Accessing new magnetic regimes by tuning the ligand spin-orbit coupling in van der Waals magnets

Understanding and tuning magnetism in van der Waals-type metal thiophosphates

Over the past two decades, significant progress in two-dimensional (2D) materials has invigorated research in condensed matter and material physics in low dimensions. While traditionally studied in three-dimensional systems, magnetism has now been extended to the 2D realm. Recent breakthroughs in 2D magnetism have attracted substantial interest from the scientific community, owing to the stable magnetic order achievable in atomically thin layers of the van der Waals (vdW)-type layered magnetic materials. These advances offer an exciting platform for investigating related phenomena in low dimensions and hold promise for spintronic applications. Consequently, vdW magnetic materials with tunable magnetism have attracted significant attention. Specifically, antiferromagnetic metal thiophosphates MPX3 (M = transition metal, P = phosphorus, X = chalcogen) have been investigated extensively. These materials exhibit long-range magnetic order spanning from bulk to the 2D limit. The magnetism in MPX3 arises from localized moments associated with transition metal ions, making it tunable via substitutions and intercalations. In this review, we focus on such tuning by providing a comprehensive summary of various metal- and chalcogen-substitution and intercalation studies, along with the mechanism of magnetism modulation, and a perspective on the development of this emergent material family.

Self-Stacked 1T-1H Layers in 6R-NbSeTe and the Emergence of Charge and Magnetic Correlations Due to Ligand Disorder

The emergence of correlated phenomena arising from the combination of 1T and 1H van der Waals layers is the focus of intense research. Here, we synthesize a self-stacked 6R phase in NbSeTe, showing perfect alternating 1T and 1H layers that grow coherently along the c-direction, as revealed by scanning transmission electron microscopy. Angle-resolved photoemission spectroscopy shows a mixed contribution of the trigonal and octahedral Nb bands to the Fermi level. Diffuse scattering reveals temperature-independent short-range charge fluctuations with propagation vector qCO = (0.25 0), derived from the condensation of a longitudinal mode in the 1T layer, while the long-range charge density wave is quenched by ligand disorder. Magnetization measurements suggest the presence of an inhomogeneous, short-range magnetic order, further supported by the absence of a clear phase transition in the specific heat. These experimental analyses in combination with ab initio calculations indicate that the ground state of 6R-NbSeTe is described by a statistical distribution of short-range charge-modulated and spin-correlated regions driven by ligand disorder. Our results demonstrate how natural 1T-1H self-stacked bulk heterostructures can be used to engineer emergent phases of matter.

Self-doped flat band and spin-triplet superconductivity in monolayer 1T-TaSe2-xTex

Two-dimensional van der Waals materials have become an established platform to engineer flat bands which can lead to strongly-correlated emergent phenomena. In particular, the family of Ta dichalcogenides in the 1T phase presents a star-of-David charge density wave that creates a flat band at the Fermi level. For TaS2and TaSe2this flat band is at half filling leading to a magnetic insulating phase. In this work, we theoretically demonstrate that ligand substitution in the TaSe2-xTexsystem produces a transition from the magnetic insulator to a non-magnetic metal in which the flat band gets doped away from half-filling. Forx∈[0.846,1.231]the spin-polarized flat band is self-doped and the system becomes a magnetic metal. In this regime, we show that attractive interactions promote three different spin-triplet superconducting phases as a function ofx, corresponding to a nodal f-wave and two topologically-different chiral p-wave superconducting phases. Our results establish monolayer TaSe2-xTexas a promising platform for correlated flat band physics leading to unconventional superconducting states.

2D Heterostructure of CoCl2 /Co3 O4 Built for Strong Enhanced Magnetism

As a remarkable structure, 2D magnetic heterojunctions have attracted researchers' attention owing to their controlled manipulation in the electronic device. However, successful fabrication as well as modulation of their structure and compound remain challenging. Herein, a novel method is designed to obtain a CoCl2 /Co3 O4 heterojunction on Si/SiO2 substrate with the assistance of supercritical CO2 (SC CO2 ), and the as-fabricated sample has significantly increased coercivity and saturation magnetization, which is 11 times higher than pure Co3 O4 . Further, it can be found that the CO2 pressure has the decisive effect on the saturation magnetization of the sample. Therefore, it suggests that the tunable electronic-magnetic device can be anticipated to be obtained in the future.

