Non-local detection of spin dynamics via spin

Non-local detection of spin dynamics via spin rectification effect in yttrium iron garnet/SiO2/NiFe trilayers near simultaneous ferromagnetic resonance Wee Tee Soh, Bin Peng, and C. K. Ong Citation: AIP Advances 5, 087184 (2015); doi: 10.1063/1.4930079 View online: http://dx.doi.org/10.1063/1.4930079 View Table of Contents: http://scitation.aip.org/content/aip/journal/adva/5/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Observation of spin rectification in Pt/yttrium iron garnet bilayer J. Appl. Phys. 117, 17C725 (2015); 10.1063/1.4914962 The role of the non-magnetic material in spin pumping and magnetization dynamics in NiFe and CoFeB multilayer systems J. Appl. Phys. 117, 163901 (2015); 10.1063/1.4918909 Localized excitation of magnetostatic surface spin waves in yttrium iron garnet by shorted coaxial probe detected via spin pumping and rectification effect J. Appl. Phys. 117, 153903 (2015); 10.1063/1.4918668 Spin current injection by spin Seebeck and spin pumping effects in yttrium iron garnet/Pt structures J. Appl. Phys. 111, 07C513 (2012); 10.1063/1.3676239 Amplification of spin waves in yttrium iron garnet films through the spin Hall effect Appl. Phys. Lett. 99, 192511 (2011); 10.1063/1.3660586

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AIP ADVANCES 5, 087184 (2015)

Non-local detection of spin dynamics via spin rectification effect in yttrium iron garnet/SiO2/NiFe trilayers near simultaneous ferromagnetic resonance Wee Tee Soh,1,a Bin Peng,2 and C. K. Ong1 1

Center for Superconducting and Magnetic Materials, Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117551 2 State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China

(Received 6 May 2015; accepted 20 August 2015; published online 31 August 2015) The spin rectification effect (SRE), a phenomenon that generates dc voltages from ac microwave fields incident onto a conducting ferromagnet, has attracted widespread attention due to its high sensitivity to ferromagnetic resonance (FMR) as well as its relevance to spintronics. Here, we report the non-local detection of yttrium iron garnet (YIG) spin dynamics by measuring SRE voltages from an adjacent conducting NiFe layer up to 200 nm thick. In particular, we detect, within the NiFe layer, SRE voltages stemming from magnetostatic surface spin waves (MSSWs) of the adjacent bulk YIG which are excited by a shorted coaxial probe. These non-local SRE voltages within the NiFe layer that originates from YIG MSSWs are present even in 200 nm-thick NiFe films with a 50 nm thick SiO2 spacer between NiFe and YIG, thus strongly ruling out the mechanism of spin-pumping induced inverse spin Hall effect in NiFe as the source of these voltages. This long-range influence of YIG dynamics is suggested to be mediated by dynamic fields generated from YIG spin precession near YIG/NiFe interface, which interacts with NiFe spins near the simultaneous resonance of both spins, to generate a non-local SRE voltage within the NiFe layer. C 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4930079]

The spin rectification effect (SRE),1–4 which involves the nonlinear coupling between the microwave current and microwave field-induced oscillating magneto-resistance to produce rectified dc voltages in metallic ferromagnetic films, has attracted much interest recently due to its high sensitivity to spin dynamics. In addition to ferromagnetic resonance (FMR),5–11 spin waves12,13 and domain wall resonance14 studies, its powerful phase-resolving15–18 capability enables the direct probing of the electromagnetic phase difference between microwave magnetic and electric fields that paves the way for novel phase-resolving applications previously limited to the infrared-optical regime. In context to the field of spintronics, especially for metallic ferromagnetic (FM)/normal metal (NM) bilayers in spin pumping mediated inverse spin Hall effect (ISHE)19–28 and spin torque transfer (STT)29,30 measurements, SRE can sometimes prove to be cumbersome as it often mixes with the signal of interest, and thus a good understanding of SRE is advantageous to avoid potential pitfalls. A noteworthy system in the present context is the insulator FM/metallic FM bilayer (i.e yttrium iron garnet (YIG)/NiFe), where unambiguous separation of SRE and ISHE signals would allow for the quantification of spin Hall effect in ferromagnetic NiFe.31–33 More interestingly as recently demonstrated,32 both SRE and ISHE signals may be enhanced at the simultaneous resonances of both FM layers in this system, which were suggested to be due to two physical mechanisms: (i) ac spin-pumped current from metallic FM enhances the FMR of the adjacent insulating FM, and vice versa (Fig. 1(a)),

a Corresponding author: Wee Tee Soh, E-mail: [email protected]

