Emission of THz Radiation from Co20Fe60B20/Pt Spintronic Thin Films with Varying Microstructural Properties

Abstract

The ability to generate and control pulses of terahertz (THz) radiation has revolutionised a multitude of critical technologies, from medical imaging [1] to the detection of explosives [2]. Furthermore, sub-picosecond pulses of THz radiation have future application in the ultrafast control of electron spin states [3] and picosecond magnetisation switching in ferromagnets [4], ferrimagnets [5] and antiferromagnets [6]. To exploit the full potential of THz technology, broadband emitters are required which can generate radiation over a spectral range of 1-10 THz [7]. Spintronic emitters, consisting of ferromagnetic (FM)/non-magnetic (NM) heavy metal thin films, have been shown to produce THz pulses with large electric field amplitudes and gapless bandwidths of up to 30 THz [8]. This THz emission is produced when the spintronic structure is excited by femtosecond laser pulses, demonstrating their potential as low-cost sources of broadband radiation.Whilst it has been well established that particular combinations of FM/NM materials, such as Co20Fe60B20/Pt, produce pulses of THz radiation with high electric field amplitudes (300 kV/cm) [8], a detailed understanding of the role that material properties contribute to the generation process of THz radiation has yet to be fully explored in order to inform the future design of optimum performance spintronic emitters. In particular, post deposition annealing of CoFeB based structures has been shown to enhance the emission of THz radiation by up to a factor of 3 [9,10] further increasing the potential utility of these emitters. This enhanced electrical field amplitude has been attributed to two possible effects: 1) diffusion of Boron into the heavy metal layer, leading to an increased number of scattering centres and a reduced hot electron relaxation length, λNM [9]; or 2) local crystallisation of CoFeB leading to an increase in mean free path of hot electrons travelling through the FM layer [9,10].In order to explore the electric field enhancement, we present a systematic study which investigates the effects of inducing defects in the NM layer and crystallisation of the FM/NM interface upon the emission of THz radiation from Co20Fe60B20 (2.5 nm)/Pt (3 nm) spintronic thin films. The films have been grown onto fused silica substrates using magnetron sputtering where the base pressure was better than 5 x10-8 Torr. We present results from two series of films. In the first series, the dc sputtering power for Pt was varied between 25 W and 100 W in order to control defects in the Pt layer [11]. In the second series, the films were annealed post deposition between 200°C and 400°C to induce crystallisation at the CoFeB/Pt interface. For all films, the Co20Fe60B20 layer was sputtered at 100 W dc power to enable direct comparison. The structural properties of Co20Fe60B20/Pt films were characterised via X-ray diffraction (Fig. 1a) and atomic force microscopy (Fig. 1b-e), from which the crystalline ordering and the surface roughness of the Pt were observed to change with varying sputtering power (series 1) and annealing temperature (series 2). X-ray reflectivity analysis of Co20Fe60B20/Pt films (series 2) shows a decrease in density of the Pt layer and increase in interfacial roughness of the CoFeB layer, indicating diffusion at the CoFeB/Pt interface with post deposition annealing above 200°C. We have used terahertz time-domain spectroscopy to measure the electric field amplitude of the emitted THz pulses from Co20Fe60B20/Pt films (see Fig. 2). Initial analysis shows that interfacial interdiffusion has a significant influence on the THz electric field amplitude. A full analysis of our results will be presented at the conference. **