Indium-tin-oxide (ITO), known as transparent electrode material, shows small spin Hall effect. However, by preparing the nonmagnetic metal (NM: Cu or Ag)/ITO interfaces, we observed an efficient charge-spin interconversion due to the Edelstein effect by means of damping modulation and spin pumping measurements. The estimated spin current conductivity for charge-to-spin conversion in NM/ITO interfaces is larger than Pt and β-Ta. Furthermore, the spin-to-charge conversion coefficient λ in Ag/ITO interfaces becomes 0.35 nm, which is comparable with that of the Ag/Bi interface with large Rashba spin splitting. These experimental results imply that a heavy element like Bi is not necessary to realize an efficient spin-charge interconversion at the interface. By the combination of the experimental results of the interconversion effect, we found that the spin ejection time from the interface is as fast as the momentum relaxation time. As a result, we realized not only efficient spin current generation but also suppression of magnetic damping in the contacted ferromagnetic layer. These findings will help us to understand the physics of interfacial charge-spin conversion and provide a new perspective to material research of spin current generator for low-consumption spintronics devices.

Magnetization control with low-power consumption can be achieved by a spin current induced spin torque. In the spintronics field, one of the common ways to generate the spin current is the spin Hall effect (SHE), which is caused by spin-orbit coupling (SOC).1 By utilizing the SHE in Pt and β-Ta, several magnetization controls such as magnetization reversal and auto-oscillation have been demonstrated.2,3 Recently, charge-spin conversion phenomena at the surface state with spin splitting such as the Rashba interface and topological insulator have been intensively studied as an alternative to SHE.4–9 The interfacial charge-spin conversion effect called the Edelstein effect (EE) is induced by spin-momentum locking at the surface state.10 The coefficient of charge-to-spin conversion using direct EE (DEE) and spin-to-charge conversion using inverse EE (IEE) in the Rashba interface is characterized by the conversion parameters, qJS3D/JC2D and λJC2D/JS3D, in which

q=αR/(ћτDEEνF2)
(1)

and

λ=αRτIEE/ћ,
(2)

where αR is the Rashba parameter, ћ is Dirac’s constant, and τDEE and τIEE are spin relaxation time at the outside and inside of the surface state, respectively, as shown in Fig. 1.11–14 Thus, the conversion coefficient q and λ is proportional to 1/τDEE and τIEE, respectively. The τDEE corresponds to τej which is the spin ejection time from the interface to FM layer. The τIEE corresponds to τp which is the momentum relaxation time at the interface.14 Actually, at a long τIEE system due to long τp in the oxide interface such as the LaAlO3/SrTiO3 interface, highly efficient spin-to-charge conversion has been reported,15 while there are few reports about charge-to-spin conversion due to DEE at the Rashba interface. It might be caused by longer τej than τp. In this sense, it is expected that metallic interfaces like Ag/Bi and Cu/Bi2O3 are more comfortable to efficient charge-to-spin conversion than oxide interfaces because the τej in the metallic system can be shorter than that of the oxide interface.

FIG. 1.

Spin relaxation process during charge-spin conversion at the Rashba interface. (a) Schematic image of Rashba spin split due to inversion symmetry breaking at the metal/oxide interface. (b) Two types of spin relaxation processes, where τIEE and τDEE are spin relaxation time at the inside and outside of the surface state.

FIG. 1.

Spin relaxation process during charge-spin conversion at the Rashba interface. (a) Schematic image of Rashba spin split due to inversion symmetry breaking at the metal/oxide interface. (b) Two types of spin relaxation processes, where τIEE and τDEE are spin relaxation time at the inside and outside of the surface state.

Close modal

Here, we investigated the charge-to-spin conversion at NM/Indium-tin-oxide (ITO) interfaces, where SOC of indium and tin are much smaller than bismuth.17 Nevertheless, we succeed in the observation of efficient charge-to-spin conversion in the NM/ITO interface by means of damping modulation measurements. The estimated spin current conductivity is larger than Pt and β-Ta. Furthermore, we observed an efficient spin-to-charge conversion at the same interfaces. The interfacial conversion coefficient λ in Ag(Cu)/ITO interfaces becomes 0.35 (0.17) nm, which is comparable with that of Ag/Bi interfaces with large Rashba spin splitting.6,17 These experimental results imply that a heavy element like Bi is not necessary to realize an efficient spin-charge conversion at the interface. By the combination of both experimental results, we found that the spin ejection time τej is comparable with the momentum relaxation time. As a result of such fast spin ejection from the interface, we realized not only efficient spin current generation but also suppression of magnetic damping constant in the contacted ferromagnetic layer. Its suppression directly contributes to the decrease in critical current density for the operation of spin current based magnetization control devices.3 Thus, our findings provide a new perspective for the development of the spin current generator in low-power consumption devices by using material interfaces.

