Electrical generation and detection of spin waves in polycrystalline YIG/Pt grown on silicon wafers

We studied the magnetic properties of polycrystalline Y3Fe5O12 (YIG) thin films (less than 100 nm) deposited on thermally oxidized silicon wafer by magnetron sputtering and followed by the post-annealing process. Our ferromagnetic resonance (FMR) results demonstrate that sputtering at room temperature combined with the post-annealing treatment can be an efficient method to achieve large-area (inch scale) and highly uniform YIG thin films with a low damping constant α ∼ 7 × 10−3 on cheap oxidized Si wafer. Furthermore, our spin pumping experiments demonstrate that the polycrystalline YIG/Pt system has a good spin mixing conductance, where spin current can be effectively injected into the adjacent Pt layer from YIG through the interface. Then the electrical detection of magnetic properties (e.g., spin waves) of insulating YIG film can be achieved via the inverse spin Hall effect of Pt. The electrical detection of spin waves in the large-area polycrystalline YIG/Pt on silicon wafer may help to develop new spintronic devices (e.g., magnon-based devices) by utilizing the complementary metal-oxide-semiconductor (CMOS) technology.

However, to easily integrate the YIG-based spintronic devices with silicon-based semiconductor electronics on one chip and to reduce the cost, the explorations of the spin-related properties (e.g., spin-wave, spin current) of polycrystalline YIG films deposited on the conventional semiconductor wafers (e.g., silicon wafer) is imperative for the development of multifunctional magnon-based devices. Although a few of previous works studied the magnetic properties and microstructure of the YIG films deposited on Si/SiO 2 or quartz substrates and found that thin YIG films were easy to crack during the post-annealing procedure due to the thermal expansion and the mismatch between YIG film and substrates [18][19][20][21], it is still lack of the essential investigations of the electrical generation, detection of spin waves and pure spin-related transport properties in the magnetic heterostructure consisted of polycrystalline YIG and spin Hall materials (e.g., Pt, W) deposited on Si/SiO 2 or quartz substrates.
In the present work, the crack-free polycrystalline YIG films thinner than 100 nm were obtained by using the ex-situ post-annealing method to crystallize the amorphous YIG films deposited on thermally oxidized silicon substrate by radio frequency (rf ) magnetron sputtering at room temperature. Based on the analysis of the surface morphology, the static and dynamic magnetization of YIG films at different annealing temperatures, the optimal annealing condition was determined. The deterioration of saturation magnetization and the damping coefficient with the decrease of film thickness was also confirmed. Additionally, our spin pumping experiments showed that the magnetization precession excited by the external rf microwave signal in these YIG films could efficiently pump spin current into the adjacent Pt layer and generate a distinct direct current (dc) voltage due to the inverse spin Hall effect (ISHE). The demonstrated electrical detection of spin waves in the polycrystalline YIG/Pt system is an essential step towards integrating YIG-based magnonics and silicon electronics.

Experimental procedure
The amorphous YIG films with different thicknesses were firstly deposited on Si/SiO 2 substrates by rf magnetron sputtering with 50 W power at 4×10 −3 Torr with high purity argon (99.999%), and then crystallized through ex-situ post-annealing procedure in air. The film structures and crystalline phases were characterized by using x-ray diffraction (XRD) with Cu Kα 1 radiation. The thickness, the surface morphology, and saturation magnetization of polycrystalline films were determined by spectroscopic ellipsometry (SE), scanning electron microscope (SEM)/atomic force microscopy (AFM), and vibration sample magnetometer (VSM), respectively. Ferromagnetic resonance (FMR) spectra of the YIG films was performed by using a broadband coplanar waveguide (CPW) with the simultaneous magnetic field and rf excitation under the pulse modulation. All the static and dynamic magnetic properties were performed at room temperature.

