Î²-W System (CL

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Capping layer (CL) induced anti-damping in CL/Py/#-W system (CL: Al, #-Ta, Cu, #-W) Nilamani Behera, Puspendu Guha, Dinesh K. Pandya, and Sujeet Chaudhary ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06991 • Publication Date (Web): 18 Aug 2017 Downloaded from http://pubs.acs.org on August 19, 2017

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ACS Applied Materials & Interfaces

Capping layer (CL) induced anti-damping in CL/Py/β-W system (CL: Al, β-Ta, Cu, β-W) Nilamani Behera,1 Puspendu Guha,2 Dinesh K. Pandya,1 and Sujeet Chaudhary1* 1

Thin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India 2

Institute of Physics, Sachivalaya Marg, Bhubaneswar-751005, India

ABSTRACT: For achieving ultrafast switching speed and minimizing dissipation losses, the spin-based data storage device requires a control on effective damping (αeff) of nanomagnetic bits. Incorporation of interfacial anti-damping spin orbit torque (SOT) in spintronic devices therefore has high prospects in enhancing their performance efficiency. Clear evidence of such an interfacial anti-damping is found in Al capped Py(15 nm)/β-W(tW)/Si (Py=Ni81Fe19 and tW = thickness of β-W), which is in contrast to the increase of αeff (i.e., damping) usually associated with spin pumping as seen in Py(15 nm)/β-W(tW)/Si system. Because of spin pumping, the interfacial spin mixing conductance (g↑↓) at Py/β-W interface and spin diffusion length (λSD) of β-W are found to be 1.63(±0.02)×1018 m-2 (and 1.44(±0.02)×1018 m-2), and 1.42(±0.19) nm (and 1.00(±0.10) nm) for Py(15 nm)/β-W(tW)/Si (and βW(tW)/Py(15 nm)/Si) bilayer systems, respectively. Other different non-magnetic capping layers (CL), namely β-W(2 nm), Cu(2 nm), and β-Ta(2,3,4 nm) were also grown over same Py(15 nm)/βW(tW). However, increase of anti-damping is seen only in β-Ta(2,3 nm)/Py(15 nm)/β-W(tW)/Si. This decrease in αeff is attributed to interfacial Rashba like SOT generated by non-equilibrium spin accumulation subsequent to spin pumping. Contrary to this when interlayer positions of Py(15 nm) and β-W(tW) were interchanged irrespective of the fixed top non-magnetic layer, an increase of αeff was observed, which ascribes to spin pumping from Py to β-W layer. KEYWORDS: ultrafast switching, effective damping, spin pumping, interfacial Rashba like SOT, non-equilibrium spin accumulation

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1. INTRODUCTION Utilization of pure spin current in spintronic devices is a major challenging task in the field of spintronics.1-4 For successful operation, the spin-based devices require ultrafast switching speed for data processing, low energy dissipation loss, and high thermal stability.5 To achieve the first two, control of effective damping of a nanomagnet is very essential.5 Recently, a new mechanism of inverse Rashba Edelstein effect (IREE) has been identified that occurs during spin pumping from ferromagnetic (FM) layer to non-magnetic (NM) layer near the interface in FM/NM system.6-8 This IREE mechanism, which is just opposite of the Rashba mechanism, arises near/at the interface due to non-equilibrium spin-accumulation subsequent to spin pumping in presence of strong spin-orbit coupling (SOC).6-8 During spin pumping, pure spin current is generated from the precession of FM layer’s magnetization (M) during the ferromagnetic resonance (FMR) in FM/NM system.2-4 This spin current accumulates in the NM layer thereby creating a non-equilibrium spin density near FM/NM interface which eventually gives rise to a charge current.6-9 In the presence of strong SOC of the NM layer, the interaction between FM layer’s magnetization and the interfacial charge current acts like an additional SOT torque on M which acts opposite to the damping torque,10 and ultimately precesses the magnetization with high speed. Incorporation of interfacial anti-damping in spintronic devices leads to the enhancement in the performance efficiency of the device. Thus, the spin pumping mechanism provides the straight forward approach of using spin current to efficiently explore the utility of interfacial Rashba effect8 which helps in lowering the critical current needed for magnetization switching, spin torque nano-oscillators (STNO), and other similar applications.5,11,12,13 Therefore, the charge current generated by the spin current can be useful for potential applications in next generation spintronics devices, viz. magnetic random access-memory (MRAM), magnetic data storage, spin based logic devices, etc.3,5,14,15


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The efficiency of spin pumping mechanism is controlled by an interfacial spin mixing conductance, ↑↓.4,16-19 The spin current transfer efficiency strongly depends upon the transparency (T) of the FM/NM interface.1,3 For better T, the spin flip probability parameter, ∈ (which is the ratio of spin injection to spin flip scattering time, i.e. τel/τsf) should be greater than 0.1 (i.e., ∈ > 0.1).18-20 This holds for the case of FM/Pt17 and FM/Pd20 bilayer systems. On the other hand, when ∈ < 0.1, as seems to be the case of FM/NM with NM=β-Ta, β-W, Cu or Al,18-20 the pure spin-accumulation occurs in NM layer and creates a non-equilibrium spin density () near the interface in FM/NM system.5,18-20 In such cases, T is small as compared to the case when∈ > 0.1.1,3 In the present study, we are interested in the 2nd choice of material systems having the properties like ∈ < 0.1, in which the spin-accumulation eventually results in non-equilibrium spin density near the interface in NM layer. This non-equilibrium plays an important role for the generation of RSOT by IREE.6-10,21 It can eventually enable speeding up the magnetization precession of the FM layer.21-23 Thus, can act both as anti-damping like SOT as well as field like SOT10,21,22 on the magnetization precession. This has been demonstrated both in isolated FM semiconductors like Ga1-xMnxAs as well as in couple of FM/NM bilayer systems.23,24 The strength of this anti-damping SOT directly depends upon the SOC of NM layer and vary inversely with the strength of FM exchange interaction in FM layer.23 Recently, it has been reported that Rashba SOT is strengthened in the presence of oxidation at interface in the structures like Pt/FM(Py or Co40Fe40B20)/Oxide(AlOx or MgO or TaOx).25-27 The main reason behind this enhancement of SOT is the breaking of the structural inversion symmetry at the oxide/FM interface which enhances the spin-accumulation.24-27 For spin-pumping, Platinum has been very widely investigated as NM layer in FM/NM bilayers.1-4,16-17 The β-W possesses high spin orbit interaction (SOI) value of 0.027 Ry (comparable to 0.030 Ry in Pt),28-29 high spin Hall angle (SHA) of -0.33 (much larger than 0.06 in Pt30-33,16,17 and 3 ACS Paragon Plus Environment