For this study, the //ferromagnetic metal (FM)/NM/ITO tri-layer and //FM/ITO bilayer were prepared. The double slash indicates the contacting side with substrate. As FM and NM layers, 5-nm-thick NiFe (Ni80Fe20) and 10-nm-thick Cu (or Ag) were evaporated on a Si wafer substrate covered with 200 nm of thermally oxidized SiO2. A 200 nm-thick-ITO layer was deposited by using a dc-sputtering system. The weight ratio of the ITO (In2O3/SnO2) target was 90% vs. 10%, which has a high light-transmission property. So, ITO is widely used as transparent electrodes for solar cells, displays for touch panels, and so on.18 The resistivity of Cu, Ag, and ITO films were 10 μΩ cm, 9 μΩ cm, and 0.75 mΩ cm at room temperature. First, we investigated the charge-to-spin conversion effect by means of the magnetic damping modulation technique.19,20 Figure 2(a) shows the schematic image of the sample structure consisting of a multi-layer wire and a Ti/Au coplanar wave guide (CPW). The wires are fabricated by optical lithography and a lift-off method. The width and length of wires are 20 × 60 µm2. The CPW is deposited on both ends of the wire so that the wire elements act as a part of the waveguide. When the rf current Irf and the direct current Idc are applied to the sample, ferromagnetic resonance (FMR) in the NiFe layer is excited by a local magnetic field due to Irf. The Idc plays the role of modulation of the damping torque by a spin torque, as shown in Fig. 2(a). During measurements, an external static magnetic field Hex is applied with an angle θ = 45° and 225°. All of the measurements were performed at room temperature.

FIG. 2.

Charge-to-spin conversion measurements by means of magnetic damping modulation. (a) Spin torque-ferromagnetic resonance (ST-FMR) measurement circuit and sample structure. Yellow (white) arrows in NiFe layer correspond to spin torque (damping torque). (b)–(d) ST-FMR spectrum without dc current input frequency and power is 8 GHz and 20 dBm. (e)–(g) Modulation of magnetic damping constant (Δαeff) due to spin current in //NiFe/ITO, //NiFe/Cu/ITO, and //NiFe/Ag/ITO.

FIG. 2.

Charge-to-spin conversion measurements by means of magnetic damping modulation. (a) Spin torque-ferromagnetic resonance (ST-FMR) measurement circuit and sample structure. Yellow (white) arrows in NiFe layer correspond to spin torque (damping torque). (b)–(d) ST-FMR spectrum without dc current input frequency and power is 8 GHz and 20 dBm. (e)–(g) Modulation of magnetic damping constant (Δαeff) due to spin current in //NiFe/ITO, //NiFe/Cu/ITO, and //NiFe/Ag/ITO.

Close modal

The amount of damping modulation Δαeff due to the spin current can be expressed as follows:

Δαeff=sinθ(Hex+MS/2)μ0MStFMћJS2e,
(3)

where e is the electron charge, μ0 is the permeability in vacuum, MS is the saturation magnetization, tFM is the FM layer thickness, θ is the static magnetic field angle from the longitudinal direction of the wire, and ћJS/2e is the spin current density. Figures 2(e)–2(g) show the modulation results of the magnetic damping constant (Δαeff). In the //NiFe/ITO bilayer, we could not observe the finite variation of Δαeff. On the other hand, in the //NiFe/Ag/ITO tri-layer, we observed a clear variation of Δαeff, as shown in Fig. 2(g). The direction of Δαeff is reversed by changing the Hex direction from 45° to 225°, which is caused by switching of the direction of the spin torque. The slope of damping modulation implies that the sign of charge-to-spin conversion in Ag/ITO is the same with that of β-Ta.2 Interestingly, by the insertion of Cu instead of Ag, the amount of modulation becomes about half, even though SHE in Cu is comparable with that of Ag.

Second, the inverse effect, i.e., spin-to-charge conversion measurement by means of the spin pumping method, has been performed on devices shown in Fig. 3(a). The spin pumping sample consists of a multi-layer wire and a CPW separated from the wire by a 200-nm-thick Al2O3 insulating layer. The width and length of the wires are 10 × 200 µm2 which are placed beside the signal line of CPW. When the rf current is applied in the CPW, the FMR is excited by the rf field (Hrf). As a result of FMR, the spin current is injected into the NM layer, the ITO layer, and their interfaces. The injected spin current gives rise to an electric dc voltage Vout through the inverse spin Hall effect (ISHE) and IEE. The static magnetic field Hex was applied in the in-plane direction. All measurements were performed at room temperature.

