France



Creating a dissipation-less pure spin current and being able to detect it, have been one of the biggest challenges that held the spintronics community for decades. In the beginning of 1970’s, Dyakonov et al.&lt;sup&gt;1&lt;/sup&gt;, made a big break through in this aspect by discovering the Spin Hall Effect (SHE), that made it possible to create a pure spin current by sending an electrical current in non-magnetic heavy metals like Platinum (Pt), this effect relies on the Spin-Orbit Coupling (SOC) interaction which links the spin degree of freedom to the orbital degree of freedom resulting in a preferential directional scattering for electrons of different spins.&lt;br/&gt;Recently another type of SOC have been discovered, which relies on the Rashba interaction&lt;sup&gt;2,3&lt;/sup&gt;, this effect stems for a joint action of the SOC and an electrical built-in potential in two-dimensional electrons systems (2 DES) existing at interfaces of two different materials. A promising system to study the spin to charge conversion is the interface between two band gap insulators like the interface between LaAlO&lt;sub&gt;3 &lt;/sub&gt;/SrTiO&lt;sub&gt;3 &lt;/sub&gt;(LAO/STO) &lt;sub&gt; &lt;/sub&gt;, where a 2DES is created due a polar discontinuity at the interface&lt;sup&gt;4,&lt;/sup&gt;&lt;sup&gt;5&lt;/sup&gt;&lt;sup&gt;,&lt;/sup&gt;. This 2 DES at the interface of LAO/STO shows a very strong Rashba spin-orbit interaction raising from the breaking of the inversion symmetry. It appears that due to the high dielectric constant in STO below 30K&lt;sup&gt;6&lt;/sup&gt;, we are able to tune our Rashba splitting using an electric field&lt;sup&gt;7&lt;/sup&gt;. In this work we showed that the Rashba splitting in LAO/STO depends on the angular dependence of the injected spins, so we are able to retrieve an important information regarding the symmetry of the orbitals that are participating in the conversion signal&lt;sup&gt;8&lt;/sup&gt;.

**References:**

&lt;sup&gt;1&lt;/sup&gt;&lt;sup&gt; &lt;/sup&gt;M. I. Dyakonov International Journal of Modern Physics BVol. 23, No. 12n13, pp. 2556-2565 (2009).&lt;br/&gt;&lt;sup&gt;2&lt;/sup&gt;&lt;sup&gt; &lt;/sup&gt;E I Rashba 1959 Fiz. Tverd. Tela 1 407 (E I Rashba 1959 Sov. Phys.-Solid State 1 368).&lt;br/&gt;&lt;sup&gt;3&lt;/sup&gt;&lt;sup&gt; &lt;/sup&gt;Linding Yuan et al., Nature Communications volume 10, Article number: 906 (2019).&lt;br/&gt;&lt;sup&gt;4&lt;/sup&gt; Thiel S, Hammerl G, Schmehl A, Schneider C W and Mannhart J Science 313 1942 (2006).&lt;br/&gt;&lt;sup&gt;5&lt;/sup&gt; Cantoni C et al Adv. Mater. 24 3952 (2012)&lt;br/&gt;&lt;sup&gt;6&lt;/sup&gt; H. E. Weaver 7 J. Phys. Chem. Solids Pergamon Press. Vol. 11. pp. 274-277 (1959).&lt;br/&gt;&lt;sup&gt;7&lt;/sup&gt; A. D. Caviglia et al., Phys. Rev. Lett. 104, 126803 – Published 26 March (2010).&lt;br/&gt;&lt;sup&gt;8&lt;/sup&gt; J.-Y. Chauleau et al. , EPL, vol 116, num 1 (2016)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/c6f6ee6f6e30005c8a72604e2db48f29.jpg)
Fig: Angular dependence of the DC IEE conversion signal for a 5-monolayer-thick LAO, for different positive back gate voltages.



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 Spin to charge conversion at LaAlO3/SrTiO3 interface states

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The Tenth Peer Review Congress features 3 days for presentations of new research into peer review and all aspects of scientific publication, bias, quality of reporting, and information access and dissemination.

Creating a dissipation-less pure spin current and being able to detect it, have been one of the biggest challenges that held the spintronics community for decades. In the beginning of 1970’s, Dyakonov et al.1, made a big break through in this aspect by discovering the Spin Hall Effect (SHE), that made it possible to create a pure spin current by sending an electrical current in non-magnetic heavy metals like Platinum (Pt), this effect relies on the Spin-Orbit Coupling (SOC) interaction which links the spin degree of freedom to the orbital degree of freedom resulting in a preferential directional scattering for electrons of different spins. Recently another type of SOC have been discovered, which relies on the Rashba interaction2,3, this effect stems for a joint action of the SOC and an electrical built-in potential in two-dimensional electrons systems (2 DES) existing at interfaces of two different materials. A promising system to study the spin to charge conversion is the interface between two band gap insulators like the interface between LaAlO3 /SrTiO3 (LAO/STO) , where a 2DES is created due a polar discontinuity at the interface4,5,. This 2 DES at the interface of LAO/STO shows a very strong Rashba spin-orbit interaction raising from the breaking of the inversion symmetry. It appears that due to the high dielectric constant in STO below 30K6, we are able to tune our Rashba splitting using an electric field7. In this work we showed that the Rashba splitting in LAO/STO depends on the angular dependence of the injected spins, so we are able to retrieve an important information regarding the symmetry of the orbitals that are participating in the conversion signal8.

