Electric Hidden Unconventional Collinear Magnetism in 2D $\mathcal{P}\mathcal{T}$-Symmetric Antiferromagnets

Electric hidden unconventional collinear magnetism, in which a perpendicular electric field (${\mathcal{E}}_z$) lifts the $\mathcal{P}\mathcal{T}$-enforced degeneracy and selectively releases the hidden crystallographic constraints in conventional 2D antiferromagnets to drive emergent magnetic phases, has remained largely unexplored. Here we establish a complete symmetry classification of all 88 collinear spin layer groups (cSLGs) compatible with conventional $\mathcal{P}\mathcal{T}$-symmetric antiferromagnets, and show that the ${\mathcal{E}}_z$ drives the system into three distinct classes of unconventional collinear magnetism: type-IV magnetism (Class-I, 23 cSLGs), altermagnetism (Class-II, 34 cSLGs), and fully compensated ferrimagnetism (Class-III, 31 cSLGs). We illustrate these categories using the most common and feasibly exfoliable 2D candidate materials. These include Sr${}_2$CaCr${}_2$P${}_2$O${}_9$ and Sr${}_2$CaCr${}_2$As${}_2$O${}_9$ (Class-I), BaMnGe and MnPS${}_3$ (Class-II), and Sr${}_3$Fe${}_2$O${}_5$Cl${}_2$ and bilayer MnBi${}_2$Te${}_4$ (Class-III). We further demonstrate that the ${\mathcal{E}}_z$ generates distinctly different spin-splitting fingerprints across the three classes, yielding distinct spin-dependent responses: the electric Hall effect in Class-I, spin-polarized transport in Class-II, and the electric reversible magneto-optical Kerr effect in Class-III. Our work not only closes a complete internal cycle from conventional to unconventional collinear magnetism in 2D antiferromagnets, but also establishes a viable all-electrical compensated magnetism platform and control scheme for future spintronic applications.