Terahertz spectroscopy for all-optical spintronic characterization of the spin-Hall-effect metals Pt, W and Cu₈₀Ir₂₀

Abstract

Identifying materials with an efficient spin-to-charge conversion is crucial for future spintronic applications. In this respect, the spin Hall effect is a central mechanism as it allows for the interconversion of spin and charge currents. Spintronic material research aims at maximizing its efficiency, quantified by the spin Hall angle and the spin-current relaxation length . We develop an all-optical contact-free method with large sample throughput that allows us to extract and . Employing terahertz spectroscopy and an analytical model, magnetic metallic heterostructures involving Pt, W and Cu80Ir20 are characterized in terms of their optical and spintronic properties. The validity of our analytical model is confirmed by the good agreement with literature DC values. For the samples considered here, we find indications that the interface plays a minor role for the spin-current transmission. Our findings establish terahertz emission spectroscopy as a reliable tool complementing the spintronics workbench. Figure 1. Schematic of the experiment. (a) Terahertz emission experiment. A femtosecond nearinfrared pump pulse excites electrons in both the ferromagnetic (FM, in-plane magnetization ) and non-magnetic (NM) metal layer. Due to the asymmetry of the heterostructure, a spin current is injected from the FM into the NM material where it is converted into an in-plane charge current by the inverse spin Hall effect (ISHE). The sub-picosecond charge-current burst leads to the emission of a terahertz (THz) pulse into the optical far-field. (b) Terahertz transmission experiment. A THz transient is incident onto either the bare substrate or onto the substrate coated by a thin metal film. By comparing the two transmitted waveforms and , the metal conductivity at THz NM THz pulse FM Femtosecond pump M j s j c ISHE (a) (b) Metal Substrate frequencies is determined.Figure 2. Typical THz emission raw data and sample characterization. (a) THz emission signal measured from a C40F40B20(3 nm)|Pt(3 nm) bilayer for two opposite orientations of the sample magnetization (± ). (b) Normalized pump-power dependence of the THz signal amplitude (RMS) for one orientation of the sample magnetization. (c) Pump-light absorptance, transmittance and reflectance as function of the Pt-layer thickness. (d) Frequency-dependent THz conductivities measured by THz transmission experiments (black and red dots) along with fits obtained by the Drude model (black and red solid lines).

Ultrafast terahertz magnetometry

“…This includes the pioneering observation of THz emission from an optically pumped Ni/Cr nanostructure made by one of us, Beaurepaire et al 15 , which was initially assigned to the process of laser-driven transient demagnetization M(t) in Ni. Our present understanding 17,19,31 , however, clearly points to the electric dipole nature of this THz emission 15 via the ISHE mechanism, enabled by the superdiffusion of laser-excited spin-polarized electrons from Ni to Cr, and resulting in the THz-emitting in-plane charge current in the Cr layer. As such, even after more than two decades since the first experiments on ultrafast magnetism 6 , and after more than a decade since the pioneering observations of THz emission from magnetic materials 15,16 , the fundamental process of laser-driven transient demagnetization in ferromagnetic metals has not been identified so far in a clear, unambiguous and calibrated manner, to the best of our knowledge.…”

THz spintronic emitters: a review on achievements and future challenges

“…The THz signal amplitude seems to scale with the intrinsic spin Hall conductivity of the used NM layer [5], see Figure 4. The THz emission was even used to estimate the relative spin Hall angle [21,29]. The prominent choice at the moment is Pt since it has provided so far the highest signal compared to W [21], Cu 80 Ir 20 [21], Ta [29], Ru [29,30], Ir [29], Pd [30], Pt 38 Mn 62 [5].…”

“…Alternatively, free-space electro-optical sampling (EOS) is used for the detection of THz radiation from spintronic emitters [20,21]. In order to be able to detect frequencies well above 10 THz very thin ZnTe crystals are used as a detector in combination with extremely short laser pulses.…”

Terahertz Emission Functionality of High‐Temperature Superconductors and Similar Complex Systems

“…2020, 8,1900892 into nonmagnetic Au/Ru cap layer. [133,134] However, the electrons will stay in the Au layer for a short period.…”

“…2020, 8,1900892 mentation of THz emission spectroscopy to 2D systems is a viable and credible approach to realizing such ultrafast optical control. [128][129][130][131][132][133][134][135][136][137][138][139][140][141] In the specific area of active metamaterials research, combining the electronic and magnetic control with the optical THz functionality of metamaterials promises several new possibilities for the technological utility of THz light-matter interaction. [136,137] Thus, material research by and for THz technology is one of the most exciting research fields where we can study the intrinsic nature of materials and devices.…”