Sascha Keller

Lack of correlation between the spin-mixing conductance and the inverse spin Hall effect generated voltages in CoFeB/Pt and CoFeB/Ta bilayers

We investigate spin pumping phenomena in polycrystalline CoFeB/Pt and CoFeB/Ta bilayers and the correlation between the effective spin mixing conductance

Optimized Spintronic Terahertz Emitters Based on Epitaxial Grown Fe/Pt Layer Structures

g^{\uparrow\downarrow}_{\rm eff}

Relative weight of the inverse spin-Hall and spin-rectification effects for metallic polycrystalline Py/Pt, epitaxial Fe/Pt, and insulating YIG/Pt bilayers: Angular dependent spin pumping measurements

and the obtained voltages generated by the spin-to-charge current conversion via the inverse spin Hall effect in the Pt and Ta layers. For this purpose we measure the in-plane angular dependence of the generated voltages on the external static magnetic field and we apply a model to separate the spin pumping signal from the one generated by the spin rectification effect in the magnetic layer. Our results reveal a dominating role of anomalous Hall effect for the spin rectification effect with CoFeB and a lack of correlation between

Spin-pumping through a varying-thickness MgO interlayer in Fe/Pt system

g^{\uparrow\downarrow}_{\rm eff}

Spintronic terahertz emitters based on epitaxial-grown Fe/Pt Bilayers (Conference Presentation)

and inverse spin Hall voltages pointing to a strong role of the magnetic proximity effect in Pt in understanding the observed increased damping. This is additionally reflected on the presence of a linear dependency of the Gilbert damping parameter on the Pt thickness.

Determination of the spin Hall angle in single-crystalline Pt films from spin pumping experiments

We report on generation of pulsed broadband terahertz radiation utilizing the inverse spin hall effect in Fe/Pt bilayers on MgO and sapphire substrates. The emitter was optimized with respect to layer thickness, growth parameters, substrates and geometrical arrangement. The experimentally determined optimum layer thicknesses were in qualitative agreement with simulations of the spin current induced in the ferromagnetic layer. Our model takes into account generation of spin polarization, spin diffusion and accumulation in Fe and Pt and electrical as well as optical properties of the bilayer samples. Using the device in a counterintuitive orientation a Si lens was attached to increase the collection efficiency of the emitter. The optimized emitter provided a bandwidth of up to 8 THz which was mainly limited by the low-temperature-grown GaAs (LT-GaAS) photoconductive antenna used as detector and the pulse length of the pump laser. The THz pulse length was as short as 220 fs for a sub 100 fs pulse length of the 800 nm pump laser. Average pump powers as low as 25 mW (at a repetition rate of 75 MHz) have been used for terahertz generation. This and the general performance make the spintronic terahertz emitter compatible with established emitters based on optical rectification in nonlinear crystals.

Study of fully epitaxial Fe/Pt bilayers for spin pumping by ferromagnetic resonance spectroscopy

We quantify the relative weight of inverse spin Hall and spin rectification effects occurring in RF-sputtered polycrystalline permalloy, molecular beam epitaxy-grown epitaxial iron and liquid phase epitaxy-grown yttrium-iron-garnet bilayer systems with different capping materials. To distinguish the spin rectification signal from the inverse spin Hall voltage the external magnetic field is rotated in-plane to take advantage of the different angular dependencies of the prevailing effects. We prove that in permalloy anisotropic magnetoresistance is the dominant source for spin rectification while in epitaxial iron the anomalous Hall effect has an also comparable strength. The rectification in yttrium-iron-garnet/platinum bilayers reveals an angular dependence imitating the one seen for anisotropic magnetoresistance caused by spin Hall magnetoresistance.

Separation of the two-magnon scattering contribution to damping for the determination of the spin mixing conductance

The spin-pumping mechanism is probed through a tunnelling MgO interlayer in Fe/Pt bilayers. We show by ferromagnetic resonance technique and spin-pumping experiments that spin currents can tunnel through the MgO interlayer for thicknesses up to 2 nm and can produce significant voltages in the Pt layer. The electrical detection of spin-pumping furthermore reveals the critical role of rectification and shunting effects on the generated voltages. The non-zero spin current transport through a few monolayers of an insulating interlayer might initiate further studies on the role of very thin oxides in spin-pumping experiments.

Efficient Terahertz Generation Using Fe/Pt Spintronic Emitter Excited at 800 nm and 1550 nm

The inverse spin Hall effect (ISHE) can be used to generate broadband terahertz (THz) radiation. This has been demonstrated recently [1 – 3]. We report on efficient generation of pulsed broadband terahertz radiation utilizing the inverse spin hall effect in Fe/Pt bilayers on MgO and sapphire substrates. The magnetic and nonmagnetic layers were epitaxially grown on MgO and sapphire substrates. The emitter was optimized with respect to layer thickness, growth parameters, substrates and geometrical arrangement. Using the device in a counterintuitive orientation a hyperhemispherical Si lens was attached to increase the collection efficiency of the emitter. In this arrangement multiple reflections of the THz pulses from the substrate surfaces are avoided as the metallic layers act as an antireflection coating [4]. The experimentally determined dependence of the THz signal on the layer thicknesses was in qualitative agreement with simulations of the ISHE in the Fe-Pt bilayer. An optimum layer thicknesses of 2 nm and 3 nm were found for Fe and Pt, respectively. The optimized emitter provided a bandwidth of up to 8 THz for both the sapphire and MgO substrates which was mainly limited by the GaAs photoconductive antenna used as detector. The dynamic range reached 60 dB for the MgO substrate at a frequency of 1.5 THz. The pulse length was as short as 220 fs for a pump pulse length of the 800 nm pump laser of about 50 fs. In the case of MgO substrates strong THz absorption of MgO reduced the dynamic range above 3 THz considerably. Average pump powers as low as 25 mW (at a repetition rate of 80 MHz) have been used for terahertz generation. This and the general performance makes the spintronic terahertz emitter compatible with established emitters using nonlinear generation methods. References [1] T. Seifert et al., Nature Photonics 10, 483 (2016) [2] D. Yang, et al., Advanced Optical Materials. doi:10.1002/adom.201600270 (2016) [3] Y. Wu, Adv. Mater. doi:10.1002/adma.201603031 (2016) [4] J. Kroll et al., Optics Express 15, 6552 (2007)

Efficient Terahertz Generation Using Fe/Pt Spintronic Emitters Pumped at Different Wavelengths

We report on the determination of the spin Hall angle and the identification of the role of the interface roughness for the inverse spin Hall effect (ISHE) in ultra-clean, defect-reduced epitaxial Pt films. By applying vector network analyzer ferromagnetic resonance spectroscopy to a series of single crystalline Fe (12 nm) /Pt (t