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The development of small and simple room-temperature devices capable of generating and/or receiving resonant signals in a &quot;THz-gap&quot; still stands a significant challenge. However antiferromagnetic spintronics promises to be the right direction for the research and development of THz technology.&lt;br/&gt;&lt;br/&gt;Recently [1], [2], it has been theoretically proposed to use active AFM generators for the detection of external THz-frequency signals via the mechanism of injection-locking of such a signal to the oscillations generated by a DC-current-driven AFM/HM THz generator.&lt;br/&gt;&lt;br/&gt;An alternative way to develop quasi-passive AFM/HM-based detectors [3] is to use the fact that resonance eigenfrequencies of the AFM dynamic modes (standing AFMR modes) lie in the THz frequency range. It has been shown theoretically in [3], that a dielectric AFM having bi-axial anisotropy, such as NiO, can be used for the resonance quadratic rectification of a linearly-polarized AC spin current of THz-frequency, and could have a sensitivity in the range of 10&lt;sup&gt;2&lt;/sup&gt;-10&lt;sup&gt;3 &lt;/sup&gt;V/W.&lt;br/&gt;&lt;br/&gt;In this work [4], we propose the concept of an electrically tunable resonance detector of THz-frequency signals based on an antiferromagnetic/heavy metal bilayer. The conversion of a THz-frequency input signal into DC voltage is done using the inverse spin Hall effect in a bilayer. An additional bias DC current in the HM layer can be used to vary the effective anisotropy of the AFM, and, therefore, to tune the AFMR frequency. The proposed hetero-structure works as a resonance-type quadratic detector which can be tuned by the bias current in the range of at least 10% of the AFMR frequency.&lt;br/&gt;&lt;br/&gt;In addition, we discuss experimental results obtained during spin-pumping experiments with antiferromagnetic/heavy metal bilayer (NiO/Pt). The results were obtained in the HFML-FELIX laboratory where we have access to intense THz radiation free-electron laser source as well as high DC magnetic fields up to 38T.

**References:**

[1] O. Gomonay, T. Jungwirth, and J. Sinova. &lt;i&gt;Physical Review B&lt;/i&gt; 98.10 (2018)&lt;br/&gt;[2] R. Khymyn, V. Tiberkevich, and A. Slavin, Book of abstracts “Intermag – 2018”, p. 795, (2018)&lt;br/&gt;[3] R. Khymyn, V.Tiberkevich, and A.Slavin, &lt;i&gt;AIP Advances&lt;/i&gt; 7.5 (2017)&lt;br/&gt;[4] A. Safin&lt;i&gt;, &lt;/i&gt;V. Puliafito&lt;i&gt;, &lt;/i&gt;M. Carpentieri&lt;i&gt;, &lt;/i&gt;G. Finocchio&lt;i&gt;, &lt;/i&gt;S. Nikitov&lt;i&gt;, &lt;/i&gt;P. Stremoukhov&lt;i&gt;,&lt;/i&gt; A. Kirilyuk&lt;i&gt;, &lt;/i&gt;V. Tyberkevych&lt;i&gt;, and &lt;/i&gt; A. Slavin &lt;i&gt;Appl. Phys. Lett.&lt;/i&gt; 117, 222411 (2020)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/8912bbefc6eefba483c89d307904df9b.jpg)
Schematic view of the THz-frequency resonance detector based on the AFM-Pt structure.



INTERMAG 2021

 Electrically Tunable Detector of THz-Frequency Signals Based on an Antiferromagnet

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The development of small and simple room-temperature devices capable of generating and/or receiving resonant signals in a "THz-gap" still stands a significant challenge. However antiferromagnetic spintronics promises to be the right direction for the research and development of THz technology. Recently [1], [2], it has been theoretically proposed to use active AFM generators for the detection of external THz-frequency signals via the mechanism of injection-locking of such a signal to the oscillations generated by a DC-current-driven AFM/HM THz generator. An alternative way to develop quasi-passive AFM/HM-based detectors [3] is to use the fact that resonance eigenfrequencies of the AFM dynamic modes (standing AFMR modes) lie in the THz frequency range. It has been shown theoretically in [3], that a dielectric AFM having bi-axial anisotropy, such as NiO, can be used for the resonance quadratic rectification of a linearly-polarized AC spin current of THz-frequency, and could have a sensitivity in the range of 102-103 V/W. In this work [4], we propose the concept of an electrically tunable resonance detector of THz-frequency signals based on an antiferromagnetic/heavy metal bilayer. The conversion of a THz-frequency input signal into DC voltage is done using the inverse spin Hall effect in a bilayer. An additional bias DC current in the HM layer can be used to vary the effective anisotropy of the AFM, and, therefore, to tune the AFMR frequency. The proposed hetero-structure works as a resonance-type quadratic detector which can be tuned by the bias current in the range of at least 10% of the AFMR frequency. In addition, we discuss experimental results obtained during spin-pumping experiments with antiferromagnetic/heavy metal bilayer (NiO/Pt). The results were obtained in the HFML-FELIX laboratory where we have access to intense THz radiation free-electron laser source as well as high DC magnetic fields up to 38T.

**References:**

[1] O. Gomonay, T. Jungwirth, and J. Sinova. Physical Review B 98.10 (2018) [2] R. Khymyn, V. Tiberkevich, and A. Slavin, Book of abstracts “Intermag – 2018”, p. 795, (2018) [3] R. Khymyn, V.Tiberkevich, and A.Slavin, AIP Advances 7.5 (2017) [4] A. Safin, V. Puliafito, M. Carpentieri, G. Finocchio, S. Nikitov, P. Stremoukhov, A. Kirilyuk, V. Tyberkevych, and A. Slavin Appl. Phys. Lett. 117, 222411 (2020)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/8912bbefc6eefba483c89d307904df9b.jpg)
Schematic view of the THz-frequency resonance detector based on the AFM-Pt structure.