**References:**

1 M. I. Dyakonov International Journal of Modern Physics BVol. 23, No. 12n13, pp. 2556-2565 (2009). 2 E I Rashba 1959 Fiz. Tverd. Tela 1 407 (E I Rashba 1959 Sov. Phys.-Solid State 1 368). 3 Linding Yuan et al., Nature Communications volume 10, Article number: 906 (2019). 4 Thiel S, Hammerl G, Schmehl A, Schneider C W and Mannhart J Science 313 1942 (2006). 5 Cantoni C et al Adv. Mater. 24 3952 (2012) 6 H. E. Weaver 7 J. Phys. Chem. Solids Pergamon Press. Vol. 11. pp. 274-277 (1959). 7 A. D. Caviglia et al., Phys. Rev. Lett. 104, 126803 – Published 26 March (2010). 8 J.-Y. Chauleau et al. , EPL, vol 116, num 1 (2016)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/c6f6ee6f6e30005c8a72604e2db48f29.jpg)
Fig: Angular dependence of the DC IEE conversion signal for a 5-monolayer-thick LAO, for different positive back gate voltages.

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The spin current generated from the spin Hall effect of the heavy metal layer can exert spin orbit torques (SOTs) on the adjacent ferromagnetic layer and further produce current-induced SOT-driven magnetization switching. However, for the ferromagnetic layer with in-plane magnetic anisotropy (IMA), probing such switching phenomenon typically relies on tunneling magnetoresistance of three-terminal devices[1-3], differential planar Hall resistance in Hall-bar devices[4], or Kerr effect imaging approach[5]. In this work, we provide a reliable and all-electrical scheme to characterize both the damping-like SOT (DL-SOT) and field-like SOT (FL-SOT) in micron-sized heterostructures with IMA. Firstly, for magnetic heterostructures containing heavy metals, there exist current-induced in-plane spin Hall effective fields (Hyeff) and unidirectional magnetoresistance (UMR) that will modify their anisotropic magnetoresistance (AMR) behavior (Fig.1). The electrical detection of SOT-driven magnetization switching (Fig.2 (b) and (d)) in W/CoFeB heterostructures is demonstrated by means of AMR measurement under such influences. Through switching measurements with various pulse widths, the effective DL-SOT efficiency (|ξDL| ≈ 0.32) can be extracted for Hall-bar devices with 2.0 nm ≤ tCo-Fe-B ≤ 3.5 nm, which is fairly consistent with the values for thin W layer obtained by other approaches[3, 6-8]. The effective FL-SOT efficiency ( (|ξFL| ≦ 0.05) ) can be obtained by AMR loops shift measurements and is found to be slightly enhanced by inserting a MgO layer on top of CoFeB. Moreover, we demonstrate that for a micro-sized device with a well-defined easy axis (along y-direction), as prepared from an 8-inch CMOS-compatible fabrication facility, the detection of SOT switching can also be achieved by the UMR readouts. Our studies suggest that this pump (DL-SOT) -probe (SHF-modified AMR or UMR) approach allows for characterizing key parameters of SOT switching from IMA heterostructures without lengthy device fabrication processes and complicated measurement protocols.

**References:**

[1] M. Cubukcu, O. Boulle, M. Drouard, et al., Appl. Phys. Lett., Vol. 104, 042406 (2014). [2] L. Liu, C.-F. Pai, Y. Li, et al., Science, Vol. 336, 555 (2012). [3] C.-F. Pai, L. Liu, Y. Li, et al., Appl. Phys. Lett., Vol. 101, 122404 (2012). [4] G. Mihajlović, O. Mosendz, L. Wan, et al., Appl. Phys. Lett., Vol. 109, 192404 (2016). [5] S. Shi, S. Liang, Z. Zhu, et al., Nat. Nanotechnol., Vol. 14, 945-949 (2019). [6] R. Bansal, G. Nirala, A. Kumar, et al., Spin, Vol. 08, 1850018 (2019). [7] Q. Hao, G. Xiao, Phys. Rev. Appl., Vol. 3, 034009 (2015). [8] S. Mondal, S. Choudhury, N. Jha, et al., Phys. Rev. B, Vol. 96, 054414 (2017).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/e2f0127374b5a84b126e71ea2a9a7422.jpg)
Fig. 1. Representative shifted AMR loops measured from a W(3)/CoFeB(1.8) sample with dc currents Idc = ±3mA.

![](https://assets.underline.io/uploads/markdown_image/1/image/5edd50eccb0c9cfaf3b331791e9cd810.jpg)
Fig. 2. (a, c) Measurement sequence for the switching experiment and (b, d) current-induced SOT switching results of a W(3)/Co40Fe40B20(1.8) Hall-bar sample with opposite sense currents Isense = +1 mA and Isense = -1 mA.

 Detection of Spin-Orbit Torques for In-plane Magnetized Heterostructures by Utilizing Spin Hall Effective Field