Theoretical prediction of two-dimensional ferromagnetic Mn2X2 (X = As, Sb) with strain-controlled magnetocrystalline anisotropy

Two-dimensional (2D) magnetic materials with large and tunable magnetocrystalline anisotropy (MCA) provide unique opportunities to develop various spintronic devices. We, herein, propose an experimentally feasible 2D material platform, Mn2X2 (X = As, Sb), which is a family of intrinsic ferromagnet. Using first-principles calculations, we show that 2D Mn2X2 (X = As, Sb) with a robust ferromagnetic ground state exhibits not only a large perpendicular magnetic anisotropy (PMA), but also significant strain-driven modulation behaviors under external biaxial strain. The analysis of the results demonstrates that the dominant contribution to the change of MCA of Mn2As2 and Mn2Sb2 primarily arises from the Mn and Sb atoms, respectively. Moreover, we reveal that the underlying origin is the competitive mechanism for the spin-orbit coupling (SOC) between different orbitals and spin channels. These findings indicate that 2D Mn2X2 (X = As, Sb) provides a promising material platform for the next generation of ultra-low energy memory devices.

Controlling the 2D Magnetism of CrBr3 by van der Waals Stacking Engineering

The manipulation of two-dimensional (2D) magnetic order is of significant importance to facilitate future 2D magnets for low-power and high-speed spintronic devices. van der Waals stacking engineering makes promises for controllable magnetism via interlayer magnetic coupling. However, directly examining the stacking order changes accompanying magnetic order transitions at the atomic scale and preparing device-ready 2D magnets with controllable magnetic orders remain elusive. Here, we demonstrate the effective control of interlayer stacking in exfoliated CrBr3 via thermally assisted strain engineering. The stable interlayer ferromagnetic (FM), antiferromagnetic (AFM), and FM-AFM coexistent ground states confirmed by the magnetic circular dichroism measurements are realized. Combined with the first-principles calculations, the atomically resolved imaging technique reveals the correlation between magnetic order and interlayer stacking order in CrBr3 flakes unambiguously. A tunable exchange bias effect is obtained in the mixed phase of FM and AFM states. This work will introduce new magnetic properties by controlling the stacking order and sequence of 2D magnets, providing ample opportunities for their application in spintronic devices.

Visualization of Moiré Magnons in Monolayer Ferromagnet

Two-dimensional magnetic materials provide an ideal platform to explore collective many-body excitations associated with spin fluctuations. In particular, it should be feasible to explore, manipulate, and ultimately design magnonic excitations in two-dimensional van der Waals magnets in a controllable way. Here we demonstrate the emergence of moiré magnon excitations, stemming from the interplay of spin-excitations in monolayer CrBr₃ and the moiré pattern arising from the lattice mismatch with the underlying substrate. The existence of moiré magnons is further confirmed via inelastic quasiparticle interference, showing the appearance of a dispersion pattern correlated with the moiré length scale. Our results provide a direct visualization in real-space of the dispersion of moiré magnons, demonstrating the versatility of moiré patterns in creating emergent many-body excitations.

Emergence of electric-field-tunable interfacial ferromagnetism in 2D antiferromagnet heterostructures

Van der Waals (vdW) magnet heterostructures have emerged as new platforms to explore exotic magnetic orders and quantum phenomena. Here, we study heterostructures of layered antiferromagnets, CrI₃ and CrCl₃, with perpendicular and in-plane magnetic anisotropy, respectively. Using magneto-optical Kerr effect microscopy, we demonstrate out-of-plane magnetic order in the CrCl₃ layer proximal to CrI₃, with ferromagnetic interfacial coupling between the two. Such an interlayer exchange field leads to higher critical temperature than that of either CrI₃ or CrCl₃ alone. We further demonstrate significant electric-field control of the coercivity, attributed to the naturally broken structural inversion symmetry of the heterostructure allowing unprecedented direct coupling between electric field and interfacial magnetism. These findings illustrate the opportunity to explore exotic magnetic phases and engineer spintronic devices in vdW heterostructures.

