Ultrafast spin dynamics in magnetic/nonmagnetic two-dimensional interfaces: Insights from <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>Cr</mml:mi> <mml:msub> <mml:mi>X</mml:mi> <mml:mn>3</mml:mn> </mml:msub> <mml:mo>/<

Ultrafast spin injection across magnetic/nonmagnetic (FM/NM) interfaces represents a pivotal mechanism for spintronic applications, yet the governing principles of interlayer-mediated spin dynamics remain elusive. Herein, we systematically investigate femtosecond-scale spin transfer in $\mathrm{Cr}{X}_{3}/\mathrm{W}{Z}_{2} (X=\mathrm{Br},\phantom{\rule{0.28em}{0ex}}\mathrm{I};\phantom{\rule{0.28em}{0ex}}Z=\mathrm{S},\phantom{\rule{0.28em}{0ex}}\mathrm{Se})$ van der Waals heterostructures through combined first-principles calculations and nonadiabatic molecular dynamics simulations. Our results demonstrate that asymmetric interlayer charge transfer, accompanied by spin-flip processes, mediates ultrafast (9--27-fs) spin injection from FM $\mathrm{Cr}{X}_{3}$ to NM $\mathrm{W}{Z}_{2}$ layers. Crucially, the magnetic moment persistence in NM layers exhibits material-dependent lifetimes $(\mathrm{CrB}{\mathrm{r}}_{3}/\mathrm{WS}{\mathrm{e}}_{2}&gt;\mathrm{CrB}{\mathrm{r}}_{3}/\mathrm{W}{\mathrm{S}}_{2}&gt;\mathrm{Cr}{\mathrm{I}}_{3}\mathrm{WS}{\mathrm{e}}_{2}&gt;\mathrm{Cr}{\mathrm{I}}_{3}/\mathrm{W}{\mathrm{S}}_{2})$ on picosecond timescales. This behavior is governed by three key factors: (i) bandgap-dependent nonadiabatic coupling, where ${\mathrm{CrBr}}_{3}$ systems exhibit prolonged retention due to their larger energy gaps; (ii) spin-orbit coupling and electron-phonon coupling synergistically modulate spin-flip and transfer dynamics; and (iii) built-in electric fields that selectively accelerate or decelerate the interlayer charge transfer. These insights offer a comprehensive framework for understanding interfacial spin dynamics and provide design principles for tailoring spin lifetimes in two-dimensional heterostructure-based spintronic devices.