Tuning terahertz emission generated by anomalous Nernst effect in ferromagnetic metal

Despite intensive research, the mechanism determining the terahertz (THz) emission of the ferromagnetic (FM) metallic monolayers remains elusive. Here, we report on the results of a systematic investigation on the THz emission generated by pumping Ni80Fe20 monolayers on Al2O3 substrates with a femtosecond laser. We found solid evidence that the THz emission is dominated by the anomalous Nernst effect (ANE), in which a transient spin-polarized charge current can be induced by an ultrafast electron temperature gradient on the picosecond timescale, outputting THz emission. We found a polarity reversal of the THz waveform after the introduction of a SiO2 buffer layer to the sample and found that, based on ultrafast temperature simulation, it was a consequence of direction reversal of temperature gradient. Comparing the THz emission of different FM monolayers further confirms that the THz polarity also strongly depends on the sign of the ANE coefficient. These phenomena unambiguously indicate that the ANE plays a decisive role in the process of THz emission. The present work shows the importance of ultrafast spin caloritronics for a spintronic THz emitter. The principle demonstrated here can be applied to other FM metallic materials.