Pure Spin Transport in YIG Films with Amorphous‐to‐Crystalline Transformation

Magnetic insulators, especially Y3Fe5O12 (YIG), are considered promising candidates for spin‐based device applications due to their ultralow damping, high spin injection efficiency, and long‐distance spin propagation. However, these intriguing features are widely studied based on crystallization YIG films. Pure spin phenomena, like spin transport in YIG films with structural evolution, remain unclear. Herein, pure spin transportation is systematically investigated in the sandwich structure formed by YIG, the inserted layer‐nominal YIG (n‐YIG) with a varied crystalline structure and heavy metal Platinum (Pt). By applying ferromagnetic resonance (FMR)‐driven inverse spin Hall effect (ISHE) measurement, the detected ISHE voltage signal presented a strong correlation with the thickness of n‐YIG and its crystalline phase. A significant increasement in spin transportation is obtained for the crystallized n‐YIG via a high‐temperature annealing. These results demonstrate that pure spin current is transported availably in the structural evolution of YIG films. Furthermore, the element‐specific X‐ray absorption spectroscopy (XAS) and X‐ray magnetic circular dichroism (XMCD) spectra on the n‐YIG films showed a distinction for the crystallized n‐YIG which indicates that the spin propagation is correlated to its magnetic order. These findings are instructive for low‐dissipation spin‐based devices.


Introduction
Pure spin current, which is considered as a flow of spin angular momentum without flow of any accompanying net charge current and stray Oersted field, is conducive to realizing spin-based devices with minimum power dissipation. [1,2]Thus, the exploration of the pure spin current, such as its generation, detection, and transmission, has attracted great interests recently. [3,4]ithin them, efficient spin transport, as a key process in the DOI: 10.1002/apxr.20230014710][11][12][13][14] In these conducting materials, spin is transported by the diffusion of the conduction electrons.Alternatively, spin can be carried by magnetic quasiparticles in insulating films, such as magnons or spin waves. [15]18][19][20] Thus, understanding spin transport in magnetic insulators has attracted lots of interests in spin community.
Experimentally, spin transport mediated by magnon has been demonstrated in insulating materials, such as ferrimagnets (like YIG), [10,11] antiferromagnets (like NiO, Cr 2 O 3 , -Fe 2 O 3 ), [21][22][23][24][25] and even paramagnets (like Gd 3 Ga 5 O 12 (GGG)). [26,27]Apparently, previous studies have focused on crystalline films and only a few reported are about spin transport in amorphous films which also furnish various magnetic orders. [28,29]Very recently, Wesenberg et al. presented spin transport in amorphous YIG over distances of many micrometers via nonlocal transport measurements. [30]After that, Gomez-Perez et al. showed a contrasting result and exhibited no evidence of long-distance spin transport in amorphous with a similar procedure, and the detected signal was attributed to leakage current. [31]Subsequently, Yang et al. performed the spin transport measurement based on the ferromagnetic resonance (FMR)induced spin pumping and inverse spin Hall effect (ISHE) with both lateral and vertical geometries.The experimental results manifested amorphous YIG does not possess long-distance spin transport and only an influence on the damping enhancement of the adhere layers.The authors claimed the leaked spin rectification voltage gave a dominant contribution to the detected signal rather than spin transport. [32]Therefore, a systematic investigation of pure spin current transport in the amorphous YIG films (as grown) and the recrystallization YIG films (abbreviated n-YIG and distinguished with ordered YIG films) without parasitic signals is still lacking.
In this study, to demonstrate an efficient spin transport in the structural evolution of YIG films prepared with the same ratio of Y, Fe, and O atoms as ordered YIG, we apply the FMR-induced spin pumping and ISHE method in a sandwich structure YIG/n-YIG/Pt.This method is widely used for the spin current generation and detection in ferromagnetic and non-magnetic multilayers.YIG film is chosen due to its high spin injection efficiency and insulating properties to avoid parasitically electrical signals.Pt acts as the spin detector due to its very strong spin-orbit interaction.The detected voltage presents a strong correlation with the thickness and the crystalline state of n-YIG film, respectively.An apparent signal can be still observed as the thickness of n-YIG is up to 33 nm for any crystalline state.As recrystallized n-YIG films with different annealing temperatures, the detected signal presents an increasement trend which is related to its magnetic order state formed gradually via post-annealed process.It is further confirmed by the distinct X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) spectra on the n-YIG films.Thus, an efficient spin transport in n-YIG films is obtained by reducing mostly the possible parasitic signal.

