The all-electrical realization of highly spin-polarized currents and their efficient conversion into pure spin currents remains a fundamental challenge in spintronics. Here, we report a spin-axis dynamic locking (SADL) effect in altermagnets that pins the high and dynamically robust spin polarization to the crystalline axes: an in-plane electric field along one principal axis drives a highly spin-up-polarized current, whereas along the orthogonal axis, it generates a symmetry-enforced, equal-magnitude spin-down current. Consequently, applying an electric field diagonally yields a transverse pure spin current, reaching 100% charge-to-spin conversion in the ideal limit. Mechanistically, SADL originates from a spin-split tent-state band structure whose Lifshitz transitions delimit an open-Fermi-line regime. The momentum-separated Fermi lines carry orthogonal nonzero winding vectors, producing a pronounced velocity contrast while suppressing ordinary backscattering to dynamically stabilize the axial spin selectivity. High-throughput first-principles screening confirms SADL in broad materials. Notably, monolayer ${\mathrm{Cr}}_2{\mathrm{WSe}}_4$ and synthesized bulk ${(\text{BaF})}_2{\mathrm{Mn}}_2{\mathrm{Se}}_2\mathrm{O}$ exhibit efficiencies close to the ideal limit, paving the way for ultra-low-power, reconfigurable spintronic devices where the spin states are governed solely by electric field orientation.

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• Electrical conductivity
• Fermi surface
• Flat bands
• Spintronics
• Magnetic systems
• Boltzmann theory
• First-principles calculations
• High-throughput calculations
• Tight-binding model