FIG. 3.

Spin-to-charge conversion measurements. (a) Spin pumping measurement circuit and sample structure injected spin current convert to charge current by inverse spin Hall effect and inverse Edelstein effect. (b) Output signal spectrum in //NiFe/ITO, //NiFe/Cu/ITO, and //NiFe/Ag/ITO at 7 GHz. Inset on lower-right is the spectrum in //NiFe/ITO bilayer. (c) Input power dependence of output signals in //NiFe/ITO, //NiFe/Cu/ITO, and //NiFe/Ag/ITO. (d) Enhancement ratio between Iout in //NiFe/NM/ITO and Iout in //NiFe/ITO, respectively. (e) Field angle dependence of the output signal.

FIG. 3.

Spin-to-charge conversion measurements. (a) Spin pumping measurement circuit and sample structure injected spin current convert to charge current by inverse spin Hall effect and inverse Edelstein effect. (b) Output signal spectrum in //NiFe/ITO, //NiFe/Cu/ITO, and //NiFe/Ag/ITO at 7 GHz. Inset on lower-right is the spectrum in //NiFe/ITO bilayer. (c) Input power dependence of output signals in //NiFe/ITO, //NiFe/Cu/ITO, and //NiFe/Ag/ITO. (d) Enhancement ratio between Iout in //NiFe/NM/ITO and Iout in //NiFe/ITO, respectively. (e) Field angle dependence of the output signal.

Close modal

Figure 3(b) shows the output spectrum in //NiFe/ITO, //NiFe/Cu/ITO, and //NiFe/Ag/ITO samples, when the angle of the applied magnetic field Hex is θ = 90° and 270°. In the case of //NiFe/ITO bilayer film, a tiny output signal was observed. On the other hand, in the tri-layer films [i.e., the inserted NM layer (Ag or Cu) between NiFe and ITO], significantly enhanced output signals were observed. The Iout of the vertical axis correspond to Vout/Rsample, where Vout and Rsample are the detected dc voltage and sample resistance, respectively. The sign change of Iout is caused by magnetic field reversal; its behavior is a typical feature of the spin-to-charge conversion induced by the spin pumping technique.21 The sign of Iout indicates the sign of the spin-to-charge conversion effect, which is same as the charge-to-spin conversion in the //NiFe/NM/ITO tri-layer film. Figure 3(c) shows the input rf power Pin dependence of Iout. In all the samples, Iout increases linearly with Pin as expected for the spin-to-charge conversion induced by spin pumping with a small precession cone angle. The tiny signal in the //NiFe/ITO bilayer is reasonable magnitude of small spin Hall angle in bulk-ITO.22,23 Figure 3(d) shows the enhancement ratio between Iout in //NiFe/NM/ITO and Iout in //NiFe/ITO. By insertion of the Ag (Cu) layer, Iout was enhanced 42 (20) times larger than that for the //NiFe/ITO bilayer. Figure 3(e) shows the field angle dependence of the output signal. To remove the influence of the thermo-electric background and anisotropic magnetoresistance (AMR) effect, the magnitude of the spin-charge conversion signal was estimated by ISC = [V(+Hr) − V(−Hr)]/2Rsample, where V(+Hr) and V(−Hr) are the output voltages at positive and negative resonance fields.24 The field angle dependence can be fitted well by sin θ, which corresponds to ISHE and/or IEE at the isotropic spin splitting state. Thus, the experimental results of both damping modulation and spin pumping measurements clearly indicate that the insertion of the NM layer between NiFe and ITO layers plays a crucial role in the large enhancement of the charge-spin interconversion effect.

