Ferromagnetism usually only occurs in materials containing elements that form covalent 3d and 4f bonds. Its occurrence in pure carbon is therefore surprising, even controversial. A systematic magnetic force microscope study indicates that ferromagnetism in graphite is the result of localized spins that arise at grain boundaries.

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Room-temperature ferromagnetism in graphite driven by two-dimensional networks of point defects

We report the connection between the stacking order and magnetic properties of bilayer CrI3 using first-principles calculations. We show that the stacking order defines the magnetic ground state. By changing the interlayer stacking order, one can tune the interlayer exchange interaction between antiferromagnetic and ferromagnetic. To measure the predicted stacking-dependent magnetism, we propose using linear magnetoelectric effect. Our results not only gives a possible explanation for the observed antiferromagnetism in bilayer CrI3 but also have direct implications in heterostructures made of two-dimensional magnets.

Stacking-dependent magnetism in bilayer CrI3

The discovery of intrinsic ferromagnetism in ultrathin two-dimensional van der Waals crystals opens up exciting prospects for exploring magnetism in the ultimate two-dimensional limit. Here, we show that environmentally stable CrSe2 nanosheets can be readily grown on a dangling-bond-free WSe2 substrate with systematically tunable thickness down to the monolayer limit. These CrSe2/WSe2 heterostructures display high-quality van der Waals interfaces with well-resolved moire superlattices and ferromagnetic behaviour. We find no apparent change in surface roughness or magnetic properties after months of exposure in air. Our calculations suggest that charge transfer from the WSe2 substrate and interlayer coupling within CrSe2 play a critical role in the magnetic order in few-layer CrSe2 nanosheets. The highly controllable growth of environmentally stable CrSe2 nanosheets with tunable thickness defines a robust two-dimensional magnet for fundamental studies and potential applications in magnetoelectronic and spintronic devices.

Van der Waals epitaxial growth of air-stable CrSe2 nanosheets with thickness-tunable magnetic order

We report on the observation of ferromagnetism in epitaxial thin films of the topological insulator compound Bi2Se3 with chromium doping. The structural, magnetic, and magnetoelectrical properties of Bi2Se3 were investigated for Cr concentrations up to 10%. For a Cr content up to ∼5% the films are of good crystalline quality, with the lattice parameter a decreasing and the lattice parameter c increasing with increasing Cr concentration. The Curie temperature reached a maximum TC = 20 K for 5.2% Cr. Well-defined ferromagnetic hysteresis in the magnetization and in the magnetoresistance was also observed in these films.

Ferromagnetism in thin-film Cr-doped topological insulator Bi₂Se₃

A grand family of two-dimensional (2D) materials and their heterostructures have been discovered through the extensive experimental and theoretical efforts of chemists, material scientists, physicists, and technologists. These pioneering works contribute to realizing the fundamental platforms to explore and analyze new physical/chemical properties and technological phenomena at the micro-nano-pico scales. Engineering 2D van der Waals (vdW) materials and their heterostructures via chemical and physical methods with a suitable choice of stacking order, thickness, and interlayer interactions enable exotic carrier dynamics, showing potential in high-frequency electronics, broadband optoelectronics, low-power neuromorphic computing, and ubiquitous electronics. This comprehensive review addresses recent advances in terms of representative 2D materials, the general fabrication methods, and characterization techniques and the vital role of the physical parameters affecting the quality of 2D heterostructures. The main emphasis is on 2D heterostructures and 3D-bulk (3D) hybrid systems exhibiting intrinsic quantum mechanical responses in the optical, valley, and topological states. Finally, we discuss the universality of 2D heterostructures with representative applications and trends for future electronics and optoelectronics (FEO) under the challenges and opportunities from physical, nanotechnological, and material synthesis perspectives.

2D Heterostructures for Ubiquitous Electronics and Optoelectronics: Principles, Opportunities, and Challenges.

Two-dimensional (2D) Cr-based layered and non-layered materials such as CrI3, Cr2Ge2Te6, Cr2S3, CrSe, and CrOX (X = Cl and Br) have attracted considerable attention due to their potential application in spintronics. Despite few experimental studies, theoretical studies reported that 2D chromium dichalcogenide (CrS2) materials show unique properties such as valley polarization, piezoelectric coupling, and phase dependent intrinsic magnetic properties. Here, we report for the first time the synthesis of 2D layered CrS2 flakes down to the monolayer via the chemical vapor deposition (CVD) method, its phase structures and electronic properties. We observed the 2H, 1T, and 1T′ phases coexisting in CVD grown monolayer CrS2. The formation of 1T′ phases from 1T phases is described by dimerization of metal atoms at room temperature according to our molecular dynamics studies. The coexistence of 1T and 1T′ phases with 2H phases is referred to as the 1T and 1T′ puddling phenomenon. We theoretically showed that the monolayer 2H-CrS2 is a direct bandgap semiconductor with a gap of approximately 0.95 eV predicted by the PBE functional, while the 1T- and 1T′-CrS2 are metallic and semi-metallic with approximately 10 meV gap, respectively. Furthermore, 2H CrS2 exhibits nonmagnetic semiconducting properties while for ferromagnetic spin configuration, the 1T and 1T′ CrS2 show magnetic characteristics with 0.531μB and 2.206μB magnetic moment per Cr atom respectively, for ferromagnetic spin configuration as predicted from DFT+U calculation. Importantly, CrS2-based field-effect transistors exhibit a p-type behavior. Our study would stimulate further exploration of 2D layered CrS2 with astonishing properties and open up a whole new avenue for the urgent need for developing multifunctional 2D materials for nanoelectronics, valleytronics, and spintronics.

Electronic properties of polymorphic two-dimensional layered chromium disulphide.

Recent breakthroughs in two-dimensional van der Waals magnetic materials and emerging applications

MoS2 and Perylene Derivative Based Type‐II Heterostructure: Bandgap Engineering and Giant Photoluminescence Enhancement

Theoretical Study of Interfacial and Electronic Properties of Transition Metal Dichalcogenides and Organic Molecules Based van der Waals Heterostructures

Yahya Khan

Papers

Electronic properties of polymorphic two-dimensional layered chromium disulphide.

Recent breakthroughs in two-dimensional van der Waals magnetic materials and emerging applications

MoS2 and Perylene Derivative Based Type‐II Heterostructure: Bandgap Engineering and Giant Photoluminescence Enhancement

Theoretical Study of Interfacial and Electronic Properties of Transition Metal Dichalcogenides and Organic Molecules Based van der Waals Heterostructures

Interfacial Properties for a Monolayer CrS 2 Contact with Metal: A Theoretical Perspective