Magnetization dynamics and spin-transport in a topological insulator/ferromagnetic metal bilayer system.*

We have studied magnetization dynamics and spin-transport phenomenon in the topological insulator/ferromagnetic metal heterostructures here. The precessing magnetization of the metallic ferromagnet (N i80F e20) pumps spin current into the topological insulator (BiSbT e1.5Se1.5) layer due to a phenomenon called the spin-pumping effect. As a result of spin pumping, fast relaxation in the ferromagnet results in the broadening of ferromagnetic resonance linewidth (?H) and enhancement of the damping coefficient of a ferromagnet occurs. The calculated values of spin-transport parameters like spin-mixing conductance (g ↑↓ eff ), and spin current density (j 0 S) in our experiment confirm successful spin injection into the BiSbT e1.5Se1.5 layer. Low-temperature measurements have also been accomplished to understand the effect of the surface and/or bulk state of the topological insulator (TI) in the effective spin-electricity conversion. At low temperatures where the surface state of BiSbTe1.5Se1.5 dominates, the variation of ?H and damping coefficient due to spin pumping(αSP) are more prominent. It indicates a stronger surface state attribution to the dynamics of the ferromagnetic magnetization than the bulk state of the TI.