Strain Engineering of Epitaxial Pt/Fe Spintronic Terahertz Emitter

Abstract

Spin-based terahertz (THz) emitters, utilising the inverse spin Hall effect, are ultra-modern sources for the generation of THz electromagnetic radiation. To make a powerful emitter having large THz amplitude and bandwidth, fundamental understanding in terms of microscopic models is essential. This study reveals important factors to engineer the THz emission amplitude and bandwidth in epitaxial Pt/Fe emitters grown on MgO and MgAl 2 O 4 (MAO) substrates, where the choice of substrate plays an important role. The THz amplitude and bandwidth is affected by the induced strain in the Fe spin source layer. On the one hand, the THz electric field amplitude is found to be larger when Pt/Fe is grown on MgO even though the crystalline quality of the Fe film is superior when grown on MAO. This is because of the larger defect density, smaller electron relaxation time and lower electrical conductivity in the THz regime when Fe is grown on MgO. On the other hand, the bandwidth is found to be larger for Pt/Fe grown on MAO and is explained by the uncoupled/coupled Lorentz oscillator models. This study provides an insightful pathway to further engineer metallic spintronic THz emitters in terms of proper choice of substrate and microscopic properties of the emitter layers.

Direct evidence of terahertz emission arising from anomalous Hall effect

“…A THz time-domain spectrometer was used to measure the THz emission from the CoFeB films 15,28,29 . A Spectra-Physics Tsunami (Ti: sapphire) was used as a laser source, which delivers pulses of ∼ 55 fs duration (bandwidth ∼ 12 nm, central wavelength ∼ 800 nm, and maximum output energy ∼ 10 nJ) at a repetition rate of 80 MHz.…”

Influence of misfit strain on the physical properties of Fe thin films

“…The growth of thin Fe layers on MgAl 2 O 4 has been shown to result in a better crystal quality of the Fe layers, as compared to when grown on MgO 16 . This has been used for investigations of the effects of crystal quality on Gilbert damping 16 , THz emission 17,18 as well as on the magnetic properties of Fe in Fe/MgAl 2 O 4 /Fe (001) 15,[19][20][21][22] . Crystalline Fe has a fourfold in-plane magnetocrystalline anisotropy.…”