arXiv subjects

Weiyi Pan

Publications and source records attributed to Weiyi Pan.

16 recordsLinked to original sources

arXiv2026

Dimensional crossover and local strain induced deflection of the spin spiral state in multiferroic NiI2

Low-dimensional multiferroics hold great promise for integrated magnetoelectric devices. Spin spiral state has recently been shown to induce ferroelectricity in single-layer van der Waals (vdW) material NiI2. However, how this state evolves and can be tuned towards the two-dimensional limit remain unclear. Here, we combine spin-polarized scanning tunneling microscopy, layer-by-layer film growth, and multi-scale theoretical modeling to investigate the spin spirals in NiI2 thin films. As the film thickness increases from 1 to 7 monolayers, we observed a continuous increase of spin-spiral wavelength and a rotation of wavevector from near [110] to [1-10] direction, which evidences a dimensional crossover primarily driven by enhanced interlayer exchange energy. Moreover, we find that the film wrinkles can cause deflection of the spin spiral wavevector, which is caused by local curvature induced modification of exchange interactions. Our findings establish thickness and local strain as two tuning methods for engineering non-collinear helical magnetism and accompanied electric polarization in vdW multiferroics.

cond-mat.mes-hall↗

arXiv2026

Ferroelectric-controllable spin-orbit torque in two-dimensional multiferroic heterostructure

Spin-orbit torque (SOT), which enables electrical control of magnetization, plays a crucial role in the development of next-generation spintronic devices. Realizing SOT in two-dimensional van der Waals systems, together with achieving efficient nonvolatile manipulation via ferroelectricity, would be highly beneficial for the implementation of tunable logic devices with enhanced storage density. In this work, based on first-principles calculation and using a multiferroic Fe$_{3}$GeTe$_{2}$/In$_{2}$Se$_{3}$ heterostructure as a representative example, we demonstrate that switching the ferroelectric polarization of the In$_{2}$Se$_{3}$ layer induces a pronounced modification in the magnetization-dependent distribution of torkance within the heterostructure. Specifically, when the magnetization is in the plane, where the torque is maximal, reversing the polarization of In$_{2}$Se$_{3}$ from upward to downward enhances the total torkance to more than 150% of its original value. This substantial variation primarily originates from the polarization-induced modulation of the $z$ component of the time-reversal-odd torkance, which is mainly associated with an approximately 233% change in the atomic-resolved torque contributed from the middle Fe layer in Fe$_{3}$GeTe$_{2}$ layer. Further analysis reveals that the electronic states near $\Gamma$ on the Fermi surface undergo significant reconstruction upon polarization switching, which is responsible for the observed variation in the time-reversal-odd torque. Our results not only provide new insights into the functional potential of van der Waals multiferroic heterostructures, but also offer a viable strategy for achieving electrically tunable SOT, paving the way for future programmable spintronic devices.

cond-mat.mes-hall↗

arXiv2026

Electric Field Induced Multi-Space Topological Phase Transitions in Janus Monolayer MnBi2Se2Te2

Manipulating coexisting multi-space topological states within a single material is a critical frontier for low-power spintronic applications, for which perpendicular electric-field gating offers a highly precise tuning mechanism. Using first-principles calculations and atomistic spin simulations, it is demonstrated that Janus monolayer MnBi2Se2Te2 can exhibit simultaneous momentum-space and real-space topological phase transitions under an external electric field. Specifically, the electric field drives momentum-space transitions across a topologically trivial state (C = 0), a low-Chern-number quantum anomalous Hall state (C = -1), and a high-Chern-number state (C = 2). Concurrently, by actively tuning the competition between the Dzyaloshinskii-Moriya interaction and magnetic anisotropy, the electric field induces real-space magnetic transitions from a uniform ferromagnetic state to an isolated skyrmion state, and ultimately to a skyrmion-spiral domain coexistence phase. Electric-field-induced variations in both the QAHE and magnetic textures may give rise to multiple topological phase transitions, involving distinct topological regimes: a purely k-space topology, RK-joint skyrmions, and pure real-space skyrmions. These findings establish a powerful material platform and efficient route for the synergistic control of coexisting topological orders, making it highly promising for next generation multi-functional spintronic devices.