We discuss the origin of enhancement of the charge-spin interconversion effect by insertion of the NM layer. From previous studies, there are three possible reasons: (1) impurity in the NM layer, (2) oxidation of the NM layer, and (3) NM/ITO or NiFe/NM interfaces. The former two reasons are the origin of enhancement of bulk SHE in the NM layer and the latter is the interfacial conversion. First, we consider the influence of the impurity effect. Cu alloys doped with heavy elements like Ir and Bi have a large spin Hall angle due to skew scattering at the impurities.25,26 To obtain a large spin Hall angle in these Cu alloys, it is necessary to dope until the resistivity in the Cu alloy becomes several times larger than non-doped Cu. In our NM/ITO samples, the increase of resistivity in the NM layer is less than 10% which varies within the range of the experimental error bar. So, the influence of impurity can be negligible. Then, we consider the contribution of oxidization in the NM layer. More recently, it has been reported that the spin Hall angle of Cu is increased drastically by oxidation.27 This phenomenon also requires that the resistivity in the Cu layer should be changed dramatically by several orders of magnitude. The reported sign of the spin Hall angle in oxidized-Cu is opposite to that of our NM/ITO samples. Thus, we concluded that the contribution of bulk-SHE due to impurity and oxidation can be negligible in our samples. Recently, a novel spin current generation mechanism at FM/Ti interfaces has been reported.28 However, the spin-to-charge current conversion and spin torque at NiFe/Ag and NiFe/Cu interfaces have not been observed in the previous studies.6,27,29 Thus, we guess that the contribution in NiFe/Ag(Cu) interfaces is small for our experimental results. Therefore, the observed enhancement of the interconversion effect in //NiFe/NM/ITO tri-layer samples is likely to be caused by EE at the NM/ITO interface.

Table I shows charge-spin interconversion coefficients at NM/ITO interfaces. In the case of charge-to-spin conversion, coefficient q is defined as q = JS3D/JC2D, where JS3D and JC2D are the spin current density generated from the interface and the input charge current density at the NM/ITO interface, respectively. The values of JS3D can be estimated from damping modulation measurements by using Eq. (3). JC2D was calculated from the conductivity ratio in each layer, assuming that the conductivity at the interface is the same as that at the NM layer. The estimated q in //NiFe/Ag(Cu)/ITO becomes 0.061 (0.021) nm−1. To compare with transition metals, we estimated the spin current conductivity σS, which is defined as σS= θSHσNM in SHE and σS= qσint in EE, where θSH, σNM, and σint are the spin Hall angle, conductivity of spin Hall material, and conductivity at the interface, respectively.4,7,8 The estimated spin current conductivity σS in //NiFe/Ag(Cu)/ITO becomes 0.67 (0.22) (106 Ω−1 m−1), the values being larger than those of the transition metals of Pt and β-Ta, as shown in Fig. 4.

TABLE I.

Charge-spin interconversion coefficient at NM/ITO interfaces. Spin-to-charge conversion coefficient λ, charge-to-spin conversion coefficient q, spin current conductivity, spin relaxation times, and spin ejection ratio η at NM/ITO interfaces.

Interfacesλ (nm)q (nm-1)σS (M Ω-1m-1)τIEE (fs)τDEE (fs)η
Cu/ITO 0.17 0.021 0.21 4.8 4.7 0.51 
Ag/ITO 0.35 0.061 0.68 6.8 2.3 0.74 
Interfacesλ (nm)q (nm-1)σS (M Ω-1m-1)τIEE (fs)τDEE (fs)η
Cu/ITO 0.17 0.021 0.21 4.8 4.7 0.51 
Ag/ITO 0.35 0.061 0.68 6.8 2.3 0.74 
FIG. 4.

Spin current conductivity Yellow bars show the spin current conductivity σS. σS in NM/ITO is higher than that of transition metals (Pt, β-Ta).

FIG. 4.

Spin current conductivity Yellow bars show the spin current conductivity σS. σS in NM/ITO is higher than that of transition metals (Pt, β-Ta).

Close modal

The conversion coefficient for spin-to-charge conversion λ can be estimated by λ = JC2D/JSSP = Iout/(wJSSP), where w and JSSP are the wire width and injected spin current density due to spin pumping. The injected spin current density JsSP is given by

JsSP=2eћ×ћgeffγe2(Hrf)2Msγe+(Msγe)2+(2ω)28πδF/N/O2(Msγe)2+(2ω)2,
(4)

where Hrf and ω are the applied rf field and the angular frequency. The strength of Hrf is determined by precession cone angle measurements.29,geff is the effective spin mixing conductance which is calculated by geff=γω4πMstFM(δFM/NM/ITOδFM/NM/Al2O3)gμB, where δFM/NM/ITO and δFM/NM/Al2O3 are the damping constants for //NiFe/NM/ITO and //NiFe/NM/Al2O3, respectively. The estimated λ in //NiFe/Ag(Cu)/ITO became 0.35 (0.17) nm. These values are comparable with those of Cu/Bi2O330,31 and Ag/Bi interfaces,6 even though SOC in indium is much smaller than that in bismuth.16 These experimental results indicate that a heavy element is not necessary to induce the efficient charge-spin interconversion at the NM/oxide interface.

