CONDENSED MATTER PHYSICS All-optical detection of ... · The plethora of works in the emerging field of spin-orbitronics (18) have revealed that interfacial transport observed in

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CONDENSED MATTER PHYS I CS

1Department of Condensed Matter Physics and Material Sciences, S. N. Bose Na-tional Centre for Basic Sciences, Block JD, Sector III, Salt Lake, Kolkata 700106,India. 2Department of Physics and Nanotechnology, SRM Institute of Scienceand Technology, Kattankulathur 603203, Tamil Nadu, India.*Corresponding author. Email: [email protected]

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All-optical detection of interfacial spin transparencyfrom spin pumping in b-Ta/CoFeB thin filmsS. N. Panda1, S. Mondal1, J. Sinha1,2, S. Choudhury1, A. Barman1*

Generation and utilization of pure spin current have revolutionized energy-efficient spintronic devices. Spinpumping effect generates pure spin current, and for its increased efficiency, spin-mixing conductance and in-terfacial spin transparency are imperative. The plethora of reports available on generation of spin current withgiant magnitude overlook the interfacial spin transparency. Here, we investigate spin pumping in b-Ta/CoFeBthin films by an all-optical time-resolved magneto-optical Kerr effect technique. From variation of Gilbert dampingwith Ta and CoFeB thicknesses, we extract the spin diffusion length of b-Ta and spin-mixing conductances. Conse-quently, interfacial spin transparency is derived as 0.50 ± 0.03 from the spin Hall magnetoresistance model for theb-Ta/CoFeB interface. Furthermore, invariance of Gilbert damping with Cu spacer layer thickness inserted betweenb-Ta and CoFeB layers confirms the absence of other interface effects including spin memory loss. This demonstratesa reliable and noninvasive way to determine interfacial spin transparency and signifies its role in generation of purespin current by spin pumping effect.

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INTRODUCTIONDevelopment of advanced spintronic (1–4) devices with minimalpower dissipation has attracted great interest in recent times. Themajor goal of modern spintronics is to harness pure spin current(5, 6) to enable more efficient information processing through non-volatility, rapid switching, and energy-efficient on-chip integrationof magnetic bits in memory devices. Since pure spin current does notinvolve net flow of charge, Joule heating and stray Oersted fieldeffects can be avoided in spin current–based devices (7). Pure spincurrent or flow of spin angular momentum can originate from thespin-dependent scattering in heavy metals such as spin Hall effect(8, 9), Rashba-Edelstein effect (10, 11), spin pumping (12, 13),electrical injection from ferromagnet (FM) in a nonlocal geometry(14, 15), and spin caloritronic effect (16, 17). The fundamental conceptof spin pumping is depicted as follows: Precessing spins in FM transferangular momentum to the conduction electrons of adjacent non-magnetic (NM) layer. This can act as a sensitive probe for many bulkand interface spin-orbit effects. The extent of spin pumping is calculatedfrom the modulation of the Gilbert damping parameter; the latter playsan important role in determining the switching efficiency of spin trans-fer torque–based spintronic devices. The plethora of works in theemerging field of spin-orbitronics (18) have revealed that interfacialtransport observed in spin Hall effect (8, 9), Rashba-Edelstein effect(10, 11), spin-Seebeck effect (19), spin-Nernst effect (20), etc. is high-ly influenced by spin conductance at the interface. Interfacial spintransparency (21), as a function of spin-mixing conductance (22),effectively determines the extent of spin current diffused throughthe NM/FM interface. The role of transparency in a Pt-basedinterface, while determining the amount of spin Hall effect, has beenreported in recent studies (21). Later, the influence of spin transpar-ency on spin pumping effect has been studied in Co2FeAl/b-Tainterface with electrical detection technique (23). However, to thebest of our knowledge, investigation with the perspective ofdetermining spin-mixing conductance and interfacial spin transpar-

ency by all-optical excitation and detection technique is missing inthe literature. In addition, determination of interfacial spin transpar-ency of technologically important b-Ta/CoFeB is also absent in theliterature. Notably, b-Ta has a large spin Hall angle, in addition tobeing a good spin sink material and cost effective in comparison toPt. On the other hand, CoFeB is technologically important becauseof high spin polarization, exhibition of large tunnelmagnetoresistancewhen used as a ferromagnetic electrode in a magnetic tunnel junction,and low intrinsic Gilbert damping. The presence of boron at theinterface between b-Ta and CoFeB makes this system intriguing, assome of the earlier studies suggest that a small amount of Boron helpsin achieving sharp interface, although excess Boron leads to contam-ination at the interface.

