Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals

Abstract

The realization of long-range ferromagnetic order in two-dimensional van der Waals crystals, combined with their rich electronic and optical properties, could lead to new magnetic, magnetoelectric and magneto-optic applications. In two-dimensional systems, the long-range magnetic order is strongly suppressed by thermal fluctuations, according to the Mermin-Wagner theorem; however, these thermal fluctuations can be counteracted by magnetic anisotropy. Previous efforts, based on defect and composition engineering, or the proximity effect, introduced magnetic responses only locally or extrinsically. Here we report intrinsic long-range ferromagnetic order in pristine CrGeTe atomic layers, as revealed by scanning magneto-optic Kerr microscopy. In this magnetically soft, two-dimensional van der Waals ferromagnet, we achieve unprecedented control of the transition temperature (between ferromagnetic and paramagnetic states) using very small fields (smaller than 0.3 tesla). This result is in contrast to the insensitivity of the transition temperature to magnetic fields in the three-dimensional regime. We found that the small applied field leads to an effective anisotropy that is much greater than the near-zero magnetocrystalline anisotropy, opening up a large spin-wave excitation gap. We explain the observed phenomenon using renormalized spin-wave theory and conclude that the unusual field dependence of the transition temperature is a hallmark of soft, two-dimensional ferromagnetic van der Waals crystals. CrGeTe is a nearly ideal two-dimensional Heisenberg ferromagnet and so will be useful for studying fundamental spin behaviours, opening the door to exploring new applications such as ultra-compact spintronics.

The 2017 Magnetism Roadmap

“…Proximity-induced ferromagnetism or a large SHE could help overcome this hurdle, and would also enable valley manipulation by means of electrical spin injection. Magnetism in 2D van der Waals crystals [28,29] may allow the electric and magnetic field control of the magnetic anisotropy and novel magneto-optic devices.…”

Electrically Controllable Magnetism in Twisted Bilayer Graphene

“…Some of them have been shown to exhibit considerable tunability through doping, gating, stacking, and strain. Unfortunately, very few 2D crystals have been found to exhibit intrinsic magnetism [21,22], let alone magnetic frustration and potential spin-liquid phases.In this work we predict that twisted graphene bilayers could be a notable exception, realizing a peculiar magnetism on an effective triangular superlattice, and with exchange interactions that may be tuned by an external electric bias. We show that, at a mean-field level, spontaneous magnetization of two different types may develop for small enough twist angles θ ≲ 2°as a consequence of the moiré pattern in the system.…”

“…Some of them have been shown to exhibit considerable tunability through doping, gating, stacking, and strain. Unfortunately, very few 2D crystals have been found to exhibit intrinsic magnetism [21,22], let alone magnetic frustration and potential spin-liquid phases.…”

Room Temperature Uniaxial Magnetic Anisotropy Induced By Fe‐Islands in the InSe Semiconductor Van Der Waals Crystal

“…The extended family of vdW crystals includes graphene, hexagonal boron nitride, transition metal dichalcogenides, and many others. Although a variety of semiconductor crystals and stacks has been demonstrated, the available structures are nonmagnetic, weakly magnetic, or magnetic only at low temperature 13. Here, we show that InSe, which is nonmagnetic in its pristine form, becomes magnetic following the incorporation of Fe‐atoms during the growth of InSe by the Bridgman method.…”