Scaling the Altermagnet GdAlSi beyond the Monolayer Limit

Altermagnets, an emerging class of magnetic materials, combine zero net magnetization with strong spin-based phenomena that makes them highly attractive for spintronics. Practical applications require altermagnets to be brought to the nanoscale; however, this area is scarcely explored experimentally. Recently, films of the Weyl altermagnet GdAlSi have been studied down to a single monolayer to reveal symmetry breaking and thickness-dependent evolution of its properties. Here, we show that the structure of GdAlSi allows for further miniaturization, making 1/2 monolayer a basic building block for construction of 2D spintronic materials. Epitaxy of 1/2 monolayer of GdAlSi results in a film with nearly compensated magnetic moments; it exhibits a semiconducting behavior, in contrast to the metallic monolayer and the bulk. The semiconductor demonstrates colossal negative magnetoresistance, exceeding that in a monolayer by more than an order of magnitude. The anomalous Hall effect in 1/2 monolayer of GdAlSi follows the universal scaling relation for magnetic semiconductors. The fabricated material is seamlessly integrated with Si, thus holding high promise for ultracompact spintronics.