n-YIG Films Growth and its Properties
The n-YIG films were deposited on the crystalline YIG films on GGG substrate using radio frequency (RF) magnetron sputtering (more details in Experimental Section).To obtain the structural evolution of n-YIG films, a series of as-grown films are annealed with various annealing temperatures (T a = 200, 300, 400, 500, 600, 800 °C), respectively.Both the as-grown and annealing YIG films with T a = 800 °C presented a smooth surface as revealed by atomic force microscopy (Please refer to Supplementary S1, Supporting Information).Then we first characterized the crystal structures of the as-grown and annealed n-YIG films by using X-ray diffraction (XRD).The XRD data was shown in Figure 1a for a series of n-YIG films with varying T a .As the T a was lower than 600 °C, the absence of the diffraction peaks corresponding to crystal YIG indicated the amorphous properties of the n-YIG films.After n-YIG films were annealed at a higher temperature (T a = 600, 800 °C), the diffraction peaks corresponding to YIG (444) appeared which suggested n-YIG films were recrystallized, consistent with the previous reported. [33,34]In addition, to further investigate the microstructure of the n-YIG films, transmission electron microscopy (TEM) measurements were performed for n-YIG films with as-grown and annealed at 600 °C, respectively.These results are presented in Figure 1b,c.In Figure 1c, a clear halo pattern (left bottom panel) and a disorder image (right panel) are visible for as-grown n-YIG film which reflects its amorphous structure. [31,32,35]After annealing the n-YIG film with elevated temperature being 600 °C, a phase transition from amorphous-to-crystalline was observed where a sharp interface between YIG/GGG, and a visible diffraction pattern [35] was presented in Figure 1c.Besides, the HR-TEM images of the annealed YIG film with a higher T a = 800 °C are presented in Supplementary S2 (Supporting Information).The result was also consistent with the XRD data mentioned above.It suggests that as-grown YIG films can be completely recrystallized to form high-quality quasi-epitaxial films via post-annealing processing.Moreover, to investigate systematically the magnetic properties of the reconstructed YIG films, we performed the angle-dependent FMR measurements for the annealing YIG film with T a = 600 °C (as presented in Supplementary S3, Supporting Information) and summarized the previously reported (Table S1, Supporting Information).A relatively weak magnetic anisotropy was obtained and the magnetic parameters were comparable with the reported.

Spin Pumping Measurement in As-Grown n-YIG Films
To investigate the pure spin transport in as-grown n-YIG films, we performed the FMR-induced spin pumping and ISHE measurement in YIG/n-YIG(as-grown with varying thickness (t a-YIG ))/Pt tri-layers.[38][39] Pt is commonly chosen as the spin detector due to its strong spin-orbit coupling.Figure 2a presents a schematic of the experimental setup for the FMRdriven spin pumping and ISHE measurement.Under microwave excitation, the magnetization of the YIG film processes and a pure spin current (J S ) is accumulated at the interfaces between YIG and as-grown n-YIG films.Then it can be diffused across the as-grown n-YIG films at a limited range of t a-YIG and approaches the interfaces between as-grown n-YIG films and Pt.The decayed spin current is injected into the Pt layer and then converts into an electrically detectable charge current (J C ) via the ISHE through J C = (2e/ℏ)  SH J S × , where  denotes the spin polarization direction of J S and is parallel to the external magnetic field, and ℏ is the reduced Planck constant,  SH is the spin Hall angle. [3]A positive spin pumping-induced ISHE voltage V ISHE is obtained in YIG/n-YIG(t a-YIG = 6, 10, 13, 18, 23, 30, 33 nm)/Pt(5 nm), exhibiting a symmetrical Lorentzian lineshape when the external magnetic field H ext is along the x-axis (+H ext ) as shown in Figure 2b.Reversing the field direction changes the sign of the detected voltage, which indicates its pure spin current origin (Figure S4b, Supporting Information). [34,40]Apparently, a noticeable decrease in the V ISHE values is observed as increasing the thickness of the inserted layers (as grow n-YIG films).Figure 2c summarizes the t a-YIG -dependent spin pumping-induced V ISHE (H r ) (the detected voltage obtained at the resonance field H r ).A significant value can be obtained when t n-YIG < 15 nm, a relatively weakened signal for t n-YIG > 15 nm as shown in the insets.The decay behavior for the detected voltage which reflects pure spin current is diffused due to spin accumulation at the interface between as-grown n-YIG and ordered YIG film.Besides, to distinguish the contribution of the detected voltage signals from the inserted and annealed n-YIG films and the other parasitic signal, we detected the ISHE voltage signal by sweeping a wide range of magnetic fields and replaced the inserted n-YIG films with GGG films (please refer to the Supplementary S5, Supporting Information).A negligible signal was obtained which suggested the detected signal originated from the spin transportation across the inserted n-YIG films.

