Studying spintronic emitters using terahertz emission spectroscopy

Abstract

In this thesis spintronic emitters have been studied using THz emission spectroscopy. First, optimization of bilayer Co20Fe60B20/Pt spintronic emitters was performed to maximize the emitted THz radiation. This was accomplished through a systematic variation of Co20Fe60B20 and Pt layer thicknesses. The optimal combination was identified at 2 nm for both layers. The broadband and gapless THz spectrum generated was found to reach 23 THz, as verified by measuring absorption of polystyrene at a frequency of 16 THz. Additionally, differences in the bandwidth of the THz emission from bilayer FM/Pt and Co20Fe60B20 /NM spintronic emitters were demonstrated. Various FM materials were investigated, including Ni80Fe20, Co20Fe60B20, Fe and Co, in which the emitted THz bandwidth ranged from 11 THz to 12 THz. These differences were attributed to the different demagnetization dynamics, characterised by unique demagnetization time constants. The NM materials studied included W, Pt, Ru and Au, in which the bandwidth ranged from 10 THz to 13 THz. This difference was attributed to the varying spin transports, governed by the materials carrier lifetime and spin-scattering times. For a more comprehensive understanding of the conversion from spin to charge and the role of the bilayer interface in THz emission, temperature dependent THz emission spectroscopy was performed on bilayer FM/Pt emitters. Through the analysis of temperature-dependent impedance and spin current, it was found that the temperature-dependent spin-Hall angle was influenced by the properties of the FM/Pt interface, dominating the THz emission. It was further demonstrated that a spintronic THz emitter can be driven by a chirped-pulse-beating scheme to generate narrowband THz pulses, with continuous tuning of the frequency and linewidth by simply adjusting the laser chirp and/or the time delay between chirped pulses. As supported by model calculations, it was found that temporal shaping of the drive laser pulses could be exploited to manipulate the ultrafast demagnetization dynamics in the emitter, modulating the spin-polarized current in the ferromagnetic layer to produce a multi-cycle THz emission. These results pave the way to future applications utilising spintronic THz emitters for both broadband and narrowband applications. Moreover, they provide a detailed understanding of spin generation and spin-to-charge conversion within spintronic emitters. This work also highlights the importance and validity of THz emission spectroscopy for analysing spintronic heterostructures.

Date of Award	1 Aug 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Darren Graham (Supervisor), Paul Nutter (Supervisor) & Thomas Thomson (Supervisor)