Role of Cu Layer on the Enhancement of Spin-to- Charge Conversion in Py/Cu/Bi2Se3

doi:10.21203/rs.3.rs-494900/v1

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Role of Cu Layer on the Enhancement of Spin-to- Charge Conversion in Py/Cu/Bi₂Se₃

https://doi.org/10.21203/rs.3.rs-494900/v1

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The enhancement of spin-to-charge conversion in Py/Cu/Bi₂Se₃ is achieved when increasing the Cu layer thickness up to 7nm. The conversion rate is studied using spin pumping technique. The inverse IEE length λ_IEE is found to increase up to ∼2.7nm when 7nm Cu layer is introduced. Interestingly, maximized λ_IEE is obtained when the effective spin mixing conductance (and thus J_s) is decreased due to the Cu insertion. Monotonic increase of λ_IEE with decreasing J_s suggests that IEE relaxation time τ is enhanced due to additional tunnelling barrier (Cu) that limits the interface transmission rate. The results have shown the importance of interface engineering in Py/TI magnetic heterostructure, which is the key factor for optimizing spin-to-charge conversion efficiency.

Hard Condensed-matter Physics

Soft Condensed-matter Physics

Computational Physics

spin-to-charge conversion

topological insulator

spin pumping

Manipulation of spin current by electrical charges or voltages is one of the key subjects for new generation of spintronic devices. Three dimensional (3D) topological insulators (TI) are great candidate for spintronic applications, since their spin-momentum locked properties enable spin accumulation on the surface by passing electrical charge current through the TI channel.^1,2 In addition, the degree of spin polarization can be further manipulated by controlling the Fermi level.^3,4 Owing to the spin momentum locking, 3D spin current density J_s injected onto TI surface will produce 2D charge density J_c on TI surface states (SS), so-called inverse Edelstein effect (IEE). The inverse IEE length λ_IEE is determined as J_c/J_s and can be experimentally probed using spin pumping technique.^5,6,7,8

Numerous studies have been reported for determining the spin-to-charge conversion efficiency in 3D TI.^5,9,10,11 Particularly, giant spin hall angle (SHA) as large as ∼0.43 was reported in Bi₂Se₃ that attributed to the enhanced spin current by the surface states and then converted into dc-voltage due to the bulk inverse spin hall effect.⁹ However, large variations of SHA was found, which is an order of magnitude difference and the authors ascribed such variation to the non-uniformity of interface quality.⁹ On the other hand, Wang et al. observed the dominant role of surface states in spin-to-charge conversion, despite the unavoidable conducting bulk in Bi₂Se₃.⁵ The effective spin mixing conductance was not increased monotonically although the thickness of Bi₂Se₃ varied from 2 QL to 60 QL, suggesting surface states dominated mechanism.⁵ Obviously, the spin pumping characteristic are important parameters for investigating the spin-to-charge conversion mechanism in 3D TI, where controlling the interfacial properties are necessary steps.

Cu is widely used to control the spin transmissivity in the multilayer devices.^12,13,14 Du et al. demonstrated that the insertion of Cu layer between Y₃Fe₅O₁₂ (YIG) and W substantially improved the spin current injection into W, while similar insertion between YIG and Pt degraded the spin current.¹³ The authors reported the quantitative analysis and found that the spin transport efficiency in heterostructures depends on spin conductance of each constituents and their interfaces.¹³ Similar results were also reported by Deorani et al. where the effect of Cu interlayer on spin mixing conductance indeed material dependent (Pt versus Ta).¹⁴ Recently Cu layer was deposited on TI film in order to eliminate the proximity induced ferromagnetism in the spin-orbit torque (SOT) devices.^1,15 Despite the fact that Cu is the most common used spacer layer in the spintronic devices, there is still lacking of quantitative studies about the role of Cu insertion on spin-to-charge conversion in TI that measured based on spin pumping mechanism. In this work, we fabricated trilayer structure of Py/Cu/Bi₂Se₃ (Fig. 1) and studied the spin pumping characteristic by varying the Cu layer thickness. Cu layer is used to protect the TI surface from exchange interaction with Py. Our results imply that Cu also acts as the barrier to spin transmission into TI film. More importantly, spin-to-charge conversion efficiency was enhanced due to the introduction of Cu barrier. The related mechanism is discussed in this work.

