Van der Waals magnets have long been constrained by two major limits: they usually work only at low temperatures, and their best magnetic properties are typically found in multilayer structures rather than in true monolayers. Researchers have now pushed past both barriers with Co-doped Fe₅GeTe₂ (FCGT), first showing room-temperature ferromagnetism in thicker films and then extending ferromagnetic order to about 200 K in epitaxial monolayers — a major advance for 2D spintronics.

The work addresses a central challenge in the field. Two-dimensional vdW magnets are attractive because their atomic-scale thickness could enable ultracompact spintronic devices with sharp interfaces and highly tunable magnetic behavior. But most studies have relied on micrometric flakes exfoliated from bulk crystals, which offer limited control over thickness and lateral size. Developing methods to grow atomically thin layers over large areas has therefore become essential, especially since long-range magnetic order in true monolayers has remained difficult to achieve and Curie temperatures in the thinnest vdW magnets are generally far below room temperature.

The team began by synthesizing Co-doped Fe₅GeTe₂ multilayer films by molecular beam epitaxy on centimeter-scale areas. These films showed ferromagnetism well above room temperature, reaching roughly 370 K. They then moved to the strictly 2D limit, growing FCGT monolayers on Ge(111) substrates. At the optimal Co concentration, the monolayers remained ferromagnetic up to about 200 K, setting a record for 2D vdW magnets.

To understand why the Curie temperature rises, the researchers used synchrotron-based X-ray magnetic circular dichroism (XMCD) to separately probe the magnetic moments on Fe and Co atoms. The result was unexpected: the Co dopants that drive the TC increase are only weakly magnetic themselves. Density functional theory confirmed this picture and showed that Co nevertheless strengthens the magnetic moments of neighboring Fe atoms and locally enhances exchange interactions, matching the XMCD data.

A key advantage is that Co substitutes for Fe atoms rather than occupying the van der Waals gaps. As a result, Co doping can raise the Curie temperature regardless of thickness, including in the thinnest 2D layers. Beyond clarifying the physics of these materials, the study opens a path toward integrating 2D magnets into complex spintronic architectures through epitaxial growth.

The work involved the Topological spintronics and Theory and simulation teams, with collaboration from ESRF, SOLEIL and IRIG-MEM. Funding came from ANR projects ELMAX and NEXT, FLAG-ERA MNEMOSYN, and PEPR SPIN (SPINMAT). The findings were published as “High Temperature Ferromagnetism in Epitaxial Monolayers of Co-doped Fe5GeTe2” by J. Courtin, F. Ibrahim et al. in Physical Review Letters 137, 046703 (2026), with open access available at hal-05682327. Contact: Frédéric Bonell and Mairbek Chshiev.

Van der Waals magnets are typically limited to low-temperature and multilayer form. Here, we first demonstrated room-temperature ferromagnetism in thicker Co-doped Fe₅GeTe₂ films, then achieved ferromagnetism up to ∼200 K in epitaxial monolayers, a breakthrough for two-dimensional spintronics.

(a) Atomically resolved image of an FCGT monolayer by scanning transmission electron microscopy. (b) Variation of the Curie temperature as a function of the Co content. A maximum is observed when one in five Fe atoms is replaced by Co.

Two-dimensional (2D) van der Waals (vdW) magnets are highly sought after for their potential in spintronics. Their 2D nature could enable building ultracompact devices with sharp interfaces at the atomic level, leading to enhanced and tunable spintronic functionalities. However, these materials are mostly studied in the form of micrometric flakes exfoliated from bulk crystals, with little control on their thickness and lateral dimensions. Therefore, developing methods able to fabricate atomically thin layers on large areas has become critical. Moreover, achieving long-range magnetic order in true monolayers has remained a significant challenge, as vdW magnets generally exhibit Curie temperatures (TC) well below room temperature in their thinnest form.

Our work began by successfully synthesizing Co-doped Fe₅GeTe₂ (FCGT) multilayer films on centimeter-scale areas using molecular beam epitaxy. These films exhibited ferromagnetism well above room temperature, up to ∼370 K. Building on this, we then focused on the strictly 2D regime, growing monolayers of FCGT on Ge(111) substrates. For an optimum concentration in Co, these monolayers demonstrated ferromagnetism up to ∼200 K, a record for 2D vdW magnets.

In order to understand the origin of the TC enhancement, we performed synchrotron experiments that allowed us to resolve the magnetic moments on Fe and Co atoms separately, with a technique called X-ray magnetic circular dichroism (XMCD). Surprisingly, these measurements revealed that the Co dopants responsible for the increase in TC were only weakly magnetic. Our density functional theory calculations confirmed this observation. They also showed that, despite their weak magnetic moment, the Co dopants strengthen those of neighboring Fe atoms and locally boost the magnetic exchange interaction, in good agreement with XMCD observations.

The good news is that Co dopants substitute Fe atoms instead of intercalating in the vdW gaps. This means that Co doping is able to enhance the TC whatever the thickness of the material, even in ultimately thin 2D layers. This work not only improved our understanding of these novel materials but also opened new avenues for integrating 2D magnets into complex spintronic devices by epitaxial growth.

Teams: Topological spintronics, Theory and simulation

Collaboration: ESRF, SOLEIL, IRIG-MEM

Funding: ANR (ELMAX, NEXT), FLAG-ERA MNEMOSYN, PEPR SPIN (SPINMAT)

Further reading: High Temperature Ferromagnetism in Epitaxial Monolayers of Co-doped Fe5GeTe2, J. Courtin, F. Ibrahim et al., Phys. Rev. Lett. 137, 046703 (2026).

Open access: hal-05682327

Contact: Frédéric Bonell and Mairbek Chshiev

The post High-temperature ferromagnetism in 2D appeared first on Spintec.