technical paper

THz spintronics emitters represent today novel sources for broadband emission using nanometer-scaled materials. We present and model the THz spintronic emission based on the spin Hall effect in 3d metals and on the inverse Rashba-Edelstein effect in the optimized topological insulator Bi1-xSbx. 1. Introduction Terahertz (THz) spintronic emitters are promising candidates for covering the so-called THz gap as they offer broadband emission going up to 30 THz [1]. Conventionally, they consist in nanometer-thin heterostructures composed by a ferromagnetic layer and a heavy metal from the 3d-5d family which, owing to their high spin-orbit coupling (SOC), allow spin-to-charge conversion (SCC) from a pure transient spin-current coming from the ferromagnetic layer into a transverse transient charge current. Efforts have been made recently to increase the output emission power of such devices by layer thickness engineering in order to optimize spin-to-charge interconversion and to reduce both THz absorption in metallic layers and spin current relaxation in thick ferromagnets. In this study, we evidence the processes timescales at stake in the emission properties of metallic THz spintronic emitters using Finite Domain Time Domain (FDTD) simulations. We evidence the main figure of merit which will permit to optimize the next generation of THz emitters. We then turn towards an alternative type of THz spintronic emitters based on topological insulators. We discuss the role of surface states in the conversion mechanisms and highlight promising THz emission in topological insulator Bi1-xSbx. 2. Optimization of THz metallic spintronic emitters based on FDTD modelling Conventional THz spintronic emitters are based on ferromagnet/heavy metal heterostructures such as Co/Pt, W/CoFeB/Pt or Fe/Pt. Shining infrared femtosecond laser pulses on the heterostructure induces a transient spin current diffusing from the ferromagnet to the heavy metal. Through spin-to-charge conversion due to the inverse spin Hall effect (ISHE), a THz wave is generated. The direction of the THz emission depends on the direction of the magnetization: the polarization can thus be easily tuned by small external magnetic field. The amplitude of the THz emission depends strongly on the amplitude of the interconversion mechanism, which is related to skew-scattering and side-jump effects in heavy metals. In first approximation, the amplitude of the emission is thus given by the spin Hall angle θSHE = jc/js, ratio between the incoming spin current and the generated charge current given by jc ∝ θSHE (js x σ) where σ is the spin accumulation orientation [2,3]. Here, we provide a comprehensive understanding of the THz emission processes using FDTD modelling, including spin-dependent timescales such as spin-flip rates in heterostructures. We obtain a good agreement between experiments obtained by THz time-domain spectroscopy (THz-TDS) and performed simulations as shown in Fig. 1. We also evidence the crucial role of the interface quality between the heavy metal and the ferromagnet, as spin backflow or interfacial spin-flip may decrease the conversion. We thus propose a figure of merit η ∝ g↑↓σSHElsfHM which defines the THz emission strength in metallic heterostructures. This product includes the spin-mixing conductance g↑↓ illustrating interface quality, spin-Hall conductivity σSHE linked to interconversion strength and spin diffusion length in the conversion material lsfHM proportional to spin-current relaxation. We believe that our results will help optimizing the next generation of THz spintronic emitters. 3. Towards surface-states mediated THz emission using topological insulators Alongside ISHE-based emitters, topological insulators (TI) are studied here as an alternative source for THz emitters. They present conductive surface-states which allow interfacial interconversion via inverse Rashba-Edelstein effect (IREE). In these systems, strong spin-to-charge conversion is expected owing to i) the Fermi velocity of interfacial carriers and ii) the insulating behaviour of the material bulk, reducing possible THz absorption in the heterostructure. Such systems have been proven to show emission in the THz range, for example with topological insulator Bi2Se3 [4]. We report in Fig. 2 emission features from Bi1-xSbx/Co interface. Experimental data are taken with standard electro optic sampling (EOS) using ZnTe(111) 500 μm thickness. Emission performance of Bi0.79Sb0.21(15)/Co(4) is about the same order of magnitude as Co(2)/Pt(4) single-ISHE conversion state-of-the-art emitter. As recovered on IREE-based emitter, THz emission phase is reversed as applied magnetic field direction is reversed. Topological insulators here illustrated by Bi1-xSbx are thus suitable candidates for strong output THz emitters.

**References:**

[1] Seifert, T. et al., “Efficient metallic spintronic emitters of ultrabroadband terahertz radiation”, Nature Photon 10, 483–488 (2016). [2] Dang T. H., Rongione E. et al., “Ultrafast spin-currents and charge conversion at 3d-5d interfaces probed by time-domain terahertz spectroscopy”, Appl. Phys. Rev. 7 (2020) [3] Hawacker J., Rongione E. et al., “Correlation between spin-mixing-conductance, THz emission and spin-memory-loss at magnetic/heavy metal interfaces”, to be submitted [4] Wang, X. et al., “Ultrafast Spin-to-Charge Conversion at the Surface of Topological Insulator Thin Films”, Adv. Mater. 30, 1802356 (2018).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/b0838de1ea6769af462c232ef2eef21b.jpg)
Fig. 1 – Comparison between experimental measurements (blue curve) and FDTD simulations (red curve) for Co(2)/Pt(4) metallic THz emitter considering an infrared excitation pulse duration about 100 fs.

![](https://assets.underline.io/uploads/markdown_image/1/image/b977da06608df08ed43adbd351f5d2ed.jpg)
Fig. 2 – Bi0.79Sb0.21/Co emission alongside metallic ISHE-based emitters for comparison (left). Magnetic field dependence of Bi0.79Sb0.21/Co emission: phase reversal with field direction change proves magnetic origin of the emission (right). Thicknesses in brackets are given in nanometers.

 Efficiency of THz spintronic emitters: from spin-Hall effect in 3d metals to surfaces states in topological insulators INVITED

In recent years, the magnon-phonon coupling in magnetostrictive materials has gained renewed interest due to emerging spintronic and magnonic applications, such as low power consumption microwave devices. Linear and parametric acoustically driven spin waves (ADSW) were studied in various hybrid magnon-phonon structures containing piezoelectric and ferro (ferri) magnetic layers [1-4]. Previously [5-7], we have demonstrated the piezoelectric excitation of linear and parametric ADSW in the gigahertz frequency range and their detection by the combination of spin pumping and inverse spin Hall effect (ISHE) in the spintronic hybrid High overtone Bulk Acoustic wave Resonator (HBAR) with the structure ZnO – YIG– GGG–YIG – Pt (Fig.1a). Here, the features of phonon-magnon interconversion in the phononic cavity mentioned above are studied in detail. The pump frequency fp , power P, and magnetic field H dependences of the ISHE voltage UISHE as well as resonance frequencies fn are experimentally and theoretically studied. As a result of magnetoelastic interaction, bulk acoustic waves (BAW) cavity modes excite magnons in the YIG films either at frequencies fn or at half resonance frequencies (when the threshold power is exceeded). When specifying a certain frequency range, depending on the value of the applied magnetic field, either direct excitation of linear ADSWs or parametric excitation (in low magnetic fields) is observed. In the first case the important features of ADSW and acoustic spin pumping are determined by double resonance: the magnetoelastic (MER) in YIG films and BAW resonance of the entire resonator structure. Electrical detection of ADSWs occurs while simultaneously measuring the electrical response of the ZnO transducer and the dc voltage signal on the Pt stripe. The dependences of the resonance frequencies fn and UISHE on the field H and the frequency f are correlated with each other (Fig. 1b). The fitting of the theoretical and experimental dependences allows us to determine a number of magnetic and magnetoelastic parameters of YIG and field dependencies of the resonance frequencies: magnetoelastic fMER (H) and ferromagnetic fFMR(H). For the frequency range around 2.4 GHz, the magnetic field corresponding to the ferromagnetic resonance is HFMR ≈ 534Oe. In the second case when the field decreases below HFMR, the HBAR spectrum fn is practically independent of the magnetic field, but the clearly detectable voltage UISHE (f, H) is significantly field dependent. The signal from the parametric ADSW was observed when: (i) the pump frequency coincided with one of the HBAR resonant frequencies, (ii) half of the pump frequency coincided with one of the spin wave frequencies for a given magnetic field (see Fig.2a), and (iii) the pump power exceeded a threshold, depending on the field (see Fig 2b). It can be seen from Fig. 2 the field of the maximum voltage is close to the critical field Hc = 184 Oe, at which fFMR(Hc) = fp/2. It indicates that UISHE(Hc) originates from the detection of parametric ADSW with wave number q = 0. The voltage signal is observed in the field range below 432 Oe. This field correlates with the value Hc1 = fp /(2γ) in Fig. 2a. Above this field, the excitation of any parametric spin wave is impossible; therefore, the voltage signal is suppressed. But at the higher fields, the linear excitation of ADSW takes place at the HFMR, at which fFMR(HFMR) = fp. In Fig. 2, the field Hc2= Hc1/2=fp/(4γ) corresponds to the upper limit on H for the possible decays of parametric magnons with the frequency fp/ 2 into two secondary parametric ones at a frequency fp/4. Measurements with a smaller field step show that, in some field ranges, the UISHE (H) dependence has additional features, which can be explained by parametric excitation of various groups of magnons. For example, in a field near Hc3 = 100 Oe, the nonmonotonic behavior of the UISHE (H) dependence is apparently due to the magnon confluence process, for which the condition fFMR (Hc3) = fp /3 is satisfied [8]. For certain ranges of fields and powers, the data of Fig. 2b can be fitted as UISHE (P, H) ~ (P-Pth (H))1/2, where the fitting parameter Pth (H) ≈ 0.4 mW can be interpreted as a threshold power. We explain the low threshold obtained by the high efficiency of electric power transmission into the acoustic pump confined in the phononic cavity - high-Q hybrid HBAR. This work was carried out in the framework of the State task 0030-2019-0013 "Spintronics" and with partial support of the RFBR (Project No. 20-07-01075).