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β-Ta (-0.15)31,34) and higher ST efficiency than Pt and/or β-Ta.32,35 However, since spin flip probability parameter ∈ in β-W is smaller than 0.1, the spin pumped current in β-W is expected to cause spin-accumulation in β-W in very close proximity to the β-W /FM interface. This is likely to cause the flow of spin angular momentum back to the FM layer, i.e., spin back-flow.16-20 Although, the forward flow of spin angular momentum is always larger than the backward flow, but the net spin pumping contribution in β-W eventually is expected to be suppressed due to the diffusive spin accumulation near the interface.19,20,36,37 The effect of spin back-flow on the overall spin pumping process is lowering of the αeff which is taken into account via the effective spin-mixing conductance g↑↓(eff) instead of usual spin-mixing conductance g↑↓. Thus compared to Pt, the large values of SHA and ST efficiency together with the low αeff make β-W a potential candidate material in achieving low current magnetization switching35 desirable in STNO devices12,13 and ST microwave excitation in three terminal magnetic tunnel junctions.11 In this communication, we investigate a detailed and conclusive study of the effect of varying the thickness of β-W layer tW on the αeff in the two (uncapped) bilayer systems, namely Py(15 nm)/β-W(tW)/Si [hereafter referred to as PWS] and β-W(tW)/Py(15 nm)/Si [hereafter referred to as WPS] by using in-plane FMR measurement technique. Thereafter, a comparative study of the influence of the presence of a capping layer (CL) of different materials [Cu(2 nm), β-W(2 nm), βTa(2,3,4 nm) or Al(2,4 nm)] over the PWS and WPS systems on the αeff has been done so as to naturally propose the scheme for tuning the damping constant by tailoring the interfacial spin pumping mechanism. 2. MATERIALS AND METHODS The various capped and uncapped bilayer film samples of the two series (PWS and WPS) were deposited at room temperature on SiO2/Si substrates (SiO2 is the native oxide layer on Si) by 4 ACS Paragon Plus Environment

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employing pulsed dc magnetron sputtering technique using 99.99% pure metal targets. In all these, the thickness of the β-W layer has been carefully varied, tW = 0.5, 1, 1.5, 2, 3, 4, 5, 6, 8, 10, 12, and 20 nm. The base pressure was 4×10-7 Torr and during sputtering, pressure was maintained at ~3.4×10-3 Torr (for Py), ~3.0×10-3 Torr (for β-Ta), and ~1×10-2 Torr (forβ-W, Al, and Cu). For the growth of β-W layer, a slow deposition rate ∼ of 0.01 nm/sec was ensured to get the desired metastable β-phase (A-15 cubic) of W,30-32,38 whose formation was confirmed from the x-ray diffraction (XRD) and resistivity measurements. The in-plane magnetization of β-W/Py thin films was measured by physical property measurement system (PPMS) (Model Evercool-II from Quantum Design Inc). The XRD patterns on these thin films have been recorded using X’Pert-Pro x-ray diffractometer with Cu-Kα (1.54 Å) source to study the phase purity and orientation aspects of the β-W and Py thin films. The thickness of the individual layers and interface roughness were accurately determined by x-ray reflectivity (XRR) measurements. The microstructure, interface as well as thickness analyses of β-Ta(3 nm), β-W(2 nm) and Al(2 nm) capped PWS multilayer thin films are performed by using cross sectional high resolution transmission electron microscopy (HRTEM) (model# JEOL JEM 2010 (UHR Pole piece)), with electron beam energy of 200 keV. The cross sectional specimen for HRTEM was prepared by mechanical polishing, dimpling, and ion milling (conventional method). The ferromagnetic resonance field and the linewidth ∆ were measured by using broadband lock-in-amplifier based ferromagnetic resonance (LIA-FMR) technique with the help of a vector network analyzer (VNA) in an in-plane magnetic field configuration employing a coplanar waveguide (CPW) (see Figure1a and ref. 39 for details). For studying the surface composition, we have performed the x-ray photoelectron spectroscopic (XPS) measurements by using SPECS make system which uses non monochromatic Al-Kα (1486.61eV) source and hemispherical energy analyzer (pass energy of 40 eV with a resolution of ~0.3eV).



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3. RESULTS AND DISCUSSION Figure 1b (top panel) shows the XRD patterns recorded at a glancing angle of incidence of 1o on the indicated uncapped and capped PWs and WPS bilayers, as well as W thin films (thicknesses 38 and 52 nm) grown on Si/SiO2 substrates. The formation of highly meta-stable β-W phase (A-15 type) is established from (i) the presence of all the peaks of this phase for the both W films (38 and 52 nm) at corresponding known 2θ values, (ii) absence of (110), (200), (211) peaks of more stable but undesirable α-W(bcc) phase. This was established based on comparing the experimental data with the well-known JCPDS (joint committee on powder diffraction system) file data #47-1319 for β-W, and #04-0806 for α-W (The bottom panel depicts their relative intensities and respective 2θlocations). We found that the metastable β-W phase exists up to a thickness of ~50 nm, consistent with studies reported on 27 nm thin metastable β-W films grown at room temperature on Si.30,31 Figure 1b also shows the XRD patterns recorded on 38 nm thin β-W, and uncapped and capped bilayers. In all of these cases, the main β-W peaks (210) and (321) are clearly visible. The other peaks either have very low intensity or could not be detected possibly due to the low thickness of W layer. The (111) peak of Py(15) at 2θ∼44.0o might have merged with (211) peak β-W at 2θ∼43.9o.40 The resistivity ρW of the β-phase W thin films is estimated ~433.0(±17.0) µΩ.cm from the fitting of the fit of the measured sheet resistance (R□)-vs.-tW data of β-W(tW)/Si (see Figure 1(c)) by using the ρ

expression of □ = Ω. The agreement of the estimated ρW-value with the reported reports

further confirmed that W thin films indeed grew in the desired β(A-15) phase.30-32 To gain the insight about the interface-width, density and the exact thicknesses of the individual layers, the XRR profiles recorded on these samples were simulated by appropriately modeling the samples (Figure 2). The interface-width, layer density and layer thicknesses obtained


