Computational design of two‐dimensional magnetic materials

The prediction of two-dimensional PbN: opened bandgap in heterostructure with CdO

The development of two-dimensional (2D) materials has received wide attention as a generation of optoelectronics, thermoelectric, and other applications. In this study, a novel 2D material, PbN, is proposed as an elemental method using the prototype of a recent reported nitride (J. Phys. Chem. C 2023, 127, 43, 21,006-21014). Based on first-principle calculations, the PbN monolayer is investigated as stable at 900 K, and the isotropic mechanical behavior is addressed by the Young's modulus and Poisson's ratio at 67.4 N m-1 and 0.15, respectively. The PbN monolayer also presents excellent catalytic performance with Gibbs free energy of 0.41 eV. Zero bandgap is found for the PbN monolayer, and it can be opened at about 0.128 eV by forming a heterostructure with CdO. Furthermore, the PbN/CdO is constructed by Van der Waals interaction, while the apparent potential drop and charge transfer are investigated at the interface. The PbN/CdO heterostructure also possesses excellent light absorption properties. The results provide theoretical guidance for the design of layered functional materials.

Surface Stability and Exfoliability of Non-van der Waals Magnetic Chromium Tellurides

Exfoliation of two-dimensional (2D) magnetic materials from non-van der Waals (non-vdW) materials has attracted increasing attention because it provides a great platform for the construction of 2D magnetic materials. For non-vdW magnetic chromium tellurides with high Curie temperatures, their few-layer samples show promising applications in the field of spintronics. However, there is still no consensus on whether the surface structures of few-layer chromium tellurides should be terminated by Cr or Te atoms. By calculating the surface and exfoliation energy, we find that which structure is more stable depends greatly on the value of the chemical potential of Te atoms, and the few-layer sample with a Cr-terminated surface is easier to exfoliate than that with both Te-terminated surfaces. Finally, we propose that different exfoliated structures can be identified by using the atomic number ratio of Cr to Te and the average magnetic moment of Cr atoms in few-layer samples.

Recent Progress in Strain Engineering on Van der Waals 2D Materials: Tunable Electrical, Electrochemical, Magnetic, and Optical Properties

Strain engineering is a promising way to tune the electrical, electrochemical, magnetic, and optical properties of 2D materials, with the potential to achieve high-performance 2D-material-based devices ultimately. This review discusses the experimental and theoretical results from recent advances in the strain engineering of 2D materials. Some novel methods to induce strain are summarized and then the tunable electrical and optical/optoelectronic properties of 2D materials via strain engineering are highlighted, including particularly the previously less-discussed strain tuning of superconducting, magnetic, and electrochemical properties. Also, future perspectives of strain engineering are given for its potential applications in functional devices. The state of the survey presents the ever-increasing advantages and popularity of strain engineering for tuning properties of 2D materials. Suggestions and insights for further research and applications in optical, electronic, and spintronic devices are provided.

Large Orbital Magnetic Moment in VI3

The existence of the V³⁺-ion orbital moment is an open issue of the nature of magnetism in the van der Waals ferromagnet VI₃. The huge magnetocrystalline anisotropy in conjunction with the significantly reduced ordered magnetic moment compared to the spin-only value provides strong but indirect evidence of a large V orbital moment. We used the unique capability of X-ray magnetic circular dichroism to determine the orbital component of the total magnetic moment and provide a direct proof of an exceptionally sizable orbital moment of the V³⁺ ion in VI₃. Our ligand field multiplet simulations of the XMCD spectra in synergy with the results of DFT calculations agree with the existence of two V sites with different orbital occupations and OM magnitudes in the ground state.

Breaking through the Mermin-Wagner limit in 2D van der Waals magnets

The Mermin-Wagner theorem states that long-range magnetic order does not exist in one- (1D) or two-dimensional (2D) isotropic magnets with short-ranged interactions. Here we show that in finite-size 2D van der Waals magnets typically found in lab setups (within millimetres), short-range interactions can be large enough to allow the stabilisation of magnetic order at finite temperatures without any magnetic anisotropy. We demonstrate that magnetic ordering can be created in 2D flakes independent of the lattice symmetry due to the intrinsic nature of the spin exchange interactions and finite-size effects. Surprisingly we find that the crossover temperature, where the intrinsic magnetisation changes from superparamagnetic to a completely disordered paramagnetic regime, is weakly dependent on the system length, requiring giant sizes (e.g., of the order of the observable universe ~ 1026 m) to observe the vanishing of the magnetic order as expected from the Mermin-Wagner theorem. Our findings indicate exchange interactions as the main ingredient for 2D magnetism.

