Review

Two-dimensional magnetic crystals and emergent heterostructure devices

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Science  15 Feb 2019:
Vol. 363, Issue 6428, eaav4450
DOI: 10.1126/science.aav4450

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The ultimate in thin-film magnetism

The alignment of the magnetic properties of atoms gives rise to a wealth of simple and exotic properties that can be exploited. As the dimension of the material is reduced, such that the atoms are in a single monolayer, it was widely believed that thermal fluctuations overwhelm and prevent magnetic ordering. Gong and Zhang review the developments that have followed the recent discovery of magnetism in two-dimensional materials. Recognizing that magnetic anisotropy can be used to induce stable magnetism in atomic monolayers, they provide an overview of the materials available and the physical understanding of the effects and then discuss how these effects could be exploited for widespread practical applications.

Science, this issue p. eaav4450

Structured Abstract

BACKGROUND

The electron can be considered as a tiny magnet, with two opposite poles defining its magnetic field associated with the spin and orbital motion. When such minuscule magnets are collectively aligned as a result of the inherent coupling, ferromagnetism emerges. However, ferromagnetism had long been believed to hardly survive in two-dimensional (2D) systems because of the enhanced thermal fluctuations revealed by the Mermin-Wagner theorem. The recent discovery of 2D magnetic crystals showed that magnetic anisotropy could stabilize the long-range magnetic order by opening up an excitation gap to resist the thermal agitation. Two-dimensional magnetic crystals constitute ideal platforms to experimentally access the fundamental physics of magnetism in reduced dimensions. In contrast to the traditional magnetic thin films, 2D materials largely decouple from the substrates, allow electrical control, are mechanically flexible, and are open to chemical functionalization. These attributes make 2D magnets accessible, engineerable, and integrable into emergent heterostructures for previously unachieved properties and applications such as atomically thin magneto-optical and magnetoelectric devices for ultracompact spintronics, on-chip optical communications, and quantum computing.

ADVANCES

Magnetism has been explored in 2D materials for more than a decade. Magnetic moments have been created through defect engineering based on vacancies, adatoms, boundaries, and edges; band structure engineering, assisted by density functional theory calculations, has raised possibilities of 2D magnetism in, for instance, gated bilayer graphene and doped GaSe; the proximity effect has been applied to imprint spin polarization in 2D materials from magnetic substrates. However, these prior efforts centered on extrinsically induced magnetic response.

In early 2017, the first observations of long-range magnetic order in pristine 2D crystals were reported in Cr2Ge2Te6 and CrI3. Both are magnetic insulators, yet with distinct magnetic properties. In contrast, 2D Fe3GeTe2 was recently proven to be a magnetic conductor. Itinerant magnets and magnetic insulators possess diverse application perspectives. Molecular beam epitaxial growth of 2D magnets has been reported for Fe3GeTe2, VSe2, MnSex, and Cr2Ge2Te6. The typical Curie temperatures of 2D magnets are much lower than those of their 3D counterparts. However, this does not fundamentally exclude the possibility of high-temperature 2D magnets. Efforts toward this goal have shown promise.

When van der Waals (vdW) magnets contact nonmagnetic materials, time reversal asymmetry could be introduced in the original nonmagnets, likely leading to, for example, valley polarization in transition metal dichalcogenides or quantum anomalous Hall states in topological insulators. However, it should be noted that 2D magnets’ properties are susceptible to the contacting materials. Stacking vdW magnets with dissimilar materials could enrich the landscape of emergent phenomena by causing, for example, heterostructure multiferroicity, unconventional superconductivity, and the quantum anomalous Hall effect.

Two-dimensional spintronic and magnonic devices have begun to emerge. Spin-orbit torque has been generated while spin-polarized current is injected from 2D materials (e.g., WTe2) into magnetic substrates; conversely, a spin wave has been pumped from magnetic substrates into 2D materials for spin-charge conversion. Magnetic tunnel junctions with 2D magnets (e.g., CrI3) as tunneling barriers exhibit giant tunneling magnetoresistance at low temperatures. New concepts of spin field-effect transistors based on 2D magnets have been reported as well.

OUTLOOK

Most currently available 2D magnets rely on mechanical exfoliation and only work at low temperatures. The wafer-scale synthesis of 2D magnets that operate above room temperature is a prerequisite for the development of practical applications. In the longer run, the monolithic integration of such 2D magnets with other functional materials is crucial for practical scalability. Spintronic devices require efficient electrical modulation of 2D magnets, long-distance transport of spins or spin waves, and efficient tunneling and injection of spins at various junctions. The practical development of low-power spintronic devices needs to be compatible with the existing complementary metal-oxide semiconductor technology (e.g., impedance match and affordable power supply). Furthermore, the exotic spin textures, quantum phases, and quasiparticles in 2D magnetic crystals and hetero-interfaces could lead to new ways of computation and communication. We envision that successive breakthroughs in 2D magnets could usher in a new era of information technologies with exciting applications in computing, sensing, and data storage.

Two-dimensional magnetic crystals: The atomically thin crystalline hosts of magneto-optic and magnetoelectric effects.

2D magnetic crystals, including 2D ferromagnets (left) and 2D antiferromagnets with diverse intra- and interplane magnetic configurations (right), can exhibit a plethora of magneto-optic and magnetoelectric effects. The red and blue protrusions in the atomic Lego depict the opposite local spins in magnetic layers.

Abstract

Magnetism, originating from the moving charges and spin of elementary particles, has revolutionized important technologies such as data storage and biomedical imaging, and continues to bring forth new phenomena in emergent materials and reduced dimensions. The recently discovered two-dimensional (2D) magnetic van der Waals crystals provide ideal platforms for understanding 2D magnetism, the control of which has been fueling opportunities for atomically thin, flexible magneto-optic and magnetoelectric devices (such as magnetoresistive memories and spin field-effect transistors). The seamless integration of 2D magnets with dissimilar electronic and photonic materials opens up exciting possibilities for unprecedented properties and functionalities. We review the progress in this area and identify the possible directions for device applications, which may lead to advances in spintronics, sensors, and computing.

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