2158-3226/2015/5(8)/087184/7

5, 087184-1

© Author(s) 2015

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087184-2

Soh, Peng, and Ong

AIP Advances 5, 087184 (2015)

FIG. 1. (a) Spin-pumped currents jsNiFe and jsYIG from NiFe and YIG respectively produced at their simultaneous ferromagnetic resonance, whose ac components enhances the spin precessions of the other adjacent layer via spin torque. (b) Long range dynamic fields hdNiFe and hdYIG due to NiFe and YIG spin precessions respectively at simultaneous resonance, which enhances the spin precessions of the other adjacent layer. (c) Measurement setup of shorted coaxial probe on NiFe film patterned onto YIG substrate of thickness 0.5 mm, with applied static field H and dynamic microwave excitation field h predominantly in the above specified direction. Voltage leads across the NiFe film simultaneously measure the induced dc voltages.

and (ii) dynamic field from insulating FM spin precession enhances the FMR of the metallic FM layer and vice versa (Fig. 1(b)). Clearly, this condition of simultaneous enhancement of SRE and ISHE signals must occur in the vicinity of the respective FMR of each layer in order for their respective susceptibilities to overlap and influence each other, which was achieved in Hyde et al.’s work32 by an out-of-plane field based on the differing anisotropies of the FM layers. While Hyde et al. showed that both SRE and ISHE signals can be enhanced near the simultaneous resonances of both layers, the quantitative contributions from each possible mechanism (i) and (ii) are still unclear since SRE and ISHE signals could not be clearly separated in that case. In our work, we achieved simultaneous resonance in YIG (bulk)/NiFe (film) bilayers by adjusting an in-plane magnetic field and making use of their different saturation magnetizations Ms as well as the coupling between YIG and NiFe to shift the FMR field. Under near simultaneous resonance, in addition to the self-SRE voltages of NiFe, we detect dc voltages characteristic of YIG magnetostatic surface waves (MSSWs) that persist even in thick NiFe flims (200 nm) and with the presence of 50 nm-thick SiO2 spacer between YIG and NiFe (that blocks spin pumping from MSSWs). Note that the insulating YIG does not directly produce dc signals, thus any detected voltages must be produced within the NiFe layer only. This means that mechanism (ii) is likely operative for this case and that the observed MSSWs voltages are due to its generated SRE in NiFe only which is mediated by long range dynamic fields produced by spin precession. Our results clearly demonstrate the non-local detection of YIG spin dynamics via SRE (non-local SRE) in an adjacent NiFe film which could even be hundreds of nm thick.

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087184-3

Soh, Peng, and Ong

AIP Advances 5, 087184 (2015)

