Two-dimensional magnetic crystals and emerging heterostructure devices

CONTEXT

The electron can be considered as a small magnet, with two opposite poles defining its magnetic field associated with spin and orbital motion. When such tiny magnets are collectively aligned due to the inherent coupling, ferromagnetism appears. However, it has long been believed that ferromagnetism hardly survived in two-dimensional (2D) systems because of the increased thermal fluctuations revealed by Mermin-Wagner's theorem. The recent discovery of 2D magnetic crystals has shown that magnetic anisotropy can stabilize the long range magnetic order by opening an exciting range to resist thermal agitation. Two-dimensional magnetic crystals are ideal platforms for experimentally accessing the fundamental physics of magnetism in small dimensions. Unlike traditional thin magnetic films, 2D materials largely decouple from substrates, allow electrical control, are mechanically flexible, and are open to chemical functionalization. These attributes make 2D magnets accessible, adaptable and integrable into emerging heterostructures for properties and applications not yet achieved, such as magneto-optical and magneto-magnetic thin-film devices for ultracompact spintronics, optical on-chip communications, and the like. quantum computing.

ADVANCES

Magnetism has been explored in 2D materials for over a decade. Magnetic moments have been created through fault engineering based on vacancies, adatoms, boundaries and contours. the band structure engineering, assisted by functional density theory calculations, has opened up 2D magnetism possibilities in, for example, blocked bilayer graphene and doped GaSe; The proximity effect was applied to the spin polarization printing in 2D materials from magnetic substrates. However, these earlier efforts focused on the extrinsically induced magnetic response.

In early 2017, the first observations of a long-range magnetic order in primitive 2D crystals were reported in Cr.₂Ge₂You₆ and CrI₃. Both are magnetic insulators, yet with distinct magnetic properties. In contrast, 2D Fe₃GeTe₂ has recently been proven to be a magnetic conductor. Itinerant magnets and magnetic insulators have various application perspectives. The molecular beam epitaxial growth of 2D magnets has been reported for the Fe₃GeTe₂, VSe₂, MnSe_Xand Cr₂Ge₂You₆. Curie temperatures typical of 2D magnets are much lower than those of their 3D counterparts. However, this does not fundamentally exclude the possibility of 2D magnets at high temperatures. Efforts in this direction have been promising.

When van der Waals magnets (vdW) come into contact with non-magnetic materials, time inversion asymmetry could be introduced into the original non-magnets, which would probably lead, for example, to a polarization of valley in transition metal dichalcogenides or abnormal quantum Hall states in topological insulators. However, it should be noted that the properties of 2D magnets are sensitive to the materials in contact. The stacking of vdW magnets with different materials could enrich the landscape of emerging phenomena by causing, for example, the heterostructure's multi-heterostructure, unconventional superconductivity and quantum anomalous Hall effect .

Two-dimensional spintronic and magnetic devices have begun to emerge. A spin-orbit pair has been generated while spin-polarized current is injected from 2D materials (eg, WTe).₂) in magnetic substrates; conversely, a spin wave was pumped from magnetic substrates into 2D materials for spin charge conversion. Magnetic tunnel junctions with 2D magnets (eg, CrI₃) like tunnel barriers, have a gigantic tunnel magnetoresistance at low temperatures. New concepts of spin field effect transistors based on 2D magnets have also been reported.

PERSPECTIVE

Most of the 2D magnets currently available rely on mechanical exfoliation and only work when temperatures are low. The scaled synthesis of a slice of 2D magnets operating above ambient temperature is a prerequisite for the development of practical applications. In the longer term, the monolithic integration of such 2D magnets to other functional materials is crucial for their scalability. Spintronic devices require efficient electrical modulation of 2D magnets, transport of spins or spin waves over long distances, as well as efficient tunneling and spin injection at different junctions. The practical development of low power spintronic devices must be compatible with existing complementary metal oxide semiconductor technology (eg, impedance matching and affordable power supply). In addition, exotic spin textures, quantum phases and quasi-particles in 2D magnetic crystals and hetero-interfaces could lead to new methods of computation and communication. We anticipate that successive breakthroughs of 2D magnets could pave the way for a new era of information technology, with exciting applications in the areas of computing, data sensing and storage.