Orbital-selective magnetic transitions induced by Janus symmetry breaking in MA2Z4 monolayers.

Janus engineering has emerged as an effective strategy for tailoring the electronic and magnetic properties of two-dimensional materials through controlled symmetry breaking. Here, using first-principles calculations, crystal orbital Hamilton population (COHP) analyses, and Monte Carlo simulations, we systematically investigate the symmetry-driven magnetic evolution in MA2Z4 monolayers by comparing pristine α-phase TiSi2N4 and VGe2N4 with their Janus counterparts. Pristine TiSi2N4 is a nonmagnetic semiconductor with fully spin-compensated N-2p states, whereas symmetry breaking in Janus-TiSiN3 induces robust ferromagnetism with a predicted Curie temperature of 126 K. Detailed orbital analyses reveal that this emergent magnetism originates from spin-polarized nonbonding N-pz states generated by dangling bonds induced by symmetry breaking, which drive Stoner instability under an asymmetric crystal field. In contrast, pristine VGe2N4 is an intrinsic ferromagnetic semiconductor with a high Curie temperature of 395 K, arising from localized V-3d states; however, the Janus transformation completely quenches this magnetism. Orbital-resolved COHP analyses demonstrate that enhanced anisotropic p-d hybridization significantly delocalizes the V-3d electrons, broadens the d bands, and collapses the exchange splitting, leading to magnetic quenching. These findings propose a microscopic framework wherein Janus symmetry breaking can activate p-orbital magnetism or suppress d-orbital magnetism based on the specific orbital character. This provides a promising theoretical strategy for the design of 2D spintronic devices.