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from the simulated fits for the various samples are presented in Table S1 (see supporting information (S.I.) File). The XRR pattern of W(38 nm)/Si thin film matched satisfactorily with that of the β-W phase reported in the literature.38 It suggests the formation of metal oxide layer only on the top of the bilayers, i.e., WOx~2-3 nm, NiO~0.5-1 nm, CuOx~1.0-1.5nm, AlOx~0.7-1.0 nm, TaOx~2.0 nm, of Py layer due to the surface oxidation. The average width of the top CL(tCL)/Py(15 nm) interface and the bottom Py(15 nm)/β-W(tW) interface in all these multilayers is found to be approx. ∼0.5 nm. In addition, a good match between the estimated layer thickness and the nominal thickness is also clearly evident. To gain further insight about the microstructure, interface and thickness of each individual layers, we performed cross sectional HRTEM measurements on these multilayer thin films. Figures 3(a, b, and c) show the cross sectional HRTEM images of W capped PW i.e., β-W(2 nm)/Py(15 nm)/β-W(3 nm)/Si, Al capped PWS, i.e., Al(2 nm)/Py(15 nm)/β-W(3 nm)/Si, and Ta capped PWS i.e., β-Ta(3 nm)/Py(15 nm)/β-W(3 nm)/Si multilayer thin films. The images suggest that the growth of all layers in these samples is found to be spatially coherent such that the film thickness is nearly uniform everywhere. The thickness of each individual layer almost matches with the thickness expected on the basis of the simulated fit of XRR profiles of corresponding samples. The growth of

β-W (thickness of 3 & 2 nm in seed as well as top), Al (2 nm), and β-Ta(2 nm) layers in all three samples is amorphous in nature and this might be due to the very low thickness. Similar amorphous nature of thinner cap layers was also reported by others.38 The interface of these thin films is understandably not very sharp due to the amorphous growth of the β-W (2 & 3 nm), β-Ta(3 nm) and Al(2 nm) adjacent to the Py layer. The Py layer is found to be polycrystalline in nature having different oriented planes consistent with the XRD findings. Some of planes with different orientations are indicated in Figure 3 by round circles in yellow color. Moiré fringes which result 7 ACS Paragon Plus Environment


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from the superposition of two different oriented planes are also observed in the Py layer. Figures 3(a) and 3(c) clearly confirm the presence of naturally formed protective oxides of β-Ta and β-W layers as was also discussed above in the XRR section. However, the presence of top oxide layer of Al in Al(2 nm)/Py(15 nm)/β-W(3 nm)/Si (see Figure3(b)) could not be clearly evidenced. As will be discussed later, the XPS study revealed the presence of thin Aluminum oxide layer on top of Al(2 nm)/Py(15 nm)/β-W(3 nm)/Si multilayer structure. The spin dynamic response of these multilayer thin films was studied by performing the inplane FMR measurements in the presence of applied external dc-magnetic field, Hdc. The Hdc was varied from high (higher than the saturation magnetization state of Py/β-W bilayers) to low value at constant microwave frequency in the broadband frequency range of 5-11 GHz. From these recorded in-plane FMR spectra shown in Figure 4(a), the FMR resonance field Hr and resonance linewidth ∆H was obtained by fitting with the derivative of Lorentzian function. In Figure 4(b), the f-vs.-Hr variation for one representative sample from six different series of samples is presented, viz, uncapped PWS and WPS, PWS capped with Al(2 nm), Cu(2 nm), W(2 nm) and Ta(3 nm). The effective saturation magnetization 4πMeff and the surface/interface anisotropic energy constant (Ks) were determined from the fitting (solid lines) of the observed f-vs.-Hr data by using in-plane Kittel’s Equation (1):41

γ  f = H r ( H r + 4π M eff )  2 2π  1

(1)

Here γ (=1.856×1011 (Hz/T)) is the gyromagnetic ratio, and g (=2.1) is the spectroscopic splitting factor. The saturation magnetization 4πMS=1066.1(±7.8) mT and KS=0.50(±0.03) mJ/m2 of Py layer were obtained from the linear fit of 4πMeff-vs.-1/tPy data (see Figure 4c) with the expression


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4 = 4 −

40,42,43

for the β-W(5)/Py(tPy=3-36 nm)/Si system. The 4πMS value

calculated from the fitting of FMR data is found to match quite well with the value of ∼841.8 emu/cc (∼1058 mT) of Py(15 nm)/W(3 nm)/Si obtained from VSM-PPMS measurement as well as with the reported values.40 The dependence of 4πMeff on tW in some PWS and WPS bilayers capped with different layer is shown in S.I. file (Figure S1). The 4πMeff values of Py(15 nm)/β-W(tW)/Si are slightly higher (∼3-5%) than that of bare Py(15 nm) thin film within the error of measurement, and are also very close to the 4πMS value (i.e., 1058 mT) of the Py layer. This suggests a mild presence of magnetic proximity effect in Py induced by the adjacent β-W layer.44 For studying the FMR induced spin pumping mechanism in presence of β-W layer in Py/βW(tW) system, it is necessary to inspect the nature of the observed frequency dependence of ∆H in both capped and uncapped Py/β-W(tW) systems. Figure 5 shows the comparison of ∆H-vs.-f plots of uncapped and different capped Py/β-W bilayers. It can be observed that ∆H increases linearly with f in all these samples, which indicates that the damping of the magnetization precession is mostly governed by intrinsic magnon electronic (ME) phenomena (also known as spin orbit interaction (SOI) phenomena).45-47 From this linear behavior of ∆H-vs.-f, Gilbert’s damping parameter α and magnetic inhomogeneity ∆H0 were determined as fitting parameters by using the equation:46

∆ H = ∆ H0 +

4π α f

γ

(2)

The ∆H0 is related with the growth quality of the films. For better film quality, the ∆H0 should approach to zero. The 2nd term of the Equation (2) depicts the ME contribution arising from SOI in FM material, which is purely intrinsic in nature. In the present case, the observed values of ∆H0 for all Py/β-W thin films capped with different NM layers remain around 0.5 mT within error of 9 ACS Paragon Plus Environment


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measurement (see S. I. file, Figure S2). This value being low compared to the reported values6,8 of ~1.0 mT indicates that the layers are of acceptable film quality. To study the role of interface contribution on spin pumping phenomena, the 1/tPy dependence of αeff in β-W(5 nm)/Py(tPy=3-36 nm)/Si system is presented in Figure 5(b). We find that the αeff -vs.-1/tPy plot is linear and follows the equation,45,47,48

αeff (tPy) = αB + αS (1/tPy)

(3)

Here αB and αS are the bulk and interface/surface contributions to the Gilbert’s damping. The intercept αB is found to be 0.0065(±0.0002) and 0.0298(±0.0035) nm for β-W(5 nm)/Py(tPy=3-36 nm)/Si system. The observed value of αS is quite high and it suggests that there is a significant interfacial spin transport through the spin pumping.45,47,48 For determining the total interfacial spin pumping contribution to damping in the presence of