Figure 2a shows the dc voltage as a function magnetic field H that measured at excitation frequency of 3GHz for sample Py/Cu(7nm)/Bi₂Se₃. The results for other frequencies can be found in Figure S2. The voltage signals consisting of symmetric (V_s) and antisymmetric (V_as) parts, which can be isolated by fitting the measured voltage (data curve) with the form

Here Hr is the FMR resonant field and ΔH is the line width of the signal. V_s is attributed to the injected spin current whereas V_as originated from anisotropic magnetoresistance (AMR) or anomalous Hall effect (AHE). Similar fitting is also done for other samples and V_s is extracted as shown in Figure 2b. It was found that V_s is larger in the presence of Cu layer. FMR experiments were also conducted as shown in Figures 2c and 2d. Figure 2d presents the frequency dependence of FMR line width for samples with different Cu thickness. The linear fitted slope is larger for bilayer and trilayer samples (Cu0, Cu3 and Cu7 denote t_Cu=0, 3, 7nm respectively) in comparison to single Py, indicating spin current injected into Bi₂Se₃, resulting in the broadening of the FMR line width and thus larger damping constant a. Interestingly, the a_Py/Cu/TI was found decreases from 0.01262 to 0.01185 when increasing the thickness of Cu layer up to 7nm.

The resistance of the multilayer samples R_d were measured using four probe method. J_c is determined as J_c= I_c/w = V_s/wR_d, where w and I_c are the width of the sample and charge current as shown in Figure 3a. We used the standard analysis of spin pumping on TI^5,6,7 to evaluate the spin-to-charge conversion J_c/J_s. Spin mixing conductance G_eff^¯ which is used to illustrate the efficiency of generating spin current is extracted using Equation (1):

where M_s is saturation magnetization of Py, t_Py is thickness of Py and g is Landé factor and u_B is the Bohr magneton. M_s is calculated using Kittel formula from f vs. H_r (Figure 2c). Da = a_Py/Cu/TI - a_Py and is determined by analysing DH_pp vs. f as shown in Figure 2d. For the spin current densities that injected across the interface due to spin pumping, Equation (2) is utilized as shown below:

in which g is gyromagnetic ratio, w(=2pf) and h_rf are frequency and amplitude of microwave magnetic field respectively. The calculated J_s is presented in Figure 3b. By dividing J_c with J_s, spin-to-charge conversion efficiency J_c/J_s can be determined as shown in Figure 3c.

Figure 3a plots the J_c against t_Cu. There is an optimized J_c at thickness of 3 and 7nm. In contrast, J_s is reduced when 3 and 7nm Cu were incorporated. The trend of J_s vs t_Cu is consistent with the changes of a_Py/Cu/TI, where effective damping constant is found decreases when 3 and 7nm Cu were introduced. Interestingly, a maximized J_c/J_s is observed at t_Cu=7nm, where J_c/J_s reaches ~2.7nm. This result suggests that the optimization of J_c/J_s could be related to the decrease of J_s due to the Cu insertion. To investigate the possible reason for the J_c/J_s enhancement, we plot J_c/J_s as a function of effective spin mixing conductance G_eff^¯ as shown in Figure 4a. Various G_eff^¯ values were obtained by changing the Cu layer thickness. Large J_c/J_s is obtained at low value of G_eff^¯ (and thus minimum J_s as shown in Figure 3b). We further examined the J_c vs. G_eff^¯ as shown in Figure 4b. There is no enhancement of J_c with increasing G_eff^¯, revealing the spin-to-charge mechanism may not be dominated by the bulk spin hall effect (SHE).¹⁴ Hence, we here suggest that the spin-to-charge conversion in our Py/Cu/TI system arises from inverse Edelstein effect, where the origin is the spin-momentum locked surface states of TI layer, same interpretation as other literatures.^5,11

Low G_eff^¯ indicates strong spin backflow and spin memory loss (spin absorption) at the high SOC interface.^16,17 Both factors are relevant in this Py/Cu/TI trilayer system. If we examine G_eff^¯ (Py/Cu/TI) at various t_Cu, as presented in Figure 4c, except for Py/Cu(3nm)/TI and Py/Cu(7nm)/TI, samples Py/TI, Py/Cu(9nm)/TI and Py/Cu(11nm)/TI exhibit G_eff^¯~1.25´10¹⁹m^-2, which is the typical value for metal-metal interfaces.^18,19 As reported by Du et al., the effective spin mixing conductance of trilayer system (FM/Cu/NM, FM denotes ferromagnetic while NM denotes nonmagnetic material) is determined by the serial contribution of the two interfaces (FM/Cu and Cu/NM) and the spin resistance of Cu.¹³ Here we refer FM to Py while NM to TI film. One of the reason for the lower G_eff^¯ compared with G_Py/TI^¯ may be resulted from smaller spin conductance g_Cu/TIof Cu/TI than that of G_Py/TI^¯, similar to the case in Cu/Pt.^13,14 However, since G_eff^¯ » G_Py/TI^¯ at t_Cu ³9nm, here we assume Cu/TI and Py/Cu present similar quality with G_Py/Cu^¯ » g_Cu/TI. Thus, by assuming degree of spin absorption at Cu/TI interfaces are similar in all cases, we suggest that the reason for lower G_eff^¯ of 3nm and 7nm Cu-based trilayer samples could be due to the strong spin accumulation at this ultrathin regime.¹³ Stronger spin backflow occurs, as compared to t_Cu³9nm, which eventually leads to reduction of G_eff^¯.