In an NM/FM bilayer, there are other mechanisms of dissipationof spin angular momentum at the interface than interfacial spintransparency, which may affect the magnitude of spin pumping, i.e.,spin memory loss (24), Rashba effect (11), two-magnon scattering(25), interfacial band hybridization (26), etc. However, for the sakeof energy-efficient device fabrication, the interface in the engineeredstructure should have high spin transmission probability. So, it is im-perative to get deep insight of the mechanisms involved for optimizingthe efficiency of generation and transfer of pure spin current.

Here, we have performed time-resolved magneto-optical Kerr ef-fect (TR-MOKE) measurements (27) to explore the effect of spinpumping phenomena in a b-Ta/CoFeB bilayer system.Measurementsof spin pumping effect performed so far by electrical excitation anddetection techniques, such as spin-torque ferromagnetic detectiontechnique (FMR) and spin Hall magnetoresistance methods requireextremely delicate microfabrication. Extraction of magnetic dampingfrom FMR linewidth measurement, where the excitation of multiplemodes and the effect of impurity scattering center, may lead to in-homogeneous line broadening, resulting in an overestimation of damp-ing values. Here, we have reliably probed spin pumping and interfacialspin transparency using a noninvasive all-optical method without therequirement of a complicated microfabrication procedure. Magneticdamping can be directly extracted from the decaying amplitude oftime-resolved Kerr rotation data, free from any experimental artifacts.In the case of multimodal oscillation, the time-resolved data can be ap-propriately analyzed to precisely extract damping of individual modes.

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From the modulation of damping with Ta thickness, we have deter-mined the intrinsic spin-mixing conductance of b-Ta/CoFeB inter-face (which does not involve the backflowof spin angularmomentum)and spin diffusion length (28) of b-Ta. Later, effective spin-mixingconductance (which involves backflow of spin angular momentum)is estimated from the dependence of damping on FM layer thickness.Using a spin Hall magnetoresistance model (29), we have calculatedinterfacial spin transparency of b-Ta/CoFeB. We further investigatethe possible effects of other interface phenomena, including spinmemory loss, by incorporating a thin Cu spacer layer between theb-Ta and CoFeB layers. The negligible modulation of damping withCu spacer layer thickness confirms the dominance of spin pumpinggenerated pure spin current and its efficient transport in this system.

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RESULTSFigure 1A shows the x-ray diffraction patterns of Sub/Ta(t)/Co20Fe60B20(3 nm)/SiO2(2 nm) heterostructures at the glancingangle of 1°. The formation of a highly textured b-Ta phase is estab-lished from the presence of very intense (002) peaks of the b-Ta phaseat a 2q value of ~33.5°. We have not observed any signature of a-Taphase, which generally appears for 2q value of ~38.5°, in our experi-mental thickness regime. The d value obtained from the b-Ta peak at33.5° corresponds to ~2.6 Å which ensures the growth of the Ta thinfilms in the desired tetragonal b phase having a preferential orienta-tion of (002) planes (30).

Furthermore, we measured the thickness-dependent resistivity ofthe Ta layer from the heterostructures. Charge current was appliedalong the length of the sample, and the experiment was performed inconstant current mode. The sheet resistance (Rs) of the film stack as afunction of Ta thickness is plotted in Fig. 1B. The result has beenfitted with the parallel resistor model (29). We have obtained the sta-ble phase of b-Ta over the whole experimental thickness regime witha constant resistivity (rTa) of 248 ± 24 mW∙cm, and for CoFeB, this

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(rCoFeB) is found to be 139 ± 13 mW∙cm, which is very close to thevalues reported in the literature (31).

