Giant Spin Splitting and Anisotropic Spin Polarization in 2D Altermagnet Cr₂O

In this work, we theoretically propose a novel 2D nonvan der Waals (non-vdW) monolayer Cr₂O, exfoliated from the bulk CrO along the [001] direction. Combining first-principles calculations, spin group symmetry analysis, and Monte Carlo simulations, we reveal that Cr₂O has exceptional altermagnetic metallic characteristics, featuring a record nonrelativistic spin splitting of 1.83 eV, a high Néel temperature of 507 K, and full spin polarization along specific Γ-X/Γ-Y paths. The spin splitting feature in Cr₂O exhibits an anisotropic d-wave-like pattern, arising from nontrivial spin-group symmetry operations $[C_2\parallel {\overline{C}}_{4z}^{\pm }]$ that govern direction-dependent spin polarization. Moreover, Boltzmann transport calculations show that the spin polarization ${SP}_{\hat{n}}$ reaches up to 95.2% along principal axes and follows a cosine-like angular dependence in the 2D plane. It remains above 80% across a wide energy range, ensuring robust spin filtering. These results establish Cr₂O as a promising platform for spin-symmetry-engineered transport in 2D non-vdW altermagnets.