The thin film samples are prepared via room-temperature rf magnetron sputtering (base pressure of 6 × 10−7 Torr) on rectangular bulk polycrystalline YIG substrates of lateral dimensions 5 mm × 10 mm and thickness of 0.5 mm. The deposited films are patterned into 2 mm × 5 mm strips of the following layered structures: YIG/NiFe (20/200 nm), YIG/Cu(10 nm)/NiFe (20/200 nm), YIG/SiO2 (10/50 nm)/NiFe (20/200 nm) and YIG/SiO2 (10/50 nm)/Pt (20 nm), where the film thicknesses are controlled by the sputtering rate and verified by a depth profile meter. The continuity of the SiO2 layers are verified by atomic force microscopy (AFM), in which the surface roughness are determined to be much smaller than the film thickness. Under microwave excitation at 18 dBm power provided by a near-field shorted coaxial probe34 (Fig. 1(c)) whose dimensions are comparable to the film area, the induced dc voltages are measured sensitively via a lock-in technique at 10 kHz modulation frequency, while sweeping the in-plane applied magnetic field. In our studied YIG/NM (Pt or Cu) systems, under the measurement setup shown in Fig. 1(c), ⃗ under FM resonance spin pumps a spin current the precession of the dynamic YIG magnetization m ⃗j s ∝ m ⃗ ⃗ ˆ via ISHE. Here, nˆ ⃗ × dm ⃗ /dt into the NM, which dissipates into a charge current  jc ∝ j s × nL 2 2 2 is an out-of-plane unit vector and L = ∆H / 4(H − Hr ) + ∆H is the Lorentzian lineshape with resonance field Hr and linewidth ∆H under an applied field H. However, for the YIG/NiFe bilayer, an additional voltage contribution due to SRE of NiFe adds to the ISHE voltage. This SRE dc current which occurs at NiFe resonance, may be approximated as ⃗jSRE ∝ ∆R| ⃗j||⃗h| (D cos Φ − L sin Φ), where ∆R is the change in resistance due to anisotropic magneto-resistance (AMR), ⃗j and ⃗h are the microwave currents fields  and magnetic  respectively with Φ being their phase difference 2 2 and D = 2∆H (H − Hr ) / 4(H − Hr ) + ∆H is the dispersive lineshape. Note that SRE is only present in conducting NiFe near its resonance condition, and thus can be separated from spin pumping-induced ISHE due to YIG provided that the resonances of NiFe and YIG modes are sufficiently far apart. Any spurious thermal voltages due to anomalous Nernst effect35 would not be detected, since such voltages are not modulated at the lock-in frequency. From the measured voltage spectra (Fig. 2(a)), we obtained and fitted the dispersion relation of the various modes (Fig. 2(b)) for the YIG/NiFe (20 nm) sample. Remarkably, we observe

FIG. 2. Voltage spectra at 4 GHz measured for (a) YIG/NiFe (20 nm) and (c) YIG/Cu (10 nm)/NiFe (20 nm) samples. Frequency dependence of the resonance field Hr of various modes for (b) YIG/NiFe (20 nm) and (d) YIG/Cu (10 nm)/NiFe (20 nm) samples. The resonance field for the Si/NiFe (20 nm) sample is also shown in (b).

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087184-4

Soh, Peng, and Ong

AIP Advances 5, 087184 (2015)

FIG. 3. Voltage spectra at 2.6 GHz measured for (a) YIG/NiFe (200 nm) and (c) YIG/Cu (10 nm)/NiFe (200 nm) samples. Frequency dependence of the resonance field Hr of various modes for (b) YIG/NiFe (200 nm) and (d) YIG/Cu (10 nm)/NiFe (200 nm) samples. The resonance field for the Si/NiFe (200 nm) sample is also shown in (b).