**References:**

1. M. Weiler, H. Huebl and F. S. Goerg, Phys. Rev. Lett., Vol.108, p.176601 (2012). 2. P. Chowdhury, A. Jander and P. Dhagat, IEEE Magn. Lett., Vol. 8, p.1 (2017). 3. H. Keshtgar, M. Zareyan and G. E. Bauer, Solid State Commun., Vol.198, p.30 (2014). 4. I. Lisenkov, A. Jander and P. Dhagat, Phys. Rev. B., Vol.99, p.184433 (2019). 5. N. I. Polzikova, S. G. Alekseev, I. I. Pyataikin., AIP Adv., Vol.8, p.056128 (2018). 6. N. Polzikova, S. Alekseev, V. Luzanov, J. Magn. Magn. Mater., Vol. 479, p.38 (2019). 7. S. G. Alekseev, S. E. Dizhur and N. I. Polzikova, Appl. Phys. Lett., Vol.117. p.072408 (2020). 8. T. B. Noack, V. I. Vasyuchka and D. A. Bozhko, Phys. Stat. Sol. b, Vol.256. p.1900121(2019).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/d9e5c5203a96b021bee099c786fa45d6.jpg)
Fig.1 (a) Schematic of hybrid HBAR for ADSW excitation and acoustic spin pumping. The rf voltage V is applied across the piezoelectric transducer sandwiched between thin-film Al electrodes. The transducer launches and detects a bulk shear acoustic waves, which propagate vertically. (b) Left panel – the fragment of HBAR spectrum, containing two resonant frequencies fn and fn-1 at zero magnetic field. Right panel –3D colour plot UISHE(f,H) . The red points correspond to positions of HBAR resonant frequency fn positions. The calculated frequencies fMER(H) and fFMR(H) are shown by black lines.

![](https://assets.underline.io/uploads/markdown_image/1/image/c6be37479dfd46f03c0afe4c66f67145.jpg)
Fig. 2. (a) Dependences of spin wave frequencies at q = 0 on the magnetic field: f = fH(0) at θ=0; f = fFMR at θ=90 deg (θ is the angle between wave vector and magnetic field), fD =fH(0)+fM /2- upper boundary of dipole (non-exchange) surface magnetostatic waves, fp = fn =2.412 GHz. (b) 3D plot represents UISHE(H, P) measured at fp = fn = 2.412 GHz. Red lines – power dependences UISHE(Hc, P) and UISHE(Hc2, P).

 Spin Waves and Spin Pumping Driven by Cavity Confined Bulk Hypersonic Waves

The use of a nonmagnetic (NM) capping layer on top of a ferromagnetic (FM) layer can efficiently increase the Gilbert damping parameter when the choice of the NM layer is a heavy metal (HM)[1][2]. Spin pumping is an efficient method for the generation of spin currents in FM/NM bilayer films where magnetization precession driven by ferromagnetic resonance (FMR) pumps spin current into NM layer via spin angular momentum transfer[3][4]. The saturation magnetization, Gilbert damping and spin mixing conductance are the critical parameters for efficient spin pumping mechanism. An anti-damping torque can be generated by the application of external DC to the NM layer due to the involvement of Rashba spin-orbit interaction [5][6]. Nevertheless, this procedure reduces the efficiency of the device operation [7]. On the other hand, lower damping value can be achieved for the smaller or comparable thickness of heavy metal NM layer to its spin diffusion length in absence of external DC[8]. In this scenario, a non-equilibrium spin accumulation occurs at the interface due to Rashba spin orbit interaction, which does not allow dissipation of spin angular momentum at the interface. As a result, an anti-damping effect can be observed in FM/HM bilayer system. Heavy metals such as Pt, Ta, etc. show high spin-orbit coupling (SOC) and possess short spin diffusion length up to few nm[9] Bilayers such as Permalloy(Py)/Tantalum(Ta) and CoFeB/Ta has been extensively studied in this context[2], [10]. CoFe alloys as a choice of FM layer provides high saturation magnetization and the composition of the CoFe alloy plays a vital role as the concentration of Co and Fe tune the saturation magnetization and coercivity[11], [12]. Therefore, a systematic variation of NM layer thickness and its effect on magnetization dynamics of CoFe alloy offers the possibility of optimization of Gilbert damping parameter and exploration of interfacial phenomena. In this work, we report a comprehensive study of static and dynamic magnetic properties of a series of sample considering Co50Fe50 and Ta as FM and NM layer respectively. A series of Co50Fe50 (20 nm)/Ta (t nm), henceforth CoFe (20)/Ta (t), samples were deposited on Si substrates at a base pressure of 2×10-7 Torr using UHV magnetron sputtering system. The thickness of the Ta layer was varied as 0, 2, 5, 10, 20, and 30nm. In-plane magnetic hysteresis measurements were carried out using magneto-optical Kerr effect (MOKE) technique with a field variation from -50 mT to 50 mT. A lock-in based FMR technique was used to study the magnetization dynamics where samples were placed on a coplanar waveguide (CPW) in flip-chip configuration. FMR responses were recorded by varying the magnetic field from -300mT to +300mT at a fixed excitation frequency in field sweep mode. The frequency was varied from 2-18 GHz with a step of 1 GHz. MOKE measurements revealed that there were almost negligible variations in coercive fields (HC) for all samples except for CoFe (20)/Ta (2). Fig.1 shows the variation of HC as a function of Ta thickness. The HC for bare CoFe and CoFe (20)/Ta (2) sample was 24 mT and 19 mT respectively. The drop in HC for CoFe (20)/Ta (2) sample may occur due to various reason such as interface strain, variation of microstructure or oxidation at the surface. FMR measurements were performed to investigate the magnetization dynamics of our samples. The linewidth (ΔH), resonance field (Hres), magnetic anisotropy field (HK), saturation magnetization (4πMs), effective Gilbert damping (αeff) and spin mixing conductance (g↑↓) were extracted by fitting the experimental spectra to the appropriate fitting equation. Hres for our samples were found invariant irrespective of Ta layer thickness, which was clear evidence of negligible extrinsic two magnon scattering contribution to the overall damping [13]–[15]. The minimum and maximum 4πMs values were obtained as 1756 mT and 1878 mT for CoFe (20)/Ta (2) and CoFe (20)/Ta (30) sample respectively. 1872 mT was obtained for bare CoFe (20) sample. HK was almost unchanged for all the samples except for CoFe (20)/Ta (2) sample where it was reduced slightly to -105 mT. In-homogenous linewidth broadening (ΔH0) for our samples was varying in between (70 ~ 90 mT). Fig.2 represents the variation of (αeff) as a function of Ta thickness. FMR measurement revealed that there was a significant reduction in αeff for CoFe (20)/Ta (2) sample i.e. 0.041 as compared to bare CoFe (20) sample i.e. 0.068. The sudden decrease in αeff can be attributed to Rashba spin-orbit interaction at the interface. The capping layer thickness of Ta in this sample is lower than the spin diffusion length of Ta which is 2.7 nm [9]. Hence, the non-equilibrium spin accumulation at the interface doesn’t dissipate their spin angular momentum in the regime of spin diffusion length, which leads to the anti-damping phenomena and ultimately reduces the damping. αeff was further increased to 0.055 for CoFe (20)/Ta (5) sample and saturated to 0.049 for the higher thickness of Ta capping layer. An anomalous trend in αeff was observed due to the absence of DC current in the NM layer and this behaviour is consistent with the earlier report on Py/Ta bilayer [8]. The maximum g↑↓ was obtained as 38.7 x 1018 m-2 for CoFe (20)/Ta (2) sample. It is expected due to the enhancement of Δα with the involvement of Rashba spin orbit interaction and higher thickness of FM layer.

