pave another path towards low-dissipation nanoelectronics.

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.

Magnetic field induced partially polarized chiral spin liquid in a transition metal dichalcogenide moiré system

Magnetic field induced partially polarized chiral spin liquid in a transition metal dichalcogenide moiré system

Chiral spin liquid phase in an optical lattice at mean-field level

Chiral spin liquid phase in an optical lattice at mean-field level

Unveiling the influence of fixed layer polarization on skyrmion nucleation and dynamics in circular geometry

Chiral magnetic skyrmion spin textures promise to be the next generation of energy-efficient computing. An effective way of skyrmion creation is essential to use as an information carrier. We report the creation of a skyrmion in a circular spin valve nanostructure using spin-polarized currents. This research investigates skyrmion nucleation under perpendicular and vortex fixed layer polarizations. The impact of Dzyaloshinskii-Moriya interaction (DMI), magnetic anisotropy, and saturation magnetization on the nucleation process has been studied for the perpendicular case. Depending on fixed layer polarization, we observed a trade-off between current density and pulse width for skyrmion nucleation. Vortex polarization enables faster nucleation at higher currents, while perpendicular polarization facilitates lower current thresholds at the expense of longer pulses. Further skyrmion dynamics have been explored under the sinusoidal magnetic fields.

Enhanced interlayer Dzyaloshinskii–Moriya interaction and field-free switching in magnetic trilayers with orthogonal magnetization

The indirect interlayer exchange coupling (IEC) between two magnetic layers holds significant importance in the field of spintronics and has been widely used in the construction of synthetic antiferromagnets. Recently, the interlayer Dzyaloshinskii–Moriya interaction (DMI), antisymmetric counterpart of IEC, has been discovered in magnetic trilayers with a heavy-metal spacer. In this study, we present an investigation on antisymmetric and symmetric counterparts of IEC in D022-Mn3Ga/Pt/Co trilayers with orthogonal magnetization. Due to the strong interlayer DMI across the entire multilayer, the symmetry of magnetic reversal process was broken, leading to an enhanced chiral exchange-bias field of 42.7 Oe in the Co layer. In addition, field-free spin–orbit torque (SOT) switching of D022-Mn3Ga layer has been realized in Hall bar devices. In-plane field dependence analysis of the SOT switching behavior reveals that the symmetric counterpart of IEC exhibits antiferromagnetic characteristics within the spacer thickness range of 2 nm ≤ tPt ≤5 nm. Moreover, the magnitude of both antisymmetric and symmetric counterparts of IEC exhibits an exponential decreasing trend with increasing tPt. These findings hold significant implications for the design and manipulation of three-dimensional chiral spin textures in the future spintronic devices.

All-electrical skyrmionic magnetic tunnel junction.

Topological whirls or 'textures' of spins such as magnetic skyrmions represent the smallest realizable emergent magnetic entities1-5. They hold considerable promise as robust, nanometre-scale, mobile bits for sustainable computing6-8. A longstanding roadblock to unleashing their potential is the absence of a device enabling deterministic electrical readout of individual spin textures9,10. Here we present the wafer-scale realization of a nanoscale chiral magnetic tunnel junction (MTJ) hosting a single, ambient skyrmion. Using a suite of electrical and multimodal imaging techniques, we show that the MTJ nucleates skyrmions of fixed polarity, whose large readout signal-20-70% relative to uniformly magnetized states-corresponds directly to skyrmion size. The MTJ exploits complementary nucleation mechanisms to stabilize distinctly sized skyrmions at zero field, thereby realizing three non-volatile electrical states. Crucially, it can electrically write and delete skyrmions to both uniform states with switching energies 1,000 times lower than the state of the art. Here, the applied voltage emulates a magnetic field and, in contrast to conventional MTJs, it reshapes both the energetics and kinetics of the switching transition, enabling deterministic bidirectional switching. Our stack platform enables large readout and efficient switching, and is compatible with lateral manipulation of skyrmionic bits, providing the much-anticipated backbone for all-electrical skyrmionic device architectures9,10. Its wafer-scale realizability provides a springboard to harness chiral spin textures for multibit memory and unconventional computing8,11.