cond-mat.mtrl-sci↗

arXiv2026

Reversible nonrelativistic magnon spin transport in ferroelastic altermagnets

Magnons in antiferromagnetic (AFM) insulators facilitate low-dissipation, stray-field-free spin transport. However, achieving nonvolatile, field-free control over magnon spin currents remains elusive. Here, based on symmetry analysis, we propose a universal mechanism for the active manipulation of magnon spin transport via ferroelastic transitions in two-dimensional (2D) altermagnets (AMs)-a class of unconventional AFMs simultaneously exhibiting compensated magnetization and nonrelativistic spin splitting. We show that these transitions effectively reorient principal crystal axes and modulate the underlying magnetic exchange anisotropy. Consequently, this magnetoelastic coupling drives nonrelativistic anisotropic spin transport that is ferroelastically switchable without the need for external magnetic fields or Berry curvature, leading to sign reversals in the spin Seebeck and spin Nernst conductivities. We validate this mechanism using first-principles calculations and spin-model analyses of an AM CoTe2 monolayer. Our findings establish a symmetry-based magnetoelastic paradigm for the nonvolatile control of magnon spin transport in 2D AMs, opening new avenues toward energy-efficient, reconfigurable AFM magnonic devices.

cond-mat.mtrl-sci↗

arXiv2026

Multistate Manipulation of Charge-Spin Conversion in Two-Dimensional Ferroelectric Bilayers

Achieving nonvolatile and multistate manipulation of charge-spin conversion, including the Edelstein effect (EE) and spin Hall effect (SHE), is crucial for high-density spintronic memory. Here, we propose a mechanism to simultaneously control both EE and SHE in two-dimensional ferroelectric bilayers, where interlayer-parallel and interlayer-antiparallel polarization configurations can coexist. Symmetry analysis shows that in the interlayer-parallel states, reversal of the total polarization switches the sign of the EE, whereas changing to an interlayer-antiparallel configuration suppresses the EE to zero, enabling electrically switchable current-induced spin accumulation among three distinct states, which could be used for ternary logic operations. Meanwhile, the magnitude of the SHE can be tuned by switching between two different classes of polarization configurations, namely interlayer-parallel and interlayer-antiparallel configurations. Using first-principles calculations, we demonstrate this mechanism in bilayer metallic ferroelectric PtBi2, where both interlayer-parallel and interlayer-antiparallel polarization configurations are energetically stable. The EE coefficient in interlayer-parallel states, which can be reversed by polarization switching, arises from competing electron- and hole-pocket contributions near the Fermi surface. The intrinsic SHE coefficients originate from spin Berry curvature that can be reshaped by polarization configuration variation and Fermi-level tuning. Our results establish ferroelectric bilayers as an all-in-one platform for electrically programmable charge-spin conversion.

cond-mat.mtrl-sci↗

arXiv2026

Magnetization-dependent and stacking-tunable Edelstein effect in two-dimensional magnet 2H-VTe2

The Edelstein effect in magnetic systems enables magnetization switching via the coupling between current-induced spin accumulation and intrinsic magnetic order, and is therefore highly promising for next-generation spintronic devices. Realizing and manipulating the Edelstein effect in two-dimensional (2D) magnetic systems is particularly desirable for achieving high-efficiency and multifunctional spintronic applications. In this work, based on first-principles calculations and symmetry analysis, we demonstrate that the Edelstein effect can intrinsically arise in the 2D in-plane ferromagnetic semiconductor 2H-VTe2, with its behavior strongly dependent on the magnetization orientation. For monolayer 2H-VTe2 with D3h crystal symmetry, under an applied current along the +x direction, only the time-reversal-even z component and the time-reversal-odd y(x) component of the spin accumulation are allowed when the magnetization is aligned along +x (+y). For ferromagnetic bilayer 2H-VTe2 in AB or BA stacking, where the crystal symmetry is reduced to C3v, additional spin components emerge with the presence of in-plane magnetization. Specifically, for magnetization along +x (+y), besides dSz_even and dSy_odd (dSz_even and dSx_odd), extra components such as dSy_even and dSz_odd (dSy_even) become allowed. Notably, these additional components can be reversibly switched by changing the stacking configuration from AB to BA via interlayer sliding. Our results not only deepen the understanding of current-induced spin accumulation in 2D magnetic systems from both symmetry and first-principles perspectives, but also identify 2H-MX2 materials as a promising platform for realizing intrinsic and tunable Edelstein effects in high-efficiency spin-orbit torque devices.

