Electrical control of spin mixing conductance in a Y$_3$Fe$_5$O$_{12}$/Platinum bilayer

TL;DR

This study demonstrates that applying a gate voltage via ionic gating can precisely tune the spin mixing conductance, spin pumping, and spin Hall angle in a YIG/Pt bilayer, highlighting the importance of interfacial charge density in spin transport.

Contribution

It introduces a method to electrically control spin transport properties in YIG/Pt bilayers using ionic gating, a novel approach for spintronic device tuning.

Findings

Spin mixing conductance tunable up to ±22% with gate voltage.

Spin Hall angle in Pt layer tunable up to ±13.6%.

Gate-dependent ferromagnetic resonance line width observed.

Abstract

We report a tunable spin mixing conductance, up to $\pm 22\%$, in a Y${}_{3}$Fe${}_{5}$O${}_{12}$/Platinum (YIG/Pt) bilayer.This control is achieved by applying a gate voltage with an ionic gate technique, which exhibits a gate-dependent ferromagnetic resonance line width. Furthermore, we observed a gate-dependent spin pumping and spin Hall angle in the Pt layer, which is also tunable up to $\pm$ 13.6\%. This work experimentally demonstrates spin current control through spin pumping and a gate voltage in a YIG/Pt bilayer, demonstrating the crucial role of the interfacial charge density for the spin transport properties in magnetic insulator/heavy metal bilayers.

Equations

$$\Delta H=\Delta H_0+\frac{\omega }{\gamma }{\alpha }_{YIG/Pt}(1+\eta V_G).$$

$$g_{\uparrow \downarrow }=\frac{4\pi M_s\gamma t_{YIG}}{g{\mu }_B\omega }(\Delta H_{YIG/Pt}-\Delta H_{YIG}),$$

$$J_{C,SP}=k{\theta }_{SH}{g}_{\uparrow \downarrow }/{\alpha }_{YIG/Pt}^2.$$

$${\theta }_{SH}={\theta }_{SH,0}(1+\zeta V_G).$$

Full text

I introduction

Spin currents in Y3Fe5O12/Platinum (YIG/Pt) bilayers have attracted much attention in the past decade due to the unique spin current transport properties in magnetic insulators and spin charge conversion in heavy metals Kajiwara2010Nature ; Lu2012PRL ; Bai2013PRL ; Hahn2013PRB ; Sun2013PRL ; Castel2014PRB ; Hyde2014PRB ; Haertinger2015PRB ; Zhou2016PRB ; Dushenko2016PRL ; Wang2016PRL ; Wesenberg2017NatPhysics ; Kapelrud2017PRB ; Keller2017PRB . Mechanisms including spin pumping at the bilayer interface Tserkovnyak2002PRL , spin diffusion Montoya2014PRL and the inverse spin Hall effect in heavy metals Saitoh2006APL , have been established to interpret the generation, transfer and conversion of spin current Tserkovnyak2005RMP . This understanding has advanced the development of spintronic devices, as evidenced by, for example, nano oscillators. However spintronic devices based on spin pumping require additional control beyond the basic generation and detection of spin current Igor2004RMP . In this regard improvement of spin current transport at the interface of the YIG/Pt bilayer is the kernel toward the application of spin current due to spin pumping.

The efficiency with which the spin current crosses a bilayer interface is characterized by the spin mixing conductance, $g_{\uparrow\downarrow}$, which is a constant for a given sample and is usually measured by comparing the ferromagnetic resonance (FMR) line width in a bilayer device to that of a bare YIG layer. Some attempts have been made to change this interfacial spin transport. For example, a thin NiO Du2013PRL ; Wang2014PRL or CrO Qiu2018NatMaterials layer inserted at the interface has been reported to enhance or suppress the spin current, respectively. Additionally, Pt alloyed with Al or Au was demonstrated to enhance the spin transfer torque at the interface Nguyen2016APL . Importantly, such previous works hint that the conductivity in the Pt layer may change the boundary at the interface, hence changing the spin transport properties as well as the spin mixing conductance.

To control the spin transport properties of bilayer samples a gate voltage can be used to control interfacial boundary conditions. Interestingly gate voltage techniques, originally developed to control the carrier density in semiconductors, have recently been applied to thin metal films Wang2012NatMaterials . In this context the addition of an ionic gate has been shown to enlarge the field effect in metallic films due to the huge number of ions accumulated at the interface under certain bias voltage conditions Yamada2011Science . As a result it has been reported that the carrier density and anomalous Hall effect in Pt may be modulated Shimizu2013PRL ; Liang2018PRB . This brings about the interesting possibility to develop additional control techniques to manipulate spin transport at the YIG/Pt interface Guan2018AM .