clear peaks originating from the resonances of both YIG and NiFe modes. In particular, the mode corresponding to NiFe FMR, which stems from itsown SRE, can be identified via a fit of its dispersion relation to the Kittel equation f = (γ/2π) (H + Hk ) (H + Hk + 4πMeff ), with effective magnetization 4πMeff ≈ 5400 G (for bulk NiFe, saturation magnetization 4πM ≈ 10 kG), and effective anisotropy field Hk ≈ −55 Oe. For subsequent fits to NiFe FMR for the other films on YIG substrate presented here in Figs 2, 3, and 4, the extracted 4πMeff ranges from 5400-9800 G, with Hk from −38 Oe to −88 Oe. This discrepancy of 4πMeff from the bulk saturation value is likely the result of surface anisotropy as well as YIG/NiFe interfacial exchange coupling. However, a negative in-plane effective anisotropy Hk can be explained by significant out-of-plane perpendicular anisotropy which tend to lead to the formation of stripe domains for thick NiFe films (>170 nm), as reported in our earlier36 experiments. As a comparison, we present in Fig. 2(b) the FMR dispersion for single layered NiFe (20 nm) film fabricated onto Si substrate, whereby a fit of its measured FMR spectra leads to 4πMeff ≈ 9770 G and Hk ≈ 19 Oe. The YIG FMR mode is also identified with 4πMeff ≈ 1620 G and Hk ≈ 0. Furthermore, as reported in our earlier work,34 strong peaks due to magnetostatic surface spin waves (MSSWs) in YIG with wave vector k = nπ/(5 mm) can be identified (M8 and M13, where the number denotes the mode number n). The Mm mode corresponds to a superposition of YIG MSSWs with wave vectors k > 100 cm−1, since such modes occur at the same resonance field position in the spectra. One may conclude that these modes due to YIG FMR, M8, M13 and Mm are the result of spin pumping from YIG into NiFe where the spin current dissipates via ISHE into a measurable voltage. However, it is important to note that, as evidenced in Fig. 2(b), the finite-linewidth resonances of NiFe and YIG can overlap and cross one another, resulting in simultaneous resonances that may enhance both SRE and ISHE voltages based on mechanism (i) and/or (ii) described above, therefore the origin of the peaks may not be as clear. To investigate further, we first insert a Cu spacer between the FM layers with results shown in Fig. 2(c) for the YIG/Cu (10 nm)/NiFe (20 nm) trilayer. We observe two clear modes: one due to SRE at NiFe FMR and the other can be identified as the Mm mode of YIG MSSWs. (Fig. 2(d)) Since Cu is essentially transparent to spin currents due to its long spin diffusion length, the occurrence of the Mm mode is not unexpected, though other MSSW modes appear to be absent. This

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087184-5

Soh, Peng, and Ong

AIP Advances 5, 087184 (2015)

FIG. 4. (a) Voltage spectra measured for YIG/SiO2 (10 nm)/NiFe (20 nm) at 2.5 GHz. (b) Frequency dependence of the resonance field Hr of various modes for YIG/SiO2 (10 nm)/NiFe (20 nm). (c) Voltage spectra measured for YIG/SiO2 (50 nm)/NiFe (200 nm) sample at 4 GHz and for the (d) YIG/SiO2 (10 nm)/Pt (20 nm) and YIG/SiO2 (50 nm)/Pt (20 nm) samples at 3 GHz.

is most likely due to the decrease in voltage signal from the shunting effect of Cu layer that cause other MSSW modes (which is much weaker than the Mm mode) to be less resolvable in the spectra. The effect of the Cu spacer does not distinguish between mechanisms (i) or (ii) at near simultaneous FMR since neither one nor the other is suppressed, but simply confirms our results for the YIG/NiFe (20 nm) sample, and acts as a control later when a SiO2 spacer is used instead. Next, it is instructive to use a thicker NiFe film instead, since it is known31,33 that its spin diffusion length λ d is small (∼2 nm) and therefore a thicker NiFe film would lead to a much diminished ISHE signal, while the NiFe SRE remains approximately independent on film thickness as long as it is thinner than the microwave skin depth (∼0.2 µm at 2.5 GHz). Intriguingly, we find that the voltage signals from YIG MSSW modes remain as strong even for NiFe films up to 200 nm thick (Fig. 3(a)), while conventional ISHE theory with VISHE ∝ (1/d) tanh (d/2λ d ) (d is the NiFe thickness) would have predicted a decrease of ISHE voltage by at least one order of magnitude (note that voltage also further decreases with increasing d due to decreased film resistance) and so would not have been observable. We confirm these results with measurements from a YIG/Cu (10 nm)/NiFe (200 nm) sample (Fig. 3(c), 3(d)), where a Cu spacer is added, that shows these YIG MSSW modes remain. Thus, the observed YIG MSSW modes in the spectra cannot be explained by conventional ISHE theory for spin current diffusion into NiFe and an alternative explanation is needed. The presence of YIG MSSW voltages that persist even for thick NiFe films suggests that some sort of long-range dynamic interaction between YIG MSSWs and the adjacent NiFe layer might be responsible. We therefore make use of SiO2, a non-magnetic insulator, as a spacer, to suppress all short-range exchange interactions and thus spin pumping between YIG and NiFe. Surprisingly, we find that the voltage signals from YIG MSSW modes persist for the YIG/SiO2 (10 nm)/NiFe (20 nm) sample (Fig. 4(a), 4(b)), while YIG MSSW modes can be clearly seen even for the YIG/SiO2 (50 nm)/NiFe (200 nm) sample (Fig 4(c)). A more detailed angular measurement to distinguish whether these signals are due to spin pumping or SRE could not be done here when MSSWs are involved since their wave vectors (and thus resonance field and amplitude) also changes with angular orientation of applied field, which complicates the issue. However, the presence of