β-W in uncapped PWS and WPS bilayers, and also in PWS bilayer capped with Al(2,4 nm), Cu(2 nm), W(2 nm) and β-Ta(2,3,4 nm), it is necessary to study the intrinsic interfacial spin mixing conductance g↑↓. In addition, the determination of spin diffusion length (λSD) of β-W layer is also needed, because at tW >λSD the spin current loses its spin coherence and spin accumulation cannot result.19,20 Therefore, we investigated the β-W layer thickness dependence αeff (i.e., αeff-vs.-tW) of the multilayer thin films in different configurations.48 Figures 6a and 6b show the comparative study of αeff(tW)-vs.-tW behavior among the uncapped PWS, WPS, and β-W(2 nm) capped PWS systems. It shows that with the increase of β-W layer thickness from 1 to 4 nm, αeff increases in all the cases, i.e., irrespective of the fact that Py layer is in contact with seed, top or both the β-W layers. Thereafter, αeff slightly decreases with tW at higher thicknesses. Although for β-W, values of


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SHA and SOC are high,29-31 but the initial increase of αeff with tW is not significant, unlike in the case of Pt/Py1,3,16,17,35 or Pd/Py20 systems. As discussed above, this might be due to the fact that for

β-W,∈ < 0.1, which results in the spin back-flow due to interfacial non-equilibrium spin accumulation. As a result, further spin pumping to the β-W layer slows down. The back-flow spin " ⁄λ# 19,20 current, indicated by . . Here is the pure spin current without , is given by = !

spin back flow. The factor 2 in exponent appears since the pure spin current travels twice distance (i.e., forward and backward in NM metal). It is evident that since decays exponentially with %& , spin back flow effects are dominant when the NM layer is ultrathin (≤ λSD). Thus, in presence of spin back-flow, the net spin current across the interface is given by (%(%)*) = (1 − ! " ⁄λ# ). Whenever > , there will be still a net transfer of spin angular momentum to the

NM layer, and one would expect an enhancement in the Gilbert’s damping due to finite spin pumping contribution, i.e., αS. Thus, in the present case, the higher αeff value in PWS bilayers, as compared to the WPS bilayers might be due to the additional spin pumping from the Py layer to the very thin surface layer of antiferromagnetic NiO formed on top of Py. Also, in contrast to PWS bilayers, the presence of WOX on top of β-W(tW) in WPS bilayer might induce anti-damping like torque because of high SOT nature.30-32,34,35 However, this effect is expected to be dominant only at very small tW regime. The interfacial spin mixing conductance g↑↓ is determined from the αS-contribution (see Figure 6c) associated with spin pumping by using the equation,49,50 . = ./ = µ0

↑↓ (3" 456 ⁄λ# ) 78

(4)

@

Here, 90 (= 9.274 × 10"7 A) is the Bohr magneton. The additional bracketed term (1 − e"CD ⁄λEF ) on the right hand side of equation (4) appears here in order to account for the spin back11 ACS Paragon Plus Environment


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flow effect, where fraction of the pure spin current gets reflected back to Py (FM) layer from the Air/β-W interface (in WPS) and β-W/SiO2 interface (in PWS). As discussed in preceding para, the fraction of spin current that returned back to Py(FM) layer adds angular momentum (previously lost due to spin pumping) to Py layer. This leads to effectively decrease the spin mixing conductance, thus the effective spin mixing conductance is given by g↑↓(eff) = ↑↓ (1 − e" ⁄λ# ). From the plots of αSP-vs.-tW for PWS and WPS bilayers, the fitted values of g↑↓ and λSD are found to be 1.63(±0.02)×1018 m-2 and 1.44(±0.02)×1018 m-2, and 1.42(±0.19) and 1.00(±0.10) nm, respectively. The fitted values of g↑↓ are one order magnitude lower as compared to Pt/Py system, understandably due to the spin back-flow effect from β-W layer.13-14,39 It may be pointed out that our observed λSD values are lower than the value of 3.30±0.3 nm reported by Hao et al. in W/Co40Fe40B20 system.31 From the ongoing discussion, it is concluded that the spin pumping mechanism is occurring in presence of β-W layer irrespective of the fact whether it is on top or below the Py layer in the bilayer system. However, in case of β-W(2 nm) capped PWS (see Figure 6d), the αSP contribution associated with spin pumping due to both seed and cap β-W layers. The exponential increase in the overall damping, i.e., ./ (%(%)*) with tW is mainly due to seed layer thickness variation. Thus, in β-W(2 nm) capped PWS system, the observed plot of αeff(tW)-vs.-tW is fitted by using equation: 49,50 ./ (%(%)*) = ./ (G)H − I/KL/M!!N(0)) + ./ (G)H − I/KL/M!!N(%& ))…...(5) Here, the 1st term ./ (G)H − I/KL/M!!N(0)) is a constant term because it accounts for the spin pumping from Py layer to the capping β-W (2 nm) layer in β-W(2 nm) capped PWS. As explained above, its magnitude is governed by equation (4). On the other hand, the 2nd term in RHS of equation (5) accounts for the seed layer thickness-dependent spin pumping effects. Thus, in β-W(2 12 ACS Paragon Plus Environment

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nm) capped PWS system, the observed plot of αeff(tW)-vs.-tW is fitted by using Equation (5). The consistency

of

the

analysis

is

remarkable

since

the

./ (G)H − I/KL/

M!!N(0))=0.0023(±0.00005), which is almost equivalent to our observed value 0.0021(±0.0002) for WPS bilayer with tW=2 nm. The values of g↑↓( G)H − I/KL/M!!N(%& )) and λSD as determined as fitting parameters from the equation (5) are found to be 0.60(±0.05)×1018 m-2 and 2.36(±0.91) nm, respectively. The net spin mixing conductance can be summed up from the individual spin mixing conductance contributions from the cap and seed β-W layers as: g↑↓(total) ≈ g↑↓( G)H − I/KL/M!!N(0)) +g↑↓(G)H − I /KL/M!!N(%& ))49≈ 2(±0.07)×1018 m-2. However, in the case of Al(2 nm) capped PWS (see Figure 7a), the αeff shows a reverse trend i.e., it decreases with increase in tW (i.e., anti-damping) in sharp contrast to the β-W(2 nm) capped PWS system. Further, the αeff decreases until tW=5 nm, above which it slightly increases to 0.0081 at tW=8 nm, and thereafter it remains constant for higher tW up to 20 nm. Similar behavior is also observed for Al(4) capped PWS system (see S. I. file Figure S3b). However, αeff starts showing increasing behavior again when β-W layer is inserted between Al(2 nm) and Py layer i.e. in Al(2 nm) capped WPS system (see Figure 7d). Thus whenever β-W is in contact with Py, the role of top Al layer on spin pumping in Al(2 nm) capped WPS system is negligible due to low SOC constant and high λSD (∼330 nm at RT51) for Al and it merely acts like a protecting capping layer over β-W layer.36,45 But in the reverse case, when Al is in contact with Py, i.e., in the Al(2 nm) capped PWS system, the suppression of spin pumping mechanism is seen with increase in seed β-W layer thickness up to 5 nm. This issue of the suppression of spin pumping mechanism is discussed in later section.