In this work we applied a gate voltage to a YIG/Pt bilayer using an ionic gate technique to modulate the charge accumulation at the bilayer interface. We experimentally observed that the FMR line width is both enhanced and suppressed depending on the polarity of the gate voltage. By compare the FMR line width in the bilayer to that of the bare YIG thin film, we found a gate tunable spin mixing conductance in the YIG/Pt bilayer. Additionally, the observation of a shift in the FMR resonance field towards both high and low fields, depending on the polarity of the gate voltage, allows us to rule out the Joule heating effect. This shift in the FMR resonance field indicates that the effective field of the magnetization was changed by the gate voltage which may be induced by charge accumulation at the interface. Using the Landau-Lifshitz-Gilbert equation and spin pumping theory, we evaluated the spin current and the spin Hall effect in Pt, finding a strong gate voltage dependence. Thus we have experimentally demonstrated control of the spin current transport at a bilayer interface, which will be useful for understanding spin transfer at the interfaces of magnetic insulator and heavy metal bilayers.

II EXPERIMENTAL METHODS

To fabricate the YIG/Pt bilayer a 20-nm thick YIG layer was first deposited on a GGG substrate using pulsed laser deposition. The YIG had a saturation magnetization of $\mu_{0}M_{s}$ = 0.175 T and a Gilbert damping of $\alpha_{YIG}$ = 0.00049. A 2.5-nm-thick Pt layer was then deposited on top of the YIG using magneto sputtering in which the base pressure and the sputtering pressure were $2\times 10^{-5}$ Pa and 0.68 Pa, respectively. The Gilbert damping of the bilayer was $\alpha_{YIG/Pt}$ = 0.00157 which is 3 times larger than that of the bare YIG film. The lateral dimensions of the bilayer were 5 mm $\times$ 2.5 mm, and a Hall bar was fabricated using lithography techniques for reference and Hall measurements.

To measure the spin current signal due to spin pumping the YIG/Pt bilayer was driven to FMR by microwaves applied through a coplanar waveguide beneath the sample. The microwave output power was 158 mW and the modulation frequency of the microwave power (used for lock-in measurements) was 8.33 kHz. An external magnetic field H was applied in-plane and perpendicular to the Pt strip to enhance the spin pumping signal. The spin pumping voltage was measured along the $x$-axis of the Pt layer using a lock-in technique while sweeping the magnetic field H at room temperature.

An ionic gate, composed of a composite solid electrolyte, was placed on top of the Pt layer and a gate voltage $V_{G}$ was applied between a contact (labelled as Gate) on top of the solid electrolyte, and the Pt layer. The composite solid electrolyte used for gating was prepared using 81 wt$\%$ of acrylic resin, 14.6 wt$\%$ of succinonitrile, and 4.4 wt$\%$ of cesium perchlorate. As shown in Fig. 1 (b) the sheet carrier density in the 3-nm-thick Pt layer was found to be 1.8 $\times 10^{17}$ cm*-2* when no gate voltage was applied, $V_{G}$ = 0 V, assuming the single band relation $R_{H}=1/ne$ ($e$ is the electron charge and $n$ is the carrier density). This is comparable to the reported results for Pt Shimizu2013PRL . However, as indicated by the squares and triangles for $V_{G}$ = 2 V and -2 V respectively, the Hall resistance, and hence the carrier density, changes significantly when a gate voltage is applied. For a 3-nm-thick Pt thin film it is expected that the boundary at the interface of the YIG and Pt layer is strongly dependent on the carrier density as well as the spin orbit interaction in the Pt layer as shown in Fig. 1 (c). Thus the change in boundary conditions due to charge accumulation at the YIG/Pt interface will change the interfacial spin transport properties, which was experimentally observed in the FMR measurement. In our work we defined a positive gate voltage when the carrier density was reduced and vice versa. As a reference the FMR absorption in a bare YIG film was also measured as a function of the gate voltageSuppleM . In this case the voltage induced FMR changes were found to be small and ignorable.

III RESULTS AND DISCUSSION

III.1 $V_{G}$-dependent spin mixing conductance

Figure 2 (a) shows the spin pumping voltage measured in the YIG/Pt bilayer for a variable gate voltage $V_{G}$, with the FMR absorption signal in the YIG bare layer plotted for reference. Here the FMR spectra has been shifted by the resonance field $H_{0}$ and normalized to the maximum signal amplitude, in order to highlight the FMR line width change due to the gate voltage. The significant broadening of the YIG/Pt line width, as compared to the FMR line width in the bare YIG layer, has been experimentally observed and theoretically studied previously Weiler2013PRL ; Tserkovnyak2002PRL . Here we also observe that the bilayer line width is dependent on the applied gate voltage $V_{G}$. Figure 2 (b) shows the FMR line width $\Delta H$ (half-width-half-maximum) in both the YIG and YIG/Pt samples. The Gilbert damping, determined from the gradient of the line width as a function of the microwave frequency, is enhanced in the YIG/Pt bilayer compared to that in YIG. Clearly the gate voltage applied to the YIG/Pt bilayer suppresses and enhances the Gilbert damping of the bilayer. The inhomogeneous broadening $\Delta H_{0}$ is around 0.1 mT which is one order smaller than the line width $\Delta H$ at 6.8 GHz in both the YIG/Pt and YIG samples. Furthermore $\Delta H_{0}$ was barely affected by the gate voltage. Therefore it is a good approximation to subtract the Gilbert damping directly using the line with $\Delta H$ at 6.8 GHz. Such a line width change as a function of $V_{G}$ is summarized in Fig. 2 (c), where the dashed horizontal line indicates the FMR line width of the bare YIG. In a first order approximation we assume that the line width $\Delta H$ is influenced by the gate voltage $V_{G}$, according to