cond-mat.mes-hall↗

arXiv2026

Microscopic evidence of spin-driven multiferroicity and topological spin textures in monolayer NiI2

In type II multiferroics, noncollinear spin textures are expected to induce electric polarization directly, leading to strong magnetoelectric coupling. Realizing such spin driven multiferroicity in two-dimensional systems, and elucidating the interplay between local spins and electric polarization, are of both fundamental and technological importance. Here, using vectorial spin polarized scanning tunneling microscopy, we investigated the spin-driven multiferroicity in monolayer NiI2 at atomic scale. We identify a canted spin-spiral state with fully determined spin rotation plane, accompanied by a 2Q charge modulation. At spin spiral domain walls, we discover topological spin textures that composed of meron/antimeron pairs. These textures are associated with distinct charge pattern and notable band shifts, indicating local bound charges induced by variations of ferroelectricity at domain wall. Our observations are well captured by a realistic spin model incorporating Kitaev interactions and generalized spin-current model of type II multiferroicity. The findings provide microscopic evidence of spin-driven multiferroicity in an extreme 2D system and establish a platform for low-dissipation, electric-field control of topological spin textures.

cond-mat.mes-hall↗

arXiv2026

Switching between Antiferromagnetic and Ferromagnetic Skyrmions in Two-Dimensional Magnets

Antiferromagnetic (AFM) and ferromagnetic (FM) skyrmions possess unique advantages for spintronic applications. AFM skyrmions eliminate the skyrmion Hall effect and exhibit fast dynamics, whereas FM skyrmions are easier to nucleate and manipulate. However, realizing a transition between AFM and FM skyrmions within the same two-dimensional (2D) material has remained elusive. Here, using first-principles calculations and atomistic spin simulations on the Janus monolayer Cr2Ge2Te3S3, we demonstrate that strain-driven modulation of magnetic interactions enables switching between AFM and FM skyrmion phases. A compressive strain of $-3\%$ induces an AFM ground state hosting AFM skyrmions, while a tensile strain of $+2\%$ drives the system into a FM skyrmion phase. Moreover, under an out-of-plane magnetic field, FM skyrmions are rapidly transformed into a uniform FM phase, while AFM skyrmions transform into AFM bimerons under stronger fields. These findings establish a framework for controllable transitions between topological magnetic states in a single 2D material.

cond-mat.mtrl-sci↗

arXiv2026

Tunable Edelstein effect in intrinsic two-dimensional ferroelectric metal PtBi$_{2}$

The Edelstein effect, which enables charge-to-spin conversion and is therefore highly promising for future spintronic devices, can be realized and non-volatilely manipulated in ferroelectric materials owing to their broken inversion symmetry and switchable polarization states. To date, most ferroelectric systems reported to exhibit the Edelstein effect are semiconductors, requiring extrinsic doping for functionality. In contrast, the Edelstein effect has rarely been reported in metallic ferroelectric systems, where doping is unnecessary. Using first-principles calculations, we predict that a pronounced Edelstein effect can be realized in the recently proposed intrinsic two-dimensional ferroelectric metal PtBi$_{2}$ monolayer, where the sign of the Edelstein coefficient is coupled to the direction of ferroelectric polarization through the polarization-switching-induced reversal of spin textures, thereby enabling non-volatile control of charge-spin conversion. The Edelstein effect reaches a magnitude of $10^{11}~\hbar/(\textup{A} \cdot \textup{cm})$, which is sizable compared to previously reported ferroelectric systems. Microscopically, the Edelstein effect in a PtBi$_2$ monolayer originates from competing contributions of inner Rashba-like electron pockets and outer hole pockets with opposite signs; an upward shift of the Fermi level alters their balance and can reverse the sign of the Edelstein effect. Upon applying biaxial strain, the Fermi-surface electronic structure is strongly modified, resulting in a pronounced change of the Edelstein effect: a 2 \% compressive strain suppresses the Edelstein effect by about 50 \%. Our results not only identify a promising material platform for tunable charge-spin conversion but also provide new insights into the functional potential of metallic ferroelectric systems.

cond-mat.mtrl-sci↗

arXiv2025

Switching between Skyrmions and Yoshimori Spin Spirals via Li Absorption in Janus Magnets