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087184-6

Soh, Peng, and Ong

AIP Advances 5, 087184 (2015)

such 50 nm thick SiO2 spacer would definitively suppress spin pumping, as it is well-known33,37 that even a SiO2 spacer ∼5 nm is thick enough to eliminate any ISHE signals, as is also evident from results from the YIG/SiO2/Pt samples. (Fig. 4(d)) Moreover, a closer inspection of the spectra for both samples (Fig. 4(a) and 4(c)) shows that these YIG modes appear to be enhanced near the self-SRE of NiFe FMR, and that the modes are clearly not necessarily Lorentzian (Fig. 4(c) ). While the non-Lorentzian nature of these modes were previously observed38 in YIG/NiFe bilayers and had been attributed to a competition between the spin pumping from YIG and NiFe, this effect does not apply for our case due to the presence of SiO2 spacer that blocks spin pumping. However, if we attribute a long-range dynamic field ⃗hd YIG ∝ χYIG produced by YIG precession, where χYIG is the dynamic susceptibility of the YIG mode, then the results could be explained by non-local SRE in NiFe due to this field. The dynamic field ⃗hd YIG simply adds to the external applied microwave field ⃗h within NiFe, producing a SRE current as   ⃗jSRE1 ∝ ∆R| ⃗j| |k1⃗h| (D cos Φ − L sin Φ) − |k2 χYIG| (D sin Φ + L cos Φ) , (1) where k1 and k 2 are constants. Here, we have considered an additional π/2 phase shift in Φ due to resonance of YIG modes. Since D and L are maximum near NiFe FMR and χYIG peaks near YIG mode resonance, the additional SRE contribution from ⃗hd YIG is only evident near the simultaneous resonance of NiFe and YIG modes, and occurs at the resonance of the YIG modes. Note that equation (1) can also account for the non-Lorentzian nature of the peaks due to YIG modes since Φ is not zero in general. However, the exact physical nature of this long-range dynamic field ⃗hd YIG is yet unclear and requires further research to elucidate. Notably, such long-range dynamic interactions have also been observed between YIG and paramagnetic defects in diamonds,39 that may extend to hundreds of nm. As also revealed in their experiments, while paramagnetic defects in non-magnetic insulators (for our case, in SiO2) could possibly carry spin-pumped spin currents across the YIG and NiFe layers, this effect is quite unlikely for our case since the YIG/SiO2 (10 nm)/Pt (20 nm) and YIG/SiO2 (50 nm)/Pt (20 nm) samples under similar fabrication conditions show no clear voltage peaks in the measured frequency range from 1-5 GHz (Fig. 4(d)), implying that the SiO2 spacer for these samples do not carry spin momentum into Pt via spin pumping from YIG. In summary, we have observed non-local dc voltages in NiFe films due to the spin dynamics of an adjacent YIG bulk substrate under microwave excitation. These voltages persist even in the presence of a 50 nm-thick SiO2 spacer and in 200 nm-thick NiFe films, thus rendering conventional spin pumping-induced ISHE within NiFe as the source of these voltages highly unlikely. We attribute a long-range dynamic field due to YIG spin precession, analogous to a microwave field that drives the self-SRE in NiFe, as the source of these non-local SRE voltages near the simultaneous resonance of both YIG and NiFe. Our setup for the realization of simultaneous YIG and NiFe resonance under relatively low in-plane static magnetic fields and frequencies, to observe clear YIG and NiFe modes via SRE, should provide an attractive and convenient platform to further study such dynamic couplings between FM layers.

ACKNOWLEDGEMENTS

This project is partially supported by the Asian office of Aerospace R&D under Award No. FA2386-14-1-4080, as well as project NSFC No. 61471095. 1

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AIP Advances 5, 087184 (2015)

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