A Three-Stage Magnetic Phase Transition Revealed in Ultrahigh-Quality van der Waals Bulk Magnet CrSBr

van der Waals (vdW) magnets are receiving ever-growing attention nowadays due to their significance in both fundamental research on low-dimensional magnetism and potential applications in spintronic devices. The high crystalline quality of vdW magnets is the key to maintaining intrinsic magnetic and electronic properties, especially when exfoliated down to the two-dimensional limit. Here, ultrahigh-quality air-stable vdW CrSBr crystals are synthesized using the direct solid-vapor synthesis method. The high single crystallinity and spatial homogeneity have been thoroughly evidenced at length scales from submm to atomic resolution by X-ray diffraction, second harmonic generation, and scanning transmission electron microscopy. More importantly, specific heat measurements of ultrahigh-quality CrSBr crystals show three thermodynamic anomalies at 185, 156, and 132 K, revealing a stage-by-stage development of the magnetic order upon cooling, which is also corroborated with the magnetization and transport results. Our ultrahigh-quality CrSBr can further be exfoliated down to monolayers and bilayers easily, providing the building blocks of heterostructures for spintronic and magneto-optoelectronic applications.

Control of structure and spin texture in the van der Waals layered magnet CrSBr

Controlling magnetism at nanometer length scales is essential for realizing high-performance spintronic, magneto-electric and topological devices and creating on-demand spin Hamiltonians probing fundamental concepts in physics. Van der Waals (vdW)-bonded layered magnets offer exceptional opportunities for such spin texture engineering. Here, we demonstrate nanoscale structural control in the layered magnet CrSBr with the potential to create spin patterns without the environmental sensitivity that has hindered such manipulations in other vdW magnets. We drive a local phase transformation using an electron beam that moves atoms and exchanges bond directions, effectively creating regions that have vertical vdW layers embedded within the initial horizontally vdW bonded exfoliated flakes. We calculate that the newly formed two-dimensional structure is ferromagnetically ordered in-plane with an energy gap in the visible spectrum, and weak antiferromagnetism between the planes, suggesting possibilities for creating spin textures and quantum magnetic phases.

Ultrathin Van der Waals Antiferromagnet CrTe3 for Fabrication of In-Plane CrTe3 /CrTe2 Monolayer Magnetic Heterostructures

Ultrathin van der Waals (vdW) magnets are heavily pursued for potential applications in developing high-density miniaturized electronic/spintronic devices as well as for topological physics in low-dimensional structures. Despite the rapid advances in ultrathin ferromagnetic vdW magnets, the antiferromagnetic counterparts, as well as the antiferromagnetic junctions, are much less studied owing to the difficulties in both material fabrication and magnetism characterization. Ultrathin CrTe₃ layers have been theoretically proposed to be a vdW antiferromagnetic semiconductor with intrinsic intralayer antiferromagnetism. Herein, the epitaxial growth of monolayer (ML) and bilayer CrTe₃ on graphite surface is demonstrated. The structure, electronic and magnetic properties of the ML CrTe₃ are characterized by combining scanning tunneling microscopy/spectroscopy and non-contact atomic force microscopy and confirmed by density functional theory calculations. The CrTe₃ MLs can be further utilized for the fabrication of a lateral heterojunction consisting of ML CrTe₂ and ML CrTe₃ with an atomically sharp and seamless interface. Since ML CrTe₂ is a metallic vdW magnet, such a heterostructure presents the first in-plane magnetic metal-semiconductor heterojunction made of two vdW materials. The successful fabrication of ultrathin antiferromagnetic CrTe₃ , as well as the magnetic heterojunction, will stimulate the development of miniaturized antiferromagnetic spintronic devices based on vdW materials.

The Magnetic Genome of Two-Dimensional van der Waals Materials

Magnetism in two-dimensional (2D) van der Waals (vdW) materials has recently emerged as one of the most promising areas in condensed matter research, with many exciting emerging properties and significant potential for applications ranging from topological magnonics to low-power spintronics, quantum computing, and optical communications. In the brief time after their discovery, 2D magnets have blossomed into a rich area for investigation, where fundamental concepts in magnetism are challenged by the behavior of spins that can develop at the single layer limit. However, much effort is still needed in multiple fronts before 2D magnets can be routinely used for practical implementations. In this comprehensive review, prominent authors with expertise in complementary fields of 2D magnetism (i.e., synthesis, device engineering, magneto-optics, imaging, transport, mechanics, spin excitations, and theory and simulations) have joined together to provide a genome of current knowledge and a guideline for future developments in 2D magnetic materials research.