[TABLE]

Here $\Delta H_{0}$ is the frequency independent inhomogeneous broadening, the gyromagnetic ratio $\gamma=2\pi\mu_{0}\times$28 GHz/T, the Landé factor $g=2$, $\mu_{B}$ is the Bohr magneton, $\omega$ is microwave angular frequency, $\alpha_{YIG/Pt}$ is the Gilbert damping of the YIG/Pt bilayer and $\eta$ is a proportionality factor that characterizes the influence of the gate voltage on the Gilbert damping. The units of $\eta$ are $V^{-1}$. Here we find $\eta$ = -0.038 $V^{-1}$ by fitting the line width of the YIG/Pt bilayer to Eq. (1). Such a $V_{G}$-dependent line width was also observed for various different frequencies (not shown here). The spin mixing conductance $g_{\uparrow\downarrow}$ was experimentally evaluated by comparing the FMR line width $\Delta H_{YIG/Pt}$ in the YIG/Pt bilayer to the FMR line width $\Delta H_{YIG}$ in the bare YIG layer Weiler2013PRL ,

[TABLE]

where $M_{s}$ is the saturation magnetization and $t_{YIG}$ is the thickness of the YIG layer.

By comparing the line width of FMR in the YIG/Pt bilayer to that in the YIG thin film according to Eq. (2), one can evaluate the spin mixing conductance $g_{\uparrow\downarrow}$ as summarized in Fig. 2 (d). We find that $g_{\uparrow\downarrow}$ is roughly linearly-dependent on the gate voltage $V_{G}$ as indicated by the solid line, which can be predicted by Eq. (2) using $\eta$ = -0.038 $V^{-1}$. This indicates that we have experimentally manipulated the YIG/Pt interfacial spin transport properties, which is a key issue concerning spin injection in the spintronics community. We note that here the spin mixing conductance is an effective value since spin back flow will play a role in the 2.5-nm-thick Pt Jiao2013PRL ; Du2014PRApplied .

III.2 Spin current manipulated by $V_{G}$

Since the spin mixing conductance is $V_{G}$ tunable we may expect that the spin current due to spin pumping is also controlled by the gate voltage. As evidenced by the broadened line width, the gate voltage enhanced spin mixing conductance is the key source of additional FMR damping. This leads to a reduced FMR amplitude and thus a (1/$\alpha$)2 decrease in the spin current pumped by FMR. Therefore even though the enhanced spin mixing conductance leads to a large spin current transparency at the bilayer interface, the observed spin current amplitude is reduced. Figure 3 (a) shows the spin pumping voltage at different values of $V_{G}$. The amplitude of $V_{SP}$ is enhanced by applying a positive voltage and suppressed by a negative voltage, with a total change up to $\pm$46.3% as shown in Fig. 3 (b). We also carefully examined the resistance change of the Pt layer as a function of $V_{G}$, which shows a much smaller change ($\pm$3.4%) as highlighted in Fig. 3 (c). Fig. 3 (d) displays the $V_{G}$-dependent charge current $J_{C,SP}$ which has been generated from the spin current through the inverse spin Hall effect of the Pt layer. The gate voltage dependence of the charge current can be evaluated by considering the Polder tensor for FMR in the YIG layer, the spin mixing conductance at the interface, the spin diffusion into the Pt layer, and the inverse spin Hall effect in the Pt layer. For a given microwave frequency and power the spin current produced by the FMR and injected into the Pt layer at the interface will have the form $j_{s,0}\propto g_{\uparrow\downarrow}/\alpha_{YIG/Pt}^{2}$ and therefore the charge current may be written as,

[TABLE]

