Spin Response Properties in Electronically Robust Ferromagnetic Strained CrSiSe$_3$ Monolayer under External Electric Fields
Integrating two-dimensional van der Waals magnets into field-effect spintronic devices requires robust charge stability and tunable spin responses. In this study, we investigate the electronic, topological, magnonic, and magneto-optical properties of the strain-engineered ferromagnetic CrSiSe$_3$ monolayer under out-of-plane external electric fields by using first-principles calculations. We find that for this material, the intrinsic charge sector, including the indirect band gap, charge Berry curvature, optical conductivities, and magneto-optical Kerr effect spectra, exhibits exceptional robustness against applied fields up to 0.3 V/$\r{A}$. Conversely, the spin degrees of freedom demonstrate highly sensitive tunability. Electrostatic gating significantly modulates the spin Berry-like curvature, driving a non-monotonic enhancement in the spin Hall conductivity. Furthermore, external fields effectively tune collective magnon excitations by modifying microscopic Heisenberg exchange interactions. Such coexistence of robust charge immunity and flexible spin manipulation establishes the strained CrSiSe$_3$ monolayer as a promising platform for stable spintronic devices.