The Topological Hall Effect in CoGd Films Controlled by Hydrogen Migration under Gate Voltage

AbstractThe topological Hall effect (THE) presents hump signals in the Hall resistance versus magnetic field hysteresis loop, showing promise for future spintronics due to its robust chiral magnetic textures. Here, it is shown that solid‐state protonic gating can control possible topological magnetic structures in CoGd thin films. Injecting H+ leads to sizable hump signals in the film. Magneto‐optical Kerr microscopy shows that induced hump signals in transporting measurements do not scale with magnetization, supporting topological magnetism. Successive hydrogen ion extraction completely erases the effect. Thus, topological magnetism manipulations are reversible, nonvolatile, and effective. Ab inito calculations and effective chiral spin models demonstrate that hydrogen injection remarkably enhances the Dzyaloshinskii–Moriya interaction over fourfold, stabilizing chiral structures contributing to the large THE. These findings reveal the vital role of hydrogen ions in topological magnetism and suggest that amorphous ferrimagnetic CoGd thin films are outstanding platforms for realizing controllable topological spintronics at room temperature.

Electric manipulation of the magnetization in heterostructure Pt/Co/Bi2Se3

Spin–orbit torque (SOT) can provide efficient electrical manipulation of magnetism via applying electrical current to breaking the symmetry of damping-like torque. In the heterojunction of heavy and ferromagnetic metal, Dzyaloshinskii–Moriya interaction (DMI) is one of the key ingredients for stabilizing chiral spin structures, like chiral domain walls. Meanwhile, materials with larger charge-spin conversion rates are also highly expected for the efficient SOT. In this paper, spin–orbit torque magnetic switching is observed in the perpendicularly magnetized Pt/Co/Bi2Se3 and shows relatively high efficiency with low critical switching current density of about 5 × 105 A cm−2. The SOT efficiency and DMI in perpendicularly magnetized Pt/Co/Bi2Se3 were quantitatively investigated by electrical detection of the effective spin Hall field. The DMI constant is about 2.6 mJ m−2, and the effective spin Hall angle of Pt/Co/Bi2Se3 is about 0.14. The work also demonstrates that the Bi2Se3 layer takes the main responsibility for SOT, and the Pt/Co interface is the main source of DMI in Pt/Co/Bi2Se3 structures, which makes it possible to achieve independent optimization of DMI and SOT in the Pt/Co/Bi2Se3 structure at room temperature for the advanced application of spintronic devices.

Spin-Frame Field Theory of a Three-Sublattice Antiferromagnet.

We present a nonlinear field theory of a three-sublattice hexagonal antiferromagnet. The order parameter is the spin frame, an orthogonal triplet of vectors related to sublattice magnetizations and spin chirality. The exchange energy, quadratic in spin-frame gradients, has three coupling constants, only two of which manifest themselves in the bulk. As a result, the three spin-wave velocities satisfy a universal relation. Vortices generally have an elliptical shape with the eccentricity determined by the Lamé parameters.

Scalar spin chirality induced by a circularly polarized electric field in a classical kagome magnet

Scalar spin chirality induced by a circularly polarized electric field in a classical kagome magnet

From chiral spin liquids to skyrmion fluids and crystals, and their interplay with itinerant electrons

From chiral spin liquids to skyrmion fluids and crystals, and their interplay with itinerant electrons

Homochiral antiferromagnetic merons, antimerons and bimerons realized in synthetic antiferromagnets

The ever-growing demand for device miniaturization and energy efficiency in data storage and computing technology has prompted a shift towards antiferromagnetic topological spin textures as information carriers. This shift is primarily owing to their negligible stray fields, leading to higher possible device density and potentially ultrafast dynamics. We realize in this work such chiral in-plane topological antiferromagnetic spin textures namely merons, antimerons, and bimerons in synthetic antiferromagnets by concurrently engineering the effective perpendicular magnetic anisotropy, the interlayer exchange coupling, and the magnetic compensation ratio. We demonstrate multimodal vector imaging of the three-dimensional Néel order parameter, revealing the topology of those spin textures and a globally well-defined chirality, which is a crucial requirement for controlled current-induced dynamics. Our analysis reveals that the interplay between interlayer exchange and interlayer magnetic dipolar interactions plays a key role to significantly reduce the critical strength of the Dzyaloshinskii-Moriya interaction required to stabilize topological spin textures, such as antiferromagnetic merons, in synthetic antiferromagnets, making them a promising platform for next-generation spintronics applications.

Altering Spin Distribution of Tb2Pc3 via Molecular Chirality Manipulation.

