Discoveries of intrinsic two-dimensional (2D) ferromagnetism in van der Waals (vdW) crystals provide an interesting arena for studying fundamental 2D magnetism and devices that employ localized spins. However, an exfoliable vdW material that exhibits intrinsic 2D itinerant magnetism remains elusive. Here we demonstrate that FeGeTe (FGT), an exfoliable vdW magnet, exhibits robust 2D ferromagnetism with strong perpendicular anisotropy when thinned down to a monolayer. Layer-number-dependent studies reveal a crossover from 3D to 2D Ising ferromagnetism for thicknesses less than 4 nm (five layers), accompanied by a fast drop of the Curie temperature (T) from 207 K to 130 K in the monolayer. For FGT flakes thicker than ~15 nm, a distinct magnetic behaviour emerges in an intermediate temperature range, which we show is due to the formation of labyrinthine domain patterns. Our work introduces an atomically thin ferromagnetic metal that could be useful for the study of controllable 2D itinerant ferromagnetism and for engineering spintronic vdW heterostructures.
The integration of diverse electronic phenomena, such as magnetism and nontrivial topology, into a single system is normally studied either by seeking materials that contain both ingredients, or by layered growth of contrasting materials 1-9 . The ability to simply stack very different twodimensional (2D) van der Waals materials in intimate contact permits a different approach 10,11 . Here we use this approach to couple the helical edges states in a 2D topological insulator, monolayer WTe2 12-16 , to a 2D layered antiferromagnet, CrI3 17 . We find that the edge conductance is sensitive to the magnetization state of the CrI3, and the coupling can be understood in terms of an exchange field from the nearest and next-nearest CrI3 layers that produces a gap in the helical edge. We also find that the nonlinear edge conductance depends on the magnetization of the nearest CrI3 layer relative to the current direction. At low temperatures this produces an extraordinarily large nonreciprocal current that is switched by changing the antiferromagnetic state of the CrI3. Main Text:The introduction of magnetic order into topological band structure gives rise to new phenomena such as the quantum anomalous Hall effect 1,8,9 and nonreciprocal magnetoelectric effects [5][6][7]18 . In the case of a two-dimensional topological insulator (2D TI), topology guarantees the existence of helical edge modes in which the spin is locked to momentum, causing current at the edge to be fully spin-polarized (the quantum spin Hall effect) 19 . Combining 2D TIs with magnets should therefore directly yield magnetoelectric coupling 20,21 . For example, a magnetic proximity effect may modify the spin polarization and hence the current, or the flow of current in the edge may produce a torque on the magnetization 22,23 . Since backscattering in the edge modes is suppressed by time reversal symmetry, the edge conduction should be affected by magnetic order, which will mix the two opposite-spin branches and so modify backscattering. The expected gapping of the helical edge modes by proximity with a ferromagnet is an important way to control them which, combined with induced superconductivity 24,25 , plays a role in schemes to produce Majorana modes 26 .Stacking van der Waals materials offers a simple, flexible and low-disorder approach to combining magnets with other materials 10,11 . In this work we measure transport through a 2D TI, monolayer (1L) WTe2 13-16 , stacked under the layered magnetic insulator CrI3 17,27-30 . We find that the magnetism of the CrI3 suppresses the edge conduction in the WTe2, in a manner consistent with
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