Engineering magnetic order, spin polarization, and anisotropy in CrMnXT_z Janus MXenes

Janus MXenes have gained increasing visibility because they combine the structural and charge-density asymmetry of Janus with the broad versatility of the MXene family, allowing for great tunability of properties in a single monolayer. In this work, we report a first-principles investigation of CrMnX and CrMnXT_z Janus MXenes with different functional groups, including nitrides and carbides. The relative stability of the 1T and 2H phases shows that the first structure is energetically preferred for both CrMnC and CrMnN, by 0.45 eV and 0.32 eV per formula unit, respectively. These structures are dynamically and thermally stable up to room temperature. The magnetic ground state of the pristine 1T-CrMnC and 1T-CrMnN monolayers is ferrimagnetic, which arises from the interaction between Cr and Mn atoms at different sublattices. Functionalized CrMnC and CrMnN systems become semiconducting ferromagnets, while S- and Se-based functionalization preserves the metallicity and stabilizes antiferromagnetic or ferrimagnetic phases. CrMnCCl₂ and CrMnCF₂ reach Curie temperatures of 1348 K and 1107 K, respectively. Also, the CrMnCO₂ and CrMnNO₂ MXenes favor perpendicular magnetic anisotropy, making those materials particularly interesting for constructing perpendicular magnetic tunnel junctions. Our results demonstrate that surface functionalization provides efficient control over magnetism, Curie temperature, and magnetic anisotropy in CrMn-based Janus MXenes.