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It is imperative here to see the effect of the change in capping layer material like Cu (having low SHA, SOC36) and β-Ta (having high SHA, SOC28) for the same PWS system. Figure 7b shows the comparative study of αeff –vs.-tW behavior between β-Ta(3 nm) capped with β-W(2 nm) capped PWS bilayers. The observed trend in αeff(tW) is found to be identical to that seen in case of Al(2 nm) capped PWS system (though its magnitude is little different). In order to have more information about the role of β-Ta top layer over PWS, the samples with three different thicknesses (2, 3 and 4 nm) of capped β-Ta layer were grown and the extent of anti-damping mechanism occurring at different capped β-W thicknesses was explored. It is observed that maximum lowering of αeff is seen in the case of capped β-Ta(3 nm) as compared to β-Ta (2, 4 nm) capped PWS bilayers (see Figure 7c). Further, in case of β-Ta(4 nm) capped PWS bilayer, no discernible lowering in magnitude of αeff with increase in tW is observed and the values of αeff(tW) are slightly higher compared to that measured in β-Ta(2, 3 nm) capped PWS bilayers. This trend might be associated with the increase in spin pumping at higher β-Ta thickness.39,52 Now, in the case of Cu(2 nm) capped PWS system (see S. I. file, Figure S3a), the αeff shows relatively less enhancement from 0.0082±0.0001 at tW=0.5 nm to 0.0086±0.0002 at tW=3 nm, as compared to the uncapped PWS bilayer system (Figure 6a). Thereafter, αeff exhibits a decreasing trend till tW = 6 nm, above which it remains almost unchanged. This can be ascribed to the relatively lower spin pumping in presence of Cu relative to W(2 nm) capped PWS, since Cu possesses a very low value of SHA and SOC.18,19,36,53 Here, it should be noted that no decrease in αeff(tW) was seen for Cu(2 nm) and β-W(2 nm) capped PWS bilayers unlike the case of Al(2,4 nm) or β-Ta(2,3 nm) capped PWS bilayers despite the fact that both Cu and Al have nearly same values of of SHA, and SOC strength19,20,36 that are smaller than those observed in β-W and β-Ta.30-32,34


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From the ongoing discussion, it is clear that (i) The occurrence of spin pumping mechanism is linked to increase in αeff whenever β-W(tW) layer is adjacent to the Py layer though there is net spinaccumulation near the interface discussed in introduction section. (ii) The suppression of spin pumping i.e., enhancement of anti-damping occurs when Al(2,4 nm) or β-Ta(2,3 nm) cap layer is deposited on top of Py layer, and (iii) Anti-damping is not observed when the Al(2 nm)/β-Ta(2,3 nm) cap layers are placed over β-W layer (i.e., over WPS bilayers). The mechanism behind the observed spin pumping and anti-damping behavior in these specimens is discussed in following sections. In order to understand the observed behavior of αeff-vs.-tW, we follow the theoretical model about the generation of non-equilibrium spin density due to diffusive spin accumulation from spin pumping proposed by Tserkovnyak et al.19,36 According to this model, the pure transverse spin current (and hence spin angular momentum) flows out of FM layer and gets accumulated in the nearby NM layer. As a result, the non equilibrium spin density created near the interface causes back flow of spin angular momentum (hence pure spin current, ) back to FM layer, causing a kind

of opposition effect to the enhancement of αeff.18-20,36,37,39 The samples in the present study demonstrate the direct experimental evidence of such a spin back flow. It may be noted that although SOI in β-W is comparable to Pt (and higher than that in β-Ta30-32,35) the increase in αeff with tW is not much (Figures 6a & 6b). This can be understood as the effect of back-flow of a fraction of spin angular momentum in case of uncapped and β-W capped PWS bilayers. As a result, the net spin current flow in the NM layer is actually small (compared to those reported in case of Pt/FM bilayers). This explains the observed gradual increase in αeff (Figure 6). Here, it is to be emphasized that spin back-flow is a consequence of spin accumulation at the interface since the latter is very sensitive to spin diffusion length λSD of β-W layer20. In fact, the strength of the spin 15 ACS Paragon Plus Environment


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accumulation near the interface is expected to dominate over the counter effect of SOC (which causes ISHE) only when tWλSD, the spin accumulation gets weaken in β-W layer and it results in weakening of .20 The observed nearly constant nature of αeff for all tW > 5 nm is due to loss of spin coherent state at tW >λSD.20 These results find very good experimental support from the results reported by Jiao et al. who observed higher ISHE signal in different bilayers of Py with Ta, Pt, and Pd, only in thickness regime tW>λSD..20 Thus the dependence of αeff on tW observed in Py(15 nm)/β-W(tW)/Si, βW(tW)/Py(15 nm)/Si, Cu(2 nm)/Py(15 nm)/β-W(tW)/Si, β-W(2 nm)/Py(15 nm)/β-W(tW)/Si. Al(2 nm) or β-Ta (2,3 nm)/β-W(tW)/Py(15)/Si series of thin films finds natural explanation within the purview of spin back-flow model.19,36 However, in Al(2 nm) or β -Ta(3 nm) capped PWS system, the observed decrease in αeff is completely opposite trend as compared to uncapped PWS and WPS, and beta-W(2 nm) or Cu(2 nm) capped PWS (see Figure7a, and 7b). It might be attributed to the formation of protective oxide e.g., Al2O3(∼1 nm) and β -Ta2O5(∼2 nm)39 (confirmed from XRR and later by XPS measurements) on the top of Py layer due to passivation after exposure to air in ambient atmosphere. It is very likely that the top Al2O3 (∼1 nm) and β -Ta2O5(∼2 nm) would possess the structural inversion asymmetry (SIA) on Py layer. Similar generation of SIA on FM layer is also reported by many groups in the Al and Ta capped structures.24-27 It is also well-known that in presence of such SIA the non-equilibrium experiences a IREE and generates a charge current supported by the presence of interfacial states near the interface.6-9 The charge current might cause an anti-damping SOT on the magnetization of FM layer.10 As a result, the αeff must be small compared to the case when oxide layer is absent. This


