Observation of magnon-mediated current drag in Pt/yttrium iron garnet/Pt(Ta) trilayers

Abstract

Pure spin current, a flow of spin angular momentum without flow of any accompanying net charge, is generated in two common ways. One makes use of the spin Hall effect in normal metals (NM) with strong spin-orbit coupling, such as Pt or Ta. The other utilizes the collective motion of magnetic moments or spin waves with the quasi-particle excitations called magnons. A popular material for the latter is yttrium iron garnet, a magnetic insulator (MI). Here we demonstrate in NM/MI/NM trilayers that these two types of spin currents are interconvertible across the interfaces, predicated as the magnon-mediated current drag phenomenon. The transmitted signal scales linearly with the driving current without a threshold and follows the power-law Tⁿ with n ranging from 1.5 to 2.5. Our results indicate that the NM/MI/NM trilayer structure can serve as a scalable pure spin current valve device which is an essential ingredient in spintronics.

ARTICLE

Received 12 Oct 2015 | Accepted 27 Jan 2016 | Published 2 Mar 2016

Junxue Li1,*, Yadong Xu1,*, Mohammed Aldosary1, Chi Tang1, Zhisheng Lin1, Shufeng Zhang2, Roger Lake3

& Jing Shi1

Pure spin current, a ow of spin angular momentum without ow of any accompanying net charge, is generated in two common ways. One makes use of the spin Hall effect in normal metals (NM) with strong spinorbit coupling, such as Pt or Ta. The other utilizes the collective motion of magnetic moments or spin waves with the quasi-particle excitations called magnons. A popular material for the latter is yttrium iron garnet, a magnetic insulator (MI). Here we demonstrate in NM/MI/NM trilayers that these two types of spin currents are interconvertible across the interfaces, predicated as the magnon-mediated current drag phenomenon. The transmitted signal scales linearly with the driving current without a threshold and follows the power-law Tn with n ranging from 1.5 to 2.5. Our results indicate that the NM/MI/NM trilayer structure can serve as a scalable pure spin current valve device which is an essential ingredient in spintronics.

DOI: 10.1038/ncomms10858 OPEN

Observation of magnon-mediated current drag in Pt/yttrium iron garnet/Pt(Ta) trilayers

1 Department of Physics and Astronomy, University of California, Riverside, California 92521, USA. 2 Department of Physics, University of Arizona, Tucson, Arizona 85721, USA. 3 Department of Electrical and Computer Engineering, University of California, Riverside, California 92521, USA. * These authors contributed equally to this work. Correspondence and requests for materials should be addressed to J.S. (email: mailto:[email protected]

Web End [email protected] ).

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There has been intense research interest in pure spin current transport in both conducting and insulating materials. Whether by spin pumping14, spin Seebeck effect (SSE)58

or spin Hall and inverse spin Hall effects915, pure spin current generation and detection are typically accomplished in bilayers consisting of a magnetic and a non-magnetic layer24. Either generation or detection, rarely both, is done by electrical means. Kajiwara et al.16 rst demonstrated a lateral all-electrical device in which an electrical signal can transmit through yttrium iron garnet (YIG) over a macroscopic distance (B1 mm).

The response (B1 nV) is highly nonlinear with a threshold in driving current, which was interpreted as the critical value for the spin transfer torque17,18 induced coherent precession of YIG magnetization. The effect also suggests that spin currents of two different types, that is, electronic915 and magnonic1922, are inter-convertible at the interfaces. However, the phenomenon has not been reproduced in a similar planar geometry.

Zhang et al.23,24 predicted a phenomenon with a different origin in sandwich structures, analogous to the conventional spin-valve device for spin-polarized charge currents25,26. Rather than exciting a coherent precession of magnetization, at one interface, electrons in the NM create/annihilate magnons in the MI. The individual magnon creation/annihilation does not need to overcome any threshold; therefore, the interconversion takes place at any current. Due to the long magnon decay length in MI27, this effect couples two remote electrical currents, which is called the magnon-mediated current drag. Moreover, the on and off states are controlled by the relative orientation between the polarization of the spin current and MI magnetization. In small lateral devices similar to that of Kajiwara, Cornelissen et al.28 reported both o- and 2o nonlocal current responses in the a.c. measurements (o is the frequency of a.c. current). The o-response indicates a linear nonlocal current relation, which apparently does not share the same origin as that of the effect reported by Kajiwara. Nonlocal magnetoresistance was also reported in similar device geometry29. However, it is unclear whether the reported effect indeed arises from the microscopic mechanism predicted by Zhang et al. An objective of this work is to experimentally establish the current drag effect in the sandwich geometry proposed by Zhang et al.

