Yttrium iron garnet thickness and frequency dependence of the spin-charge current conversion in YIG/Pt systems

We report the frequency dependence of the spin current emission (spin pumping) in a hybrid ferrimagnetic insulator/normal metal system as a function of the insulating layer thickness. The system is based on an yttrium iron garnet (YIG) film [0.2, 1, and $3\mu \mathrm{m}$] grown by liquid-phase epitaxy coupled with a spin current detector of platinum[6 nm]. A strong YIG thickness dependence of the efficiency of the spin pumping has been observed. The highest conversion factor $\Delta V/P_{\mathrm{abs}}$ has been demonstrated for the thinner YIG (1.79 and 0.55 mV/mW at 2.5 and 10 GHz, respectively), which is of interest for research heading towards YIG-based devices. Furthermore, we demonstrate the threshold frequency dependence of the three-magnon splitting process, which is shown to cease to exist for the thinner YIG of $0.2\mu \mathrm{m}$.

Figure 1

Frequency dependence of (a) the dc voltage from spin pumping ($V_{\mathrm{ISHE}}$), and (b) the microwave absorption determined by the scattering parameter $S_{11}$, for different values of the static magnetic field ($H$) applied in the plane of the substrate along the $x$ direction. The thickness of the YIG layer is 1 $\mu \mathrm{m}$. For each value of $H$, the excitation frequency has been swept at an rf power of 10 mW (at room temperature). $\Delta V$ and $\Delta S_{11}$ correspond to the magnitude of $V_{\mathrm{ISHE}}$ and $S_{11}$ at the resonant condition, respectively. The inset represents the measurement configuration. (1) and (2) are the electrical contacts, the gray area corresponds to the Pt layer, the green is the ${\mathrm{Al}}_2{\mathrm{O}}_3$ layer, the yellow stripe is the microwave antenna, and the brown part shows the YIG.

Figure 2

(a) Frequency dependence of the $S_{11}$ spectrum measured at $H=3$ kOe (in red) and at a saturation field $H_{\mathrm{sat}}$ (in green) equal to 4 kOe for a YIG thickness of 1 $\mu \mathrm{m}$. The measurement has been performed at an rf power of 10 mW at room temperature. (b) Frequency dependence of the microwave absorption corresponding to the difference between the $S_{11}$ spectrum for the resonance in the YIG layer ($H=3$ kOe) and from the spectrum without resonance signature ($H_{\mathrm{sat}}$). $S_{11}^{\mathrm{Baseline}}$ and $S_{11}^{\mathrm{FMR}}$ correspond to the magnitude of the microwave absorption in dBm, of the baseline and at the resonant condition, respectively.

Figure 3

Frequency dependence of (a) the microwave absorption $\Delta S_{11}$ in mW and (b) the dc voltage $\Delta V$, for different thicknesses of YIG [0.2, 1, and $3\mu \mathrm{m}$]. Vertical dashed-dotted lines correspond to the frequency cutoff of the three-magnon splitting process estimated using the dispersion relation of spin waves for a YIG thickness of 1 (red) and $3\mu \mathrm{m}$ (black). All measurements have been carried out at room temperature under an rf excitation of 10 mW.

Figure 4

(a) Frequency dependence of the conversion efficiency factor $\Delta V/P_{\mathrm{abs}}$ for different thicknesses of YIG [0.2, 1, and $3\mu \mathrm{m}$]. (b) $\Delta V/P_{\mathrm{abs}}$ normalized by its value at 3.3 GHz. The vertical dashed-dotted lines correspond to the frequency cutoff of the three-magnon splitting process, for a YIG thickness of 1 (red) and $3\mu \mathrm{m}$ (black). The green, blue, red, and black dashed curves correspond to the theoretical frequency dependence of $\Delta V/P_{\mathrm{abs}}$ from Eq. (1). The inset in (b) shows the dependence of the ratio of $\Delta V/P_{\mathrm{abs}}$ at 2.5 and 10 GHz as a function of the thickness of the YIG layer. The green dashed line shows the theoretical expected magnitude of this ratio and the black dashed line is a guide to the eye.

Figure 5

Dispersion relation of spin waves [19, 20]. (a) Dependence of the frequency $f$, as a function of the wave vector $k$, when $k\parallel H$ (BVMSW) and $k\perp H$ (MSSW) for a YIG thickness of 1 $\mu \mathrm{m}$. The arrows represent the three-magnon splitting process that creates two spin waves with a frequency corresponding to $f/2$. $f_{\min }$ gives the minimum possible frequency of the BVMSW modes. (b) Evolution of the spin-wave spectrum for different thicknesses of YIG [0.2, 1, and $3\mu \mathrm{m}$]. For increasing YIG thickness, it shows that $f_{\min }$ decreases, and finally approaches the Larmor frequency $f_H$. In both figures, the magnetic field is fixed at 150 Oe ($f=f_{\mathrm{FMR}}=1.55$ GHz). The exchange stiffness has been fixed at $D=2\times {10}^{-13}$ ${\mathrm{Oe}}^{-1}$ ${\mathrm{m}}^2$ (see Ref. [21]).