**References:**

[1] S. Azzawi et al., “Evolution of damping in ferromagnetic/nonmagnetic thin film bilayers as a function of nonmagnetic layer thickness,” Phys. Rev. B, vol. 93, no. 5, pp. 1–6, Feb. (2016). [2] E. Montoya et al., “Spin transport in tantalum studied using magnetic single and double layers,” Phys. Rev. B, vol. 94, no. 5, p. 054416, Aug. (2016). [3] Y. Tserkovnyak, A. Brataas, and G. E. W. Bauer, “Spin pumping and magnetization dynamics in metallic multilayers,” Phys. Rev. B, vol. 66, no. 22, p. 224403, Dec. (2002). [4] M. Jamali, A. Klemm, and J.-P. Wang, “Precessional magnetization induced spin current from CoFeB into Ta,” Appl. Phys. Lett., vol. 103, no. 25, pp. 1–5, Dec. (2013). [5] A. Manchon, H. C. Koo, J. Nitta, S. M. Frolov, and R. A. Duine, “New perspectives for Rashba spin–orbit coupling,” Nat. Mater., vol. 14, no. 9, pp. 871–882, Sep. (2015). [6] G. Allen, S. Manipatruni, D. E. Nikonov, M. Doczy, and I. A. Young, “Experimental demonstration of the coexistence of spin Hall and Rashba effects in β-tantalum/ferromagnet bilayers,” Phys. Rev. B, vol. 91, no. 14, pp. 1–9, Apr. (2015). [7] S. E. Barnes, J. Ieda, and S. Maekawa, “Rashba Spin-Orbit Anisotropy and the Electric Field Control of Magnetism,” Sci. Rep., vol. 4, no. 1, pp. 1–5, May (2015). [8] N. Behera, S. Chaudhary, and D. K. Pandya, “Anomalous anti-damping in sputtered β-Ta/Py bilayer system,” Sci. Rep., vol. 6, pp. 1–9, May (2016). [9] M. Morota et al., “Indication of intrinsic spin Hall effect in 4d and 5d transition metals,” Phys. Rev. B - Condens. Matter Mater. Phys., vol. 83, pp. 1–5, (2011). [10] V. Lauer, P. Pirro, B. Heinz, M. Geilen, and A. V Chumak, “The role of the non-magnetic material in spin pumping and magnetization dynamics in NiFe and CoFeB multilayer systems,” vol. 163901, pp. 1–9, (2015). [11] A. Melloul and A. Kharmouche, “Synthesis, structure and magnetic properties of CoxFe100–x thin films thermally evaporated onto Si (111) substrate,” J. Mater. Sci. Mater. Electron., vol. 30, no. 14, pp. 1–7, Jul. (2019). [12] S.-M. Ahn and G. S. D. Beach, “Crossover between in-plane and perpendicular anisotropy in Ta/Co x Fe 100- x /MgO films as a function of Co composition,” J. Appl. Phys., vol. 113, no. 17, pp. 1–3, May (2013). [13] J. Lindner et al., “Non-Gilbert-type damping of the magnetic relaxation in ultrathin ferromagnets: Importance of magnon-magnon scattering,” Phys. Rev. B - Condens. Matter Mater. Phys., vol. 68, no. 6, pp. 1–4, (2003). [14] K. Lenz, H. Wende, W. Kuch, K. Baberschke, K. Nagy, and A. Jánossy, “Two-magnon scattering and viscous Gilbert damping in ultrathin ferromagnets,” Phys. Rev. B - Condens. Matter Mater. Phys., vol. 73, no. 14, pp. 1–6, (2006). [15] N. Behera, A. Kumar, S. Chaudhary, and D. K. Pandya, “Two magnon scattering and anti-damping behavior in a two-dimensional epitaxial TiN/Py(t Py )/β-Ta(t Ta ) system,” RSC Adv., vol. 7, no. 14, pp. 8106–8117, (2017).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/5cc7f9fc85cd12c6667f92610bc761ad.jpg)
Variation of Hc values for different thicknesses of Ta.

![](https://assets.underline.io/uploads/markdown_image/1/image/c14ac435fa75d8bacc4fd5c1de9bff0a.jpg)
Effective Gilbert damping (αeff) as a function of Ta thickness.

 Effect of Ta capping layer on damping properties in Co50Fe50 thin films.

FeCo alloys have been widely studied for decades because of their unique metallic and magnetic properties. In recent years, the search of new systems adequate for spintronic devices promoted the study of bilayers consisting of a ferromagnetic film (FM) in contact with a non-magnetic conductive metal (NM). In these bilayers, when the FM is exposed to microwave radiation it can be driven to its ferromagnetic resonance (FMR) condition to produce the injection of a pure spin current across the FM/NM interface. In this framework, the search for new materials and alloys suitable for the fabrication of FM/NM systems, from which an intense pure spin current could be obtained, is one of the main challenges of today’s spintronics. With the purpose of increasing the intensity of the pure spin current density, J0s(sp), which is inversely proportional to the damping parameter (α) of the ferromagnet (J0s(sp)∼1/α2)[1] during the spin pumping, the search of materials with low damping constants has been very active during the last years. A recent work by M. Schoen et al. [2] reported that FexCo100-x polycrystalline alloys present an ultra-low magnetic damping near x = 75 ( 20 Oe at 10 GHz), which make them promising candidates for the efficient injection of spin currents. Our results highlight the influence of the sputtering conditions (deposition power and temperature) on the magnetic properties of Fe80Co20/Ta bilayers deposited on MgO(001) and Si(001) which have been studied using ferromagnetic resonance and magneto-optic Kerr-effect techniques. We found that for the studied fabrication conditions the samples deposited on MgO(001) presented [100] epitaxial growth with a cubic anisotropy field ∼300 Oe, while those sputtered on Si(001) grow polycrystalline with a small uniaxial anisotropy. Moreover, the deposited samples on MgO grow with the axis [100] of the Fe80Co20 rotated by 45○ from the axis [100] of the MgO substrate (this was verified using XRD techniques). We have found that the optimal growth conditions to obtain a minimum in the damping constant and films with cubic symmetry are 13 - 16 W of sputtering power and single crystal MgO substrates. Substrate heating during sputtering at temperatures of 150 ○C also helps to increase the quality of the films. Ferromagnetic resonance measurements at different excitation frequencies (Fig. 1) have determined exceptionally low damping values for ferromagnetic conductors (α∼3×10-3), which makes the Fe80Co20/Ta system an excellent candidate for future applications in spintronic devices. From the dependence of the FMR linewidth on different excitation frequencies, we also found, different relaxation mechanisms act in easy and hard magnetization directions (Fig. 1). From the analysis of the dependence of the FMR linewidth on the orientation of the applied magnetic field and the excitation frequency, we were able to separate the contribution of the different relaxation mechanisms (Gilbert damping, two magnon scattering and mosaicity) to the linewidth and to explain quantitatively the observed behavior (Fig. 2). We made simulations to estimate the mosaicity and obtained relatively small values, consistent with a good quality of the samples. Finally, we also estimated the strength of the two magnon scattering which agrees with values reported in Fe-rich alloys [3]. The models proposed in this work were corroborated for different excitation frequencies. Our present results are very promising for the development of spintronic devices with improved functionalities. This work was partially supported by Conicet under Grant PIP 201501-00213, ANPCyT Grant PICT 2013-0401, and U.N. Cuyo Grant 06/C556 all from Argentina. We also received financial support through the international cooperation project between Argentina and Belgium MINCyT-FWO FW/15/01-VS.041.16 N.