In this work, we rst successfully fabricate Pt/YIG/Pt(Ta) trilayer structures using pulsed laser deposition in combination with sputtering and post-growth annealing. Below 220 K, the trilayers are sufciently insulating and we observe nonlocal

responses when an in-plane magnetic eld is swept or rotated. We nd that the polarity of the nonlocal signal is opposite to each other between Pt/YIG/Pt and Pt/YIG/Ta, indicating the spin current origin of the effect. By investigating the effect at low temperatures, we show that the power-law dependence of the nonlocal signal is consistent with the prediction for the magnon-mediated current drag effect.

ResultsNonlocal device structure. Our NM/MI/NM trilayer device structure is schematically shown in Fig. 1a,b. The MI is a thin YIG lm sandwiched by either two identical NM lms or dissimilar NM lms. Via the spin Hall effect, a charge current (Jinjected) generates a pure spin current owing in the z-direction

with the spin polarization (r) parallel to the y-direction. The conduction electrons in the bottom NM interact with the localized moments of the MI via the s-d exchange interaction at the interface, resulting in the creation or annihilation of magnons in the MI accompanied by spin-ips of conduction electrons in the bottom NM layer. Due to the nature of the s-d exchange interaction24,30, that is, Hsd Jsd P r M, where Jsd is exchange

coupling strength, when the magnetization of the MI (M) is collinear with r, magnons are created or annihilated depending on whether M is parallel or anti-parallel to r. As such, the interaction creates a non-equilibrium magnon population and spin accumulation in the MI which drives magnon diffusion. The excess/decient magnons are then converted to a spin current in the top NM layer by the reverse process, which is converted to a charge current (Jinduced) again in the top NM

layer via the inverse spin Hall effect. When r>M, there is no non-equilibrium magnon population or spin accumulation and the spin current is absorbed by the MI. Consequently, there is no induced spin or charge current in the top NM layer (Fig. 1b). Remarkably, one can switch on and off the magnon creation/annihilation process by controlling the relative orientation between M and r. Conceptually, this structure functions as a valve for pure spin current.

Field-dependent nonlocal response in trilayer devices. Since both Pt and Ta have strong spinorbit coupling with opposite signs in their spin Hall angle15,31,32, we have fabricated three Pt (5 nm)/YIG (80 nm)/Pt (5 nm) devices and two Pt (5 nm)/YIG (80 nm)/Ta (5 nm) reference samples, which were deposited

a b

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[afii9846] [afii9846]

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Figure 1 | Schematic illustration of spin current valve. (a) The transmission of spin current is switched on. Magnetization (M) of magnetic insulator (MI) oriented collinearly with the spin polarization r (//y) of the pure spin current in the bottom normal metal (NM) layer generated by the spin Hall effect with an electric current Jinjected. The spin-ip scattering of conduction electrons at the bottom NM/MI interface can create (M || r) or annihilate (M || r)

magnons. A non-equilibrium magnon population extends to the top MI/NM interface, and the spin angular momentum carried by magnons is transferred to conduction electrons in the top NM layer. The pure spin current owing perpendicular to the NM layer is then converted to a charge current (Jinduced) via

the inverse spin Hall effect. (b) The transmission of spin current is switched off. M is perpendicular to the spin polarization r of the spin current. In this geometry, the sd exchange interaction between conduction electrons and local magnetic moments does not excite magnons in the MI. Consequently, there is no spin accumulation at the top MI/NM interface or induced charge current in the top NM layer.

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a b

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Figure 2 | Msesaurement geometry and eld-dependent nonlocal signal. (a) Atomic force microscopy image of 80 nm YIG lm grown on 5 nmPt. (b) Schematic illustration of the experimental set-up. Ib is the current applied to the bottom Pt layer, and Vnl is the nonlocal voltage measured at the top layer along the Ib direction. The applied in-plane magnetic eldH makes an angle y with the y-axis which is in plane and perpendicular to the current direction (x). (c) The eld dependence of the nonlocal signal for H along Ib, that is, y 90. The inset shows the optical image of

GGG/Pt/YIG/Pt device. (d) The eld-dependent nonlocal signal with H perpendicular to Ib, that is, y 0. In both c and d, solid (empty) blue

squares and empty red circles represent Vnl for 1.5 mA ( 1.5 mA) and

0.0 mA bottom current, respectively.

