Above-room-temperature ferromagnetism in large-area van der Waals heterostructures

Two-dimensional (2D) magnetic materials are promising building blocks for the realization of novel, ultra-compact devices for spintronics. Moreover, combining them with other 2D crystals such as graphene is a very attractive route to realize hybrid material systems exhibiting integrated magnetic and electronic functionalities. They are known as van der Waals (vdW) heterostructures, since the different 2D materials are bonded to each other via weak vdW forces, resulting in the formation of a so-called vdW gap at their interface. Interestingly, besides the individual properties of each 2D crystal within the vdW heterostructure, phenomena such as proximity-induced exchange and spin-orbit coupling taking place at the vdW heterointerfaces offer unique opportunities to tailor properties via a layering design.

Such vdW heterostructures are commonly fabricated via stacking of relatively small (micrometer sizes) flakes exfoliated from bulk crystals. Despite their utility for fundamental studies, exfoliation-based methods are not compatible established device fabrication schemes. Thus, in order to maximize their potential for integration in next generation technologies, the development of bottom-up scalable synthesis allowing to realize high-quality vdW heterostructures is highly demanded.

In this work, we could demonstrate large-area synthesis of Fe5-xGeTe2/graphene vdW heterostructures exhibiting high structural quality and sharp vdW interfaces. For this, we employed molecular beam epitaxy to realize thin films of Fe5-xGeTe2 (FGT, with x ~ 0) - a 2D ferromagnetic metal with highly tunable properties - directly on single-crystalline graphene (on SiC). Importantly, magnetic and transport studies reveal that ferromagnetic order with a perpendicular magnetic anisotropy (PMA) persists up to 390 K in our material. The Both PMA and magnetic transition temperatures above room temperature (i.e. above 300 K) are critical for future applications in spintronics, and have not been observed for similar heterostructures fabricated with flakes. Last but not least, the epitaxial graphene under FGT continues to exhibit quantum features in its electronic properties, attesting to the conservation of its high electronic quality. These results represent an important advance beyond non-scalable flake exfoliation and stacking methods, thus marking a crucial step towards the implementation of ferromagnetic FGT/graphene vdW heterostructures in applications like all-2D logic/multiplexer devices in which FGT acts as the spin injector/detector and graphene as the spin transport channel.

Authors: Hua Lv, A. da Silva, A. I. Figueroa, C. Guillemard, I. Fernández Aguirre, L. Camosi,L. Aballe, M. Valvidares, S. O. Valenzuela, J. Schubert, M. Schmidbauer, J. Herfort, M. Hanke, A. Trampert, R. Engel-Herbert, M. Ramsteiner, and J.M.J. Lopes
Title: Large-area synthesis of ferromagnetic Fe_5-xGeTe₂/graphene van der Waals heterostructures with Curie temperature above room temperature
Source: Small 19, 2302387 (2023)
DOI: 10.1002/smll.202302387