Chiral magnetic textures have attracted considerable attention owing to their topological properties and potential applications in spintronic devices. Here, we employ first-principles calculations together with atomic spin dynamics simulations to explore the switching between skyrmions and Yoshimori-type spin spirals induced by Li adsorption in Janus two-dimensional (2D) CrTeSe. We show that selective Li adsorption on either the Se- or Te-terminated surface stabilizes distinct magnetic phases: Li adsorption on the Se side favors a Yoshimori-type spin spiral, whereas adsorption on the Te side stabilizes the skyrmionic state. This contrast originates from site-dependent modifications of exchange interactions, magnetic anisotropy (MA), and the Dzyaloshinskii-Moriya interaction (DMI). In addition, the response of magnetic textures to out-of-plane magnetic fields differs strongly between the two systems. These results demonstrate that surface adsorption provides an effective strategy for reversible control of chiral magnetic states in 2D magnets, while also offering fundamental insights into the competing interactions that govern the stability of skyrmions and Yoshimori spin spirals. Our findings highlight the potential of Janus 2D materials as a versatile platform for engineering tunable spintronic devices.

cond-mat.mtrl-sci↗

arXiv2025

Tunable Chern Insulator States with Coexisting Magnonic and Electronic Topology in 2D Honeycomb Kitaev Ferromagnets

The coexistence of topological magnons and electrons in magnetic materials presents a compelling route toward developing low-dissipation, multifunctional spintronic devices. However, material systems enabling their simultaneous realization and control remain largely unexplored. Here, we propose the coexistence and concurrent tunability of magnonic and electronic Chern insulator phases in Kitaev magnets and use MnBr$_{3}$ monolayer as a prototype. We find the significant Kitaev interaction in MnBr$_{3}$ induces the magnonic Chern insulator phase, manifesting as the magnon thermal Hall effect. Concurrently, MnBr$_{3}$ exhibits the quantum anomalous Hall effect driven by its electronic Chern insulator phase. Crucially, we demonstrate that these dual topological phases can be simultaneously controlled by reorienting the in-plane spins with an external magnetic field. Our findings not only deepen the fundamental understanding of spin excitations in Kitaev magnets but also provide a promising platform for exploring the interplay between electronic and magnonic topology.

cond-mat.mes-hall↗

arXiv2025

Long-Range Spin-Orbit-Coupled Magnetoelectricity in Type-II Multiferroic NiI$_2$

Type-II multiferroics, where spin order induces ferroelectricity, exhibit strong magnetoelectric coupling. However, for the typical 2D type-II multiferroic NiI$_2$, the underlying magnetoelectric mechanism remains unclear. Here, applying generalized spin-current model, together with first-principles calculations and a tight-binding approach, we build a comprehensive magnetoelectric model for spin-induced polarization. Such model reveals that the spin-orbit coupling extends its influence to the third-nearest neighbors, whose contribution to polarization rivals that of the first-nearest neighbors. By analyzing the orbital-resolved contributions to polarization, our tight-binding model reveals that the long-range magnetoelectric coupling is enabled by the strong $e_g$-$p$ hopping of NiI$_2$. Monte Carlo simulations further predict a Bloch-type magnetic skyrmion lattice at moderate magnetic fields, accompanied by polar vortex arrays. These findings can guide the discovery and design of strongly magnetoelectric multiferroics.

cond-mat.mtrl-sci↗

arXiv2024

Strain-Induced Intrinsic Antiferromagnetic Skyrmions in Two-Dimensional Janus Magnets

Antiferromagnetic (AFM) skyrmions, which are resistant to both the skyrmion Hall effect and external magnetic perturbations, are expected to be promising candidates for next-generation spintronics devices. Despite being observed in bulk materials and synthetic AFM layered systems, the existence of intrinsic AFM skyrmions within single magnetic layers, which offer potential advantages for spintronic device fabrication, has remained elusive. In this work, taking monolayer CrSi(Te,Se)$_{3}$ as a representative system, we demonstrate the emergence of intrinsic AFM skyrmions in two-dimensional Janus magnets. It is found that under moderate compressive strain, the interplay between considerable Dyzaloshinskii-Moriya interaction and the strain-induced AFM Heisenberg exchange interaction in monolayer CrSi(Te,Se)$_{3}$ would give rise to the emergence of intrinsic AFM skyrmions assembled from AFM spin spirals. Moreover, the application of an external magnetic field could trigger the emergence of AFM merons as well as a canted AFM state. Our findings propose a feasible approach for achieving intrinsic AFM skyrmions in realistic systems, which paves the way for developments in AFM topological spintronics devices.