The current induced spin-orbit-torque (SOT) [1-4] is the key physical parameter describing the generation of magnetization dynamics in bilayers composed by a heavy metal (HM) layer and a ferromagnetic (FM) layer. Such bilayers, especially when associated with a magnetic tunnel junction at the FM layer (then forming the so called 3-terminal device), are considered as promising elements to be applied in novel magnetic memories, microwave generators and even bio-inspired types of computing [1]. Therefore, the robust and accurate determination of the SOT in HM/FM bilayers is of high relevance for optimizing future applications. In HM/FM bilayers the SOT is mainly caused by the spin Hall effect (SHE) of the HM layer, by which a spin current is injected into the FM layer. 3-terminal devices with application as magnetic memories adopt mainly FM films with perpendicular magnetic anisotropy, while for sensor applications also the configuration with in-plane magnetic anisotropy is of interest. The determination and quantification of the SOT can be performed in both cases by different techniques. While in a 3-terminal device the magnetization dynamics of observed via the tunnel magnetoresistance effect, in HM/FM bilayers other methods can be employed, such as the SOT-ferromagnetic resonance (FMR), the harmonic Hall voltage measurement and the spin Hall magnetoresistance (SMR). The SMR is a straightforward method when the FM layer is an insulator, while with metallic FM, in which the testing electric current in not confined in the HM layer only, it is accompanied by additional effects, such as the anisotropic magnetoresistance or the anomalous Nernst effect [5]. Therefore, it is important to analyse the measurement results by accurate models taking into account all aspects. The dominant theoretical approach for SHE-SOT is the drift-diffusion model considering spin accumulation at the interface between the FM and HM by introducing a spin mixing conductance. However, the agreement with experiments may be poor if spin dissipation at the interface (spin memory loss, spin dependent scattering, etc.) is disregarded [2]. In this paper we analyze transport measurements on FM/HM bilayers as a function of the HM thickness by employing a non-equilibrium thermodynamic approach to SHE-SOT [6] with the aim to reach a semi-quantitative agreement. The study was performed on HM/FM bilayers with HM=Pt (wedge 4-19nm), Ta (wedge 4-19nm), and FM=Fe60Co20B20 (2nm) prepared at Singulus Technologies AG. All samples were annealed for 2h at 310°C in a magnetic field of 1T in order to develop an in-plane magnetic anisotropy (y axis). For the transport measurements the films were patterned into µm-size Hall bars/crosses (along x axis) by optical lithography and ion beam etching with Ti/Au contact pads. Magneto-optic Kerr effect (MOKE) and SMR measurements were performed following the protocol of [5], while SOT-FMR and harmonic Hall voltage measurements and analysis were based on references [3,4]. Fig.1 shows the comparison between MOKE loops and the SMR signal. The loop along the y axis is more rectangular and indicates the magnetic easy anisotropy axis. The SMR signal is much more pronounced with the field along the x axis, while it shows a small peak close to the coercivity when the field is along the y axis. With MOKE the anisotropy was determined and a dependence on HM thickness was observed. Fig.2 presents the summary of the SOT-FMR and harmonic Hall voltage measurements for HM/FM bilayers with different thickness of HM. FMR analysis enabled determination of effective magnetization and magnetization damping (α = 0.015 for Pt and 0.0075 for Ta), while anomalous Hall effect and harmonic Hall measurements resulted in SOT efficiencies dependence vs. HM thickness. While damping-like SOT efficiency for Ta reaches ξDL-Ta = -0.35 and ξDL-Pt = 0.15 for Pt, with high resistivity difference (ρTa &gt; 200 µΩcm and 24 &lt; ρPt &lt; 52 µΩcm) taken into account, the spin Hall conductivity for Pt of σSH-Pt = 2.9 (105 S/m) exceeds the one for Ta σSH-Ta = 1.7 (105 S/m) and is among the highest values reported [8]. The HM thickness dependence of the SMR is analyzed by considering both the finite conductance at the interface [7] (an approach valid for small thicknesses) and thermodynamic approach [6] (valid for larger thicknesses). The results are compared with the harmonic Hall voltage measurements the SOT and the SOT efficiency and discussed in relation to the key parameters as the spin Hall angle and the spin diffusion length. Acknowledgements: WS acknowledges National Science Centre Grant No. UMO-2015/17/D/ST3/00500, Poland. Microfabrication was performed at the Academic Centre for Materials and Nanotechnology of AGH.

**References:**

[1] B. Dieny et al. Nature Electronics 3, 446 (2020) [2] A. Manchon et al., Rev. Mod. Phys. 91, 035004 (2019) [3] M. H. Nguyen et al., Phys. Rev. Lett., 116, 126601 (2016) [4] C. F. Pai et al. Phys. Rev. B., 92, 064426 (2015) [5] Y.-T. Liu et al Phys. Rev. Applied 13, 044032 (2020) [6] H. Nakayama et al., Phys. Rev. Lett. 110, 206601 (2013) [7] V. Basso et al., J.Phys.D: Appl.Phys., 51, 214006 (2018) [8] L. Zhu et al. Phys. Rev. Applied 10, 031001 (2018)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/16c07a1ee87bd7446aea9196a14c3567.jpg)
Fig. 1. a) MOKE loop on Hall bar device with Pt thickness tPt=6.12 nm, under applied field parallel (black) or perpendicular (red) to the Hall bar. Inset1: Hall bar. Inset 2: variation of the MOKE loop’s coercive field with different Pt thickness. b) SMR measurement on the same tPt=6.12 nm device, with applied field parallel (darkgreen, lightgreen) and perpendicular (blue, red) to the Hall bar, with opposite electric current flows.

![](https://assets.underline.io/uploads/markdown_image/1/image/0c428b9a17006ee2e71123e986a38c75.jpg)
Fig. 2. Magneto-transport measurement results of HM/FM bilayers: (a) SOT-FMR for tTa = 4.4 nm buffer with different excitation frequency (f) symmetric shape of the signal indicate dominance of the damping-like SOT, (b) f vs. in-plane magnetic field (H) for different HM with tHM ≈ 5 nm, fitting based on Kittel formula enables effective magnetization calculation, (c)-(d) damping-like SOT efficiency dependence on tHM.

