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.
We performed comprehensive studies of current transport across SWCNT-Si heterojunctions, considered as a promising component for advanced photodetectors. Low-doped n-type Si was used as a substrate and SWCNT films were deposited on its top by a wet method out of solutions. We collected current-voltage (I-V) characteristics of the heterojunctions in the 78-300 K temperature range under dark conditions. In the forward bias, the I-V curves exhibited two regimes, namely, the “low” and “high” voltage regimes. We applied the Cheung–Cheung method to evaluate the height of the Schottky barrier, the series resistance, and the ideality factor, for both regimes. For tested samples, the ideality factor is very well fitted with the T-1/2 dependence. The slope of this dependency for the “high” voltage regime decreases with the increase of the SWCNT concentration, what agrees with the Card–Rhoderick model that the slope in this regime should be inversely proportional to the density of states at the SWCNT/SiO2 interface, which in turn is proportional to the SWCNT concentration. The crossover voltage between the two voltage regimes decreased linearly with the temperature for all our samples.
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