The atomic force microscopy (AFM) images for Sub/Ta(t)/Co20Fe60B20(3 nm)/SiO2(2 nm) samples are investigated to studysurface topography, as shown in Fig. 1C. From these images, we ob-tained the average topographical roughness for the samples with t =0, 2, 4, 6, 10, 15, and 20 nm as listed in Table 1.

The roughness values vary a little when measured at various re-gions of space of the same sample. Overall, the topographical rough-ness is found to be substantially small for all the samples. Because ofthe small thicknesses of the thin film heterostructures, presumably,the interfacial roughness will show its imprint on the topographicalroughness. We thus conclude that the average interfacial roughness,if any, present in these heterostructures is very small and is similar inall samples.

The principle behind the determination of spin pumpingAlong with the local damping, which arises because of energy dissipa-tionwithin the electron and phonon of FM itself, nonlocal damping inthe NM/FM system when magnetic energy is dissipated from FM toadjacent NM layer can be present. The optically induced magnetiza-tion precession in the FM layer causes the generation of spin current atthe NM/FM interface. These spins carry angular momentum to theadjacent NM layer, which acts as a spin sink by absorbing the spincurrent after traversing the spin diffusion length and leads to an en-hancement of the Gilbert damping parameter. This phenomenonis known as spin pumping and can be described by the modifiedLandau-Lifshitz-Gilbert equation as given below

dmdt

¼ �gðm�Heff Þ þ a0 m� dmdt

� �þ gVMs

Is ð1Þ

where g = gmB/ћ is the gyromagnetic ratio, Heff is the effective mag-netic field, a0 is the intrinsic Gilbert damping constant, V is the vol-ume, and Ms is the saturation magnetization of the FM. The totalspin current Is consists of the dc current I0s that does not exist inour case, current due to pumped spins from the FM Ipump

s , and cur-rent returned back to the FM (backflow current) Ibacks .

Is ¼ I0s þ Ipumps þ Ibacks ð2Þ

Those spins that are pumped out can either accumulate at theinterface or relax through spin-flip scattering, causing a flow of angu-lar mometum from FM to NM layer through the NM/FM interface.The NM layer does not always act as a perfect spin reservoir becauseof the spin accumulation effect, whichmainly causesIbacks . This backflowof spin current toward FMsolely depends on the spin diffusion length of

Fig. 1. Determination of structural phase and topographical roughness ofthe sample. (A) X-ray diffraction patterns measured at 1° grazing angle incidencefor different Ta thicknesses. Peaks corresponding to the b phase of Ta are shownwith a dashed line. (B) Variation of sheet resistance with Ta thickness. The solidline is fit. (C) Measurement of topographical properties of samples with AFM.

Table 1. The average topographical roughness values obtainedwith AFM for Sub/Ta(t)/Co20Fe60B20(3 nm)/SiO2(2 nm) samples withdifferent Ta thicknesses.

t (nm)

Roughness (nm)

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the adjacent NM layer. On the other hand, the flow of spin angular mo-mentum through the NM/FM interface is quantified by spin-mixingconductance.

The theoretical framework described by Tserkovnyak et al. (12, 13)includes a backflow factor of spin angular momentum during relaxa-tion of injected spins into the NM layers, which is given as

b ¼ 2pG↑↓

ffiffiffie3

rtanh

tl

� �� ��1

ð3Þ

where e is the ratio of the spin-conserved to spin-flip scattering times(spin-flip probability), which is material dependent

e ¼ ðlellÞ2=3 ð4Þ

where lel and l are the mean free path and spin diffusion length,respectively.

Nonlocal damping at the NM/FM interface directly depends onspin-mixing conductance. It can be of two types: (i) intrinsic spin-mixing conductance (G↑↓), which does not consider the backflow factor,and (ii) effective spin-mixing conductance (Geff), which considers thebackflow of spin angular momentum (32). G↑↓ describes the electronicconductance property of channels in the interface between NM andFM, where NM thickness is kept much longer than its spin diffusionlength so that no backflow can occur. Its dependence on the Gilbertdamping parameter and NM thickness is given by (33, 34)

Geff ¼ G↑↓ 1� e�2tl

� �¼ 4pdMeff

gmBðaeff � a0Þ ð5Þ

Da ¼ aeff � a0 ¼G↑↓ 1� e�

2tl

� �gmB

4pdMeffð6Þ

Here, the factor of 2 in the exponential term signifies the distancetraversed by the spins inside the NM layer due to the reflection fromthe NM/air interface, which is assumed to be the perfect reflector.