 Spin Hall magnetoresistence and spin orbit torque efficiency in (Pt,Ta)/FeCoB bilayers

Spin current control has attracted increasing attention from the viewpoint of fundamental research and application owing to their attractive performances; zero Joule heating, nano scale devices, and expansion of dimension. As typical phenomena we investigated the spin Seebeck effect (SSE) and the spin Hall magnetoresistance (SMR) in the bilayer consisting of a magnetic insulator Y3Fe5O12 (YIG) and a heavy metal (Pt). Since the spin current is originated from the diffusion phenomenon, the signals, that is, spin Seebeck coefficient and MRxx of SMR, are generally small. Therefore, it is important to increase the efficiencies of spin current generation and propagation for practical use. We observed significant enhancement of SSE and SMR values by inserting 0.3nm or 0.6 nm layers of magnetic materials with different composition (Cr, Ni80Fe20, Co90Fe10, Fe50Co50, and Fe) at the YIG/Pt interface [1]. The SSE and SMR showed the mostly same dependences on the insertion layer composition and thickness as shown in Fig. 1, indicating that there is the common reason of SSE and SMR enhancement. To find the reason, we investigated the magnetism of Pt on the insertion layers and the insertion layers themselves. It is known that the magnetization is induced in the Pt layer when Pt contacts the ferromagnetic metals including Fe or Co. However, it has not been clarified whether Pt on the sub-nm-thick magnetic layer is magnetized. In order to investigate the Pt magnetic property, we carried out X-ray magnetic circular dichroism (XMCD) at the BL39XU of SPring-8. The Pt thickness was decreased to 1 nm in order to emphasize the signal from Pt near the interface with the insertion layers. As a result, it was confirmed that there is merely negligible moment in Pt for all samples with and without insertion layers by XMCD. Next, we investigated the magnetism of the sub-nm insertion layers by generalized magneto-optical ellipsometry (GME). Although the magnetic materials with sub-nm thickness does not show magnetization at room temperature in general, there is the possibility that magnetization is induced when ultra-thin magnetic layer is deposited on the ferrimagnetic YIG. However, it is difficult to measure the magnetization of sub-nm insertion layers deposited on the YIG by using macroscopic measurement such as VSM or SQUID because the magnetization signal of the insertion layers is hidden by that of YIG. Compared to this, the surface magnetic information can be extracted by GME analysis. It was found that the magnetic insertion layers of 0.3 nm and 0.6 nm did not exhibit a measurable magnetization. On the contrary, when the thickness of insertion layers is increased to 1 nm, the magnetization becomes obvious and SSE and SMR were decreased. Taking it into consideration, the state of losing magnetic order in the insertion layers seems to increase the spin current propagation from YIG to Pt. In summary, the SSE and SMR was enhanced by the insertion of thin magnetic material layers, which did not result from a net interface magnetization in Pt and the insertion layers. On the contrary, the paramagnetic state is considered to have an important role to enhance the spin current related phenomena.

**References:**

[1] T. Niimura et al., Phys. Rev. B 102 094411 (2020).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/5d801144e5ba2cac759e99351e9a6aa2.jpg)
Spin Seebeck coefficient SSSE (a) and MRxx of SMR (b) for YIG/insertion layer/Pt where the insertion layer material is Cr, Ni80Fe20, Co90Fe10, Fe50Co50, or Fe.

 Spin Current Enhancement by Inserting Ultra-thin Magnetic Layer at Interface between YIG and Pt INVITED

While classical spintronics has traditionally relied on ferromagnetic metals as spin generators and spin detectors, a new approach called spin-orbitronics exploits the interplay between charge and spin currents enabled by the spin-orbit coupling in non-magnetic systems, in particular using the spin Hall effect. However, the interconversion efficiency of the direct and inverse spin Hall effect is a bulk property that rarely exceeds ten percent, and does not take advantage of interfacial and low-dimensional effects otherwise ubiquitous in spintronics. In this contribution, we report the observation of spin-to-charge current conversion in strained mercury telluride, using spin pumping experiments at room temperature. We show that a HgCdTe barrier can be used to protect the HgTe topological surface states, leading to high conversion rates, with inverse Edelstein lengths up to 2.0±0.5 nm. These measurements, associated with the temperature dependence of the resistivity, suggest that these high conversion rates are due to the spin momentum locking property of HgTe surface states [1]. We then focus on the SrTiO3 (STO)-based 2D electron system, presenting experiments performed on NiFe/Al/STO heterostructures. We investigate the nature of the spin-to-charge conversion through a combination of spin pumping, magnetotransport, spectroscopy and gating experiments, finding a very highly efficient spin-to-charge conversion, with inverse Edelstein lengths beyond 20 nm. More importantly, we demonstrate that the conversion rate can be tuned in amplitude and rate by a gate voltage. We then discuss the amplitude of the effect and its gate dependence on the basis of the electronic structure of the 2DES and highlight the importance of a long scattering time to achieve efficient spin-to-charge interconversion. Finally, we harness the electric-field-induced ferroelectric-like state of SrTiO3 to manipulate the spin–orbit properties of the two-dimensional electron gas. Using spin pumping techniques and a back-gate voltage (cf. figure), we efficiently convert spin currents into positive or negative charge currents, depending on the polarization direction [3]. This non-volatile effect opens the way to the electric-field control of spin currents and to ultralow-power spintronics, in which non-volatility would be provided by ferroelectricity rather than by ferromagnetism.

**References:**

[1] P. Noel et al., Phys. Rev. Lett. 120, 167201 (2018) [2] D. C. Vaz et al., Nature Materials 1-7 (2019) [3] P. Noël et al., Nature, 580(7804), 483-486 (2020)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/3c004d9b544e1474370e93e56b3a4a2b.jpg)
Left: Scheme of the spin-pumping technique. A rf field is used to induce magnetization precession in the NiFe layer. This leads to the injection of a pure spin current Js into the 2D electron gaz at the surface of STO. This spin current is then converted into a charge current Jc, which can be detected using a voltmeter. The back gate is used to pole the STO. Right: For a given spin current, the produced charge currrent depends in a non-volatile way of the gate voltage.

