Atomic-Scale Visualization of Multiferroicity in Monolayer NiI2

Multicomponent Magneto-Orbital Order and Magneto-Orbitons in Monolayer VCl3

Van der Waals monolayers featuring magnetic states provide fundamental building blocks for artificial quantum matter. Here, we establish the emergence of a multicomponent ground state featuring magneto-orbital excitations of the 3d2-transition metal trihalide VCl3 monolayer. We show that monolayer VCl3 realizes a ground state with simultaneous magnetic and orbital ordering by using density functional theory. Using first-principles methods we derive an effective Hamiltonian with intertwined spin and orbital degrees of freedom, which we demonstrate can be tuned by strain. We show that magneto-orbitons appear as the collective modes of this complex order and arise from coupled orbiton magnon excitations due to the magneto-orbital coupling in the system. Our results establish VCl3 is a promising 2D material to observe emergent magneto-orbital excitations and provides a platform for multicomponent symmetry breaking.

Shear-Mediated Stabilization of Spin Spiral Order in Multiferroic NiI2

Type-II multiferroicity from non-collinear spin order is recently explored in the van der Waals material NiI2. Despite the importance for improper ferroelectricity, the microscopic mechanism of the helimagnetic order remains poorly understood. Here, the magneto-structural phases of NiI2 are investigated using resonant magnetic X-ray scattering (RXS) and X-ray diffraction. Two competing magnetic phases are identified. Below 60 K, an incommensurate magnetic reflection (q ≈ [0.143,0,1.49] reciprocal lattice units) is observed which exhibits finite circular dichroism in RXS, signaling the inversion symmetry-breaking helimagnetic ground state. At elevated temperature, in the non-polar phase (60 K < T < 75 K), a distinct q ≈ [0.087,0.087,1.5] magnetic order is observed, attributed to a collinear incommensurate (CI) state. The first-order CI-helix transition is concomitant with a structural transition characterized by a significant interlayer shear, which drives the helimagnetic ground state as evidenced by a mean-field Heisenberg model including interlayer exchange and its coupling to the structural distortion. These findings identify interlayer magneto-structural coupling as the key driver behind multiferroicity in NiI2.

Realizing Intralayer Magnetoelectric Coupling in Two-Dimensional Frustrated Multiferroic Heterostructures

Recent studies have demonstrated the ability to switch weakly coupled interlayer magnetic orders by using electric polarization in insulating van der Waals heterostructures. However, controlling strongly coupled intralayer magnetic orders remains a significant challenge. In this work, we propose that frustrated multiferroic heterostructures can exhibit enhanced intralayer magnetoelectric coupling. Through first-principles calculations, we have investigated a heterostructure composed of MnBr2 and Nb3I8, wherein there is a competition between frustrated intralayer magnetic orders within the MnBr2 and interlayer magnetic coupling via a unique spin-local field effect. As a result, manipulating the vertical electric polarization of the Nb3I8 layer successfully controls the ground-state intralayer magnetic order in the frustrated MnBr2 layer, inducing transitions between zigzag antiferromagnetic and ferromagnetic orders. Our findings offer a novel approach to controlling intralayer spin structures, paving the way for advancements in spintronic applications in a single atomic layer, which cannot be achieved by interlayer magnetoelectric coupling.

Recent Advances in Two-Dimensional Ferromagnetic Materials-Based van der Waals Heterostructures

Two-dimensional (2D) ferromagnetic materials are subjects of intense research owing to their intriguing physicochemical properties, which hold great potential for fundamental research and spintronic applications. Specifically, 2D van der Waals (vdW) ferromagnetic materials retain both structural integrity and chemical stability even at the monolayer level. Moreover, due to their atomic thickness, these materials can be easily manipulated by stacking them with other 2D vdW ferroic and nonferroic materials, enabling precise control over their physical properties and expanding their functional applications. Consequently, 2D vdW ferromagnetic materials-based heterostructures offer a platform to tailor magnetic properties and explore advanced spintronic devices. This review aims to provide an overview of recent developments in emerging 2D vdW ferromagnetic materials-based heterostructures and devices. The fabrication approaches for 2D ferromagnetic vdW heterostructures are primarily summarized, followed by a review of two categories of heterostructures: ferromagnetic/ferroic and ferromagnetic/nonferroic vdW heterostructures. Subsequently, the progress made in modulating magnetic properties and emergence of various phenomena in these heterostructures is highlighted. Furthermore, the applications of such heterostructures in spintronic devices are discussed along with their future perspectives and potential directions in this exciting field.

Orientation-selective spin-polarized edge states in monolayer NiI2

Spin-polarized edge states in two-dimensional materials hold promise for spintronics and quantum computing applications. Constructing stable edge states by tailoring two-dimensional semiconductor materials with bulk-boundary correspondence is a feasible approach. Recently layered NiI2 is suggested as a two-dimensional type-II multiferroic semiconductor with intrinsic spiral spin ordering and chirality-induced electric polarization. However, the one-dimensional spin-polarized edge states of multiferroic materials down to monolayer limit has not yet been studied. We report here that monolayer NiI2 was successfully synthesized on Au(111) by molecular beam epitaxy. Spin-polarized scanning tunneling microscopy/spectroscopy experiments visualize orientation-selective spin-polarized edge states in monolayer NiI2 islands. By performing first-principles calculations, we further confirm that spin-polarized edge states are selectively aligning along the Ni-terminated edges rather than the I-terminated edges. Our result will provide the opportunity to tune edge states by selected orientation and to develop spintronic devices in two-dimensional magnetic semiconductors.

