Two-dimensional (2D) altermagnets have recently emerged as promising candidates for spintronic applications due to their intrinsic spin splitting in the absence of spin-orbit coupling (SOC). This often leads to the oversight of SOC effects in altermagnets containing heavy elements. In this work, we perform a comprehensive first-principles investigation of monolayer ${\mathrm{V}}_2{\mathrm{Te}}_2\mathrm{O}$, a 2D altermagnet containing heavy Te atoms, with a particular focus on its strain-tunable spin and valley properties in the presence of SOC. We reveal that its Néel-type antiferromagnetic ground state exhibits a unique spin-valley locking protected by the diagonal mirror symmetry, which can be tuned not only by applying uniaxial or biaxial strain, but also by the strain-induced change of easy magnetization axis with respect to the mirror plane. When SOC is included, the system exhibits rich phenomena such as a half-semiconducting state, large valley polarization, and 100% spin polarization. Accordingly, we propose a tunnel magnetoresistance (TMR)-like device constructed solely by monolayer ${\mathrm{V}}_2{\mathrm{Te}}_2\mathrm{O}$ and operated by strain, in which a high TMR ratio is expected due to the spin and valley selection mechanism. Furthermore, monolayer ${\mathrm{V}}_2{\mathrm{Te}}_2\mathrm{O}$ maintains a high Néel temperature exceeding 300 K both without strain and under several representative strains, confirming its potential applications for practical room-temperature spintronic and valleytronic devices.

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• Valley degrees of freedom
• 2-dimensional systems
• Altermagnets
• First-principles calculations