on (110)-oriented Gd3Ga5O12 (GGG) substrates (see Methods section for fabrication details). The inset of Fig. 2c shows an optic image for a GGG/Pt/YIG/Pt device. As illustrated in Fig. 2b, the bottom Pt layer is used to inject current Ib, while the top layer, either Pt or Ta, functions as a detector to measure the induced current or the nonlocal voltage Vnl. An in-plane H is either swept in a xed direction or rotated with a continuously varying angle of y measured from the y-direction. We nd that the high-quality

YIG/Pt interface is essential to the observation of the spin current transmission. As shown in Fig. 2a, the morphology of a YIG lm tracks the atomically at terraces of the GGG (110) surface33 in spite of a layer of Pt in between. The excellent interface quality is veried by both the SSE5 (see Supplementary Note 5 and Supplementary Fig. 6) and the spin Hall magnetoresistance (SMR)34,35 (see Supplementary Note 3 and Supplementary Fig. 3) in the same devices. The 80-nm-thick YIG lms are nearly insulating but have small leakage at high temperatures. However, the resistance between the top and bottom NM layers increases exponentially as the temperature (T) decreases (see Supplementary Note 2 and Supplementary Fig. 2), and exceeds 20 GO at and below 220 K. Therefore, all the nonlocal measurements were performed below 220 K to avoid any parasitic signal from the small leakage current. In Vnl, we remove a non-zero background signal that exists even at Ib 0.

Figure 2c,d plot the eld dependence of Vnl at 220 K. When H

is swept along Ib, that is, y 90 (Fig. 2c), Vnl is a constant at

Ib 0 (red). However, at Ib 1.5 mA, Vnl shows a clear

hysteresis with two positive peaks tracking the coercive elds of the YIG lm, indicating that Vnl is closely related to the magnetization state of YIG. As the Ib is reversed, Vnl also reverses the sign. In principle, a sign reversal can occur if there is a nite leakage current owing in the top layer. Through the magnetoresistance, this current can produce a hysteretic voltage signal. Estimating from the leakage current, we nd that the

relative change in Vnl due to this effect is at least three orders of magnitude smaller than the observed nonlocal voltage signal (see Supplementary Note 4 and Supplementary Figs 4 and 5). Therefore, we exclude the leakage current as the source of the nonlocal signal. Note that Vnl (1.5 mA) is the same as Vnl (0 mA) at the saturation state (H4200 Oe) when r>M, suggesting that magnon creation/annihilation is totally suppressed. For the eld sweeps with y 0 (Fig. 2d), r is

collinear with M at high elds, interface magnon creation/ annihilation results in a full current drag signal. Clearly, Vnl

( 1.5 mA) is different from Vnl (0 mA) at the saturation elds

and reverses the sign when Ib reverses. It is interesting to note that Vnl (1.5 mA) differ from Vnl (0 mA) at the coercive elds. One would expect them to be the same since the average magnetization should point to the x-direction at the coercive elds, which would correspond to the saturation states for y 90

in Fig. 2c. This discrepancy can be explained by the multi-domain state of YIG in which the actual M is distributed over a range of angles around y 90, and the collinear component of M turns

on the magnon channel and yields a nonzero Vnl. To investigate the phenomenon in the single-domain state, we perform the following experiments.

Angle-dependent nonlocal response of single-domain YIG. Figure 3a presents Vnl in GGG/Pt/YIG/Pt as a function of y between M and r at 220 K, as illustrated in Fig. 2b. The 80 nm (110)-oriented YIG grown on Pt has a well-dened uniaxial anisotropy with an anisotropy eld o200 Oe. The applied magnetic eld (1,000 Oe) is sufciently strong not only to set YIG into a single-domain state, but also to rotate M with it. For all positive Ib (solid symbols), Vnl exhibits maxima at y 0 and

180 (M collinear with r), but minima at y 90 and 270

(M>r). Vnl changes the sign as Ib is reversed (empty symbols). At y 90 and 270, the nonlocal signal for Ib coincides with

Vnl (0 mA), further validating that the spin current is in the off state when M>r. Similar angular dependent measurements are also taken on a GGG/Pt/YIG/Ta device and the results are depicted in Fig. 3d. For the same measurement geometry and the same polarity of Ib, we nd that Vnl of GGG/Pt/YIG/Ta has the opposite sign to that of GGG/Pt/YIG/Pt, which is just expected from the opposite signs in their spin Hall angle. The Vnl

sign difference here is another piece of critical evidence for the magnon-mediated mechanism, as opposed to other extrinsic ones such as leakage.