Results and discussion
To obtain the optimum crystallization condition, we investigated the influence of annealing temperature on the structure and properties of films by ex-situ post-annealing the same batch of as-grown amorphous YIG films in air at different temperatures. After post-annealing, the thickness, the crystal structure, the microstructure, and the morphology of YIG films were characterized by SE, XRD, AFM, and SEM. Figure 1 shows the XRD patterns of the YIG target and the 65 nm YIG films annealed at 800°C, 850°C, and 900°C for 1 h in air, with peaks indexed by Miller indices based on cubic space group (Ia3d). Based on the XRD analysis, the lattice constants of annealing films are obtained: 1.2332 nm (800°C), 1.2331 nm (850°C), and 1.2336 nm (900°C), where the thin films have the smaller lattice constant than the bulk YIG (1.2340 nm), which demonstrates that the substrate has the compressive strain on the thin films.
The surface morphology of the YIG films was further characterized by AFM and SEM. In figures 2(a)-(c), the SEM images (10 μm×10 μm) show that the YIG films have a relatively smooth surface without any cracks, and all films are dense and contain regular surface grains. With the increase of the annealing temperature up to 900°C, the film surface shows the enhanced agglomeration of grains, and the grain size gradually increases, which were further confirmed by the AFM.  films annealed at different temperatures. The root-mean-square (RMS) surface roughness of the annealing films is 10.1 nm (800°C), 12.7 nm (850°C) and 13.5 nm (900°C), respectively, which are about 10 times larger than RMS surface roughness of the as-grown films (1.28 nm). The results of AFM further confirmed the SEM results that the grains grow with increasing annealing temperature. The film quality, such as the crystallization, the composition, and the roughness, can affect the properties of films. To explore the annealing temperature effect on the magnetic properties of the films, we further performed static and dynamic magnetic measurements of the YIG films.
The static magnetic properties of YIG films were characterized by VSM at room temperature. Figure 3 shows the field-dependent magnetization curves (M-H) of 65 nm thin YIG films with three different annealing temperatures. The magnetization quickly saturates even at a low field of 200 Oe. Such a small value of the coercive field indicates the soft magnetic properties of all YIG films with fewer defects and inhomogeneities. The coercivity (H C ) in the inset of figure 3 was obtained from the M-H hysteresis loops. In figure 3, the saturation magnetization (M S ) of YIG films is much smaller than that of the bulk (139.3 emu cm −3 ), consistent with the previous reports [22][23][24]. For the YIG film annealed at 800°C, the coercivity H C and the saturation magnetization M S are 32 Oe and 73.0 emu cm −3 , respectively. As the annealing temperature increases to 900°C, H C drops to 25 Oe, and M S increases to 82.4 emu cm −3 , which can be attributed to the increase of the grain size  with the corresponding decrease of the magnetic anisotropy [25], indicating that the higher annealed temperature leads to the better quality of the YIG films. The static magnetic properties show that thin film has a smaller M S and a higher H C than that of the bulk (M S ∼ 140 emu cm −3 , H C ∼1 Oe) [2], which is related to the surface and interfacial effect of thin films. The deterioration of the magnetic properties of thin YIG films may hamper its application in the field of the spintronic devices.
The dynamic magnetic properties of YIG films were further investigated by broadband FMR spectroscopy. The typical differential FMR spectra of YIG films with different annealed temperatures are shown in figures 4(a)-(c). The FMR curve is a combination of symmetric and antisymmetric Lorentzian functions, as following where F S , and F A represent the symmetric and antisymmetric factor, H is the magnitude of the external magnetic field, H res is the resonance field, and ΔH is the corresponding linewidth of FMR. The coupling between the YIG film and the CPW will cause a partial mixture of the real and imaginary components of the susceptibility, which will induce a symmetric signal F S . By using equation (1), the best FMR fitting gives very small F S , which is over 100 times smaller than the F A . After fitting the FMR spectrum in figures 4(a)-(c), ΔH and H res were also extracted for films with different annealing conditions in figures 4(d) and (e), respectively. In figure 4(d), the linewidth DH as a function of the resonance frequency f can be fitted by: where γ/(2π) (2.8 MHz Oe −1 ) is the gyromagnetic ratio, ΔH 0 and α represent the inhomogeneous broadening and the Gilbert damping coefficient, respectively. Achieving the low damping coefficient α is essential for the application of the YIG thin film. By fitting the data of figure 4(d) by using equation (2), the damping coefficient α was obtained and shown in figure 4(f). As the annealing temperature increases, α decreases from 0.0091 to 0.0072, indicating the better quality of films