Speaker
Description
Transport of spin angular momentum and the conversion of the resulting spin current to a charge current are fundamental operations required for future spin-electronic devices. To push these operations to the terahertz (THz) frequency range, we use femtosecond laser pulses (of ~10 fs duration) to excite prototypical F|N bilayers consisting of a ferromagnetic metal F and a nonmagnetic metal N [1]. We recently found that the optical excitation induces a transient spin voltage in F that launches a spin current from F to N [1]. At the same time, the spin current is converted into an in-plane-oriented charge current that gives rise to the emission of an electromagnetic pulse with frequencies extending into the THz range. This effect is interesting for studying ultrafast spin transport and spin-to-charge-current conversion (S2C) and to build efficient ultrabroadband THz emitters [1].
We first study THz emission from CoFeB|Pt stacks as a function of temperature. The temperature dependence of the THz emission amplitude is very sensitive to the growth procedure (sputtering vs epitaxial growth) of the sample. Analysis of our data strongly indicates that the THz spin voltage probes the local Curie tem¬perature TC,IF of CoFeB regions close to the CoFeB/Pt interface (IF). The magni¬tude of TC,IF is the lower the higher the degree of interface alloying is.
Second, we measure THz emission from Co|TI structures, where TI is a three-dimensional topological insulator such as Bi2SnTe4 and Bi2Te3. Our THz emission data reveal a substantially slower charge current in Co|TI as compared to Co|Pt reference samples. We are able to consistently explain our results by a non-instan-taneous S2C in the TI. As microscopic mechanism, we suggest the inverse Rashba-Edelstein effect because it requires the build-up of a spin accumulation at the Co/TI interface. The two results demonstrate a pronounced interface sensitivity of THz emission spectroscopy.
Finally, we use W|CoFeB|Pt stacks for efficient generation of THz pulses. By applying a time-varying external magnetic field, we are able to modulate the direction of the CoFeB magnetization and, thus, the direction of the emitted linearly polarized THz pulses [2]. We are able to demonstrate modulation up to 30 kHz with a modulation depth >99%, which is highly promising for applications in low-noise THz ellipsometry [3].
References
[1] T. Seifert et al., Appl. Phys. Lett. 120, 180401 (2022) (Review)
[2] O. Gueckstock et al., Optica 8, 1013-1019 (2021).
[3] L. Nadvorník et al., Phys. Rev. X 11, 021030 (2021).
| Abstract Number (department-wise) | PC 07 |
|---|---|
| Department | PC (Wolf) |
