  By developing a bidirectional-pump terahertz (THz) emission spectroscopy, we
reveal an anomalous Nernst effect (ANE) induced THz emission in a single
ferromagnetic film. Based on the distinctive symmetry of the THz signals, ANE
is unequivocally distinguished from the previously attributed ultrafast
demagnetization and anomalous Hall effect mechanisms. A quantitative method is
established to separate the different contributions, demonstrating a
significant ANE contribution that even overwhelms other competing mechanisms.
Our work not only clarifies the origin of the ferromagnetic-based THz emission,
but also offers a fertile platform for investigating the ultrafast magnetism
and THz spintronics.