annealed at a higher temperature. Figure 4(e) shows the relation between the resonance frequency f and the resonance field H res , which can be well fitted by the Kittel equation where M eff is the effective magnetization. For YIG films, M eff can be obtained by fitting the field dependence of resonance frequency with the corresponding M S obtained from VSM measurements above. Figure 4(g) shows that, with the increase of the annealing temperature, the saturation magnetization M S slowly increases, while effective magnetization M eff rapidly increases. The effective magnetization M eff can be written as [24]: where H a is the magnetic anisotropy field. The obtained H a is shown in figure 4(h). With the increase of annealing temperature, the H a of YIG film decreases rapidly, which demonstrates the higher annealing temperature tends to release a part of compression strain from substrate, consistent with structure characterization analysis. Furthermore, the thickness dependence of the magnetic properties of the YIG film is also investigated. The in-plane magnetic hysteresis loops of YIG films with three different thicknesses (48 nm, 57 nm, and 65 nm) annealed at 900°C are measured and shown in figure 5. With the thickness increase from 48 nm to 65 nm, M S increases from 39.8 emu cm −3 to 82.4 emu cm −3 ; while H C decreases from 35 Oe to 25 Oe. Saturation magnetization M S is deteriorated by reducing the thickness of film due to the appearance of component diffusion/segregation, oxygen vacancies, strains, and the defects at the surface/interface, which has been observed and confirmed by many previous studies on various complex oxides [26,27]. Therefore, with decreasing thickness, the drop of M S is related to the degeneration of magnetization at the interface and surface.
We also performed FMR measurements on YIG films with different thicknesses to explore their dynamic properties, as shown in figures 6(a)-(c). Following the previous methods, the resonance field H res and linewidth ΔH were extracted from FMR fitting by equation (1), and shown in figures 6(d) and (e). These extracted data can further provide the damping a and the effective magnetization M eff , which were summarized in figures 6(f) and (g). Figure 6(g) shows that M S and M eff of the films increase with the increasing of film thickness. The in-plane anisotropy field H a is determined from M eff and M S , as shown in figure 6(h). H a for 65 nm YIG film is about half of that for 48 nm film. Figure 6(f) shows that the Gilbert damping a significantly drops from 0.0138 to 0.0072 when the film thickness increases from 48 nm to 65 nm, consistent with the behavior of the thickness-dependent linewidth for the films in figure 6(d). One can expect that thicker films hold fewer defects and less influence from the interface between YIG and substrate. And thinner YIG films with a larger Gilbert damping is related to defects at the surface and interface, which gives rise to the additional damping due to the two-magnon scattering [26,28,29].
Since the spin mixing conductance  g eff governs various spin-dependent phenomena such as spin pumping, SMR and spin Seebeck effect [30][31][32], it is essential to experimentally determine  g eff of polycrystalline YIG/Pt films grown on the silicon substrate. For our Si/SiO 2 /YIG/Pt samples, the Pt layer was finally deposited at room temperature after the post-annealing of polycrystalline YIG film. Based on the previous theoretical studies [10,11,33] where g is the Landé factor, m B is the Bohr magneton, t YIG is the thickness of magnetic material. To quantitatively obtain  g , eff we systematically measured FMR of bare YIG (65 nm) and YIG (65 nm)/Pt (7 nm) at different excitation frequencies and extracted their linewidth using the equation (1), as shown in figure 7(a). Compared with the bare YIG film, the linewidth of YIG/Pt is significantly increased due to spin current generated at the  (5), the spin mixing conductance  g eff of polycrystalline YIG/Pt deposited on Si/SiO 2 can be calculated to be 5.2×10 18 m −2 . For direct comparison, we also performed the same FMR measurement to extract Gilbert damping for the 0.92 μm thick YIG film epitaxially grown on GGG substrate GGG/YIG(0.92 μm) by LPE method with and without Pt (7 nm) layer, and found that the hybrid system of Pt and single-crystal YIG has the higher  g eff ∼6.6×10 19 m −2 as expected, which is also 10 times higher than that of the previously reported thin YIG epitaxial film system GGG/YIG (19 nm)/Pt (20 nm) grown by PLD [34]. There are several reasons for the decrease of  g eff in Si/SiO 2 /YIG (65 nm)/Pt (7 nm). Based on equation (5), the much lower saturation magnetization M S ∼80 emu cm −3 (less than 60% of M S ∼140 emu cm −3 in GGG/YIG) and the higher spin current loss in the YIG/Pt interface can reduce  g . eff Besides, the extrinsic damping enhancement factors such as the strong two-magnon scattering and the spin memory loss induced by the interfacial spin-orbit coupling will directly result in the incorrect evaluation of  g , eff which has also been verified by several recent works [35][36][37][38].
