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

Figures

  • 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.

  • Fig. 1 Fundamental physical parameters and spin wave excitations in ferromagnets of different dimensionalities.

    (A and B) In a collinear magnet, exchange interaction and magnetic anisotropy are fundamental parameters. Exchange interaction arises from electrons’ antisymmetric wave function and is governed by coulombic interaction under the Pauli exclusion principle. Exchange interaction between spins can be directly established (red dashed line 1) or indirectly mediated by conduction electrons (green ball with dashed lines labeled 2) or intermediate anions (orange ball with dashed lines labeled 3) such as O2–. While spins are aligned, there is usually a preferred orientation, which means magnetic anisotropy. Magnetic anisotropy has a variety of sources such as magnetocrystalline anisotropy, shape anisotropy, and stress anisotropy. (C to F) In a 2D isotropic Heisenberg ferromagnet, there will be massive excitations of magnons at nonzero temperatures because of the absence of a spin wave excitation gap, the abrupt onset of magnon density of states (DOS), and the diverging Bose-Einstein statistics at zero energy; the result is collapse of long-range magnetic order. The presence of uniaxial magnetic anisotropy (UMA) opens up the spin wave excitation gap to resist the thermal agitations of magnons, leading to the finite Curie temperature. As the system evolves from 2D to 3D, the magnon DOS spectrum changes from a step function to a gradually increasing function with zero DOS at the threshold of excitation. Therefore, in 3D systems, UMA (related to the spin wave excitation gap) is not a prerequisite for the presence of finite-temperature long-range magnetic order.

  • Fig. 2 Schemes to induce magnetism in nonmagnetic 2D materials.

    Point defects such as vacancies and adatoms in 2D materials are accompanied by defect states and local magnetic moments. (A) STM topography of graphene with carbon vacancies induced by Ar+ ion irradiation (17). Scale bar, 5 nm. (B) Schematic of local magnetic moments in graphene decorated by an individual hydrogen adatom (small white ball at center) (18). The same spin-polarized state extends a few nanometers in carbon sites of the same sublattice, but the opposite spin-polarized state occupies the other carbon sublattice. (C) Magnetization versus magnetic field parallel to the fluorinated graphene planes (23). Dots are experimental data; solid lines are fitting curves based on Brillouin function. No trace of ferromagnetism was found in both fluorinated graphene and defective graphene with vacancies at liquid helium temperatures. (D) Schematic of a graphene field-effect transistor fabricated on YIG, a magnetic insulator (40). (E) Schematic of a graphene field-effect transistor covered by a deposited thin film of EuS, a magnetic insulator (41). Nonmagnetic 2D materials can be made magnetic by physically contacting magnetic materials through proximity effect. (F) Calculated Mexican-hat band dispersion in electrically biased Bernal stacked bilayer graphene (34). The diverging electronic DOS at the edge of the Mexican hat potentially results in ferromagnetic Stoner instability.

  • Fig. 3 Representative 2D magnetic crystals.

    (A to C) Optical image, Kerr image, and dimensionality effect of few-layer Cr2Ge2Te6 exfoliated on SiO2/Si (6). Scale bars, 10 μm. (D and E) Atomic structure of CrI3 and thickness-dependent Kerr signal hysteresis loop of graphite-sandwiched 2D CrI3 (15). In (D), orange arrows represent the ferromagnetically coupled spin magnetic moments. In (E), red and blue vertical arrows represent spin-up and spin-down magnetic moments, respectively. (F and G) Atomic structure of Fe3GeTe2 and thickness-dependent normalized remanent anomalous Hall resistance of 2D Fe3GeTe2 on Al2O3 thin film, which was prepared by thermally evaporating Al in oxygen pressure of 10−4 mbar on Fe3GeTe2 bulk crystal, followed by multiple steps of transfer and exfoliation (62). (H and I) STM image of sub-monolayer VSe2 grown on HOPG by MBE and magnetic hysteresis of mostly monolayer VSe2 grown on MoS2 by MBE (75). Scale bar, 20 nm. (J and K) Atomic structure of vdW MnSe2 and out-of-plane magnetic hysteresis of MnSex, one monolayer on average, grown on GaSe by MBE (76). In (J), orange arrows represent the ferromagnetically coupled spin magnetic moments. In (K), red and blue curves represent the half of the hysteresis loop while magnetic field is swept from positive to negative and from negative to positive, respectively.