The decrease of G_eff^¯ seems have strong correlation to the spin-to-charge conversion efficiency. The next question is how such condition could increase the J_c/J_s? Here we defined J_c/J_s as l_IEE= n_ft where n_fis Fermi velocity of TI surface states and t is IEE relaxation time. As shown in the Figure 4d, t is modified due to the tunnelling current into TI, which is determined by momentum relaxation time t_p and interface tunnelling time t_t as shown in Equation (3):²⁰

where l_mf = n_ft_p = mean free path TI. According to this model, we proposed that the monotonic increase of l_IEE with decreasing G_eff^¯ attributed to the modification of IEE relaxation time t due to additional tunnelling barrier (Cu) that limits the interface transmission rate (1/t_t).^20,21 l_IEE is always lower than l_mf due to the correction factor of (1+2t_p/t_t). It becomes clear that one can increase l_IEEby reducing 1/t_t, which can be done by introducing tunnelling barrier in between Py and TI layer. Using l_IEE(t_Cu=7nm) =2.7nm and based on our previous ARPES result, n_f=5.7´10⁵ m/s,²² we find t~4.7fs, which is same order of magnitude as Bi/Ag²³ and a-Sn/Ag⁶ interfaces. Our extracted l_IEE(=2.7nm) is higher than 0.1-0.4nm in Bi/Ag Rashba interface,²³ 2.1nm and 2nm in TI SS of a-Sn/Ag⁶ and HgTe/HgCdTe⁷ respectively. We attribute the enhancement to the insertion of Cu tunnelling barrier. Although more theoretical calculation might be needed, our works indicate the importance of interface engineering to enhance the spin-to-charge conversion. This method could also be applied to other high SOC interfaces for obtaining high spin-to-charge conversion based on the inverse Edelstein effect, which is essential for spin current detector and other novel application such as broadband terahertz emitter.^24,25

In conclusion, we investigated the spin-to-charge conversion in Py/Cu/Bi₂Se₃ using spin pumping technique. Enhancement of J_c/J_s with increasing t_Cu is observed at room temperature, where J_c/J_s ∼2.7nm is achieved when 7nm of Cu layer is inserted. We proposed that the enhancement is attributed to the additional Cu interlayer as tunnelling barrier that modified the relaxation time at the interface. This work has provided a viable route for enhancing spin-to-charge conversion efficiency of TI, which is crucial for spin functional device applications.

Bi₂Se₃ films with 10nm thickness were synthesized using molecular beam epitaxy (MBE) method. The as-grown Bi₂Se₃ were in situ capped with 2nm Se film that used as protecting layer. The sample was then transferred to pulse laser deposition (PLD) chamber for depositing Cu and subsequently NiFe (Py) layer at room temperature. Before the depositions, Se layer was decapped in PLD chamber at 183°C for 1 hour. A series of trilayer samples were prepared by varying Cu thickness, ranging from 3 to 11nm. Bilayer of Py/Bi₂Se₃ was also prepared for comparison study. Py thickness was fixed at 17 nm. 1 nm of Al film was deposited on Py as capping layer. To evaluate spin-to-charge conversion, spin pumping technique was utilized (Fig. 1). Spin current was generated in Py via its ferromagnetic resonance (FMR) condition and injected into Bi₂Se₃, either with or without passing through the Cu layer (-z direction) (Fig. 1b). DC voltage was measured in x-direction and the produced charge current could be evaluated. All the measurements were done at room temperature.

Acknowledgments

This work was supported by the Ministry of Science and Technology, R.O.C. (Grants No MOST 107-2119-M-006-016-MY3).

Author contributions

S. H. S. and C.-W. C. designed the experiment flow, analyzed data, and wrote the main manuscript. Y.-X. H. grew the samples and performed the measurement. Y.-C. C. and V. V. M. provided useful discussion. J.-C.-A. H. financed the funding for this research.

Competing financial interests: The authors declare no competing financial interests.

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No competing interests reported.

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Role of Cu Layer on the Enhancement of Spin-to- Charge Conversion in Py/Cu/Bi₂Se₃

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