The spin transmission probability of the NM/FM interface may bedetermined from the spin backflow, which is linked to Geff. Therefore,the reduction of spin transmission probability can be explained by in-termixing and disorder at the interface. The interfacial spin transpar-ency (T) between two layers takes into account all these effects thatlead to the electrons being reflected instead of being transmitted atthe NM/FM interface and controls the flow of spin angular momen-tum across the interface. Furthermore, it is known that T depends onboth intrinsic and extrinsic interfacial factors, such as band structuremismatch, Fermi velocity, interface imperfections, etc. (21, 35). To findthe transparency of channels, we followed a spin Hall magneto-resistance model (29) where the spin current density that diffuses intothe NM layer is smaller than the actual spin current density generatedvia the spin pumping in the FM layer. This can be linked to the ef-fective spin-mixing conductance (Geff) by the following relation (21)

T ¼ Geff tanhð t2lÞ

Geff coth tl

� �þ h2le2r

ð7Þ

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where r is the resistivity and l is the spin diffusion length of theNM layer.

All-optical investigation of magnetization dynamicsFigure 2A shows the schematic of the spin pumping mechanism alongwith the experimental geometry. Figure 2B shows the time-resolvedKerr rotation data for the Sub/Co20Fe60B20(3 nm)/SiO2(2 nm) sampleat H = 1.73 kOe, which consists of three different temporal regimes.When a femtosecond laser excites the sample, a sharp drop in themagnetization is observed immediately after zero delay, which corre-sponds to ultrafast demagnetization (regime I). Regime II correspondsto the fast remagnetization due to the spin-lattice relaxation, and regimeIII consists of slower relaxation along with magnetization precession.The slower relaxation is due to heat diffusion from the lattice to thesurrounding volume. We are mainly interested in extraction of decaytime from the damped sinusoidal oscillation about a bias magnetic fieldand its modulation due to the spin pumping effect. The red line in Fig.2B corresponds to the biexponential background present in the preces-sional data in regime III. We subtract this background from the rawdata and fit the resulting data using the damped harmonic function

MðtÞ ¼ Mð0Þe�ðttÞ sinðwt þ φÞ ð8Þ

where t is the decay time, φ is the initial phase, and w = 2pf, with fbeing the precessional frequency. From the fit, we estimate the effec-tive damping, aeff, using the expression

aeff ¼ 1gtðH þ 2pMeff Þ ð9Þ

where H is the applied bias magnetic field and Meff is the effectivemagnetization. The bias field dependence of precessional frequencycan be fitted with the Kittel formula mentioned below

f ¼ g2p

ðHðH þ 4pMeff ÞÞ1=2 ð10Þ

where g = gmB/ħ and g is the Landé g factor. From the fit,Meff and g aredetermined as fitting parameters. For these film stacks, we obtainedeffective magnetization, Meff ~ 1200 ± 100 emu/cm3 and g = 2.0 ±0.1. The comparison between Meff obtained from the dynamic mea-surement and Ms from vibrating sample magnetometry (VSM)

Fig. 2. Schematic of spin pumping phenomena and representative time-resolved data. (A) Experimental geometry. (B) Time-resolved Kerr rotation datafor Sub/Co20Fe60B20(3 nm)/SiO2(2 nm) sample at applied field, H = 1.73 kOe, areshown. The three different temporal regimes are indicated in the graph.

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measurement is described systematically in the SupplementaryMaterials with varying FM and NM layer thicknesses. For almostall the film stacks investigated in this work, Meff is found to be closeto the saturation magnetization Ms, which indicates that the interfaceanisotropy is small in these heterostructures.