 Spin to charge conversion in the topological insulator HgTe and in STO-based two-dimensional electron gas INVITED

Spin currents generated by the spin Hall effect in e.g. heavy metals like tungsten (W) are interesting candidates for next generation information storage applications. The spin Hall effect has intrinsic and extrinsic contributions of which the latter can be controlled by adding impurities into the material. This enhances the spin Hall angle (SHA) θSH, which is the ratio of charge to spin current density [1]. When considering energy efficient spin current generation, it is necessary to also consider the spin Hall conductivity, which is the SHA divided by the charge resistivity. While W in its high resistive phase (β-W) has the highest spin Hall angle among pure heavy metals, it has a relatively low spin Hall conductivity (SHC). In contrast, α-W is a low resistivity phase with a low SHA but high SHC [2]. In this work, we investigated the changes in SHA and SHC in α-W with copper (Cu) impurities. We chose to measure the SHA with spin-torque ferromagnetic resonance (ST-FMR), as it also provides information about the ferromagnetic effective damping αeff. The effective damping parameter is essential when optimizing the stack for energy efficient applications because it is inversely proportional to the critical switching current density. Multilayer stacks of Ti(2)|Fe(5)|Cu100-xWx(5)|Ti(10) were fabricated with the Cu100-xWx layers deposited via co-sputtering of Cu and W at different RF and DC-powers to obtain alloys of different concentrations. The ST-FMR measurements were carried out for 6 different frequencies (10-15 GHz) and at each frequency the field was swept from 0 to 5 kOe. The measured DC voltage contains a symmetric and anti-symmetric Lorentzian function, whose prefactors can be used to obtain the spin Hall angle. We found that the SHA increases with increasing W concentration until it reaches a maximum at around 60% W. The SHA changes from 0.054 ± 0.004 to 0.20 ± 0.02 , which corresponds to an enhancement of 270%. For the SHC, the trend is similar, with an increase of 120%. Results are shown in Fig 1 (a) and (b), respectively. As the resistivity also increases with impurity content, the SHC shows a smaller enhancement than the SHA. The large SHA at 60% W could be attributed to a high crystallinity as revealed by XRD measurements. As ST-FMR offers the determination of the effective damping, we can determine the ratio of αeff/θSH, which is proportional to the critical magnetization switching current density [3]. We find that it increases only slightly with increasing W concentration (Fig 2 (a)). At 60% W, αeff/θSH decreases by a factor of 4 as compared to pure W. The ratio of αeff/θSH almost follows the inverse of the SHA (Fig 2 (b)), suggesting that the change in damping affects the ratio less than changes in SHA. Our findings here reveal that the Cu40W60 alloy has excellent spin-orbit torque switching characteristics, allowing it to be utilized in energy-efficient spintronic devices.

**References:**

 [1] G.D.H. Wong et al., Scientific Reports., 10(1), p. 9631 (2020) [2] W.-B. Liao et al., physica status solidi (RRL) – Rapid Research Letters., Vol. 13(11), p. 1900408 (2019) [3] K.-S. Lee et al., Applied Physics Letters., Vol. 102(11), p. 112410 (2013)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/571fb38d111111037937e37f59f7b175.jpg)
Fig.1: (a) spin Hall angle and (b) spin Hall conductivity for different W concentration. Dashed lines serve as guide-to-the-eye.

![](https://assets.underline.io/uploads/markdown_image/1/image/8b9d087e6e96d3344b780e708625e6c9.jpg)
Fig.2: (a) effective damping (b) ratio of effective damping and spin Hall angle, which gives information about the qualitative behaviour of the critical switching current for in-plane magnetization. Dashed lines serve as guide-to-the-eye.

 Spin Hall conductivity enhancement of tungsten by copper alloying

The spin Hall effect in heavy-metal thin films is routinely employed to convert charge currents into transverse spin currents and can be used to exert torque on adjacent ferromagnets. Conversely, the inverse spin Hall effect is frequently used to detect spin currents by charge currents in spintronic devices up to the terahertz frequency range. Numerous techniques to measure the spin Hall effect or its inverse were introduced, most of which require extensive sample preparation by multi-step lithography. To enable rapid screening of materials in terms of charge-to-spin conversion, suitable high-throughput methods for measuring the spin Hall angle are required. Here [1], we compare two lithography-free techniques, terahertz emission spectroscopy [2] and broadband ferromagnetic resonance [3], to standard harmonic Hall measurements [4] and theoretical predictions [5] using the binary-alloy series AuxPt1-x as benchmark system. Despite being highly complementary, we find that all three techniques yield a spin Hall angle with approximately the same x-dependence, which is also consistent with first-principles calculations. Quantitative discrepancies are discussed in terms of magnetization orientation and interfacial spin-memory loss.