Two-dimensional spin-gapless semiconductors: A mini-review

In the past decade, two-dimensional (2D) materials and spintronic materials have been rapidly developing in recent years. 2D spin-gapless semiconductors (SGSs) are a novel class of ferromagnetic 2D spintronic materials with possible high Curie temperature, 100% spin-polarization, possible one-dimensional or zero-dimensional topological signatures, and other exciting spin transport properties. In this mini-review, we summarize a series of ideal 2D SGSs in the last 3 years, including 2D oxalate-based metal-organic frameworks, 2D single-layer Fe₂I_2, 2D Cr₂X₃ (X = S, Se, and Te) monolayer with the honeycomb kagome (HK) lattice, 2D CrGa₂Se₄ monolayer, 2D HK Mn–cyanogen lattice, 2D MnNF monolayer, and 2D Fe₄N₂ pentagon crystal. The mini-review also discusses the unique magnetic, electronic, topological, and spin-transport properties and the possible application of these 2D SGSs. The mini-review can be regarded as an improved understanding of the current state of 2D SGSs in recent 3 years.

Robust Design of High-Performance Optoelectronic Chalcogenide Crystals from High-Throughput Computation

High-performance functional materials are the cornerstones of the continuous advance of modern science and technology, but the development of new materials is still challenging. Here, we propose a robust design strategy for novel crystalline solids based on group-theory classification and high-throughput computation, as demonstrated by the successful identification of new optoelectronic semiconductors. First, by means of theoretical group analysis and composition engineering, we obtained 78 prototypical crystal structures and built a computational materials database containing 21,060 ternary chalcogenide compounds. Our high-throughput screening of the coordination characteristics, phase stability, and electronic structures provided 97 candidate semiconductors, including 93 completely new compounds. Among them, 22 crystals with excellent dynamical and thermal stability are predicted to show high photovoltaic conversion efficiency (>30%), comparable to the currently most efficient single-junction GaAs solar cell, owing to their optimal electronic properties and outstanding optical absorption. This discovery of new chalcogenide crystals offers excellent candidates for optoelectronic applications and suggests that our design strategy is a promising way to search for unknown high-performance functional materials.

High Curie Temperature and Intrinsic Ferromagnetic Half-Metallicity in Mn2X3 (X = S, Se, Te) Nanosheets

Two-dimensional (2D) intrinsic half-metallic materials with room-temperature ferromagnetism, sizable magnetic anisotropy energy (MAE), and wide half-metallic gap are excellent candidates for pure spin generation, injection, and transport in nanospintronic applications. However, until now, such 2D half metallicity has been rarely observed in experiment. In this work, by using first-principles calculations, we design a series of such materials, namely, Mn₂X₃ (X = S, Se, Te) nanosheets, which could be obtained by controlling the thickness of synthesized α-MnX(111) nanofilm to a quintuple X-Mn-X-Mn-X layer. All these nanosheets are dynamically and thermally stable. Electronic and magnetic studies reveal they are intrinsic half metals with high Curie temperatures between 718 and 820 K, sizable MAEs with -1.843 meV/Mn for Mn₂Te₃ nanosheet, and wide half-metallic gaps from 1.55 to 1.94 eV. Above all, the outstanding features of Mn₂X₃ nanosheets make them promising in fabricating nanospintronic devices working at room temperature.

Recent Developments in van der Waals Antiferromagnetic 2D Materials: Synthesis, Characterization, and Device Implementation

Magnetism in two dimensions is one of the most intriguing and alluring phenomena in condensed matter physics. Atomically thin 2D materials have emerged as a promising platform for exploring magnetic properties, leading to the development of essential technologies such as supercomputing and data storage. Arising from spin and charge dynamics in elementary particles, magnetism has also unraveled promising advances in spintronic devices and spin-dependent optoelectronics and photonics. Recently, antiferromagnetism in 2D materials has received extensive attention, leading to significant advances in their understanding and emerging applications; such materials have zero net magnetic moment yet are internally magnetic. Several theoretical and experimental approaches have been proposed to probe, characterize, and modulate the magnetic states efficiently in such systems. This Review presents the latest developments and current status for tuning the magnetic properties in distinct 2D van der Waals antiferromagnets. Various state-of-the-art optical techniques deployed to investigate magnetic textures and dynamics are discussed. Furthermore, device concepts based on antiferromagnetic spintronics are scrutinized. We conclude with remarks on related challenges and technological outlook in this rapidly expanding field.