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explains the initial decrease in αeff(tW) observed in case of Al(2 nm) and/or β-Ta(2,3 nm) capped PWS bilayers. However, it should be noted that such a decrease in overall αeff(tW) is not seen in Al(2 nm) or β -Ta(2 nm) capped WPS bilayers (see Figure 7d), which exhibited a similar αeff(tW) behavior reported in Py(15 nm)/Ag(5 nm)/Bi(8 nm) by Zhang et al.8 These authors observed that the IREE disappeared when Ag/Bi layers were interchanged, and also when Ag was replaced with Au or Cu.6,8 Although not undertaken in our study, a similar IREE mechanism is likely to be occurring in the present case of Al(2 nm) and Ta(2 nm) capped PWS bilayers during the spin pumping from Py layer since the basic requirements6,18-20,36 of the occurrence of IREE in a multilayer, namely the presence of a non-equilibrium spin density, high SOC and high resistivity in the adjacent NM layer, are all fulfilled in them. It is worth to point out here that a similar decrease in effective damping was observed by Ozatay et al.5 in their side-wall AlOx passivated Py/Cu/Py/Pt spin valve structures. The XPS measurements were performed to substantiate the proposed explanation of decrease in effective damping behavior in Al(2 nm) and/orβ-Ta(2,3 nm) capped PWS bilayers by probing their surfaces and interfaces (Figures 8 and 9). Figure 8a, shows the XPS spectra corresponding to Al 2p3/2 recorded on Al(2 nm)/Py(15 nm)/W(2 nm)/Si sample. The formation of top Al2O3 layer is established by the presence of three de-convoluted peaks centered at binding energy (BE) values of 72.65, 73.82, and 74.88 eV. The low intensity peak centered at 72.65 eV corresponds to the metallic Al-peak. It is to be noted that the peak at 73.82 eV suggests an incomplete oxidation of Al and it is known that on complete oxidation on Py layer it approaches to BE value of 74.88 eV.54 Consistent with this, the O-1s XPS spectra (Figure 8b) also supports the formation of top Al2O3 layer.54 In Figure 8c, the two peaks appearing at BE of 851.17 and 857.07 eV correspond to the metallic Ni2p3/2 and satellite/plasmon peak of Ni-2p3/2, respectively. No evidence of oxidation of Ni (i.e., in the form of NiO and/or Ni(OH)2 which normally appear at BE values of 853.7 eV, 855.6 eV) is found,



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although as noted above there is a clear presence of passivating top AlOx layer adjacent to Py layer which is consistent with literature.5 On the other hand, the deconvolution of the XPS spectra corresponding to Fe-2p (Figure 8(d)) revealed the presence of two peaks at 705.86 and 710.79 eV BE values, which respectively correspond to the metallic Fe and γ-Fe2O3. The formation of γ-Fe2O3 and absence of any signature of the formation of anti-ferromagnetic NiO in this sample is consistent with regards to their free energy (i.e., -50.6 and -177.4 kcal/mole for NiO and γ-Fe2O3, respectively).55 Thus, the combination of top Al2O3+Al (∼1+1 nm) over Py layer is indeed responsible for the observed decreasing trend in αeff(tW) of Py layer in Al(2)/Py(15)/W(tW)/Si system (Figure 7a). Similarly, the formation of top β-Ta2O5(∼2 nm) in β-Ta/Py bilayers is also established by XPS in our previous work.39 Figure 9 presents XPS spectra corresponding to different BE ranges recorded on the W(2 nm)/Py(15 nm)/Si sample, namely (a) W-4f, (b) O-1s, (c) Ni-2p, (d) Fe-2p. Formation of oxide phase of W on top of Py is clearly evidenced from the analysis of Figures 9a & 9b. It may be stressed that consistent with XRR findings, presence of metallic W is not evidenced in XPS. No evidence for the formation of anti-ferromagnetic NiO or any of the oxides of Fe is evidenced (Figures 9c & 9d) in this sample. To further strengthening the anti-damping behavior exhibited by the Al(2 nm)/Py(15 nm)/βW(4 nm)/Si system as a result of the oxidation of top 2 nm thin Al cap, further series of samples were prepared in order to compare the effect of the different interface on top of Py(15 nm)/β-W(4 nm)/Si, i.e., with 2 nm Al metal and Al2O3 (2, 4 nm) on the resulting value of αeff. The details of these series are as under: 1. On top of Py(15 nm)/β-W(4 nm)/Si, Al2O3 is deposited by both reactive (R) sputtering of


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Al in O2 environment of partial pressure ∼2.5×10-5 Torr as well as post oxidation (P) of Al(2,4 nm) film in O2 environment of partial pressure ∼1.5×10-3 Torr for 5 min, i.e., (a)

R-Al2O3(1, 2 nm)/Py(15 nm)/β-W(4 nm)/Si, &

(b) P-Al2O3(2, 4 nm)/Py(15 nm)/β-W(4 nm)/Si. 2. As another alternative to prevent oxidation of Al, a 4 nm thin Cu layer is used to further cap the Al(tAl=2,3,4 nm)/Py(15 nm)/β-W(4 nm)/Si in situ, i.e., Cu(4 nm)/Al(tAl=2,3,4 nm)/Py(15 nm)/β-W(4 nm)/Si. 3. Lastly, another series of Cu(4 nm) capped Al(2 nm)/Py(15 nm)/β-W(tW nm)/Si samples was made in which thickness of β-W is varied (tW= 1,3,4, 6 nm) to study the effect of varying the thickness of β-W, i.e., Cu(4 nm)/Al(2 nm)/Py(15 nm)/β-W(tW=1,3,4,6 nm)/Si system. For brevity, the frequency dependence of linewidth and αeff values of these samples is presented in Supplementary Information as Figure S4 and Table S2, respectively. From Table S2, it is observed that: (i)

The αeff value of 0.0081±0.0001 for P-Al2O3 (2 nm) capped Py(15 nm)/β-W(4 nm)/Si is found to be very close to earlier observed value 0.0080±0.0001 in Al(2 nm)/Py(15 nm)/βW(4 nm)/Si, suggesting that the Al (2 nm) in the later is naturally oxidized.

