  Spintronics, a technology harnessing electron spin for information
transmission, offers a promising avenue to surpass the limitations of
conventional electronic devices. While the spin directly interacts with the
magnetic field, its control through the electric field is generally more
practical, and has become a focal point in the field of spintronics. Current
methodologies for generating spin polarization via an electric field generally
necessitate spin-orbit coupling. Here, we propose an innovative mechanism that
accomplishes this task without dependence on spin-orbit coupling. Our method
employs two-dimensional altermagnets with valley-mediated spin-layer coupling
(SLC), in which electronic states display symmetry-protected and
valley-contrasted spin and layer polarization. The SLC facilitates predictable,
continuous, and reversible control of spin polarization using a gate electric
field. Through symmetry analysis and ab initio calculations, we pinpoint
high-quality material candidates that exhibit SLC. We ascertain that applying a
gate field of $0.2$ eV/\AA~ to monolayer Ca(CoN)$_2$ can induce significant
spin splitting up to 123 meV. As a result, perfect and switchable
spin/valley-currents, and substantial tunneling magnetoresistance can be
achieved in these materials using only a gate field. These findings provide new
opportunities for generating predictable spin polarization and designing novel
spintronic devices based on coupled spin, valley and layer physics.
