A1. Experimental setup

For investigating and comparing the characteristics of ferrimagnet TbCo films depending on external conditions such as temperature, optical pump and probe wavelengths, and external magnetic field, we conducted THz emission spectroscopy (TES) and TR-MOKE experiments. Figure ^S14 shows the experimental setups for TES, static polar MOKE, and TR-MOKE. The sample was mounted on a cooper holder at the center of a vacuum chamber (Cryostation s50-MO, Montana Instruments). A fused silica window with antireflection coating for a broad wavelength range of 400–1000 nm was used to input the optical pulses, and a TPX window that provides relatively high transmission of THz waves was used to output the THz emission. The cryogenic system can control the temperature of the copper holder from 4 K to 340 K and sweep the external magnetic field from −600 mT to +600 mT through bipolar magnets in a direction parallel to the sample surface. We used optical pulses with two different wavelengths for static MOKE as well as to pump and probe the TbCo film and compare energy-level-dependent responses. Near-infrared pulses from a 1-kHz Ti:sapphire regenerative amplifier system operating at 800 nm (Spitfire Ace, Spectra Physics) and ultraviolet pulses at 400 nm generated by second-harmonic generation were used for the investigation. The pulse duration and polarization state of pulses at two different wavelengths were approximately 130 fs and horizontal (pump) at the sample position, respectively. Note that the optical pulses were spilt into three beams: an intense pump beam was used to stimulate the magnetization of the TbCo film, a TR-MOKE probe beam was spatiotemporally synchronized with respect to the pump beam at the sample position to detect the time-dependent magnetization change through MOKE measurements, and a sampling beam was used to record the time trace of the THz electric fields through the electro-optic (EO) sampling method. The radius and fluence of the pump beam were approximately 0.7 mm and 0.71 mJ/cm², respectively.

The sample was excited by the pump beam at a normal incidence angle to the sample surface. After passing the sample, the transmitted residual pump was blocked by a Si-wafer because this residual pump can distort the EO-sampling signal during the detection of THz waves. To detect the THz waves emitted from the sample, a pair of 90° off-axis parabolic mirrors was used to guide and focus onto the EO-detection crystal (a 2 mm-thick <100> ZnTe). All THz emission experiments were conducted under conditions of low humidity with dry air purging to reduce the water vapor absorption of THz waves.

To measure the IP magnetic moments of the sample (IP MOKE), the incident angle of the MOKE probe beam was kept at the maximum angle of 30° that is available in our experimental setup. The incident polarization and beam size of the probe beam were vertical and ~70 μm, respectively.