Alloying-enhanced ferromagnetic coupling in two-dimensional transition metal carbide <i>M</i>C (<i>M</i> = Cr, Mn) monolayers

Two-dimensional (2D) intrinsic ferromagnetic materials with high ferromagnetic transition temperature (Tc) are critical to the advancement of spintronic device technology. In this study, we present two stable room-temperature ferromagnetic carbides, namely, MC (M = Cr, Mn) monolayers, with Tc values of 384 and 391 K, respectively, along with strong perpendicular magnetic anisotropy (PMA). Furthermore, CrxMn8−xC8 (x=1–7) alloy monolayers are constructed. We demonstrate that the appropriate incorporation of Cr–Mn atomic pairs substantially enhances the ferromagnetic coupling, which is intimately associated with the interactions between metal atoms and their spatial arrangements within the lattice. In particular, the CrMnC2 alloy monolayer exhibits a Tc of 627 K, markedly exceeding that of the parent monolayer. This elevated Tc is attributed to the combined effect of the crystal-field energy difference introduced by the alloying process and the internal stresses arising from the lattice distortion induced by alloying, which jointly enhance ferromagnetic interactions. In addition, CrMnC2 exhibits robust PMA, in-plane magnetic anisotropy induced by structural asymmetry, and slight ferroelastic behavior. These findings offer insights on the impact of alloy engineering on 2D ferromagnetism and highlight a pathway toward high-Tc 2D ferromagnetic materials for practical room-temperature device applications.