High-efficiency Pt$_{75}$Au$_{25}$-based spintronic terahertz emitters

Abstract

Spintronic terahertz emitters (STEs) generate broadband THz radiation via ultrafast spin-charge conversion in magnetic multilayers, offering spectral coverage beyond that of photoconductive antennas and nonlinear optical crystals. Here, we demonstrate a new type of STE based on PtxAu100-x alloy that achieves significantly higher THz output power than widely used Pt-based devices. Alloy composition and layer thickness tuning yield Pt75Au25 as the optimal alloy providing a 30 % increase in THz power in CoFeB/Pt75Au25 bilayer STEs compared to the optimized CoFeB/Pt reference STE. In W/CoFeB/Pt$_{75}$Au$_{25}$ trilayer STEs, we observe a 10 % higher THz power than in the optimized W/CoFeB/Pt trilayer. The STE efficiency is reduced upon annealing for both Pt$_{75}$Au$_{25}$- and Pt-based STEs due to formation of interfacial alloys. Our results establish Pt$_{75}$Au$_{25}$ as a promising platform for high-performance STEs, where its giant spin Hall effect significantly enhances efficiency over conventional Pt-based devices.

Figures (4)

• Figure 1: (a) Schematic of a W/CoFeB/$\mathrm{Pt}_{x}\mathrm{Au}_{100-x}$ STE multilayer on a sapphire substrate, where $\overrightarrow{B}$ is the external magnetic flux density, and $\overrightarrow{M}$ is the equilibrium magnetization of the FM layer. (b) Experimental setup of THz electro-optic sampling (EOS) for STE THz emission measurements using a Ti:sapphire laser, including a beam splitter (BS), a chopper, two off-axis parabolic mirrors (OAP1, OAP2), a linear polarizer (LP), a 500 $\mu$m ZnTe crystal detector, a quarter-waveplate (QWP), a Wollaston prism (WP), and a balanced photodetector (BPDS). (c) Time-domain waveform of the emitted THz $E$-field from the W(1.8 nm)/CoFeB(1.3 nm)/$\mathrm{Pt_{75}Au_{25}}$(3.0 nm) trilayer STE measured via EOS. ${E_0}$ marks the peak-to-peak amplitude of the THz waveform. (d) Fourier transform of the THz waveform in (c) plotted as spectral amplitude versus frequency.
• Figure 2: Optimization of THz emission with respect to $\mathrm{Pt}_{x}\mathrm{Au}_{100-x}$ and CoFeB layer thicknesses: (a) THz electric field amplitude $E_0$ as a function of $\mathrm{Pt}_{x}\mathrm{Au}_{100-x}$ thickness for four Pt concentrations: $x$ = 65, 75, 85, and 100 in CoFeB(1.6 nm)/ $\mathrm{Pt}_{x}\mathrm{Au}_{100-x}$($d_\mathrm{NM}$) bilayers. $E_0$ increases with $\mathrm{Pt}_{x}\mathrm{Au}_{100-x}$ thickness up to a composition-dependent maximum, with $\mathrm{Pt_{75}Au_{25}}$ showing the highest overall $E_0$ at $d_\mathrm{NM}=3.0$ nm. (b) $E_0$ as a function of CoFeB thickness $d_\mathrm{FM}$ in W(1.8 nm)/CoFeB($d_\mathrm{FM}$)/Pt(2.1 nm) and W(1.8 nm)/CoFeB($d_\mathrm{FM}$)/$\mathrm{Pt_{75}Au_{25}}$(3.0 nm) trilayer STEs. Both systems maximize THz emission at $d_\mathrm{FM} \approx$ 1.3 – 1.4 nm, with the $\mathrm{Pt_{75}Au_{25}}$-based STEs outperforming the Pt-based STEs at any $d_\mathrm{FM}$.
• Figure 3: Effect of annealing on STE: (a) THz electric field amplitude $E_0$ as a function of annealing temperature for the optimized Pt- and PtAu-based trilayer STEs. (b) Magnetization hysteresis loops of the W(1.8 nm)/CoFeB(1.3 nm)/ $\mathrm{Pt_{75}Au_{25}}$(3 nm) STE before and after annealing at 400 °C. (c) Cross-sectional STEM-EDS elemental maps of the W(1.8 nm)/CoFeB(1.3 nm)/$\mathrm{Pt_{75}Au_{25}}$(3 nm) STE annealed at 300 °C, showing spatial distributions of Pt, Au, Co, Fe, W, Al, and O. (d, e) Normalized EDS line profiles along the film growth direction for (d) as-deposited and (e) 300 °C-annealed samples.
• Figure 4: X-ray diffraction (XRD) $\omega - 2\theta$ scan of W(6 nm)/Co$_{30}$Fe$_{70}$(6 nm)/Pt(6 nm) multilayer: (a) as-deposited and (b) annealed at 300°C for 3 hours. Red symbols are measured counts. Bragg peaks are fitted with quasi-Voigt functions (solid line) to identify annealing-induced phase transitions.