Altermagnets with spin-layer coupling (SLC) enable symmetry-protected spin splitting without net magnetization. In the SLC altermagnet $\mathrm{Ca}{\left(\mathrm{CoN}\right)}_2$ monolayer, which exhibits distinct sublayer-dependent spin and valley characteristics, we demonstrate that breaking the sublayer-exchange symmetry S via ferromagnetic substrates $({\mathrm{CoCl}}_2 \text{and} \mathrm{CoSe})$ induces robust, magnetization-tunable spin-valley splitting, with the magnitude mainly depending on the strength of interfacial interactions. First-principles calculations reveal that strong interfacial charge redistribution and exchange coupling generate spin-valley splitting for both the valence band maximum (VBM) and conduction band minimum (CBM) of $\mathrm{Ca}{\left(\mathrm{CoN}\right)}_2$. In $\mathrm{Ca}{\left(\mathrm{CoN}\right)}_2/{\mathrm{CoCl}}_2$, antiferromagnetic interfacial exchange produces large spin-valley splitting for CBM but only minor splitting for VBM, with $\mathrm{\Delta }{E}_{\mathrm{V}}=$ −0.016 and 0.019 eV corresponding to upward and downward magnetic orientations of ${\mathrm{CoCl}}_2$, respectively. In contrast, CoSe exhibits ferromagnetic interfacial exchange and pronounced interfacial charge redistribution, forming prominent Co-Se bonding and antibonding states near the Fermi level under opposite substrate magnetic orientations, resulting in larger spin-valley splitting $(\mathrm{\Delta }{E}_{\mathrm{V}}=0.075 \text{and} -0.604 \mathrm{eV})$ consistent with sublayer-resolved band shifts. Device-level spin-transport simulations of the van der Waals (vdW) tunneling junctions further confirm a magnetization-controlled, spin-valley-dependent transport, with $k$-resolved maps revealing anisotropy in transmission coefficients. These results demonstrate that vdW heterostructure engineering provides a promising route for achieving large and magnetization-tunable spin-valley splitting in spintronic and valleytronic devices based on altermagnetic materials.

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• Magnetic interactions
• Magnetotransport
• Spintronics
• Valleytronics
• Altermagnets
• Heterostructures
• van der Waals systems
• Density functional calculations