**References:**

&lt;font style="vertical-align: inherit;"&gt;&lt;font style="vertical-align: inherit;"&gt;[1] JE Gómez, B. Zerai Tedlla, NR Álvarez, G. Alejandro, E. Goovaerts y A. Butera Phys. &lt;/font&gt;&lt;font style="vertical-align: inherit;"&gt;Rev. B 90, 184401 (2014) &lt;/font&gt;&lt;/font&gt; &lt;font style="vertical-align: inherit;"&gt;&lt;font style="vertical-align: inherit;"&gt;[2] Martin AW Schoen, Danny Thonig, Michael L. Schneider, TJ Silva, Hans T. Nembach, Olle Eriksson, Olof Karis y Justin M. Shaw. &lt;/font&gt;&lt;font style="vertical-align: inherit;"&gt;Nature Physics, 12, 839 (2016) &lt;/font&gt;&lt;/font&gt; &lt;font style="vertical-align: inherit;"&gt;&lt;font style="vertical-align: inherit;"&gt;[3] K. Lenz, H. Wende, W. Kuch y K. Baberschke, Phys. &lt;/font&gt;&lt;font style="vertical-align: inherit;"&gt;Rev. B 73, 144424 (2006)&lt;/font&gt;&lt;/font&gt;

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/8afe134b414fee19314f00acd8b9d19c.jpg)
Fig.1 Frequency dependence of the resonance linewidth for the sample grown on MgO and heated at 150 ○C, for the easy (solid squares) and hard (solid circles) anisotropy axes. The fit (solid line) was made considering a linear dependence (Gilbert damping), while the dashed line represents the linear contribution for the hard direction with the addition of two magnon scattering term.

![](https://assets.underline.io/uploads/markdown_image/1/image/e46216c67f31f547286726704a685f52.jpg)
Fig.2 Angular variation of ΔHr (solid circles) and added contributions of two magnon scattering term and mosaicity (solid lines) for X-band. We also show the individual contribution of two magnon sacattering and mosaicity as dotted and dashed lines, respectively.

 Unraveling relaxation mechanisms in ultra-low damping Fe80Co20 thin films

We report the experimental observation of an excitation in a film of the ferrimagnet yttrium iron garnet (YIG), driven by the spin current generated by the giant spin Hall effect in a platinum strip with nanoscopic silver particles adjacent to the YIG film. The excitation, detected by Brillouin light scattering (BLS), has a frequency different from the thermal magnons and phonons in YIG, and does not vary with the applied magnetic field, but changes with film thickness. We interpret the BLS signal as due to phonons excited by the magnonic spin current injected into the YIG film, in a process that is the Onsager reciprocal of the spin pumping by coherent elastic waves. The observations are supported by a theory based on a process where one magnon in the magnonic spin current creates one phonon and another magnon, with conservation of energy and momentum. The theoretical value of the critical current in the metallic strip necessary to drive phonons is in order of magnitude agreement with the value measured experimentally. The experiments were performed with two samples, sample A made is with a 154 nm thick single-crystal YIG film grown by liquid-phase epitaxy onto a 0.5 mm thick [111]-oriented Gd3Ga5O12 (GGG) substrate, cut in rectangular shape with dimensions 8.0 x 1.0 mm2, and sample B made in the same way with a 93 nm thick YIG film. The bilayer samples were prepared by deposition on the YIG film of a metallic layer strip, made of Pt (3 nm)/Ag (9 nm)/Pt (3 nm) of width 150 µm, by means of DC magnetron sputtering. As shown in Ref. [1], with the appropriate deposition conditions, the strip forms nanoscopic particles of Ag in the midplane of a 6 nm thick Pt layer, that exhibits a giant spin Hall effect (SHA). A charge current in the metallic strip is converted into a spin current that flows into the YIG film as a magnonic spin current. The excitations in the YIG films are detected by back-scattering BLS using a (2x3)-pass tandem Fabry-Perot interferometer. Figure 1a shows the BLS spectrum obtained with the YIG (154 nm)/[Pt-Ag] sample, with no current in the NM strip. The spectrum was collected with 6 000 interferometer scans, with the sample under an in-plane magnetic field of 2.0 kOe. The peak denoted by M, with frequency shift of 12.6 GHz, corresponds to the Stokes scattering by thermal magnons. The inset shows the measured frequency shifts for some values of the magnetic field and the solid line represents the calculations with the magnon dispersion relation. The peak at 26 GHz, denoted by TP-GGG, is due to transverse phonons in the GGG substrate. For positive currents in the NM strip with intensity below 23 mA, there is no change in the BLS spectrum of Fig. 1(a). However, for currents above 23 mA, a new peak in the BLS spectrum shows up with frequency 7.55 GHz, with much larger intensity than the thermal peaks. Figure 1b shows the BLS spectra obtained with only 1 000 interferometer scans. The peak intensity increases with increasing current, as in Fig. 1c, and the frequency increases slightly to 7.8 GHz as the current approaches mA. For currents above this value, the BLS peak broadens and its intensity decreases due to heating effects. The thermal magnon peak cannot be seen in Fig. 1b because with 1 000 scans its amplitude is only 5 counts. The nonmagnetic character of the current driven excitation in is demonstrated in Fig. 1e showing that the frequency of the excitation does not depend on the magnetic field. A clue to the origin of the observed current-driven excitation is given by measurements in a thinner YIG film. Figure 1e shows data obtained with sample YIG/[Pt-Ag]-B, that has a YIG film with thickness 93 nm and a metallic layer as in the previous sample. By passing a current in the metallic strip of the sample, a peak emerges in the BLS spectrum for currents above a critical value smaller than in YIG/[Pt-Ag]-A, and with larger frequency, 13.5 GHz. Figure 1f shows that the frequency also does not vary with the magnetic field. The facts that the frequency of the BLS peaks increase with decreasing YIG film thickness and do not depend on the magnetic field, strongly suggest that the current driven oscillations originate from phonons. The origin of the phonon pumping observed here lies in the magnon-phonon interaction. Calculations show that the mechanism of the phenomenon reported is the generation of phonons by a magnon-phonon-magnon splitting process, driven by magnons of the magnonic spin current in the YIG film generated by the spin Hall effect in the metallic layer [2].