To directly verify the possibility of the electrical detection of spin-wave and the spin transport in the YIG films on Si/SiO 2 substrate, we patterned a 7 nm thick Pt stripe with ḿ 5 mm 200 m size on the top of the 65 nm thick YIG films by a combination of magnetron sputtering and e-beam lithography (EBL). Figure 7(b) shows the schematic illustration of the electrical detection of spin current in YIG/Pt via ISHE based on the spin pumping technique. Figure 7(c) shows the dependence of the obtained dc voltage V dc with the excitation frequency = f 6 GHz on the out-of-plane magnetic field angle q defined in figure 7(b). One can see that V dc changes its sign when the magnetic field H varies from q =  90 (along the y-axis positive direction) to q = - 90 , indicating that the dc voltage signal is related to the dc ISHE in Pt layer due to the spin pumping. Field-dependent dc voltage V dc under FMR condition of YIG can be well fitted with the symmetric Lorentzian lineshape given by [12]   [ is an ellipticity correction factor, R the resistance, w the width,q SH the spin Hall angle, l SD the spin diffusion length, and t N the thickness of Pt stripe, respectively. To better quantify the spin pumping efficiency of the Si/SiO 2 /YIG/Pt system, we further performed the compared experiments on the GGG/YIG(0.92 μm) film with the same 7 nm thick Pt stripe on its top. The V dc of GGG/YIG(0.92 μm)/Pt(7 nm) and Si/SiO 2 /YIG (65 nm)/Pt (7 nm) at 6 GHz are shown in the inset of figure 7(d). The V dc of the thick single-crystal GGG/YIG/Pt system shows a very sharp peak with a peak amplitude of 6 μV and a narrow linewidth of 12 Oe due to the ultralow damping constant and high-quality interface. By assuming the same q SH for the Pt stripe on polycrystalline and single crystal YIG films, the ratio of spin pumping voltage V dc poly and V dc single can be further obtained as following / can be calculated from the formula above. Although it is difficult to precisely calculate the absolute value of the interfacial spin mixing conductance due to unknown the actual rf power on the sample, we can obtain the ratio of the spin mixing conductance  g eff of the two systems from the measured spin pumping voltage ratio based on equation (8). According to the previous quantitative analysis, the spin pumping experiments indicate that the polycrystalline Si/SiO 2 /YIG/Pt system has a comparable  g eff with single-crystal GGG/YIG/Pt system, suggesting that the large scale polycrystalline YIG grown on the cheap Si wafer is adequate for developing YIG-based magnonics.

Conclusions
We systematically studied thickness and annealing temperature dependence of surface morphology, and static and dynamic magnetic properties of sub-100 nm YIG thin films deposited on Si substrate by magnetron sputtering and post-annealing treatment. 65 nm thick YIG film with the lowest Gilbert damping of´-7 10 3 was achieved at the 900°C annealing temperature in air. Thickness-dependent experiments show that the magnetic properties strongly degenerate with decreasing the YIG thickness, which indicates that the magnetization deterioration is related to the interfacial effects, including the defect and the composition diffusion. Furthermore, the spin injection across the YIG/Pt interface in the polycrystalline and the high-quality single-crystal YIG films were also studied by FMR and spin pumping measurements. Via the compared experiment of the spin pumping for GGG/YIG/Pt and Si/SiO 2 /YIG/Pt systems, we found that the spin mixing conductance of Si/SiO 2 /YIG/Pt is comparable with single-crystal YIG/Pt system. Our results suggest that the polycrystalline YIG/Pt film deposited on a silicon wafer is a promising system to integrate YIG-based magnonics and silicon-based electronics.