**References:**

[1] M. Meinert, B. Gliniors, O. Gueckstock et al., Phys. Rev. Appl., Vol. 14, p.064011 (2020). [2] T. Seifert, S. Jaiswal, U. Martens et al., Nat. Photonics, Vol. 10, p.483 (2016). [3] A. J. Berger, E. R. J. Edwards, H. T. Nembach et al., Phys. Rev. B, Vol. 97, p.094407 (2018). [4] K. Fritz, S. Wimmer, H. Ebert et al., Phys. Rev. B, Vol. 98, p.094433 (2018). [5] M. Obstbaum, M. Decker, A. K. Greitner et al., Phys. Rev. Lett., Vol. 117, p.167204 (2016).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/1f66821a3d8bd1f09e3567422892d41f.jpg)

 High-throughput techniques for measuring the spin Hall effect

Recently, it has been investigated that spin-orbit coupling at magnetic/nonmagnetic interface induces a spin-to-charge current conversion with different symmetry from the conventional inverse spin Hall effect (c-ISHE) [1,2]. Here, the orientation of spin current polarization can be manipulated by the direction of perpendicular magnetized material (figure) [3]. In this study, we investigate the magnetization-dependent inverse spin Hall effect (md-ISHE) in SiN(10 nm cap)/TbxCo100-x(19 nm)/Pt(1 nm)/YIG(167 nm)/SiO2 (substrate) devices due to spin Seebeck effect. We clearly observe voltage signals due to the c-ISHE and md-ISHE in our devices, which the md-ISHE polarity depends on the magnetization direction of the spin-converter layer. Moreover, the magnitude of induced charge current due to md-ISHE is observed to be almost insensitive for all TbxCo100−x samples, whereas the c-ISHE significantly depends on the composition ratio. This work was partially supported by a Grant-in-Aid for Scientific Research (C) (Grant No. 20K05307) from JSPS and the TokaiFoundation of Technology.

**References:**

[1] A. M. Humphries, T. Wang, E. R. J. Edwards, S. R. Allen, J. M. Shaw, H. T. Nembach, J. Q. Xiao, T. J. Silva, and X. Fan, Observation of spin-orbit effects with spin rotation symmetry, Nature Communications 8, 911 (2017). [2] S. C. Baek, V. P. Amin, Y.-W. Oh, G. Go, S.-J. Lee, G.-H. Lee, K.-J. Kim, M. D. Stiles, B.-G. Park, and K.-J. Lee, Spin currents and spin–orbit torques in ferromagnetic trilayers, Nature Materials 17, 509 (2018). [3] T. C. Chuang, D. Qu, S. Y. Huang, and S. F. Lee, Magnetization-dependent spin Hall effect in a perpendicular magnetized film, Phys. Rev. Research 2, 032053 (2020).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/a69fe25424a8a6b55c460a84860f73ee.jpg)
Figure (a) Schematic illustrations and results of (a) c-ISHE and (b) md-ISHE.

 Magnetization-dependent inverse spin Hall effect at perpendicular magnetized Tb-Co/Pt interface

The spin to charge interconversion arising from spin-orbit effects is an important tool for spin manipulation. The charge current to spin current conversion has attracted lots of attention as it can be used to write information through the spin orbit torque. The reciprocal mechanism, the spin to charge conversion, has also been found to be essential, in particular as it enables the reading process of magneto-electric spin-orbit devices [1, 2]. Whereas the efficiency of the interconversion is a crucial issue for applications, a consensus on the measured values seems to remain elusive, even for standard materials. The spin to charge interconversion can be for instance obtained by the Spin Hall Effect in platinum. The bulk contribution of platinum to the interconversion efficiency has been widely studied, and the interfacial contributions to the interconversion, such as the spin memory loss at the ferromagnet-platinum interface [3] and spin backflow [4], have been identified as key factors in the measurement of the spin to charge interconversion. These two contributions indeed decrease the amount of spin current flowing through the interface, possibly leading to a wrong estimation of the interconversion efficiency. Here, we will focus on the estimation of the interface resistance of the spin-orbit material/ferromagnetic interface, and of its exact role on spin to charge interconversion, showing that it has up to now been frequently wrongly overlooked. In this contribution, we measured electrically the inverse and direct spin to charge interconversion in ferromagnet/Pt nanostructures (Fig. 1) for different ferromagnetic materials. We find that the amplitude of the Spin Hall signal depends drastically on the magnetic material (CoFe, Co, or NiFe) used for the injection and the detection of the spin current. We show that the use of a conventional analysis, neglecting the interface resistance at the ferromagnet-Pt interface, gives effective spin Hall angle for Pt which are actually strongly dependent on the used ferromagnetic material. By using cross geometry measurement and finite element simulations, we succeeded in extract the interface resistance between Pt and the ferromagnetic materials. Using these resistance values, we show that in order to obtain a consistent spin to charge interconversion efficiency value in the different systems, an interfacial spin scattering asymmetry must be taken into account. This parameter, despite being frequently overlooked in spinorbitronics experiment, appears to be essential for a proper extraction of the spin to charge interconversion efficiency. Finally, it show that an important interface resistance can, contrarily with what could be expected, enhance the spin Hall signal in electrical measurements (Fig. 2). These results thus provide guidelines for the optimization of the optimization of the write and read processes using spin to charge interconversion in materials with strong spin orbit coupling.

**References:**

[1] S. Manipatruni et al. Nature Phys. 14, 338 (2018) [2] V. T. Pham, M. Cosset-Cheneau et al, arXiv:2011.12207 (2020) [3] J.-C. Rojas-Sanchez et al, Phys. Rev. Lett. 112, 106602 (2014) [4] W. Zhang et al, Nature Phys. 11, 496–502 (2015)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/6123e54e5e7c2ea0aa50c47e29d7ef35.jpg)
Sketches presenting the measurement using the ferromagnet (FM)/ spin-orbit coupling (SOC) nanostructure. The ferromagnetic electrodes allows to probe the spin accumulation and the charge-spin interconversion, and can reciprocally be used to inject a spin current in the SOC material to study the spin-charge interconversion. The studied SOC material is Pt.