Modulation of Gilbert damping parameter due tospin pumpingIn Fig. 3A, we have shown the background-subtracted time-resolveddata for Sub/Ta(t)/Co20Fe60B20(3 nm)/SiO2(2 nm), where we havevaried t from 0 to 20 nm. The intrinsic Gilbert damping (a0) of3-nm-thick CoFeB layer is found to be 0.006 ± 0.0005 for high-fieldregime, where the magnetization remains saturated. In the presence ofTa underlayer, effective damping (aeff) is found to be increased non-monotonically in the lower thickness regime, whereas it gets saturatedat higher thickness of Ta. As shown in Fig. 3B, the modulation ofdamping in the FM layer is found to be more than 40% because ofthe spin pumping in these heterostructures. The aeff shows exponen-tial dependence with Ta thickness with an asymptotic value of 0.009 ±0.0005 for t→∞. Thus, we have fitted our results with Eq. 6, where wedetermined the intrinsic spin-mixing conductance (G↑↓) = (7.22 ±0.05) × 1014 cm−2 (36, 37). Subsequently, we obtained the spin diffu-sion length (l) of Ta to be 2.44 ± 0.16 nm as a fitting parameter, whichis very close to the literature value (23). Using values for lel (about 0.5 nmfor Ta) (38) and l derived for these heterostructures, we have determinedthe spin-flip probability, e =1.4 × 10−2, fromEq. 4. For anNMmetal to bean efficient spin sink, the requirement is e≥ 1.0 × 10−2 (15). Thus, we caninfer that themodel describing the spin pumping effect is applicable to ourexperimental film stacks and that b-Ta layer acts as an efficient spin sinkhere. The backflow factor b is mainly element dependent and can beextracted from Eq. 3. We have quantified the modulation of backflowfactor (Db) to be 61% within our experimental thickness regime of1 nm ≤ t ≤ 20 nm. The spin transmission probability of the NM/FMinterface canbedetermined from the spinbackflow,which is linked toGeff.

To experimentally determine the value of Geff, we have investigatedprecessional dynamics for Sub/Ta(4 nm)/Co20Fe60B20(d)/SiO2(2 nm)samples by varying FM layer thickness as 1 nm ≤ d ≤ 13 nm at H =1.73 kOe (Fig. 4A). The aeff is observed to vary linearly with theinverse of ferromagnetic layer thickness and saturates for d > 10 nm,while the thickness of the NM layer is fixed at 4 nm (Fig. 4B). Thisis another confirmation that b-Ta is a good spin sink material re-sulting in a strong spin pumping effect (15). After fitting those data

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with Eq. 5, the effective spin-mixing conductance of the Ta/CoFeBinterface is found to be (6.92 ± 0.04) × 1014 cm−2. The variation ofGeff with different thicknesses of Ta is plotted in fig. S4. For energy-efficient applications of spin current in multilayered devices, largeinterface transparency (T) is required, and this primarily becomesassociated withGeff (t) (37). After determining the resistivity of theseheterostructures and Geff experimentally, we have found the value ofT as 0.50 ± 0.03 using Eq. 7, which is comparable with Pt/FM inter-faces (38). To the best of our knowledge, this is the first measurementof interfacial spin transparency for a b-Ta/CoFeB bilayer, and thisshows the formation of a moderately transparent interface.

There is a probability in these heterostructures to have some loss ofspin angular momentum because of interfacial depolarization, knownas spin memory loss, where spin angular momentum carried by spincurrent is not transferred to the NM but instead transferred to thelattice through interfacial spin-orbit scattering (22). So, the total trans-fer of spin current to the NM will be determined by a combined effectof interfacial spin transparency and spin memory loss, which refers tothe loss of spin information at the interface due to spin-flip scattering.In this case, the loss of spin polarization occurs because of interfacialspin-orbit scattering, whereas interfacial spin transparency is anelectronic property of a material interface and transmission of conduc-tion electrons depends on electronic band matching of two materialson either side of the interface. There can also be other interfacialeffects, such as Rashba effect, two-magnon scattering, interfacial bandhybridization, and defects, which may affect the net transfer of spincurrent to the NM. To understand the contributions of the aboveeffects in addition to the spin pumping effect, we have introduced acopper spacer layer of different thicknesses between the Ta and CoFeBlayers. Copper has very small spin-orbit coupling and spin-flip scatter-ing parameter, so it shows a very high spin diffusion length (28). Thus,a thin copper spacer layer is not expected to affect the damping of theFM layer because of the spin pumping effect but can change or elim-inate other interface effects. Consequently, if other interface effects aresubstantially present in our samples, then the introduction of copperspacer layer would notably vary the damping with the variation ofcopper (Cu) spacer layer thickness (tCu). The time-resolved Kerr ro-tation data for the Sub/Ta(4 nm)/Cu(tCu)/Co20Fe60B20(4 nm)/SiO2(2 nm) heterostructures with copper layer thicknesses of 0.4,0.6, 0.8, and 1.0 nm are presented in Fig. 5A at H = 1.73 kOe, andFig. 5B shows the plot of damping as a function of copper layer thickness.Almost no modulation of damping with the thickness of copper

