Pulsed far-infrared radiation of ferromagnetic heterojunction and its photothermal regulation

Abstract

Under illumination of a femtosecond laser pulse on the Pt/CoFe/Ta trilayer heterostructure, an impulsive spin current can be generated in the ferromagnetic layer due to the ultrafast demagnetization. The spin current is super-diffusively transported and injected into the neighboring heavy metal layers, and is converted into the transversal charge current due to the spin-orbit coupling, which is named inverse spin Hall effect. The transient charge current on a time scale of sub-picosecond gives rise to the electromagnetic radiation in the far-infrared range to the free space. In this work, we demonstrate two kinds of experiments to investigate the modulation of far-infrared emission by photo-thermal effect, which is due to the thermal energy deposed by light pulses on a short timescales. First, the amplitude of the emitted far-infrared pulse as a function of an applied magnetic field is measured, which shows a far-infrared hysteresis behavior. The coercive field of the sample obtained by far-infrared hysteresis is smaller than that obtained by the <i>M</i>-<i>H</i> hysteresis through vibrating sample magnetometer. In addition, the coercive field decreases with pump laser fluence increasing. Second, the control of spin polarization on an ultrafast timescale in the presence of a small magnetic field applied oppositely to that of the magnetization of the ferromagnetic sample. The amplitude of far-infrared time-domain signal reaches a maximum value at a pump fluence of 1.43 mJ/cm<sup>2</sup>. For the pump fluence larger than 1.43 mJ/cm<sup>2</sup>, the far-infrared pulse experiences a phase reversal. After the reversal, a decrease of the laser pump fluence cannot restore the original phase of the far-infrared pulse. The above two experimental results not only elucidate the photothermal effect of femtosecond laser pulses, but also provide a new method for controlling the far-infrared radiation pulses based on ultrafast spintronics. These results demonstrate that far-infrared emission spectroscopy can be used as an ultrafast optical method to investigate magnetic properties, such as the coercive field and anisotropy field of the samples.