Pushing an Altermagnet to the Ultimate 2D Limit: Symmetry Breaking in Monolayers of GdAlSi

Altermagnets have emerged as a class of materials combining ferromagnetic properties with vanishing net magnetization. This combination is highly promising for spintronics, especially if a material can be brought to the nanoscale. However, experimental studies of the 2D limit of the altermagnets are lacking. Here, we study epitaxial films on silicon of the Weyl altermagnet GdAlSi ranging from more than a hundred unit cells to a single unit cell. The films do not show any discernible net magnetic moments. Electron transport studies reveal a remarkable transformation. Thick films exhibit the chiral anomaly, whereas symmetry restrictions prevent observation of the anomalous Hall effect in our electron transport measurements. In ultrathin films, a spontaneous anomalous Hall effect manifests itself, indicating a nonrelativistic spin splitting. The transformation is associated with crystal symmetry breaking accompanying the 3D-to-2D crossover. The work highlights the role of dimensionality in altermagnetism and provides a platform for studies aiming at ultracompact spintronics.