**References:**

[1] O. Alves-Santos, E. F. Silva, M. Gamino, R. O. Cunha, J. B. S. Mendes, R. L. Rodríguez-Suárez, S. M. Rezende, and A. Azevedo, Phys. Rev. B 96, 060408(R) (2017). [2] J. Holanda, D. S. Maior, O. Alves Santos, A. Azevedo, and S. M. Rezende, Appl. Phys. Lett., 118, 022409 (2021).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/a24180545e1e3c110d291f895adbf99c.jpg)
Figure 1. (a) BLS spectrum of YIG (154 nm)/GGG under an in-plane magnetic field of 2.0 kOe, measured with 6 000 interferometer scans. Inset shows the field dependence of the magnon (M) peak. TP-GGG denotes the transverse phonon in GGG. (b) BLS spectra in the YIG (154 nm)/[Pt-Ag] sample under an in-plane magnetic field of 1.13 kOe, with a dc current in the [Pt-Ag] layer with intensity as indicated, collected with 1 000 interferometer scans. (c) BLS spectra in the YIG (93 nm)/[Pt-Ag] sample under an in-plane field of 1.13 kOe, with a current in the [Pt-Ag] layer of intensity as indicated, collected with 1 000 interferometer scans. (d) BLS peak intensity versus current in the YIG (154 nm)/[Pt-Ag] sample. (e) Frequency of the BLS peak with current 43 mA versus applied field in the YIG (154 nm)/[Pt-Ag] sample. (f) Frequency of the BLS peak with current 10 mA versus applied field in the YIG (93 nm)/[Pt-Ag] sample.

 Phonon Pumping by Magnonic Spin Currents: Experiments and Theory

The recent demonstration of pure spin current-induced spin-transfer torque arising from the spin Hall effect (SHE) represents an efficient route to controlling the magnetization dynamics in magnetic nanostructures. In a ferromagnet /heavy metal bilayer, a pure spin current is generated when a longitudinal charge current passes through the HM and induces a transverse spin current due to the strong spin-orbit coupling in the HM [1, 2]. The conversion of the charge current density (JC) to pure spin current density (JS) is characterized by the spin Hall angle (θSHA) in these bilayers. The pure spin current may be sufficient to excite perpetual self-oscillations in the magnetization, which can further induce spin waves in these systems. This is of particular interest for spintronics applications and has led to a new class of microwave devices called spin Hall nano-oscillators (SHNOs) [3,4]. Generating higher spin current densities through a higher θSHA is necessary to improve the performance of SHNOs and to avoid higher charge current densities. So far, studies have focused on different HMs with various spin-orbit coupling strengths to generate higher spin current---for example, the β-phase of W, Ta, or NixCu1-x. In this work, we demonstrate the compositional effect on the magnetodynamic and auto-oscillations properties of Ni100-xFex/Pt (x= 10 to 40) nanoconstriction based spin Hall nano-oscillators [5]. The devices are fabricated from Ni100-xFex(5) /Pt (6) bilayer (thicknesses in nanometers) deposited in a high vacuum magnetron sputtering chamber where Ni-Fe alloys were co-sputtered under the same conditions and from pure Ni and Fe targets, where the composition was established by varying the respective plasma powers. Two kinds of spin Hall devices were fabricated from each film: (1) 8 x 16 μm2 rectangular stripes and (2) nanoconstriction-based SHNOs with a width of 140 nm. We first discuss the ST-FMR spectra measured on rectangular stripes to determine the variation of the magnetodynamics with the Fe content. We measure an increase in the magnetization (μ0Ms) and a decrease in the Gilbert damping. In addition to that, we measure a reduction of the impact of spin-torque on the ST-FMR linewidth with the dc current (μ0ΔH/ I) as Fe content increases Fig. 1(a). This is translated into a reduction in the spin Hall angle with the increase of Fe content as shown in Fig. 1(b). The reduction in the spin Hall angle can be correlated primarily to the compositional effect: the spin Hall angle scales inversely proportional with the saturation magnetization. Note that the observed variation of spin Hall angle with increasing Fe content is also qualitatively consistent with the decrease of damping as a function of the Fe content, and can be well explained in terms of the spin transport model Ref. [6]. The suppression of spin pumping due to increased μ0Ms should result in lowering the effective spin mixing conductance and therefore spin-torque efficiency, which can be seen as a monotonic decrease of spin Hall angle with increasing Fe content in Fig. 1(b). Next, we turn to discuss the compositional effect on the characteristics of auto-oscillations in nanoconstriction-based SHNOs. We record the generated microwave power spectral density (PSD) as a function of a direct current (I) under an in-plane magnetic field μ0H = 0.05 T using a spectrum analyzer and a low noise amplifier with a +33 dB gain. The spectral characteristics of the auto-oscillations i.e. frequency, power, and linewidth were extracted from the PSD. It is interesting to note that the onset auto-oscillation frequency differs between devices, i.e. the onset frequency shifts up for Fe-rich devices due to higher magnetization. The integrated power varies with the dc current, showing a bell shape of amplitude around 1 pico-watt. A qualitative comparison between the power of devices shows that Fe-rich devices have lower output power. This is understood as the readout of devices depends on AMR and as the AMR drops in Fe-rich devices the power follows. Finally, the measured auto-oscillations have linewidths around 50 MHz. The threshold current for auto-oscillations, (Ith), is extracted from plots of p-1 vs. I, [7] as shown in the inset of Fig. 1(c). For Fe-rich devices, higher currents are required to excite auto-oscillations. The enhancement in threshold current densities for Fe-rich devices is a direct consequence of the reduction in the spin Hall angle. Our experimental results show that the dominant compositional effect is from the magnetization that plays a central role in determining the characteristics of the spin Hall devices.

**References:**

[1] J. E. Hirsch, Phys. Rev. Lett. 83, 1834 (1999). [2] M. I. D’yakonov’ et al., J. Exp. Theor. Phys.Lett.13, 467 (1971). [3] V. E. Demidov, et al. ,Nat. Mater. 3459, 1028 (2012). [4] T. Chen et al., Proc. IEEE 104, 1919 (2016). [5] M. Haidar et al., Appl. Phys. Lett. 118, 012406 (2021). [6] M. W. Keller, et al., Phys. Rev. B 99,214411 (2019). [7] A. Slavin et al., IEEE Transactions on Magnetics 45, 1875 (2009).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/bd3b1adcc571a8457bc27caad2e0ddfb.jpg)
Fig. 1 (a, b) Variation in |μ0 ΔH /I| and the spin Hall angle (θSHA) as a function of Fe composition extracted from the ST-FMR measurements. (c) Threshold current vs. Fe concentration. Inset shows how the threshold is extracted from a linear fit of p−1 vs. I at low currents (dashed line).

