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-Impact of pump wavelength on terahertz emission of a cavity-enhanced spintronic trilayer

 

Applied Physics Letters

R. I. Herapath,  S. M. Hornett,   T. S. Seifert, G. Jakob,  M. Kläui,  J. Bertolotti, T. Kampfrath,  E. Hendry

Schematic of a spintronic trilayer with added dielectric cavity, grown on 0.5 mm of sapphire (Al2O3). The near-infrared pump pulse, incident through the substrate, is partially absorbed in the metallic layers, launching a spin current from the ferromagnetic (FM) layer into the nonmagnetic (NM) layers. The inverse spin Hall effect converts this ultrashort out-of-plane spin current into an in-plane charge current resulting in the emission of THz radiation into the optical far-field. A weak in-plane magnetic field (B) determines the magnetization direction and the linear polarization of the emitted THz field.

https://aip.scitation.org/doi/abs/10.1063/1.5048297

We systematically study the pump-wavelength dependence of terahertz pulse generation in thin-film spintronic THz emitters composed of a ferromagnetic CoFeB layer between adjacent nonmagnetic W and Pt layers. We find that the efficiency of THz generation is essentially flat for excitation by 150 fs pulses with center wavelengths ranging from 900 to 1500 nm, demonstrating that the spin current does not depend strongly on the pump photon energy. We show that the inclusion of dielectric overlayers of TiO2 and SiO2, designed for a particular excitation wavelength, can enhance the terahertz emission by a factor of up to two in field.

Abstract-Néel Spin-Orbit Torque Driven Antiferromagnetic Resonance in Mn 2 Au Probed by Time-Domain THz Spectroscopy

N. Bhattacharjee, A. A. Sapozhnik, S. Yu. Bodnar, V. Yu. Grigorev, S. Y. Agustsson, J. Cao, D. Dominko, M. Obergfell, O. Gomonay, J. Sinova, M. Kläui, H.-J. Elmers, M. Jourdan, and J. Demsar

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.237201

We observe the excitation of collective modes in the terahertz (THz) range driven by the recently discovered Néel spin-orbit torques (NSOTs) in the metallic antiferromagnet ${\mathrm{Mn}}_2\mathrm{Au}$. Temperature-dependent THz spectroscopy reveals a strong absorption mode centered near 1 THz, which upon heating from 4 to 450 K softens and loses intensity. A comparison with the estimated eigenmode frequencies implies that the observed mode is an in-plane antiferromagnetic resonance (AFMR). The AFMR absorption strength exceeds those found in antiferromagnetic insulators, driven by the magnetic field of the THz radiation, by 3 orders of magnitude. Based on this and the agreement with our theory modeling, we infer that the driving mechanism for the observed mode is the current-induced NSOT. Here the electric field component of the THz pulse drives an ac current in the metal, which subsequently drives the AFMR. This electric manipulation of the Néel order parameter at high frequencies makes ${\mathrm{Mn}}_2\mathrm{Au}$ a prime candidate for antiferromagnetic ultrafast memory applications.

• 
• 
• 
• 

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-N\'{e}el spin orbit torque driven antiferromagnetic resonance in Mn 2 Au probed by time-domain THz spectroscopy

N. Bhattacharjee, A. A. Sapozhnik, S. Yu. Bodnar, V. Yu. Grigorev, S. Y. Agustsson, J. Cao, D. Dominko, M. Obergfell, O. Gomonay, J. Sinova, M. Kläui, H. -J. Elmers, M. Jourdan, and J. Demsar

https://journals.aps.org/prl/accepted/32071Y5cC871765f240317f6796a5ec9679dbb0ac

We observe the excitation of collective modes in the THz range driven by the recently discovered N\'{e}el spin-orbit torques (NSOT) in the metallic antiferromagnet Mn${}_2$Au. Temperature dependent THz spectroscopy reveals a strong absorption mode centered near 1 THz, which upon heating from 4 K to 450 K softens and looses intensity. Comparison with the estimated eigenmode frequencies implies that the observed mode is an in-plane antiferromagnetic resonance (AFMR). The AFMR absorption strength exceeds those found in antiferromagnetic insulators, driven by the magnetic field of the THz radiation, by three orders of magnitude. Based on this and the agreement with our theory modelling, we infer that the driving mechanism for the observed mode is the current induced NSOT. Here the electric field component of the THz pulse drives an AC current in the metal, which subsequently drives the AFMR. This electric manipulation of the N\'{e}el order parameter at high frequencies makes Mn${}_2$Au a prime candidate for AFM ultrafast memory applications.

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.

Abstract-Efficient metallic spintronic emitters of ultrabroadband terahertz radiation

T. Seifert,

S. Jaiswal,

U. Martens,

J. Hannegan,

L. Braun,

P. Maldonado,

F. Freimuth,

A. Kronenberg,

J. Henrizi,

I. Radu,

E. Beaurepaire,

Y. Mokrousov,

P. M. Oppeneer,

M. Jourdan,

G. Jakob,

D. Turchinovich,

L. M. Hayden,

M. Wolf,

M. Münzenberg,

M. Kläui

& T. Kampfrath 

(some author names deleted from Labels at the bottom due to blogger word limit)

http://www.nature.com/nphoton/journal/vaop/ncurrent/full/nphoton.2016.91.html

Terahertz electromagnetic radiation is extremely useful for numerous applications, including imaging and spectroscopy. It is thus highly desirable to have an efficient table-top emitter covering the 1–30 THz window that is driven by a low-cost, low-power femtosecond laser oscillator. So far, all solid-state emitters solely exploit physics related to the electron charge and deliver emission spectra with substantial gaps. Here, we take advantage of the electron spin to realize a conceptually new terahertz source that relies on three tailored fundamental spintronic and photonic phenomena in magnetic metal multilayers: ultrafast photoinduced spin currents, the inverse spin-Hall effect and a broadband Fabry–Pérot resonance. Guided by an analytical model, this spintronic route offers unique possibilities for systematic optimization. We find that a 5.8-nm-thick W/CoFeB/Pt trilayer generates ultrashort pulses fully covering the 1–30 THz range. Our novel source outperforms laser-oscillator-driven emitters such as ZnTe(110) crystals in terms of bandwidth, terahertz field amplitude, flexibility, scalability and cost.