Uniaxial strain-induced multipiezo effect in two-dimensional multifunctional altermagnet Cr2X2O and Janus Cr2XYO

Altermagnetism has attracted considerable attention in recent years. As an emerging magnetic phase, it features collinear antiferromagnetic order, yielding a zero net magnetic moment and nonrelativistic spin splitting. Using density functional theory, we reported five stable semiconducting altermagnetic monolayers: Cr2X2O and Janus Cr2XYO (X, Y = S, Se, Te, X ≠ Y). These monolayers exhibit C-paired spin–valley locking, strong out-of-plane magnetic anisotropy, a strain-tunable piezovalley response, and strain-induced magnetization assisted by carrier doping. Distinct from previously reported two-dimensional altermagnets, these five materials exhibit pronounced valley polarization under uniaxial strain, which primarily originates from the p orbitals of the chalcogen atoms. The out-of-plane piezoelectricity of Janus Cr2XYO monolayers arises from the electronegativity difference between the X and Y atoms. Among them, Cr2Te2O exhibits the largest valley polarization of 222 meV under −4% uniaxial strain, while Cr2STeO exhibits a sizable out-of-plane piezoelectric response with a d31 coefficient of 0.63 pm/V. Our results reveal a multipiezo effect in Cr2X2O and Janus Cr2XYO monolayers that integrates spin, valley, and mechanical degrees of freedom, thereby establishing a multifunctional material system for valleytronics, spintronics, and flexible piezoelectric devices.