Additionally, by using both measured conversion coefficients of q in Eq. (1) and λ in Eq. (2), we can estimate the spin ejection time τDEE. If we assume that the momentum relaxation time at the interface τIEE is equal to that in bulk-NM layers of 6.8 (4.8) fs calculated from the resistivity of the Ag (Cu) layer and vF is 6.0 × 105 m/s, τDEE becomes 2.3 (4.7) fs in Ag (Cu)/ITO samples. Here, we define the ratio η of spin ejection to the total spin relaxation by η ≡ (1/τej)/(1/τej+1/τp). In the case of Ag (Cu)/ITO, η was estimated to about 0.74 (0.51), which implies that accumulated spins at the NM/ITO interface are more likely to relax at the FM layer than at the interface.

Finally, we discuss the magnetic damping constant αeff in each film. The values of αeff of the FM layer that is in contact with a heavy metal increase with increasing spin dissipation in the adjacent layer.32,33 However, when spin relaxation is more likely to occur at the FM layer than at the interface, the increase of αeff due to spin dissipation should be suppressed because the back-flow spin current from the interface to the FM layer increases. The αeff can be estimated by measuring the frequency dependence of the linewidth (ΔH) of the FMR spectrum. The frequency dependence of ΔH is shown in Fig. 5. The ΔH exhibited a linear dependence with the frequency, and αeff was estimated to be 0.022 ± 0.002, 0.0186 ± 0.003, 0.0116 ± 0.001, 0.0114 ± 0.001, and 0.0104 ± 0.002 in //NiFe/Pt, //NiFe/ITO, //NiFe/Cu/ITO, //NiFe/Ag/ITO, and //NiFe/Cu/Al2O3 samples, respectively. The //NiFe/Cu/Al2O3 sample is the reference sample. In //NiFe/Cu/ITO and //NiFe/Ag/ITO tri-layers, extremely suppressed αeff were observed compared with the //NiFe/Pt bilayer film, despite the highly efficient charge-spin interconversion in NM/ITO interfaces. Thus, these experimental results are evidence of fast spin ejection from the interface to the FM layer.

FIG. 5.

Magnetic damping constant. (a) ΔH as a function of input rf frequency. Blue, red, and gray rectangle plots correspond to //NiFe/Pt, //NiFe/ITO, //NiFe/Cu/ITO, and //NiFe/Ag/ITO. Open circle plots represent the reference sample of //NiFe/Cu/Al2O3. (b) Magnetic damping constant in multilayers.

FIG. 5.

Magnetic damping constant. (a) ΔH as a function of input rf frequency. Blue, red, and gray rectangle plots correspond to //NiFe/Pt, //NiFe/ITO, //NiFe/Cu/ITO, and //NiFe/Ag/ITO. Open circle plots represent the reference sample of //NiFe/Cu/Al2O3. (b) Magnetic damping constant in multilayers.

Close modal

In summary, we investigated the charge-spin interconversion effect at the //NiFe/ITO bilayer and //NiFe/NM (Ag or Cu)/ITO tri-layer films by means of damping modulation and spin pumping measurements. By insertion of an NM layer between NiFe and ITO layers, the conversion efficiencies drastically increased compared with the //NiFe/ITO bilayer film. The estimated spin-to-charge conversion coefficient λ in the Ag(Cu)/ITO interface becomes 0.35 (0.17) nm, which is comparable with the large Rashba split interface such as Ag/Bi. Furthermore, we found that spin current conductivity for charge-to-spin conversion in NM/ITO interfaces becomes larger than that at Pt and β-Ta. These experimental results imply that a heavy element is not necessary to realize an efficient spin-charge conversion at the NM/oxide interface. Additionally, by the combination of interconversion measurements, we estimated the spin ejection time from the interface to the NM/FM layer, which is as fast as the momentum relaxation time. As a result, we found that the NM/ITO interface can be utilized for efficient magnetization control because of efficient spin current generation and low magnetic damping constants due to large back-flow of the spin current. These findings will help us to understand the physics of interfacial spin-charge conversion and provide a new perspective to material search for spin current generators and detectors.

This work was supported by Grant-in-Aid for Scientific Research on Innovative Area, “Nano Spin Conversion Science” (Grant No. 26103002) and Grant-in-Aid for Young Scientists (B) (Grant No. JP17K14077).

The authors declare no competing financial interests.

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