 Compositional effect on spin-wave auto-oscillation behavior of Ni100-xFex/Pt spin Hall nano-oscillators

The control of magnetization dynamics is an important issue for the development of high-frequency devices based on spintronics. There are several ways to excite magnetization dynamics. In our previous study, we reported the amplification of magnetization dynamics derived from the interaction between local magnetization dynamics and spin waves by using a slot line waveguide [1]. In this paper, we report the study on the evaluation of the interaction by measuring ST-FMR. The spin orbit torque is caused by the spin current induced by the spin hall effect in ferromagnetic and non-ferromagnetic bilayer system. The ST-FMR is originally used for the estimation of the spin orbit torque via the measurement of the spin hall magneto resistance. We found that the ST-FMR signals for the samples described below delivered information on the interaction between local magnetization dynamics and spin waves. Fig.1 shows the sample structure and measurement set up for the ST-FMR. We used yttrium iron garnet (YIG) as a ferromagnetic layer. Coplanar waveguides were prepared on the YIG film. The signal line consisted of Cu layer (200 nm) , and was partially replaced by 10nm thick Pt layer. In the separated Pt areas, additional local magnetization dynamics was excited by the spin orbit torque, emitting spin waves. The distance between Pt areas was systematically varied, leading the change of the strength of the interaction between the local magnetization dynamics and spin waves at the Pt areas. Fig.2 shows the distance between Pt areas dependence of ST-FMR signal measured at 7.0 GHz. The sample with one Pt shows symmetrical peaks. On the other hand, the samples with two Pt areas indicate that the asymmetry of the peaks changes with the distance of Pt areas. The symmetry and asymmetry of the peaks reflect the contribution of torques to the magnetization dynamics from the spin orbit torque and RF magnetic field respectively at Pt areas [2]. It should be noted that the strength of RF magnetic field was fixed for all the samples, meaning that the increase in the asymmetry was caused by an additional torque to the magnetization. Another factor is speculated from the interaction between local magnetization dynamics and spin waves because the direction of the torque for the interaction is considered the same as the direction for the RF Oersted field, leading the asymmetry of the peak for the ST-FMR measurement. The asymmetry of the curve showed strong dependence on the distance between Pt areas, reflecting the strength of the interaction. Those experimental results give a way to design the artificially controlled magnetization dynamics effectively. We thank GRANOPT Co,Ltd. for their support of YIG single crystal thin films. This work was supported in part by JSPS KAKENHI Grant Number JP18K14114. This work was supported in part by CSRN, Tohoku University. Part of this work was carried out under the Cooperative Research Project Program of the Research Institute of Electrical Communication, Tohoku University. A part of this work was carried out at Nano-Fabrication Support Laboratory, Yamaguchi University supported by "Nanotechnology Platform Project" of the Ministry of Education，Culture，Sports，Science and Technology (MEXT)，Japan.

**References:**

[1] Tetsunori Koda, Sho Muroga, and Yasushi Endo, “Synchronized excitation of magnetization dynamics via spin waves in Bi-YIG thin film by slot line waveguide” Appl. Phys. Lett. 116, 102403 (2020). [2] Luqiao Liu, Takahiro Moriyama, D. C. Ralph, and R. A. Buhrman, “Spin-Torque Ferromagnetic Resonance Induced by the Spin Hall Effect” Phys. Rev. Lett., 106, 036601 (2011).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/163119439d61e9013a174ed3d596d259.jpg)
Schematic view of measurement set up for ST-FMR measurement. A part of the signal line consists of Pt layer for inducing the spin orbit torque.

![](https://assets.underline.io/uploads/markdown_image/1/image/72e2a7835f5ae71297581d9613fbf769.jpg)
Gap of Pt areas dependence of ST-FMR signal measured at 7.0 GHz.

 Evaluation of interaction between local magnetization dynamics and spin waves measured by ST-FMR

Spintronics research concentrated its efforts on handling spin polarization current leading to the success-story of giant magnetic resistance spin valves [1-2]. More recently, pure spin currents, i.e. a spin flux without charge flux, are at the centre of interest since they do not produce Joule heating, reducing power dissipation in spintronics devices, as in STT-MRAM [3]. These applications require an increase in the emission and detection efficiencies via a full understanding of the complex physics behind pure spin currents. Spin current can be detected by inverse spin Hall effect (ISHE) that corresponds to the conversion of a spin current into a detectable charge current by measuring the voltage on a normal metal presenting a strong spin-orbit interaction. Pure spin current emission can be obtained by temperature gradients, by charge currents and by magnetization dynamics. This last mechanism permits to generate pure spin current via ferromagnetic resonance in a ferromagnetic material [4]. We will present acoustic spin pumping experiments in Co/Pt bilayers, using surface acoustic waves (SAW) to induce the magnetization precession in the Co layer taking advantage of resonant magnetoelastic coupling. We will report a peculiar behaviour of SAW-FMR assisted spin pumping in a multiferroic system where Co/Pt bilayer is in contact with a ferroelectric substrate LiNbO3 [5]. We will show that the substrate on which the bilayer is deposited plays a role in the spin current excitation processes. We will show that the electrical polarization vector affects the ISHE voltage: the inversion of the magnetization and of the SAW propagation vector modifies the spin pumping efficiency. We suggest that this polarization is an important factor in the spin pumping excitation processes. The nature of the coupling with the magnetic polarization may be due to a magnetoelectric coupling at the interface between LiNbO3 and Co, inducing non equilibrium spin density across the whole thin film [6] and affecting spin pumping efficiency.

**References:**

[1] M. N. Baibich, J. M. Broto, A. Fert, F. N. Van Dau, F. Petroff, P. Eitenne, G. Creuzet, A. Friederich, and J. Chazelas, Phys. Rev. Lett. 61, 2472 (1988). [2] J. M. George, L. G. Pereira, A. Barthelemy, F. Petroff, L. Steren, J. L. Duvail, A. Fert, R. Loloee, P. Holody, and P. A. Schroeder, Phys. Rev. Lett. 72, 408 (1994). [3] S. Maekawa, S. O. Valenzuela, E. Saitoh, and T. Kimura, Spin Current (Oxford University Press, 2017) pp. 1-520. [4] M. Weiler, H. Huebl, F. S. Goerg, F. D. Czeschka, R. Gross, and S. T. B. Goennenwein, Phys. Rev. Lett. 108, 176601 (2012). [5] P. Rovillain, R. Cardoso de Oliveira, M. Marangolo, and J.-Y. Duquesne Phys. Rev. B 102, 184409 (2020). [6] C.-L. Jia, T.-L. Wei, C.-J. Jiang, D.-S. Xue, A. Sukhov,and J. Berakdar, Phys. Rev. B 90, 054423 (2014).

**Images:**

 Nonsymmetric spin pumping in a multiferroic heterostructure using surface acoustic wave

