Ultrafast spin dynamics in magnetic/nonmagnetic two-dimensional interfaces: Insights from Cr X 3 / W Z 2 heterostructures ( X = Br , I ; Z = S , Se )

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 $(\mathrm{CrB}{\mathrm{r}}₃/\mathrm{WS}{\mathrm{e}}₂>\mathrm{CrB}{\mathrm{r}}₃/\mathrm{W}{\mathrm{S}}₂>\mathrm{Cr}{\mathrm{I}}₃\mathrm{WS}{\mathrm{e}}₂>\mathrm{Cr}{\mathrm{I}}₃/\mathrm{W}{\mathrm{S}}₂)$ 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.