An interesting feature to note here is that Vnl at y 0 and

180 shows a slight but reproducible difference that is independent of the current polarity but increases with the increasing magnitude of Ib. We attribute this phenomenon to the

SSE contribution since the joule heating in the bottom Pt layer unavoidably generates a small vertical temperature gradient, which in turn launches an upward spin current in YIG entering the top Pt (or Ta) layer. As M reverses, so does the spin polarization, which consequently produces two different SSE signal levels between y 0 and 180. Combining these two

effects, we can t the angular dependence data using:

Vnl V0 VSSE cosy VDrag cos2y 1

where V0 is an offset voltage insensitive to the magnetization orientation, VSSE is the SSE voltage amplitude and VDrag represents the amplitude of the current drag signal. The solid red curves in Fig. 3a,d t the experimental data remarkably well, and the extracted tting results are plotted in Fig. 3b,e for GGG/Pt/YIG/Pt and GGG/Pt/YIG/Ta devices, respectively. Two conclusions can be evidently drawn from these results. First, the magnitude of the current drag signal (red circles) scales linearly

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a

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Figure 3 | Angular and current dependence of nonlocal signal. (a,d) Angular dependence of nonlocal signal at different currents in the bottom Pt layer for GGG/Pt/YIG/Pt and GGG/Pt/YIG/Ta, respectively. The magnetic eld is xed at 1,000 Oe and rotated in plane. In a and d, solid symbols indicate positive Ib, and empty symbols indicate negative Ib. Red curves are least squares ts using equation (1). (b,e) Show the Ib dependence of the current drag signal (VDrag) and the spin Seebeck signal (VSSE) for GGG/Pt/YIG/Pt and GGG/Pt/YIG/Ta, respectively. (c,f) The spin Seebeck signal as a function of I2b for

GGG/Pt/YIG/Pt and GGG/Pt/YIG/Ta, respectively, the red curves are the linear ts. The error bars are from tting using equation (1) in a and d.

0

with the driving current, that is, VDragpIb. This is in stark contrast

with the highly nonlinear behaviour16. Second, the weak current dependence of the SSE contribution follows VSSE I2b

(as shown in Fig. 3c,f), which is characteristic of thermoelectric effects. Compared with usual bilayers, trilayer structures may have an enhanced SSE contribution due to the presence of the second heavy metal layer that draws an extra heat-driven spin current. Carefully designed experiments are needed to separate this effect.

Temperature dependence of nonlocal responses. According to Zhang, et al.24, the temperature dependence of the injection

interface spin convertance Gem is T

Tc

3=2, where Tc is the Curie temperature of the MI; for the detection interface, the spin convertance Gme is proportional to T

TF, where TF is the Fermi temperature of the NM layer. In the most simplied picture which is strictly applicable only for very thick lms, the current drag signal should be proportional to the product of the two spin current convertances, that is, VDragpGem GmepT5/2 The

representative angular dependence measurements below 220 K are shown in Fig. 4a,c for GGG/Pt/YIG/Pt and GGG/Pt/YIG/Ta devices, respectively. For both the samples, Ib is set at 2 mA

and H is held at 1,000 Oe. The magnitude of the current drag signal decreases progressively with decreasing temperature for both the devices. By tting Vnl using equation (1), we extract the magnitude of VDrag and VSSE shown in Fig. 4b,d. Apart from the

expected sign difference, the magnitude of VDrag in both the

devices monotonically decreases with decreasing temperature. In fact, both data sets can be well tted by a power-law VDragV0DragTn (red solid curves in Fig. 4b,d), where V0Drag is a pre-factor. The extracted exponent n is 2.21 for GGG/Pt/YIG/Pt and 1.88 for GGG/Pt/YIG/Ta, falling in the range between 1.5 and 2.5. It should be pointed out that the full picture described in ref. 24 actually contains other quantities that have weak temperature dependence. The deviation of the exponent from2.5 is fully expected if these factors are considered. On the other hand, the VSSE is found to be relatively insensitive to temperature, suggesting a completely different mechanism.

In conclusion, we experimentally establish the magnon-mediated current drag effect in NM/MI/NM trilayer structures by investigating the eld, angle, current and temperature dependences of the nonlocal signal. The spin information carried by conduction electrons and magnons in different materials can be interconverted at the interfaces. Such structures can serve as

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Figure 4 | Temperature dependence of nonlocal signal. (a,c) Angular dependence of the nonlocal signal at different temperatures for GGG/Pt/YIG/Pt and GGG/Pt/YIG/Ta, respectively. During the measurements, Ib was xed at 2 mA. The curves are vertically shifted for

clarity, the black arrows in a and c represent the magnitude scale of 2.80 and 1.98 mV, respectively. Red solid curves in a and c are the ts using equation (1). (b,d) The temperature dependence of the extracted current drag signal (VDrag) and spin Seebeck signal (VSSE) for GGG/Pt/YIG/Pt and

GGG/Pt/YIG/Ta, respectively. Red solid curves in b and d are the ts using VDragV0DragTn, here, n 2.21 for the GGG/Pt/YIG/Pt device, and n 1.88 for the GGG/Pt/YIG/Ta device.