Spin current transmission in antiferromagnetic materials is one of the intriguing topics in antiferromagnetic spintronics. The antiferromagnetic order can mediate the transmission of the spin current in the form of propagating magnons. Several groups have reported this phenomenon in varieties of antiferromagnetic materials, such as NiO, CoO, Fe2O3, FeMn, and IrMn. Most of the investigations have so far been conducted at frequencies much lower than the resonant frequency of the antiferromagnetic materials. In order to explore the physics of the phenomenon, it is crucial to extend the investigation to the THz range, at which antiferromagnets typically have a good susceptibility to electromagnetic field, because this frequency domain embraces the antiferromagnetic resonant frequencies. A suitable method for investigating antiferromagnetic dynamics could be a relatively recently developed technique, where THz radiation is generated with an ultrashort laser pulse in a heavy metal (HM) / ferromagnetic metal (FM) bilayer structure [1,2]. In this work [3], we investigated laser stimulated THz emission from Pt/CoO/FeCoB and explored the sub-picosecond pulsed spin current transmission through the antiferromagnetic CoO interlayer. We also particularly look into the polarization of the THz electromagnetic wave and reveal that the latter is influenced by the Néel vector in the antiferromagnets. Figure 1 shows the schematic illustration of the measurement set up. Magnetic field of 100 mT was applied to &lt;110&gt; direction and the CoO Néel vector initially remains in one of the [100] directions, which is an easy axis of CoO. The field cooling process then invokes a twisting of the CoO magnetic moment. The THz electromagnetic wave with x and y components and the polarization angle φ was detected by using two wire grid polarizers. The polarization of the emitted THz wave is parallel to the electric field Ec induced in the Pt layer. The polarization of the THz wave essentially tells us the polarization direction of the spin current injected into the HM layer through the antiferromagnetic layer. Figures 2(a) and 2(b) show the y component of the THz wave, Eypeak, and φ as a function of temperature. Eypeak decreases with decreasing temperature regardless of CoO layer thickness dCoO, which is associated with the dependence of the sheet conductivity of the films and the inverse spin Hall conductivity of the Pt layer [4]. On the other hand, one can notice that φ apparently increases at lower temperature and has an onset at around 200K and 300K for dCoO=2.0 and 5.0 nm, respectively, which coincide with the blocking temperature for these samples. Assuming the polarity of spin density of the ultrafast spin current induced in the FM layer is same direction as magnetization in FeCoB layer, the variations of φ for dCoO=2.0 and 5.0 nm below the blocking temperature indicates that the spin current carried by the THz magnon experiences a non-uniform scattering background, which preferentially scatters the spin orthogonal to the Néel vector gradually rotating away from the axis of magnetization in FeCoB layer. This observation is consistent with previous reports on the spin current experiments in exchange biased systems where the spin current impedance was modified by an accommodation of Néel vector twisting [5]. In summary, we found that sub-picosecond pulsed spin current induced by the femtosecond laser pulse can transmit through antiferromagnetic CoO layers. Our results not only demonstrate the picosecond magnon spin current transmission, but also the picosecond interaction of the THz magnons with the Néel vector in the antiferromagnet. This work was partially supported by JSPS KAKENHI Grant (Nos. 17H04924, 17H05181, H1803787, 19K21972, and 26103004), the Center for Spintronics Research Network (CSRN), and the Center for Science and innovation in Spintronics (CSIS). This work was also partly promoted by the Collaborative Research Program of the Institute for Chemical Research, Kyoto University. One of the authors (YS) acknowledges the Graduate Program in Spintronics (GP-Spin) at Tohoku University.

**References:**

[1] T. Seifert, et al., Nature Photon. 10, 483 (2016). [2] T. Seifert, et al., Nature Communications 9, 2899 (2018). [3] Y. Sasaki, et al., Appl. Phys. Lett. 117, 192403 (2020). [4] M. Matthiesen, et al., Appl. Phys. Lett. 116, 212405 (2020). [5] T. Moriyama, et al., Phys. Rev. Lett. 119, 267204 (2017).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/eb0b73916188b733cb3165bdf5db5783.jpg)
Figure 1. Schematic illustration of the measurement setup.

![](https://assets.underline.io/uploads/markdown_image/1/image/effab4554ad2e2d60020bae6c0b8fbbd.jpg)
Figure 2. Temperature dependence of (a) the peak intensity of Ey and (b)THz polarization angle φ. Open circle, triangle, and square show the data for films with dCoO=0, 2.0 and 5.0 nm, respectively.

 Laser pulse induced ultrafast spin current through the antiferromagnetic insulator in Pt/CoO/FeCoB

Elliptically or circularly polarized terahertz (THz) radiation plays increasingly important roles in both scientific research and practical applications, such as wireless communication, composition analysis and undamaged imaging1. Recently, the realization of highly efficient THz radiation in ferromagnetic multilayer via spin-charge conversion (SCC) brings new hope for the emerging spin THz emitter at room temperature2. However, achieving an easily-tunned THz emitter with polarization and ellipticity manipulable still be challenging. Topological insulator (TI), with strong spin-orbital coupling and spin-momentum-locked surface states, has been regarded as the most effective spin-charge convertor in spin THz emitter3. In addition, the broken inversion symmetry at surface provides topological insulator numerous fascinating nonlinear optical phenomena, the dominant one is linear photogalvanic effect (LPGE), which could yield a polarization-dependent terahertz radiation in topological insulator4. By contrast, the terahertz wave induced by SCC is polarization-independent. This difference in polarization-dependence of two terahertz components makes it possible to control them separately, inspiring a new approach in chirality-tunned terahertz emission by synthesizing the SCC-induced and LPGE-induced terahertz waves only by tailoring the polarization of incident pumping laser. Here we prepared a Bi2Te3 (10nm)/Fe (3nm) heterostructure by molecular beam epitaxy (MBE) and measure the terahertz response after illumination by a linear polarized femtosecond laser. The terahertz wave was probed by a terahertz time domain spectroscopy (THz-TDS). As shown in Figure 1a, the amplitude of the terahertz generated in Bi2Te3/Fe heterostructure is higher than single Bi2Te3 or Fe thin film, almost keeps in the same level with W/CoFeB/Pt trilayer5. The latter is usually regarded as a typical spin terahertz emitter and the result proves the high efficiency of Bi2Te3/Fe heterostructure in terahertz emission. After further careful examination, we divided the overall terahertz radiation into two components. The first is SCC-induced spin terahertz. Generated from the spin-charge conversion in the Bi2Te3/Fe interface, this component is only magnetization-dependent. Meanwhile, an ultrafast photocurrent can also originate in Bi2Te3 film due to LPGE in the inversion symmetry breaking surface, leading to a terahertz wave. Notably, the direction of the LPGE-induced terahertz is related with the polarization of incident laser, which provides a simple method to construct orthogonal terahertz components by adjusting the incident polarization. More importantly, a phase difference between these two terahertz components could be observe because of the delayed time between SCC and LPGE. After deliberately tunning the incident laser to promise both terahertz components in the same magnitude, an elliptical polarized terahertz wave appears in Bi2Te3/Fe heterostructure. Furthermore, a terahertz wave with arbitrarily tailored chirality and ellipticity could be got by tailoring the pumping laser polarization. Overall, we have demonstrated a highly efficient terahertz emission in Bi2Te3/Fe heterostructure. Compared with conventional spintronic terahertz emitter, Bi2Te3/Fe can realize terahertz wave with arbitrarily tailored chirality only by adjusting the incident polarization. The engineering of topological insulator and ferromagnet heterostructure may open up a great opportunity to study high-performance TI-based spintronic terahertz devices with easy controllability.

**References:**

(1) D. M. Mittleman, Opt. Express., Vol. 26, p.9417 (2018) (2) T. Kampfrath; M. Battiato; P. Maldonado, et al., Nat. Nanotechnol., Vol. 8, p.256 (2013) (3) X. Wang; L. Cheng; D. Zhu, et al., Adv.Mater., Vol. 30, p.1 (2018) (4) C. M. Tu; Y. C. Chen; P. Huang, et al., Phys. Rev. B., Vol. 96, p.1 (2017) (5) Y. Wu; M. Elyasi; X. Qiu, et al., Adv.Mater., Vol. 29, p.1 (2017)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/08f5430c71e14fba98452d73a3a96d90.jpg)
Fig. 1. (a) Typical THz temporal waveforms generated from Fe (3 nm), TI (10 nm), TI (10 nm)/Fe (3 nm) and W (1.8 nm)/CoFeB (1.8 nm)/Pt (1.8 nm), respectively. (b, c) Generation right and left-handed elliptical polarized THz waves by tuning incident laser polarization angle.

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