Fig. 3. Extraction of spin diffusion length and intrinsic spin-mixing conductance.(A) Time-resolved Kerr rotation data for t = 0, 1, 3, 6, 10, and 20 nm at 1.73-kOefield. (B) Modulation of damping with Ta thickness, fitted with a spin pumpingmodel to extract spin diffusion length and intrinsic spin-mixing conductance.

Fig. 4. Extraction of intrinsic Gilbert damping parameter and effective spin-mixing conductance. (A) Time-resolved Kerr rotation data for d = 1, 4, and 13 nm.(B) Variation of damping with FM layer thickness linearly fitted with a spin pumpingformalism to extract intrinsic Gilbert damping parameter and effective spin-mixingconductance.

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spacer layer is observed, which confirms that the Cu/CoFeB interface isnearly transparent (39) and that other possible interface effects arenegligible. Thus, the transparency of Ta/CoFeB is similar to that ofTa/Cu/CoFeB because of similar electronic structures. Thus, we havebeen able to measure the spin pumping effect and interfacial spin trans-parency by the all-optical TR-MOKE technique in a new thin film het-erostructure, which is very important for future spintronic application.

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DISCUSSIONWe have investigated spin pumping phenomena in b-Ta/CoFeBthin films from the modulation of Gilbert damping using an all-optical TR-MOKE magnetometer. For a stable phase of b-Ta overthe thickness range between 0 and 20 nm, we have extracted the spindiffusion length as 2.44 ± 0.16 nm and the intrinsic spin-mixing con-ductance as (7.22 ± 0.05) × 1014 cm−2 from the variation of damping asa function of Ta thickness. By considering the backflow factor in ourtheoretical model, we have extracted the effective spin-mixing con-ductance at the Ta/CoFeB interface as (6.92 ± 0.04) × 1014 cm−2 fromthe variation of damping as a function of CoFeB thickness. By fittingour data with the spin Hall magnetoresistance model, we have ob-tained the interfacial spin transparency of Ta/CoFeB as 0.50 ± 0.03for Ta thickness of 4 nm, which shows that the Ta/CoFeB interfaceis comparable with various studied heavy metal/FM interfaces suchas Pt/FM. To understand the impact of other possible interfaceeffects, which may alter the Gilbert damping apart from the spinpumping effect, we have introduced a thin copper spacer layer ofvarying thickness and found negligible modulation of damping. Thisis due to similar spin conductivity of copper and Ta and confirms theabsence of other interface effects in these structures. The low intrinsicGilbert damping parameter and the high effective spin-mixing con-ductance with moderately high transparency of the b-Ta/CoFeB bi-layer system make it a key material for spin transfer torquemagnetization switching and spin logic devices.

MATERIALS AND METHODSSample preparation and measurementsThe heterostructured thin films of Sub/Ta(t)/Co20Fe60B20(d)/SiO2(2 nm), where NM layer thickness t = 0, 1, 2, 3, 4, 5, 6, 7, 10,15, and 20 nm and FM layer thickness d = 1, 2, 3, 4, 6, 10, and13 nm, were deposited with radio frequency (rf)/dc magnetron

Panda et al., Sci. Adv. 2019;5 : eaav7200 26 April 2019

sputtering system on Si (100) wafers coated with 100-nm-thick SiO2.The depositions were done at an average base pressure of 4.0 × 10−7

Torr and an argon pressure of about 1.0 mTorr at a deposition rate of0.2 Å/s. Very slow deposition rates were chosen to have films withuniform thickness even at a very thin regime down to 1 nm. TheTa and SiO2 were deposited with an rf power of 40 and 60 W, respec-tively, while Co20Fe60B20 (CoFeB) was deposited with a dc voltage of380 V. All other deposition conditions were carefully optimized andkept almost identical for all samples. In another set of samples, weintroduced a thin Cu spacer layer in between the FM and NM layersand varied its thickness from 0.4 to 1 nm. The Cu layer was depositedat a dc voltage of 345 V, an argon pressure of 1.0 mTorr, and a dep-osition rate of 0.2 Å/s.