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pure spin current valve devices since rotating the in-plane magnetization of the MI by 90 provides a digital onoff switch of the spin current. Furthermore, such structures also provide analogue functionality since rotating the in-plane magnetization of the MI provides analogue sinusoidal modulation of the spin current. Due to the extremely low damping in the MI, transmission of the pure spin currents can occur over relatively long distances providing the functionality of a pure spin interconnect.

Methods

Fabrication. A Hall bar was rst dened in the photo-resist layer on a (110)-oriented single crystalline GGG substrate using photolithography with the channel width of 20 mm and the length of 300 mm between two voltage electrodes. Then the bottom Pt layer was deposited on the open Hall bar area by d.c. magnetron sputtering. During sputtering, argon pressure was 5 mTorr, substrate temperature was kept at 300 K, and the d.c. sputtering power was 37.5 W. The deposition rate of Pt was 0.77 s 1 and the Pt layer thickness was 5 nm. After liftoff, an 80-nm-thick

YIG lm was deposited at 450 C with O2 pressure of 1.5 mTorr by pulsed laser deposition to cover the surface of the entire sample. The as-grown YIG lm became crystallized and magnetized after rapid thermal annealing between 800 and 850 C for 200 s. We had explored a range of growth temperatures, different annealing conditions, different pulsed laser deposition rates and YIG lm thicknesses and had experienced many difculties such as YIG lm cracking, peeling off for thicker YIG lms, conducting, non-magnetic and so on before we identied the working window. The magnetic properties of YIG were investigated by vibrating sample magnetometer (see Supplementary Note 1 and Supplementary Fig. 1). The surface morphology of YIG was monitored by atomic force microscopy. Clear atomic terraces were observed and the root-mean-square roughness on terraces was B0.14 nm, indicating a very at YIG surface. The top Pt and Ta patterns were dened using standard e-beam lithography, followed by magnetron sputtering deposition and lift-off procedures. Before Pt (or Ta) deposition, 60 s argon ion milling was used to remove any polymer residues from the YIG surface. The deposition conditions for top Pt and Ta were the same as those for bottom Pt. The top Pt and Ta strips are 2 mm in width, and 70 and 60 mm in length, respectively. To generate a vertical temperature gradient in separate longitudinal Spin Seebeck effect measurements, which we conducted after the nonlocal measurements were nished, we deposited 300-nm-thick Al2O3 on top and then Cr (5 nm)/Au (50 nm), which serves as a heater for SSE measurements.

Transport measurement. For all transport measurements, the current was fed to the devices using a Keithley 2,400 d.c. current source, and the voltage was measured by a Keithley 2182A nano-voltmeter. The eld dependence measurements were carried out using a closed-cycle system, while the angular dependent measurements were performed by a physical property measurement system equipped with a rotatory sample holder. For the nonlocal measurements, the excitation current in the bottom Pt is usually no more than 2 mA; for the local magnetoresistance measurements, the current applied in top Pt and Ta was 1 mA, while the current used in bottom Pt is 10 mA. For the SSE measurements, the heating current applied in the top Au layer is 30 mA. In all the measurements, extra precaution was taken to ensure the correct polarity of both current and voltage.

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Acknowledgements

We thank Y. Barlas, N. Amos, G. Yin and S.S. Su for the technical assistance and fruitful discussions. Sandwich structure growth, device fabrication and characterization, electrical transport and thermoelectric measurements and data analyses at UCR were supported as part of the SHINES, an Energy Frontier Research Center funded by the U.S. Department of Energy, Ofce of Science, Basic Energy Sciences under Award # SC0012670. S.Z. was supported by NSF-ECCS-1404542.

Author contributions

J.S. designed and supervised the project. J.L. and Y.X. performed the device fabrication and transport measurements with the help of Z.L.. M.A. performed the pulsed laser deposition of the sandwich structures and characterization with the help of C.T. S.Z. and R.L. contributed to data analyses. J.L. and J.S. wrote the manuscript and all the authors contributed to the nal version of manuscript.

Additional information

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

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How to cite this article: Li, J. et al. Observation of magnon-mediated current drag in Pt/yttrium iron garnet/Pt(Ta) trilayers. Nat. Commun. 7:10858doi: 10.1038/ncomms10858 (2016).

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