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Area of Science:

  • Spintronics
  • Condensed Matter Physics
  • Materials Science

Background:

  • Spin pumping in ferromagnet/nonmagnetic (FM/NM) heterostructures injects spin current, enabling ultrafast, low-power spintronic devices.
  • High-order k·p theory terms induce significant Fermi contour warping in topological insulators (TIs).
  • The impact of warped topological surface states (TSS) on spin current absorption in FM/TI heterostructures remains unexplored.

Purpose of the Study:

  • Investigate the effect of warped TSS on pure spin current absorption in FM/TI heterostructures.
  • Identify mechanisms for anisotropic spin current absorption in FM/TI systems with significant warping.
  • Explore the influence of warping on Gilbert damping and ultrafast demagnetization dynamics.

Main Methods:

  • Theoretical modeling considering spin accumulation strength to identify anisotropic spin current absorption mechanisms.
  • Analysis of density of states (DOS) and bulk state contributions in Bi2Te3 films.
  • Experimental validation using ferromagnetic resonance (FMR) and time-resolved magneto-optical Kerr effect (TRMOKE).

Main Results:

  • A mechanism for anisotropic spin current absorption in FM/TI heterostructures with large warping effects was identified.
  • Warping leads to anisotropic Gilbert damping (nanosecond timescale) and nearly isotropic ultrafast demagnetization (femtosecond timescale) when TSS dominates the Fermi surface.
  • Gilbert damping anisotropy decreases with increased bulk state contribution in thicker Bi2Te3 films.

Conclusions:

  • Theoretical predictions of warping-induced spin current absorption were experimentally confirmed in Fe/Bi2Te3 heterostructures.
  • The study offers a clearer understanding of spin transfer mechanisms in FM/TI systems.
  • This work provides a foundation for the development of anisotropic spintronics.