Chemical design of monolayer altermagnets

The crystal-symmetry-paired spin-momentum locking (CSML) arisen from the intrinsic crystal symmetry connecting different magnetic sublattices in altermagnets enables many exotic spintronics properties such as unconventional piezomagnetism and noncollinear spin current. However, the shortage of monolayer altermagnets restricts further exploration of dimensionally confined phenomena and applications of nanostructured devices. Here, we propose general chemical design principles inspired by sublattice symmetry of layered altermagnet V$_2$(Se,Te)$_2$O through symmetry-preserving structural modification and valence-adaptive chemical substitutions. In total, we construct 2600 candidates across four structural frameworks, M$_2$A$_2$B$_{1,0}$ and their Janus derivatives. High-throughput calculations identify 670 potential altermagnets with N\'eel-ordered ground states, among which 91 ones exhibiting CSML Dirac cones that enable spin-polarized ultra-fast transport. These materials also feature different ground-state magnetic orderings and demonstrate diverse electronic behaviors, ranging from semiconductors, metals, half-metals, to Dirac semimetals. This work not only reveals abundant monolayer altermagnets, but also establishes a rational principle for their design, opening gates for exploration of confined magnetism and spintronics in atomically thin systems.