Pure Spin Transport in n-YIG Films with Phase Transition
To explore the effect of n-YIG crystallization state on pure spin transport, we perform the FMR-driven spin pumping and ISHE measurement on a series of YIG/n-YIG/Pt tri-layers.Six samples of as-grown n-YIG films are deposited onto the crystallized YIG films on a GGG substrate with n-YIG film thickness of 6 nm.Then the YIG/n-YIG (6 nm) films are ex-situ annealed in a tube furnace at 200, 300, 400, 500, and 600 °C in air atmosphere, respectively.By post-annealing procedure, these n-YIG films experience a recrystallization process as presented in previous XRD and TEM measurements.Then Pt layer is capped on YIG/n-YIG bilayers for FMR-driven spin pumping and ISHE measurements.The induced ISHE voltage is detectable in the whole YIG/n-YIG (6 nm)/Pt samples where n-YIG is annealed at varying T a as shown in Figure 3a.As T a ≤ 500 °C where the n-YIG films correspond to the non-crystalline state, the value of the detected signal is approximately equal.A distinguish on the FMR linewidth (ΔH) is obtained between YIG/n-YIG (as grown)/Pt and YIG/n-YIG(T a = 200, 300, 400, 500 °C)/Pt.As T a = 600 °C which the n-YIG film corresponds to the recrystallized state, both values of V ISHE and ΔH obtain a significant increasement which suggests a higher efficiency spin transport in the recrystallized n-YIG films. [36]Herein, the ΔH is a vital parameter that is widely used to obtain the damping factor () of the specified materials [38] as shown in support information (Figure S6a-d, Supporting Information).And the  contains information on the absorption of spin current at the interfaces. [3,34]Therefore, an increasement in its value is accompanied by the T a which reflects inversely a strong correlation between pure spin transport and the crystalline state of the inserted n-YIG.Moreover, to investigate the thickness dependence of the n-YIG films with varying crystalline states, we prepare the sample with the same procedure as described above.By fabricating a series of samples with a fixed thickness of the inserted n-YIG layer (t n-YIG = 6, 10, 13, 18, 23, 28, 33 nm), we anneal these samples via post-annealing process (T a = 200, 300, 400, 500, 600 °C) for each of fixed thickness.Then we perform the spin pumping measurement.A visible voltage signal is still observed for the whole sample.However, the spin-pumpinginduced V ISHE (H r ) shows a significant difference in the T a .For the non-crystalline region (T a ≤ 500 °C), a relatively weak signal can be obtained for various t n-YIG .For the crystalline region (T a > 600 °C), a giant enhancement of the V ISHE (H r ) is observed regardless of the varying t n-YIG .These results demonstrate the crystalline state of the inserted n-YIG layer which plays a vital role in pure spin propagation.