(ii) However, the observed αeff value of 0.0089±0.0002, for R-Al2O3(2 nm)/Py(15 nm)/β-W(4 nm)/Si is relatively higher as compared to the value of 0.0081±0.0001 in the P-Al2O3(2 nm)/Py(15 nm)/β-W(4 nm)/Si. We believe that this could be due to finite oxidation of 19 ACS Paragon Plus Environment


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Ni81Fe19 during reactive sputtering of Al, leading to the formation of AF NiO (Figure 2a & Table S1 in S.I. file) [39] which is known to enhance the damping. (iii) In the two P-Al2O3 capped samples, P-Al2O3(4 nm)/Py(15 nm)/β-W(4 nm)/Si, has higher

αeff value of 0.0085±0.0001 as compared to 0.0081±0.0001 in P-Al2O3(2 nm)/Py(15 nm)/β-W(4 nm)/Si, suggesting that there could be a presence of unreacted/metallic Al adjacent to Py layer instead of fully P-Al2O3. This is possible because of Al(4 nm), which might not be completely converted into AlOx during post oxidation as compared to Al(2 nm). Because of the presence of top AlOx over Py, the anti-damping mechanism is observed as is observed in naturally oxidized Al in Al(2 nm)/Py(15 nm)/β-W(tW)/Si system (Figure 7a). These comparison of FMR results (Table S2), thus, clearly point out that in the Al(2 nm)/Py(15 nm)/β-W(tW)/Si series (Figure 7a), the 2 nm thin Al cap is naturally oxidized, which is supported by XPS (Figure 8b), XRR (Figure 2e) and HRTEM (Figure 3b). To further confirm the above issue of oxidation of Al(2 nm) cap, the analysis of FMR spectra recorded on the 3rd set of samples series (c.f. Table S2) revealed that: (i) In Cu(4 nm)/Al(tAl nm)/Py(15 nm)/β-W(4 nm)/Si, the observed value of αeff, ranged in 0.0084±0.0001 to 0.0087±0.0002 is nearly constant with respect to tAl, within the error of measurement, and is higher than that in the Al(2 nm)/Py(15 nm)/β-W(4 nm)/Si, and PAl2O3(2 nm)/Py(15 nm)/β-W(4 nm)/Si as well. This confirms that the anti-damping effect arises only when Al2O3 is present adjacent to Py layer rather than metallic Al, and is indeed due to IREE.


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(ii) In Cu(4 nm)/Al(2 nm)/Py(15 nm)/β-W(tW)/Si, αeff value, which again ranged between 0.0084 to 0.0087 ±0.0002, is higher compared to that in P-Al2O3(2 nm) capped Py(15 nm)/β-W(4 nm)/Si (αeff=0.0081±0.0001) as well as in Al(2 nm)/Py(15 nm)/β-W(4 nm)/Si (αeff=0.0080±0.0001). Both these observations, therefore, clearly establish that the anti-damping mechanism observed in Al(2 nm)/Py(15 nm)/β-W(tW)/Si, (Figure 7a) is caused as a result of the formation of Al2O3 on top of Py(15 nm)/β -W(tW)/Si. 4. CONCLUSIONS In summary, the spin pumping in uncapped and capped Py(15 nm)/β-W/Si(100) bilayers system is studied as a function of thickness of the non-magnetic β-W layer (tW). Although SOC in β-W and Pt are of similar magnitude, the marginal increase observed in αeff (compared to that reported in Pt/FM) with tW due to spin pumping in the uncapped Py(15 nm)/β-W/Si [Py on top] as well as in β-W/Py(15nm)/Si [Py in bottom] is quite remarkable. This is attributed to the occurrence of spin-accumulation in β-W just near the interface with Py, since in β-W the spin flip relaxation time is higher than the spin injection time, which is contrary to the case of Pt. The interfacial spin mixing conductance (g↑↓) at the Py/β-W interface are found to be 1.63(±0.02)×1018 m-2 and 1.44(±0.02)×1018 m-2) for the uncapped Py(15 nm)/β-W(tW)/Si and β-W(tW)/Py(15 nm)/Si) bilayer systems, respectively. Also, the spin diffusion length (λSD) of β-W is found to be 1.42(±0.19) nm and 1.00(±0.10) nm) in these uncapped Py(15 nm)/β-W(tW)/Si and β-W(tW)/Py(15 nm)/Si) bilayer systems, respectively. Influence of placing a thin capping layer of different non-magnetic metals (βTa, Al, Cu and β-W) of tNM ∼2-4 nm over Py(15 nm)/β-W(tW)/Si on the interfacial damping mechanism is also investigated. A remarkable decrease in damping i.e., decrease of αeff with 21 ACS Paragon Plus Environment


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increase in tW (i.e., anti-damping) is observed in Al(2,4 nm) as well as β -Ta(2,3 nm) capped Py(15 nm)/β-W(tW) system up to tW ∼5 nm and nearly constant thereafter. This is in sharp contrast to the increase of αeff expected because of spin pumping. The study provides the presence of anti-damping like torque whose origin is ascribed to the Rashba effect. Contrary to this, an increase of damping is seen by reversing the layer order of Py and β-W(tW) while keeping the top layer Al(2 nm) or βTa(2,3 nm) layer as before. The analyses of αeff(tW) behavior in the capped and uncapped Py(15 nm)/β-W(tW) bilayers indicate that for tW >∼λSD both spin pumping and anti-damping torque diminish due to the loss of spin coherence state at higher tW. The tunability of effective damping constant reported in this work in multilayers involving the ferromagnetic Py layer suggests their high prospects in the development of spintronics devices requiring ultrafast switching speed and low energy dissipation loss.

ASSOCIATED CONTENT

Supporting Information: The Supporting Information is available free of charge via internet at http://pubs.acs.org. It contains 1. Details of XRR fitting parameters for Py(15 nm)/β-W(tW)/Si and β-W(tW)/Py(15 nm)/Si structures capped with different NM metals (β-W, Al, β-Ta, and Cu) 2. 4πMeff-vs.-tW plots for Py(15 nm)/β-W(tW)/Si and β-W(tW)/Py(15 nm)/Si bilayer series capped with different NM metals. 3. ∆H0-vs.-tW plots for Py(15 nm)/β-W(tW)/Si and β-W(tW)/Py(15 nm)/Si bilayers capped with different NM metals. 4. The αeff(tW)-vs.-tW for Cu(2 nm) and Al(4 nm) capped Py(15 nm)/β-W(tW)/Si bilayer series.


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5. Growth of Al2O3 on top of Py(15 nm)/β-W(4 nm)/Si by reactive sputtering and post oxidation technique. 6. The ∆H-vs.-f behavior for Al2O3 capped Py(15 nm)/β-W(4 nm)/Si, and Cu(4 nm) capped and Al (2-4 nm)/Py(15 nm)/β-W(4 nm)/Si multilayer thin films. 7. Effective Gilbert’s damping parameter (αeff) and magnetic inhomogeneity (∆H0) of Al2O3 capped Py(15 nm)/β-W(4 nm)/Si, and Cu(4 nm) capped and Al (2-4 nm)/Py(15 nm)/β-W(4 nm)/Si multilayer thin films. ACKNOWLEDGEMENTS: Useful discussions with Dr. Ankit Kumar and Dr. Pranaba K. Muduli are thankfully acknowledged. We would like to thank Prof. Parlapalli V. Satyam of Institute of Physics, Bhubaneswar for helping us in HRTEM characterization. The FIST-DST and MHRD, GOI are thankfully acknowledged for the XPS and PPMS measurements at IIT Delhi. Notes The authors declare no competing financial interest.

