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

The unique spin-momentum locking of the surface Dirac electrons in TI has been shown to be responsible for highly efficient spin-charge-conversion in ferromagnet (FM)/TI heterostructures. Here, we report the strongly modulated spin dynamics in yttrium iron garnet (YIG) due to the spin-momentum locked Dirac surface states in TI via ferromagnetic resonance. When the Bi-concentration x is systematically tuned in the 5 nm thick (Bi_xSb_1-x)₂Te₃ TI layer, the relative weight of the Dirac surface states varies. The Gilbert damping constant of the processing magnetization in YIG increases by one order of magnitude when the chemical potential of TI is close to the Dirac point. Remarkably, the magnetic anisotropy of YIG is also dramatically altered to favor in-plane orientation. The modification of the spin dynamics in YIG by the Dirac surface states at room temperature is much stronger than that in YIG-heavy metal heterostructures, which provides a superb knob for controlling ultrafast magnetization responses in potential nanoscale spintronic devices.