Interface‐Mediated Ultrafast Spin Transfer and Terahertz Emission in Bi 2 Se 3 /Co Heterostructures

ABSTRACT Spintronic terahertz (THz) emitters are key building blocks for next‐generation THz technologies, and topological‐insulator/ferromagnet (TI/FM) heterostructures are among the most promising platforms owing to the highly efficient spin‐to‐charge conversion promised by their spin‐momentum‐locked surface states. A central unresolved question is whether a single characteristic TI thickness can simultaneously optimize the three processes governing emitter performance: femtosecond demagnetization, picosecond damping, and ultrafast spin‐to‐charge conversion. Here, this question is addressed through a multi‐observable study of Bi 2 Se 3 /Co heterostructures, correlating time‐resolved magneto‐optical Kerr effect measurements with THz emission spectroscopy across a Bi 2 Se 3 thickness series. All dynamical and interfacial parameters, demagnetization and recovery times, Gilbert damping, spin‐orbit‐coupling indicator, spin‐lattice coupling, spin‐mixing conductance, and THz amplitude, evolve nonmonotonically and share a common extremum at 10 nm, where demagnetization accelerates from 297 to 224 fs and THz emission peaks over a ∼1.7 THz bandwidth. Structural and static magnetic characterization varies smoothly across this thickness, excluding an abrupt structural origin; the optimum instead reflects a crossover from hybridized surface states in ultrathin films to bulk‐dominated transport in thicker films. These results establish a thickness design rule and a multi‐observable screening methodology for TI/FM heterostructures toward practical high‐efficiency spintronic THz sources.