![](https://assets.underline.io/uploads/markdown_image/1/image/a874514a41b1600a02d21f56caa56179.jpg)
Evolution of the interconversion signal when changing the interface resistance RA. The parameter γ represents the amplitude of the spin scattering assymetry.

 Role of the interfacial asymmetric spin scattering at ferromagnet-Pt interfaces

Spintronic devices explore spin degree of freedom in addition to charge of an electron. The success of spin-based electronics devices depends on the injection, manipulation and detection of spin currents1. Recently, ferromagnet (FM) and heavy metal (HM) bilayer structures have received great interest due to its spin-charge interconversion. A pure spin current can be generated at the interface between FM/NM via spin pumping. In general, ferromagnetic resonance (FMR) technique is employed in spin pumping to generate pure spin currents through magnetization precession in FM layer which will be injected in to NM layer through the interface. The efficiency of the spin injection is quantitatively measured as spin-mixing conductance(g↓↑) which estimates the amount of spin current injected into the NM layer. The spin-mixing conductance depends on several parameters such as interface roughness, crystalline phase and quality of the NM layer2. The influence of crystalline phase3 and roughness4 on spin-mixing conductance have been investigated but the studies on the effect of crystallite size and quality of NM layer on g↓↑ are limited. In this work, we have investigated the effect of crystallite size of Tantalum on g↓↑ in Ta (18 nm)/Ni80Fe20 (16 nm) bilayers. The bilayer structures are deposited using magnetron sputtering technique. The crystallite size is varied by tuning the growth rate during sputtering deposition by varying the sputtering power from 60 to 120 watt as shown in figure1(a). Grazing incidence X-Ray diffraction (GI-XRD) technique is used to study the phase of Ta. The Ta films that are deposited at different deposition rates in our study have exhibited BCC (α-Ta) crystal structure as shown in figure 1(b). Crystallite size was calculated by Scherrer formula and observed that with growth rate crystallite size is improved up to 120 watts. The thickness and surface morphology of the Ta films were measured using atomic force microscopy (AFM). The measured average rms roughness for all samples is found to be below 0.6 nm. The Ta/Ni80Fe20 bilayer structures are fabricated to investigate the spin-pumping mechanism at the interface. The FMR technique is used for spin pumping in Ta/Ni80Fe20 in the range of 4 -16 GHz while sweeping magnetic field from 0 – 3000 Oe. From the FMR spectrum shown in figure 2(a) we obtained linewidth and resonance field to determine the change in damping parameter which in turn reveals the spin-mixing conductance. We have quantitatively estimated g↓↑ of our bilayer structure and observed that it is in the order of 1018 m-2. Interestingly, increase in the crystallite size of Ta in Ta/Ni80Fe20 bilayer film enhances the spin-mixing conductance as shown in the figure 2(b). It shows a better spin pumping at Ta/ Ni80Fe20 interface with larger crystallite size. Enhancement of g↓↑ with Ta crystallite size is potentially due to the less scattering in Ta layer thus reducing the spin-back flow.

**References:**

1. Fabian, J., Sarma, S. Das & Igor Zutic, Jaroslav Fabian, S. D. S. Spintronics: Fundamentals and applications. Rev. Mod. Phys. 76, (2004). 2. Tserkovnyak, Y., Brataas, A. & Bauer, G. E. W. Enhanced Gilbert Damping in Thin Ferromagnetic Films. Phys. Rev. Lett. 88, 4 (2002). 3. Kumar, A., Bansal, R., Chaudhary, S. & Muduli, P. K. Large spin current generation by the spin Hall effect in mixed crystalline phase Ta thin films. Phys. Rev. B 98, 1–6 (2018). 4. Pham, T. K. H. et al. Interface morphology effect on the spin mixing conductance of Pt/Fe3O4 bilayers. Sci. Rep. 8, 1–8 (2018).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/5310e316ff29d92a2283e7123b2ddfa0.jpg)
Figure.1: (a) Deposition rate vs Power, (b) X-ray diffraction of Ta at varied deposition rate.

![](https://assets.underline.io/uploads/markdown_image/1/image/0bad8ec71162257913fa6146e434188e.jpg)
Figure.2: (a) Ferromagnetic response of Ta/Ni80Fe20 and reference Ni80Fe20 at 9 GHz, (b) spin-mixing conductance vs deposition rate of all bilayer structure

 Effect of Ta crystallite size on spin-mixing conductance of Ta/Ni80Fe20 bilayer structure

