The two-dimensional nature of van der Waals (vdW) magnetic materials could enable the creation of ultracompact devices with sharp interfaces at the atomic scale, paving the way for enhanced and tunable spintronic functionalities. To date, these materials have been studied primarily as micrometer-scale fragments derived from bulk crystals, with limited control over their thickness and lateral dimensions. The development of methods to fabricate atomically thin layers over large areas has therefore become essential. Furthermore, vdW magnetic materials lose their spontaneous magnetization and thus, their ferromagnetic properties, at temperatures well below room temperature when in their thinnest form.
Researchers at CEA-Irig/SPINTEC have succeeded in synthesizing, via
molecular beam epitaxy*, thin films of Fe₅GeTe₂ doped with cobalt (Co), on centimeter-square areas. These films exhibit stable ferromagnetism well above room temperature (293–298 K), up to approximately 370 K. Using a germanium (Ge) substrate, the researchers then gradually reduced the thickness of the material until they obtained a single atomic layer. At an optimal Co concentration, this monolayer remains ferromagnetic up to approximately 200 K, a remarkable temperature and, to date, unprecedented for a van der Waals (vdW)-type 2D material. By combining synchrotron experiments —a technique called
X-ray circular magnetic dichroism (XMCD)*—with
ab initio calculations, the researchers were able to distinguish the magnetic contributions of iron (Fe) and cobalt (Co). They thus demonstrated that, although Co atoms are weakly magnetic, their presence enhances the magnetism of neighboring Fe atoms thanks to locally boosting the magnetic exchange interaction. The simulations also confirmed that Co atoms directly substitute for certain Fe atoms in the crystal structure. This mechanism explains why Co doping enhances the ferromagnetism of Fe₅GeTe₂, regardless of the material's thickness, down to the limit of a single atomic layer.
© CEA-Irig/SPINTEC/CONCEPTS/ F.Bonell
Figure : Left: A cobalt-doped Fe5GeTe2 monolayer, resolved at the atomic scale (transmission electron microscopy). Right: Magnetic transition temperature as a function of doping level. A maximum is observed when one out of every five iron atoms is replaced by a cobalt atom.
These results provide a better understanding of the mechanisms underlying magnetism in 2D van der Waals materials. They also open up new possibilities for their integration into next-generation spintronic devices, particularly thanks to the ability to fabricate ultrathin layers via epitaxial growth.
UMR : SPINTEC (CEA, CNRS, UGA, Grenoble INP).
Fundings : ANR (ELMAX, NEXT), FLAG-ERA MNEMOSYN, PEPR SPIN (SPINMAT).
Collaborations : ESRF, SOLEIL, Irig/MEM.