Electric Field Control Of Moiré Skyrmion Phases in Twisted Multiferroic NiI2 Bilayers

Twisted magnetic van der Waals materials provide a flexible platform to engineer unconventional magnetism. Here we demonstrate the emergence of electrically tunable topological moiré magnetism in twisted bilayers of the spin-spiral multiferroic NiI2. We establish a rich phase diagram featuring uniform spiral phases, a variety of kπ-skyrmion lattices, and nematic spin textures ordered at the moiré scale. The emergence of these phases is driven by the local stacking and the resulting moiré modulated frustration. Notably, when the spin-spiral wavelength is commensurate with the moiré length scale by an integer k, multiwalled skyrmions become pinned to the moiré pattern. We show that the strong magnetoelectric coupling displayed by the moiré multiferroic allows electric control of the kπ-skyrmion lattices by an out-of-plane electric field. Our results establish a highly tunable platform for skyrmionics based on twisted van der Waals multiferroics, potentially enabling a new generation of ultrathin topologically protected spintronic devices.

Multiferroic properties and giant piezoelectric effect of a 2D Janus WO3F monolayer

Materials possessing both ferroelectricity and ferromagnetism are regarded as ideal candidates for electronic devices, such as nonvolatile memories. Based on first-principles calculations, we systematically studied the crystal structure, electronic structure as well as magnetic, piezoelectric and ferroelectric properties of a two-dimensional van der Waals WO3F monolayer material. The WO3F monolayer was found to possess a robust square crystal structure, exhibiting exceptional stability and mechanical resilience. Magnetic characterization revealed that the material displayed a ferromagnetic state with a magnetic moment of 1μB with negligible magnetic anisotropy. In terms of ferroelectric properties, the WO3F monolayer demonstrated pronounced in-plane polarization, which is in stark contrast to its relatively weak out-of-plane polarization and indicative of anisotropic polarization behavior. Additionally, the material's piezoelectric response could be modulated through strain engineering, with its piezoelectric coefficient (d11) at 4% tensile strain, which exceeds that of the vast majority of known 2D piezoelectric materials, thus underscoring its potential for versatile multifunctional applications in diverse fields, including sensing, energy harvesting, and actuator technologies.

Coexistence of ferroelectricity and antiferroelectricity in 2D van der Waals multiferroic

Multiferroic materials have been intensively pursued to achieve the mutual control of electric and magnetic properties. The breakthrough progress in 2D magnets and ferroelectrics encourages the exploration of low-dimensional multiferroics, which holds the promise of understanding inscrutable magnetoelectric coupling and inventing advanced spintronic devices. However, confirming ferroelectricity with optical techniques is challenging in 2D materials, particularly in conjunction with antiferromagnetic orders in single- and few-layer multiferroics. Here, we report the discovery of 2D vdW multiferroic with out-of-plane ferroelectric polarization in trilayer NiI2 device, as revealed by scanning reflective magnetic circular dichroism microscopy and ferroelectric hysteresis loops. The evolution between ferroelectric and antiferroelectric phases has been unambiguously observed. Moreover, the magnetoelectric interaction is directly probed by magnetic control of the multiferroic domain switching. This work opens up opportunities for exploring multiferroic orders and multiferroic physics at the limit of single or few atomic layers, and for creating advanced magnetoelectronic devices.

Effect of metal-ligand interactions on magnetic characteristics of two-dimensional Kagome structured perthiolated coronene (PTC) metal-organic frameworks (MOFs)

In recent years, the potential applications of two-dimensional (2D) metal-organic framework (MOF) materials in fields like spintronics have drawn increasing attention. Inspired by the successful synthesis of a perthiolated coronene (PTC)-Fe MOF structure, this study explores the fine-tuning of its electronic and magnetic structure by substituting Fe elements with various transition metals. Our calculations demonstrate a substantial increase in the Curie temperature (Tc) by a factor of 5 for Co and 10 for Mn when replacing Fe. This enhancement is attributed to the elevated electron density near the Fermi level, facilitating the generation of additional itinerant electrons crucial for the Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange mechanism. However, substituting Fe with V, Cr, Ni, and Cu leads to a loss of ferromagnetic ground state. Our work enhances the understanding of the electronic and magnetic behavior of the 2D PTC-TM (transition metal) MOF family, and provides a promising avenue for engineering 2D magnetic MOF systems.

Nature of the Unconventional Heavy-Fermion Kondo State in Monolayer CeSiI

CeSiI has been recently isolated in the ultrathin limit, establishing CeSiI as the first intrinsic two-dimensional van der Waals heavy-fermion material up to 85 K. We show that, due to the strong spin-orbit coupling, the local moments develop a multipolar real-space magnetic texture, leading to local pseudospins with a nearly vanishing net moment. To elucidate its Kondo-screened regime, we extract from first-principles the parameters of the Kondo lattice model describing this material. We develop a pseudofermion methodology in combination with ab initio calculations to reveal the nature of the heavy-fermion state in CeSiI. We analyze the competing magnetic interactions leading to an unconventional heavy-fermion order as a function of the magnetic exchange between the localized f-electrons and the strength of the Kondo coupling. Our results show that the magnetic exchange interactions promote an unconventional momentum-dependent Kondo-screened phase, establishing the nature of the heavy-fermion state observed in CeSiI.