The realization of innovative and more compact electronic devices requires functional ultra-thin layers, ranging from metals to semiconductors and superconductors. Among them, van der Waals crystals, of which graphene was the precursor, present multiple attractive functionalities. However, challenges lie in our ability to synthesize them on large areas and to stabilize their magnetic properties up to room temperature.

For a long time, research on van der Waals crystals did not consider magnetism, perhaps because theory has long demonstrated that magnetism cannot exist in an isotropic two-dimensional material. However, in 2017, long-range magnetism was discovered in a material composed of a monolayer of atoms, reviving interest in this type of crystals. These can be studied in the form of sheets obtained by exfoliation of bulk crystals. However, this method of fabrication produces only micrometric flakes with random shape. Moreover, their exploitation is limited by the fact that the magnetic order remains only at very low temperatures, below 100 K. The integration of these new materials for advanced devices exploiting spin electronics requires on the one hand to maintain a magnetic order at higher temperature, called Curie temperature, and on the other hand to develop large scale synthesis methods.

In response to this challenge, IRIG researchers have been developing van der Waals thin film growth using molecular beam epitaxy (MBE) for several years. Recently, they have succeeded in growing magnetic Fe₅GeTe₂ thin films on a sapphire substrate. Thanks to various structural and chemical characterization techniques, the perfect crystallinity of these layers (Image) as well as their chemical composition and their uniformity at the square-centimeter scale have been observed, which represents a considerable progress compared to the exfoliated micrometric flakes.


Image: Fe₅GeTe₂ compound made of 4 van der Waals layers, realized by MBE (electron microscopy view). The three atomic interface gaps (van der Waals gap) are remarkably perfect.

In terms of magnetic functionality, the result obtained with Fe₅GeTe₂ is remarkable as, in the form of an ultra-thin layer of only two sheets, the Curie temperature is close to room temperature (229 K). The researchers underline the surprising presence of a magnetic order despite a negligible magnetic anisotropy, contrary to theoretical predictions. Further analysis suggests that the magnetism of this material, in particular versus temperature, deviates from the behavior expected in theory for two-dimensional layers.

This new magnetic material thus reveals an unexpected physical behavior, different from other van der Waals magnets. Furthermore, its growth by MBE paves the way to the fabrication of more complex van der Waals multilayers, as required for electronics and spintronics.