Manipulating the chirality of the spin-polarized electronic state is pivotal for understanding many unusual quantum spin phenomena, but it has not been achieved at the single-molecule level. Here, using scanning tunneling microscopy and spectroscopy (STM/STS), we successfully manipulate the chirality of spin distribution in a triple-decker single-molecule magnet tris(phthalocyaninato)bis(terbium(III)) (Tb2Pc3), which is evaporated on a Pb(111) substrate via molecular beam epitaxy. The otherwise achiral Tb2Pc3 becomes chiral after being embedded into the self-assembled monolayer films of bis(phthalocyaninato)terbium(III) (TbPc2). The chirality of the spin distribution in Tb2Pc3 is manifested via the spatial mapping of its Kondo resonance state from its ligand orbital. Our first-principles calculations revealed that the spin and molecular chirality are associated with a small rotation followed by a structural distortion of the top Pc, consistent with the experimental observation. By constructing tailored molecular clusters with the STM tip, a single Tb2Pc3 molecule can be manipulated among achiral and differently handed chiral configurations of spin distributions reversibly. This paves the way for designing chiral spin enantiomers for fundamental studies and developing functional spintronic devices.

Distinct Composition‐Dependent Topological Hall Effect in Mn2‐xZnxSb

AbstractSpintronics, an evolving interdisciplinary field at the intersection of magnetism and electronics, explores innovative applications of electron charge and spin properties for advanced electronic devices. The topological Hall effect (THE), a key component in spintronics, has gained significance due to emerging theories surrounding noncoplanar chiral spin textures. This study focuses on Mn2‐xZnxSb, a material crystalizing in centrosymmetric space group with rich magnetic phases tunable by Zn contents. Through comprehensive magnetic and transport characterizations, we found that the high‐Zn (x > 0.6) samples display THE which is enhanced with decreasing temperature, while THE in the low‐Zn (x < 0.6) samples show an opposite trend. The coexistence of those distinct temperature dependencies for THE suggests very different magnetic interactions/structures for different compositions and underscores the strong coupling between magnetism and transport in Mn2‐xZnxSb. The findings contribute to understanding topological magnetism in centrosymmetric tetragonal lattices, establishing Mn2‐xZnxSb as a unique platform for exploring tunable transport effects and opening avenues for further exploration in the realm of spintronics.

Torsional constitutive relations at finite temperature

The general form of the linear torsional constitutive relations at finite temperature of the chiral current, energy-momentum tensor, and spin energy potential are computed for a chiral fermion fluid minimally coupled to geometric torsion and with nonzero chiral chemical potential. The corresponding transport coefficients are explicitly calculated in terms of the energy and number densities evaluated at vanishing torsion. A microscopic calculation of these constitutive relations in some particular backgrounds is also presented, confirming the general structure found.

Chiral Induced Spin Selectivity.

Since the initial landmark study on the chiral induced spin selectivity (CISS) effect in 1999, considerable experimental and theoretical efforts have been made to understand the physical underpinnings and mechanistic features of this interesting phenomenon. As first formulated, the CISS effect refers to the innate ability of chiral materials to act as spin filters for electron transport; however, more recent experiments demonstrate that displacement currents arising from charge polarization of chiral molecules lead to spin polarization without the need for net charge flow. With its identification of a fundamental connection between chiral symmetry and electron spin in molecules and materials, CISS promises profound and ubiquitous implications for existing technologies and new approaches to answering age old questions, such as the homochiral nature of life. This review begins with a discussion of the different methods for measuring CISS and then provides a comprehensive overview of molecules and materials known to exhibit CISS-based phenomena before proceeding to identify structure-property relations and to delineate the leading theoretical models for the CISS effect. Next, it identifies some implications of CISS in physics, chemistry, and biology. The discussion ends with a critical assessment of the CISS field and some comments on its future outlook.

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.

Chiral spin liquids with projected Gaussian fermionic entangled pair states

Chiral spin liquids with projected Gaussian fermionic entangled pair states

Observation of Topological Hall Effect in a Chemically Complex Alloy.

The topological Hall effect (THE) is the transport response of chiral spin textures and thus can serve as a powerful probe for detecting and understanding these unconventional magnetic orders. So far, the THE is only observed in either noncentrosymmetric systems where spin chirality is stabilized by Dzyaloshinskii-Moriya interactions, or triangular-lattice magnets with Ruderman-Kittel-Kasuya-Yosida-type interactions. Here, a pronounced THE is observed in a Fe-Co-Ni-Mn chemically complex alloy with a simple face-centered cubic (fcc) structure across a wide range of temperatures and magnetic fields. The alloy is shown to have a strong magnetic frustration owing to the random occupation of magnetic atoms on the close-packed fcc lattice and the direct Heisenberg exchange interaction among atoms, as evidenced by the appearance of a reentrant spin glass state in the low-temperature regime and the first principles calculations. Consequently, THE is attributed to the nonvanishing spin chirality created by strong spin frustration under the external magnetic field, which is distinct from the mechanism responsible for the skyrmion systems, as well as geometrically frustrated magnets.