TR-MOKE technique was exploited to study the precessional mag-netization dynamics of the samples in polar Kerr geometry (shown inFig. 2A). The fundamental beam (pulse width, ~40 fs; wavelength,~800 nm; repetition rate, ~1 kHz) of an amplified laser system (Libra,Coherent) was used as probe, while a part of this beam was frequency-doubled (wavelength, ~400 nm; pulse width, ~50 fs) to be used as pumppulse. The probe and pump beams (spot sizes, 100 and 200 mm, respec-tively) fall noncollinearly on the sample to detect the polar Kerr rotationas a function of the time delay between pump and probe beams intro-duced through a variable delay generator. The sample was subjected toa magnetic field with 10° to 15° out-of-plane tilt from the sample plane,and the in-plane component of this field is referred to as the bias mag-netic field (H). This introduces a substantial demagnetizing field,which is modulated by the pump pulse to launch the precession inthe sample. The probe was placed carefully at the center of the pumpspot, so that there was no additional effect on the Gilbert dampingdue to the dissipation of energy of uniform precessional mode flowingout of the probed area. All the measurements were performed underambient conditions.

SUPPLEMENTARY MATERIALSSupplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/4/eaav7200/DC1Determination of saturation magnetization of the samplesVariation of effective spin-mixing conductance (Geff) with Ta thicknessFig. S1. Determination of saturation magnetization of Sub/Ta(t)/Co20Fe60B20(3 nm)/SiO2(2 nm).Fig. S2. Determination of saturation magnetization of Sub/Ta(4 nm)/Co20Fe60B20(d)/SiO2(2 nm).Fig. S3. Determination of saturation magnetization of Sub/Ta(4 nm)/Cu(tCu)/Co20Fe60B20(4 nm)/SiO2(2 nm).Fig. S4. Variation of effective spin-mixing conductance (Geff) with Ta thickness.Reference (40)

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Fig. 5. Variation of damping with spacer layer thickness. (A) Time-resolvedKerr rotation data for tCu = 0.4, 0.6, 0.8, and 1.0 nm. (B) Variation of dampingparameter, with spacer layer thickness, is shown. The dashed line is guide tothe eye.

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AcknowledgmentsFunding:We acknowledge financial support from S. N. Bose National Centre for Basic Sciences(SNBNCBS) under grant no. SNB/AB/18-19/211. S.N.P. and S.C. acknowledge SNBNCBS, andS.M. acknowledges DST for research fellowship. J.S. acknowledges DST for RamanujanFellowship (no. SB/S2/RJN-093/2014). Author contributions: A.B. planned and supervised theproject. J.S. and S.C. prepared the samples. S.N.P. and S.M. made the measurements andanalyses in consultation with A.B. S.N.P., S.M., and A.B. wrote the manuscript in consultationwith other co-authors. Competing interests: The authors declare that they have nocompeting interests. Data and materials availability: All data needed to evaluate theconclusions in the paper are present in the paper and/or the Supplementary Materials.Additional data related to this paper may be requested from the authors.

Submitted 14 October 2018Accepted 20 March 2019Published 26 April 201910.1126/sciadv.aav7200

Citation: S. N. Panda, S. Mondal, J. Sinha, S. Choudhury, A. Barman, All-optical detection ofinterfacial spin transparency from spin pumping in b-Ta/CoFeB thin films. Sci. Adv. 5, eaav7200(2019).

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films-Ta/CoFeB thinβAll-optical detection of interfacial spin transparency from spin pumping in

S. N. Panda, S. Mondal, J. Sinha, S. Choudhury and A. Barman

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