When the spin Hall effect (SHE) is dominated by the intrinsic (Berry phase) mechanism, the spin Hall angle θSH is proportional to resistivity [1]. Thus, alloying is a promising technique to improve θSH via resistivity control. Platinum (Pt)-based alloys have been studied intensively in particular, and large θSH were demonstrated [2]. In Pt-based alloys, dilute doping or alloying with metals with the face-centered-cubic (fcc) crystal structure is utilized to maintain the fcc long range ordering of Pt atoms. However, little work has been done on heavy doping or alloying that completely changes the Pt crystal and band structure. Here, we focused on a Pt-based alloy with orthorhombic crystal structure, YPt [3], to investigate the effect of the crystal structure change on the SHE from those of other fcc Pt-based alloys. We prepared MgAl2O4 (2.0)/YPt (tYPt)/CoPt (2) on oxidized Si substrates by magnetron sputtering with stoichiometric YPt target, where tYPt is the YPt layer thickness (the unit is nanometer). Here, the CoPt (2) layer has a laminated structure of [Pt (0.4)/Co (0.4)]2/Pt (0.4). Because the thickness of Pt layers are few times thinner than typical spin diffusion length λS of Pt and the laminated structure of CoPt is symmetric, the SHE from the Pt layers were cancelled out and negligible. We fabricated 25×50 μm2 Hall bars for second harmonics spin-orbit torque measurements [4]. We also fabricated four-terminal 60×100 μm2 Hall bars to evaluate the sample resistivity by the four-probe technique. To investigate the physical origins of the SHE in YPt, we prepared a series of samples MgAl2O4 (2.0)/YPt (11.0 - 14.5) /CoPt (2) with different Ar pressure of 0.3, 0.5, 0.7, 1.0, and 2.0 Pa during YPt sputtering deposition to control the resistivity of YPt layre ρYPt (referred as series A). The corresponding YPt thickness tYPt is 14.5, 12.8, 12.3, 13.0 and 11.0 nm, respectively. ρYPt gradually decreased from 331 to 299 μΩcm with an increase in Ar pressure from 0.3 to 2.0 Pa. These values are 1.5 - 10 times larger than those of other Pt-based alloys [2,5], indicating a dramatic change of the band structure of YPt from that of Pt. The effective spin Hall angle θSHeff of these samples were evaluated by the second harmonic technique with alternating currents at 259.68 Hz and large in-plane external magnetic fields Hext parallel with the currents, where the amplitude of the current was changed from 1 to 6 mA, and the sweeping range of Hext was between ±8.5 kOe. Fig. 1 shows θSHeff for series A with various YPt deposited at different Ar pressure, as a function of their ρYPt, where blue dots are experimental data and the red dashed line is an approximation line. A linear dependence of θSHeff on ρYPt indicates that the intrinsic and/or the extrinsic side-jump mechanisms dominate the SHE in YPt films. However, the change of θSHeff for YPt film from 0.045 to 0.081 was nearly doubled when ρYPt increased only by 10% from 299 to 331 μΩcm, hence, θSHeff cannot be explained by only the intrinsic mechanism, which requires a constant value of the effective spin Hall conductivity σSHeff=θSHeff ρYPt-1. To deconvolute the contribution of each mechanism, we fit σSHeff by αext ρYPt-2+σSHint [6], where αext is a coefficient originated from the extrinsic side-jump mechanism, and σSHint is the intrinsic spin Hall conductivity. We obtained αext = (-5±1)×10-5 Ωcm and σSHint = 700±100 Ω-1cm-1. The opposite polarity of the extrinsic and intrinsic mechanisms explains the strong sensitivity of θSHeff to ρYPt, which is a unique feature of YPt from other conventional Pt-based alloys. Next, we investigated the spin diffusion length in YPt. We prepared another series of samples with different tYPt ranging from 1.0 to 8.9 nm deposited with Ar pressure of 0.3 Pa (referred as series B). Similar to previous reports on heavy metals, ρYPt tends to increase from 330 to 384 μΩcm with a decrease in tYPt from 8.9 to 1.0 nm. The blue dots in Fig. 2 show θSHeff as a function of tYPt. θSHeff was ~0.07 for thick tYPt but increased up to ~0.09 at 2 nm, then dropped to ~0.04 at 1 nm. This behavior is originated from the tradeoff between improvement of θSHeff due to increase in ρYPt and suppression of θSHeff due to diffusion of opposite spins from the rear side at small tYPt, similar to Pt films on Ta buffers with strong thickness dependence of resistivity [7]. In our case, although ρYPt increased only 17% at small tYPt, the high sensitivity of θSHeff to ρYPt results in the peak at 2 nm. We fit the experimental data by θSHeff = θSH0(ρYPt)[1-sech(tYPt/λS)], where the resistivity dependence of θSH0(ρYPt) is given by the fitting result of σSHeff. The red solid line in Fig. 2 shows the best fit curve with λS = 0.9 nm. In Pt-based alloys with the fcc structure, λS can be calculated from their resistivity and λS of Pt assuming the Elliott-Yafet mechanism [2]. However, the estimated value of λS using the Pt-based alloy model is 0.3 nm [8], which is smaller than the obtained value of 0.9 nm for YPt. Our results show that SHE physics in YPt cannot be explained by extrapolating from conventional fcc Pt-based alloys, indicating the important influence of the band structure change on the SHE of YPt.

**References:**

[1] E. Sagasta, Y. Omori and M. Isasa, Phys. Rev. B, Vol. 94, p.060412 (2016). [2] L. Zhu, L. Zhu and M. Sui, Sci. Adv., Vol. 5, p.eaav8025 (2019). [3] N. H. Krikorian, J. Less-Common Met., Vol. 23, p.271 (1971). [4] M. Hayashi, J. Kim and M. Yamanouchi, Phys Rev. B, Vol. 89, p.144425 (2014). [5] C. Hong, L. Jin and H. Zhang, Adv. Electron. Mater., Vol. 4, p.1700632 (2018). [6] Y. Tian, L. Ye and X. Jin, Phys. Rev. Lett., Vol. 103, p.087206 (2009). [7] M.-H. Nguyen, D. C. Ralph and R. A. Buhrman, Phys. Rev. Lett., Vol. 116, p.126601 (2016). [8] Y. Wang, P. Deorani and X. Qiu, Appl. Phys. Lett., Vol. 105, p.152412 (2014).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/0846da4a52c912a52b1650d06883ba0e.jpg)
θSHeff of series A with various YPt deposited at different Ar pressure, as a function of their ρYPt. Dashed line is an approximation line.

![](https://assets.underline.io/uploads/markdown_image/1/image/12d20dd95993d249c6258c0d177acd48.jpg)
θSHeff of series B deposited at 0.3 Pa as a function of the YPt layer thickness. Solid line is a fitting curve.

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