cond-mat.mes-hall↗

arXiv2023

Chiral magnetism in lithium-decorated monolayer CrTe$_{2}$: Interplay between Dzyaloshinskii-Moriya interaction and higher-order interactions

Chiral magnetic states in two-dimensional (2D) layered noncentrosymmetric magnets, which are promising advanced spintronic materials, are usually attributed to Dzyaloshinskii-Moriya interactions (DMI). However, the role of underlying higher-order spin couplings in determining the properties of chiral spin textures has much less reported. In this work, taking the lithium-decorated monolayer CrTe$_{2}$ (monolayer LiCrTe$_{2}$) as an example, we develop a first-principles-based comprehensive spin model constructed by using the symmetry-adapted cluster expansion method. Based on this spin model, we identify the ground state of monolayer LiCrTe$_{2}$ as a chiral spin spiral state, which can further assemble macroscopic chiral labyrinth domains (LD) under zero-field conditions as well as evolve into skyrmions under a finite magnetic field. Moreover, higher-order biquadratic and three-site interactions are identified to be responsible for modulating both the size and the field stability of the spin spiral state. Our study sheds light on complex magnetic couplings in 2D magnets.

cond-mat.mtrl-sci↗

arXiv2023

Strain-induced frustrated helimagnetism and topological spin textures in LiCrTe$_{2}$

By performing first-principles calculations in conjunction with Monte Carlo simulations, we systematically investigated the frustrated magnetic states induced by in-plane compressive strain in LiCrTe$_{2}$. Our calculations support that the magnetic ground state of LiCrTe$_{2}$ crystal is A-type antiferromagnetic (AFM), with an in-plane ferromagnetic (FM) state and interlayer AFM coupling. Furthermore, it is found that compressive strain can significantly alter the magnetic interactions, giving rise to a transition from an in-plane FM to an AFM state, undergoing a helimagnetic phase. Remarkably, a highly frustrated helimagnetic state with disordered spin spirals under moderate strain arises from the competition between spiral propagation modes along distinct directions. In addition, various topological spin defects emerge in this frustrated helimagnetic phase, which are assembled from various domain wall units. These topological defects can be further tuned with external magnetic fields. Our calculations not only uncover the origin of exotic frustrated magnetism in triangular lattice magnetic systems, but also offer a promising route to engineer the frustrated and topological magnetic state, which is of significance in both fundamental research and technological applications.

cond-mat.mtrl-sci↗

arXiv2022

Tuning the magnetic anisotropy and topological phase with electronic correlation in single-layer H-FeBr$_2$

Electronic correlation can strongly influence the electronic properties of two-dimensional (2D) materials with open d- or f-orbitals. Herein, by taking single-layer (SL) H-FeBr$_2$ as a representative of the SL H-FeX$_2$ (X=Cl, Br, I) family, we investigated the electronic correlation effects in the magnetic anisotropy and electronic topology of such a system based on first-principles calculations with DFT+\textit{U} approach. Our result is that the magnetic anisotropy energy (MAE) of SL H-FeBr$_2$ shows a non-monotonic evolution behaviour with increasing electronic correlation strength, which is mainly due to the competition between different element-resolved MAEs of Fe and Br. Further investigations show that the evolution of element-resolved MAE arises from the variation of the spin-orbital coupling (SOC) interaction between different orbitals in each atom. Moreover, tuning the strength of the electronic correlation can drive the occurrence of band inversions, causing the system to undergoes multiple topological phase transitions, resulting in a quantum anomalous valley Hall (QAVH) effect. These exotic properties are universal for the SL H-FeX$_2$ (X = Cl, Br, I) family. Our work sheds light on the role of electronic correlation effects in tuning magnetic and electronic structures in the SL H-FeX$_2$ (X = Cl, Br, I) family, which could guide advances in the development of new spintronics and valleytronics devices based on these materials.

cond-mat.mtrl-sci↗