Intrinsic staggered spin-orbit torque for the electrical control of antiferromagnets - application to CrI3

Spin-orbit torque enables the electrical control of the orientation of ferromagnets' or antiferromagnets' order parameter. In this work we consider antiferromagnets in which the magnetic sublattices are connected by inversion+time reversal symmetry, and in which the exchange and anisotropy energies are similar in magnitude. We identify the staggered dampinglike spin-orbit torque as the key mechanism for electrical excitation of the Néel vector for this case. To illustrate this scenario, we examine the 2-d Van der Waals antiferromagnetic bilayer CrI₃, in the n-doped regime. Using a combination of first-principles calculations of the spin-orbit torque and an analysis of the ensuing spin dynamics, we show that the deterministic electrical switching of the Néel vector is the result of dampinglike spin-orbit torque which is staggered on the magnetic sublattices.

Two-Dimensional Magnets beyond the Monolayer Limit

Intrinsic two-dimensional (2D) magnetism has been demonstrated in various materials scaled down to a single monolayer. However, the question is whether 2D magnetism extends beyond the monolayer limit, to chemical species formed by sparse but regular 2D arrays of magnetic atoms. Here we show that sub-monolayer superstructures of Eu atoms self-assembled on the silicon surface exhibit strong magnetic signals. Robust easy-plane magnetism is discovered in both one- and two-dimensionally ordered structures with Eu coverage of half monolayer and above. The emergence of 2D magnetism manifests itself by a strong dependence of the effective transition temperature on weak magnetic fields. The results constitute a versatile platform for miniaturization of 2D magnetic systems and seed an expandable class of atomically thin magnets for applications in information technologies.

Monolayer CrCl_{3} as an Ideal Test Bed for the Universality Classes of 2D Magnetism

The monolayer halides CrX_{3} (X=Cl, Br, I) attract significant attention for realizing 2D magnets with genuine long-range order (LRO), challenging the Mermin-Wagner theorem. Here, we show that monolayer CrCl_{3} has the unique benefit of exhibiting tunable magnetic anisotropy upon applying a compressive strain. This opens the possibility to use CrCl_{3} for producing and studying both ferromagnetic and antiferromagnetic 2D Ising-type LRO as well as the Berezinskii-Kosterlitz-Thouless (BKT) regime of 2D magnetism with quasi-LRO. Using state-of-the-art density functional theory, we explain how realistic compressive strain could be used to tune the monolayer's magnetic properties so that it could exhibit any of these phases. Building on large-scale quantum Monte Carlo simulations, we compute the phase diagram of strained CrCl_{3}, as well as the magnon spectrum with spin-wave theory. Our results highlight the eminent suitability of monolayer CrCl_{3} to achieve very high BKT transition temperatures, around 50 K, due to their singular dependence on the weak easy-plane anisotropy of the material.

Polymorphism in Post-Dichalcogenide Two-Dimensional Materials

Two-dimensional (2D) materials exhibit a wide range of atomic structures, compositions, and associated versatility of properties. Furthermore, for a given composition, a variety of different crystal structures (i.e., polymorphs) can be observed. Polymorphism in 2D materials presents a fertile landscape for designing novel architectures and imparting new functionalities. The objective of this Review is to identify the polymorphs of emerging 2D materials, describe their polymorph-dependent properties, and outline methods used for polymorph control. Since traditional 2D materials (e.g., graphene, hexagonal boron nitride, and transition metal dichalcogenides) have already been studied extensively, the focus here is on polymorphism in post-dichalcogenide 2D materials including group III, IV, and V elemental 2D materials, layered group III, IV, and V metal chalcogenides, and 2D transition metal halides. In addition to providing a comprehensive survey of recent experimental and theoretical literature, this Review identifies the most promising opportunities for future research including how 2D polymorph engineering can provide a pathway to materials by design.