Spin-Selective Charge Flow Drives Ultrafast Magnetic Transitions in Type-II Bipolar Magnetic Heterostructures

In many two-dimensional van der Waals heterostructures, electronic states remain localized within individual sublayers, enabling layer-specific manipulation of charge and spin. This also holds in magnetic heterostructures such as CrSBr/CrSCl, where a type-II bipolar band alignment spatially separates spin-polarized valence and conduction states across layers. Using density-functional theory (DFT) and real-time time-dependent DFT simulations, we show that laser excitation above 4.2 eV drives an antiferromagnetic-to-ferrimagnetic transition within 40-56 fs. The transition is triggered by interlayer spin-flip charge flow: spin-up electrons in CrSBr flip into spin-down conduction states in CrSCl, selectively stabilizing residual spin polarization and inducing net magnetization. The process is highly anisotropic: b-polarized excitation promotes charge redistribution on surface Cl atoms, while a-polarized excitation confines charges near interfacial Cr atoms. Our findings establish a spin-selective interlayer charge flow, enabled by band alignment and symmetry breaking, as a general mechanism for ultrafast optical control of magnetic order in low-dimensional materials.