Abstract-Antenna-coupled spintronic terahertz emitters driven by a 1550 nm femtosecond laser oscillator

Applied Physics Letters

 U. Nandi, M. S. Abdelaziz, S. Jaiswal, G. Jakob, O. Gueckstock, S. M. Rouzegar, T. S. Seifert,   M. Kläui, T. Kampfrath,  S. Preu,

Schematic of THz emission from photoexcited FMNM bilayers, plain and microstructured. (a) A femtosecond laser pulse triggers ultrafast spin transport from the FM into the NM layer where the spin current js flowing along the z axis is converted into a charge current jc along the y direction, acting as a source of THz radiation. The direction of the in-plane magnetization of the FM layer is set along the x axis by an external magnetic field Bext. (b) Current distribution in an unstructured (plain) bilayer and (c) the STE bilayer embedded in the gap of an antenna. Note that THz current generation by the ISHE is independent of emitter type and antenna choice.

https://aip.scitation.org/doi/abs/10.1063/1.5089421

We demonstrate antenna-coupled spintronic terahertz (THz) emitters excited by 1550 nm, 90 fs laser pulses. Antennas are employed to optimize THz outcoupling and frequency coverage of ferromagnetic/nonmagnetic metallic spintronic structures. We directly compare the antenna-coupled devices to those without antennas. Using a 200 μm H-dipole antenna and an ErAs:InGaAs photoconductive receiver, we obtain a 2.42-fold larger THz peak-peak signal, a bandwidth of 4.5 THz, and an increase in the peak dynamic range (DNR) from 53 dB to 65 dB. A 25 μm slotline antenna offered 5 dB larger peak DNR and a bandwidth of 5 THz. For all measurements, we use a comparatively low laser power of 45 mW from a commercial fiber-coupled system that is frequently employed in table-top THz time-domain systems.

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

T. Seifert, N. M. Tran, O. Gueckstock, S. M. Rouzegar,
 L. Nadvornik, S. Jaiswal, G. Jakob. V. Temnov, M. Münzenberg,  M. Wolf,  M. Kläui, T. Kampfrath

https://arxiv.org/ftp/arxiv/papers/1805/1805.02193.pdf

 Identifying materials with an efficient spin-to-charge conversion is crucial for future spintronic applications. 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 𝛩 and the spin-current relaxation length 𝜆୰ୣ୪. We develop an all-optical method with large sample throughput that allows us to extract 𝛩 and 𝜆୰ୣ୪. and 𝜆୰ୣ୪. Employing terahertz spectroscopy, we characterize magnetic metallic heterostructures involving Pt, W and Cu80Ir20 in terms of their optical and spintronic properties. We furthermore find indications that the interface plays a minor role for the spin-current transmission. Our analytical model is validated by the good agreement with literature DC values. These findings establish terahertz emission spectroscopy as a reliable tool complementing the spintronics workbench.  

Abstract-Launching magnons at the terahertz speed of the spin Seebeck effect

T. Seifert, S. Jaiswal, J. Barker, I. Razdolski, J. Cramer, O. Gueckstock, S. Watanabe, C. Ciccarelli, A. Melnikov, G. Jakob, S.T.B. Goennenwein, G. Woltersdorf, P.W. Brouwer, M. Wolf, M. Kläui, T. Kampfrath

(Submitted on 3 Sep 2017)

https://arxiv.org/abs/1709.00768

Transport of spin angular momentum is an essential operation in spintronic devices. In magnetic insulators, spin currents are carried by magnons and can be launched straightforwardly by heating an adjacent metal layer. Here, we study the ultimate speed of this spin Seebeck effect with 10-fs time resolution in prototypical bilayers of ferrimagnetic yttrium iron garnet and platinum. Upon exciting the metal by a laser pulse, the spin flow is measured using the inverse spin Hall effect and terahertz electrooptic sampling. The spin Seebeck current reaches its peak within ~200 fs, a hallmark of the photoexcited metal electrons approaching a Fermi-Dirac distribution. Analytical modeling shows the spin Seebeck response is virtually instantaneous because the ferrimagnetic spins react without inertia and the metal spins impinging on the interface have a correlation time of only ~4 fs. Novel applications for material characterization, interface probing, spin-noise detection and terahertz spin pumping emerge.