  • Fig. 4 Interfacial engineering of 2D magnets.

    Magnetic properties of 2D magnets can be affected by adjacent materials via different mechanisms. The central structure depicts an interface between a 2D magnet (green) and a dissimilar material (orange). (A) Charge transfer and interfacial dipole. The orange and red balls represent electrons and holes, respectively. (B) Interfacial hybridization. The lower green bar represents a 2D magnet; the upper bar is a dissimilar material. The dumbbells represent electronic orbitals of the two materials, overlapping at the interface to hybridize. (C) Strain effect. The lower solid bar represents a stretched 2D magnet in contact with a dissimilar material; the lower dashed bar represents the relaxed 2D magnet without the top contacted material. (D) Additional superexchange interactions. The orange circles with arrows represent the elements in adjacent materials that provide additional channels to mediate the superexchange interaction between magnetic ions in 2D magnets, which are represented by the red solid balls with arrows. (E) Structural perturbation. The wavy green belt represents 2D magnets that are structurally perturbed because of contact with the adjacent materials. (F) Band renormalization. The solid curves represent the electronic, magnonic, or phononic band dispersions of 2D magnets after band renormalization with contact of the adjacent materials; the dashed curves represent the same band dispersions before band renormalization without contacting the adjacent materials. (G) Dielectric screening. Red balls with arrows represent the exchange-coupled electrons in 2D magnets; orange curves depict the electric field lines connecting electrons. The environment with higher dielectric constant ε screens the coulombic interaction more. The nature of exchange interaction as a coulombic interaction makes 2D magnets susceptible to the dielectric screening. (H) Spin-orbit coupling (SOC) proximity. By contacting heavy-element materials, the SOC in 2D magnets will be effectively modified, leading to the change of magnetocrystalline anisotropy.

  • Fig. 5 Spintronic, magnonic, and spin-orbitronic devices based on 2D magnets or magnetic heterostructures.

    (A and B) MTJ based on Fe0.25TaS2-Ta2O5-Fe0.25TaS2 (124). Iron-intercalated TaS2 is ferromagnetic, and the surface native oxide was used as an insulating spacing layer. (A) Atomic structure of Fe-intercalated TaS2. (B) Cross-section transmission electron microscopy image of the MTJ sandwich structure. (C and D) MTJ based on graphite-CrI3-graphite (126). (C) Schematic of MTJ. (D) Magnetic field–dependent tunneling conductance. (E) Schematic of graphene-YIG heterostructure for spin-charge conversion based on spin pumping (130). (F) Schematic of the spin-orbit torque measurement system, for which the core material architecture is WTe2-permalloy heterostructure (135). Inset is an optical image of the tested device. (G and H) Schematics of a spin field-effect transistor based on a bilayer A-type antiferromagnet and its predicted electrical properties (140).

  • Fig. 6 Van der Waals magnets library.

    Color code: Green, bulk ferromagnetic vdW crystals; orange, bulk antiferromagnets; yellow, bulk multiferroics; gray, theoretically predicted vdW ferromagnets (left), half metals (center), and multiferroics (right), which have not yet been experimentally confirmed; purple, α-RuCl3 (a proximate Kitaev quantum spin liquid) (144). Notes (asterisks): VSe2 has been found only in 1T-VSe2 form in experiments to date (145), although the magnetic properties of 2H-VSe2 have been widely studied by density functional theory calculations. MnSex is ferromagnetic and of vdW structure in MBE-synthesized 2D form but is antiferromagnetic in bulk (which could be either rock-salt or hexagonal structure). CrI3, although ferromagnetic in bulk, was experimentally suggested to be an A-type antiferromagnet in the 2D regime. CuCrP2Se6 does not host the electric order while being cooled down to 10 K according to experimental data (147), but the calculated ground state of CuCrP2Se6 is multiferroic with antiferroelectricity (99). MnBi2Te4 and MnBi2Se4 may exhibit ferrimagnetic features as a result of uncompensated odd-layer A-type antiferromagnets or surfaces of antiferromagnetic topological insulators. MnCl2 has a magnetic structure that has not been completely determined, which could be either antiferromagnetic or helimagnetic (142). Bulk VCl3 and VBr3 have been inferred to be weak antiferromagnets on the basis of experimental data, although detailed magnetic structures have not been determined; however, monolayer VCl3 was calculated to be ferromagnetic (146).

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