In this work, we performed temperature-dependent studies of the THz transient amplitude FeCo waveformes, from a FeCo/graphene nanobilayer sample, triggered by fs pulsed laser in the 80–400 K range. We show that a due-twofold extension, in the range 80–300 K the amplitude increases with temperature and tends to saturate above this range. This dependence contrasts sharply with the temperature dependence of the FeCo film's magnetization, which shows a typical ferromagnetic (FM) trend with Curie temperature well above 400 K. We explain this discrepancy, as the presence of an antiferromagnetic (AFM) at the FeCo/graphene interface, which is associated with the native oxide formed at the FeCo surface. The Angle Resolved X-ray Photoelectron Spectroscopy studies of a bare FeCo film revealed coexistence of the metallic [Co(0), Fe(0)] and antiferromagnetic [Co(II)O and the Fe(III)2O3] phases, at the sample surface. The observation of the exchange bias in our magnetization hysteresis loop of a FeCo film confirms presence of an FM/AFM interface layer at the FeCo surface. In summary, we conclude that the temperature dependence of the THz transient amplitude is governed by the AFM phase.
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