Here $k$ is a $V_{G}$-independent constant which depends on the microwave frequency and power, saturation magnetization of the YIG and the spin current diffusion properties of the Pt layer. Based on this analysis we can predict the $V_{G}$-dependent charge current using Eq. (3) as shown by the dashed line in Fig. 3 (d), where $k\theta_{SH}=7.64\times 10^{-24}$ $nA\cdot m^{2}$. Although the predicted dashed line does have a $V_{G}$-dependence (due to the $V_{G}$-dependent spin mixing conductance), it does not match the experimental observation. This indicates that for a given spin Hall angle, although the spin mixing conductance was enhanced, less spin current was produced than that required to generate the necessary charge current. Therefore, in order to compare with the observed $J_{C,SP}$ it is reasonable to assume that the spin Hall angle, $\theta_{SH}$ in Eq. (3), is also $V_{G}$-dependent,

[TABLE]

Here $\zeta$ defines the gate voltage dependence of the spin Hall angle and has units of $V^{-1}$ and $\theta_{SH,0}$ is the spin Hall angle for $V_{G}$ = 0 V. By combining Eqs. (3) and (4), we predict the charge current due to spin pumping as shown by the solid line in Fig. 3 (b), which matches well with the experimental data. In this calculation $k\theta_{SH,0}=7.58\times 10^{-23}$ $nA\cdot m^{2}$, which is the same as $k\theta_{SH}$ used for Eq. (3) (dashed line), and $\zeta$ = 0.034 $V^{-1}$ indicating that the spin Hall angle in the Pt layer is also tunable by the gate voltage. Therefore, we find a spin Hall angle which can be tuned by up to $\pm$13.6% as shown by the inset in Fig. 3 (d). This tendency is opposite the behaviour observed for the $V_{G}$-dependent spin mixing conductance.

III.3 $V_{G}$-dependent anisotropy field

An analogous electrically tunable anomalous Hall effect in Pt was previously reported using an ionic gate technique Shimizu2013PRL and may involve similar underlying physics, related to a strongly charge density dependent spin orbit interaction. However in previous results the gate voltage influence on the resistance and Hall effect was irreversible, whereas the spin mixing conductance control in our work can be repeated many times and may therefore be utilized to control spin current transport in future spintronic devices. We have performed spin pumping measurements in YIG(20 nm)/Pt($t$ nm) for a variety of Pt thickness $t$SuppleM . Compared to the large $V_{G}$-tunable effect in the 2.5-nm-thick-Pt sample discussed above, the spin pumping signal was greatly reduced in a 4-nm-thick-Pt sample and barely observable in a 10-nm-thick-Pt sample. These results further indicate that the effective spin mixing conductance change is due to charge accumulation at the YIG/Pt interface, which is greatly enhanced in the thin Pt samples.

The physics underlying the tunable spin mixing conductance and spin Hall angle is the change in carrier density due to the gate voltage, which will also induce a boundary change at the bilayer interface as we highlighted in Fig. 1 (c). Interestingly this also appears to induce an anisotropy change in the YIG/Pt bilayer as a function of the gate voltage. In Fig. 4 (a) we have plotted the FMR signals in the bare YIG layer and in the YIG/Pt bilayer with different gate voltages while the external magnetic field was applied in the film plane. The vertical dashed lines highlight the resonance positions. The large 6.8 mT shift to low fields which is observed in the bilayer device, compared to the bare YIG, is due to the influence of the Pt layer on the boundary conditions of the bare YIG surface. When we apply a gate voltage the FMR resonance field $H_{0}$ shifts to higher fields by 2.06 mT for $V_{G}$ = 4.0 V and to lower fields by 2.07 mT for $V_{G}$ = -4.0 V. The $V_{G}$-dependent $H_{0}$ is summarized in panel (b) and shows nearly a linear dependence. A more complex anisotropy change was observed in different samples and the mechanism is still an open question for future work, but bears an interesting analogy to the electrical field induced anisotropy changes in systems such as CoFeB/MgO Wang2012NatMaterials and CoO/Co Zhang2015nanoScale ; Wang2017nanoScale .

IV CONCLUSION

In summary, we report the modulation of the charge carrier density at the interface of a Y3Fe5O12/Platinum (YIG/Pt) bilayer using an ionic gate technique. We electrically detected the ferromagnetic resonance (FMR) at variable gate voltages, observing three major features: (1) The line width of FMR is controlled by a gate voltage, which indicates that the spin mixing conductance in the bilayer can be tuned; (2) The voltage amplitude of spin pumping is strongly dependent on the gate voltage. To model the voltage change we found that the spin Hall angle in Pt should be a function of the gate voltage; (3) The anisotropy change indicates that the boundary conditions at the interface of the bilayer are changed by the gate voltage. Thus, we experimentally demonstrated control of spin current due to spin pumping in a YIG/Pt bilayer using a gate voltage. This observation may be used to better understand spin transfer at magnetic insulator/heavy metal interfaces.

V ACKNOWLEDGMENTS

This work is supported by the ‘National Young 1000 Talents’ Program and by the National Natural Science Foundation of China (NSFC No. 11774200) grants (Lihui Bai).