AUTHOR INFORMATION

Corresponding Author *

E-mail: [email protected]



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Figure 1: (a) The schematic diagram of the coplanar waveguide (CPW) and the sample during the FMR measurement. Sample is placed (film side facing down) on top of the central signal transmission line S which is isolated from ground G. (b) X-ray diffraction patterns of W and Py/W(tW) multilayer thin films. The top panel shows the glancing angle (1o) x-ray diffraction patterns recorded on W(51.76 nm)/Si, W(38.56 nm)Si, W(20 nm)/Py(15)/Si, Py(15)/W(20 nm)/Si, Ta(2 nm)/Py(15 nm)/W(20 nm)/Si, Ta(3 nm)/Py(15 nm)/W(20 nm)/Si, and Al(2 nm)/Py(15 nm)/W(20 nm)/Si multilayers (the numbers in parenthesis are thickness in the unit of nm). The bottom panel Figure 1b shows the standard JCPDS pattern for the two phases of W namely bcc α-W (Purple colored solid lines) and A-15 (β-W) (dark yellow colored dashed lines). (c) Variation of sheet resistance (R□) with thickness of tW of β-W in β-W(tW)/Si films.



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Figure 2: X-Ray reflectivity (XRR) profiles– experimental (open blue circles) and simulated (solid red lines) of various multilayer systems, (a) Py(15 nm)/β-W(tW=2,3,4,6 nm)/Si and βW(tW=38.5 nm)/Si, (b) β-W(tW=2,3,4,6 nm)/Py(15 nm)/Si, (c) β-W(2 nm)/Py(15 nm)/βW(tW=2,3,4,6 nm)/Si (d) Cu(2 nm)/Py(15 nm)/β-W(tW=2,3,4,6 nm)/Si, (e) Al(2 nm)/Py(15 nm)/β-W(tW=2,3,4,6 nm)/Si, and (f) β-Ta(3 nm)/Py(15 nm)/β-W(tW=2,3,4,6 nm)/Si multilayer thin films.


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Figure 3: Cross sectional HRTEM images of (a) β-W(2 nm)/Py(15 nm)/β-W(3 nm)/Si, (b) Al(2 nm)/Py(15 nm)/β-W(3 nm)/Si, and (c) β-Ta(3 nm)/Py(15 nm)/β-W(3 nm)/Si multilayer thin films.



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Figure 4: (a) Representative FMR spectra for β-W(4 nm)/Py(15 nm)/Si sample recorded at different constant frequencies. Open symbols are experimental data and corresponding solid lines are Lorentzian fit. (b) f-vs.-Hr plots for Py(15 nm)/β-W(4 nm)/Si, β-W(4 nm)/Py(15 nm)/Si, Cu(2 nm)/Py(15 nm)/β-W(4 nm)/Si, Al(2 nm)/Py(15 nm)/β-W(4 nm)/Si, β-Ta(3 nm)/Py(15 nm)/β-W(4 nm)/Si, and β-W(2)/Py(15)/β-W(4 nm)/Si sample. The solid lines represent the best fitted curves using Equation (1). (c) 4πMeff-vs.-1/tPy plot for β-W(5 nm)/Py(tPy=3-36 nm)/Si. The solid line represent the best fitted straight line using the equation 4πMeff=4πMs-(2Ks/MstPy) to determine the saturation magnetization 4πMs and anisotropic constant Ks. (d) In-plane M-vs.-H loop recorded at 300K on Py(15 nm)/β-W(3 nm)/Si sample.


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Figure 5: (a) ∆H-vs.-f data (open symbols) and linear fitted plots (using Equation (2)) for Py(15 nm)/β-W(4 nm)/Si, β-W(4 nm)/Py(15 nm)/Si, Cu(2 nm)/Py(15 nm)/β-W(4 nm)/Si, Al(2 nm)/Py(15 nm)/β-W(4 nm)/Si, β-Ta(3 nm)/Py(15 nm)/β-W(4 nm)/Si, and β-W(2 nm)/Py(15 nm)/β-W(4 nm)/Si multilayer samples. (b) αeff-vs.-1/tPy behavior of the βW(5 nm)/Py(tPy=3-36 nm)/Si sample. The solid red line is the best fit to the experimental data (open symbols) by using Equation (3).



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Figure 6: The αeff-vs.-tW plots (Figures (a) and (b)), the ./ -vs.-tW plots (Figure (c)) and (d) the ./ (%(%)*)-vs.-tW plot for bilayer Py(15 nm)/β-W(tW)/Si, β-W(tW)/Py(15 nm)/Si, and trilayer β-W(2 nm)/Py(15 nm)/β-W(tW)/Si samples, respectively. Open symbols represent the experimental data. The solid red lines in Figures (c) and (d) correspond to the best fits using Equations (4) and (5), respectively. While the dashed blue lines in Figures (a) and (b) represent the bulk contribution to αeff, the dashed purple line in Figure (d) represents the spin pumping contribution from β-W(2) capped Py layer without any β-W seed layer. Samples schematics are also shown for clarity.


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Figure 7: The comparative study of αeff-vs.-tW behavior in Py(15 nm)/β-W(tW)/Si capped with different layers, (a) Al(2 nm) and β-W(2 nm) caps, (b) β-Ta(3 nm) and β-W(2 nm) caps, and (c) β-Ta(2 nm) and β-Ta(4 nm) caps. Figure (d) shows the comparison in case of Al(2 nm) and β-Ta(2 nm) capped β-W(tW)/Py(15 nm)/Si bilayers. Samples schematics are also shown for clarity.



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Figure 8: The portion of XPS spectra in different binding energy ranges corresponding to (a) Al-2p, (b) O-1s, (c) Ni-2p and (d) Fe-2p, respectively for the Al(2 nm) capped Py(15 nm)/β-W(3 nm)/Si. The open symbols are the experimental data points and the different solid lines are the component-fits to experimental data as indicated in respective figure-panels.


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Figure 9: The portion of XPS spectra in different binding energy ranges corresponding to (a) W-4f, (b) O-1s, (c) Ni-2p and (d) Fe-2p, respectively for the β-W(2 nm) capped Py(15 nm)/Si sample. The open symbols are the experimental data points and the different solid lines are the component-fits to experimental data as indicated in respective figure-panels.



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Abstract Graphic: The comparative study of αeff-vs.-tW behavior in Py(15 nm)/β-W(tW)/Si bilayer system capped with Al(2 nm), β-W(2 nm), and β-Ta(3 nm) layers.


Î²-W System (CL

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