Confirm phase reversal of THz emission with tungsten capping

Determine whether a 5 nm-thick tungsten (W) capping layer on BiFeO3 generates a phase reversal in the terahertz time-domain emission trace compared to platinum (Pt) capping, as expected for spin-to-charge emission due to the opposite sign of the spin Hall angle in W relative to Pt.

Background

The study investigates narrowband THz emission from multiferroic BiFeO3 thin films and Pt/BiFeO3 heterostructures, attributing a tunable 0.4–0.8 THz response to magneto-acoustic excitation of the electromagnon branch via strain pulses generated in the metal transducer.

To assess whether conventional spin-to-charge processes contribute, the authors examined a W/BiFeO3 heterostructure. Because W has an opposite spin Hall angle relative to Pt, a spin-to-charge dominated emitter should exhibit a phase reversal of the THz time trace. However, the measured THz signal was too weak to decisively confirm this reversal, leaving the question unresolved.

References

When capping BiFeO3 with a 5 nm-thick W layer, one would expect a phase reversal of the THz time trace, in accordance with a spin-to-charge emission process, and due to the opposite spin Hall angle of W compared with Pt. Nevertheless, the low amplitude THz response does not allow to confirm with certitude such a phase reversal (Fig. 4d).

— Phonon-tunable THz magnonic emission in multiferroic heterostructures  (2511.21596 - Massabeau et al., 26 Nov 2025) in Results, Fig. 4(d) paragraph