Dirac-Surface-State Modulated Spin Dynamics in a Ferrimagnetic Insulator at Room Temperature*

In heterostruvtures of magnetic insulator/topological insulator or MI/TI, a large enhanced spin Seebeck effect was attributed to the spin-momentum locking of the TI surface. In this work, we demonstrate dramatically modified spin dynamics of MI by the spin-momentum locked Dirac surface states of the adjacent TI. As the Bi-concentration x is systematically tuned in 5 nm thick (Bi_xSb_1-x)₂Te₃ TI film, the weight of the surface relative to bulk states peaks at x = 0.32 when the chemical potential approaches the Dirac point. At this concentration, the Gilbert damping constant of the precessing magnetization in 10 nm thick Y₃Fe₅O₁₂ MI film in the MI/TI heterostructures is enhanced by an order of magnitude, the largest among all concentrations. In addition, the MI acquires additional strong magnetic anisotropy that favors the in-plane orientation with similar Bi-concentration dependence. These extraordinary effects of the Dirac surface states along with the enhanced spin Seebeck effect distinguish TI from other materials such as heavy metals in modulating spin dynamics of the neighboring magnetic layer, which can be harnessed for spintronic applications.