X-Ray Absorption Spectroscopy Measurements
In principle, the propagation of pure spin current is correlated to the magnetic order in insulating films. [17,18]Thus, it is possible to clarify the correlation between pure spin transport and the crystalline state of the inserted n-YIG by using XAS/XMCD spectra.Figure 4a presents the element-specific XAS spectra for GGG/n-YIG films (as grown and T a = 200, 300, 400, 500 °C, from bottom to top panel) which are performed at Fe_L 3 , 2 edges by total electron yield (TEY) mode.Inset shows a sketch of experimental setup for XAS performance measured by TEY mode.The XAS signal is detected at normal incidence and a magnetic field of 0.5 T is applied along the direction of X-rays.The thicker and more colored lines correspond to the positive helicity (+).The thinner and white lines present the negative helicity (−).These experimental curves correspond to the + and - are coincide perfectly with each other for the n-YIG films (as grown and T a = 200, 300, 400, 500 °C).This result indicates an almost zero XMCD signal and then no magnetization in it. [41,42]Besides, it is further confirmed by the M-H loop of n-YIG films with varying T a obtained (Figure S7a, Supporting Information).As T a ≤ 500 °C, no visible hysteresis is observed for these n-YIG films which indicates zero magnetization in it.In contrast, as T a = 600 and 800 °C, an apparent M-H loop and very small coercivity (<2 Oe) is obtained which reflects the intrinsic character of ferrimagnetic materials (like well-defined crystalline YIG) [35] (Figure S7b in Supporting Information).Moreover, a significantly distinct XAS signal between the + and − helicity are obtained for n-YIG films with a relatively higher T a (= 580, 600, 650, and 800 °C) as presented in Figure 4b.As shown in the top panel of Figure 4b (T a = 580 °C), at the L 3 edge, the + helicity is in the edge shoulder (red line).Inversely, the − helicity occurs in the edge shoulder at the L 2 edge (black line).To isolate the XMCD ((+)-(−)) signal from the XAS, the difference value of the collected data is obtained as presented with the blue line.An opposite peak is observed at the L 3 and L 2 edge which reflects its owning magnetism in YIG film.Besides, the XMCD spectrum at the L 3 edge presents two negative peaks and one largely positive peak.In principle, two negative peaks are caused by Fe ions occupied at octahedral a sites, and one positive peak is caused by Fe ions for tetrahedral d sites as labeled. [43]A similar behavior is exhibited for the other n-YIG films (T a = 600, 650, 800 °C).Thus, using a combination XAS/XMCD spectra with XRD/TEM and M-H loop analysis, the n-YIG films annealed with varying T a have experienced a phase transformation.As T a is below 580 °C, it corresponds to an amorphous state and negligible magnetization for n-YIG films.As T a is close to and even higher than this value, the annealed n-YIG films are recrystallized and own magnetic properties.It is consistent with t n-YIG -and T a -dependent spin pumping-induced ISHE voltage signal.A high-efficiency spin transport occurs in n-YIG films (non-crystalline) at a limited t n-YIG .A giant enhancement of the detected signal is obtained in recrystallized n-YIG films at an elevated T a .

Conclusion
In conclusion, utilizing the FMR-driven spin pumping measurements, we investigated the pure spin current transport across the inserted n-YIG films with varying thicknesses and phase transformation occurring, respectively.For the amorphous n-YIG film intercalation, a detectable ISHE signal can be observed as t n-YIG is up to 33 nm.Once the n-YIG film is recrystallized completely, a giant enhancement of the detected voltage is obtained and independent of t n-YIG .A distinguished XMCD spectra is observed for recrystallized n-YIG films which is the main character of magnetic order by applying XAS/XMCD measurement.It reflects the propagation of spin current is correlated strongly to magnetic order.Our observations unambiguously demonstrate n-YIG films with different crystallization states serve as a promising material for highly efficient spin transport and provide a framework for modulation of spin transport via phase transition method.

Figure 1 .
Figure 1.a) −2 XRD scan of as grown n-YIG films and with various annealing temperature (T a = 200, 300, 400, 500, 600, 800 °C).A cross-sectional high-resolution transmission electron microscopy (HR-TEM) image of the as-grown n-YIG film b) and the annealing n-YIG film with T a = 600 °C c), respectively.Low-magnification image (at the top-left corner) and HR-TEM images (right panel).The FFT patterns from the chosen regions of the right panel are presented at the bottom-left corner.

Figure 2 .
Figure 2. a) The schematic illustration of the experimental setup for FMR-driven spin pumping and ISHE voltage measurement.b) The detected ISHE signal V ISHE as a function of the external DC magnetic field (H ext ) for the inserted YIG layers with various thicknesses (t n-YIG = 6, 10, 13, 18, 23, 28, 33, 40 nm) and without annealing process (as grown).The microwave frequency is fixed at 2 GHz.c) The amplitude of the V ISHE (H r ) (obtained at the resonance field H r ) as a function of the thickness t n-YIG.The inset presents the amplified display for the t n-YIG being 18 to 40 nm.

Figure 3 .
Figure 3. a) The detected ISHE signal V ISHE as a function of the external DC magnetic field (H ext ) for the YIG/n-YIG/Pt samples.The thickness of the inserted n-YIG layers is fixed at 6 nm and the T a is set to 200, 300, 400, 500, and 600 °C.b) The amplitude of the V ISHE (H r ) (obtained at the resonance field H r ) as a function of the T a for the YIG/n-YIG/Pt samples.The thickness of the inserted a-YIG films is varied with being 6, 10, 13, 18, 23, 28, and 33 nm.

Figure 4 .
Figure 4. a) Experimental XAS spectra of Fe element L 3 (L 2 ) edge are taken in TEY mode for the n-YIG samples.The n-YIG films are annealed with various T a (= 20, 200, 300, 400, and 500 °C).The thicker and more colored lines correspond to the positive helicity (+).The white lines present the negative helicity (−).The inset shows a sketch of the TEY mode.b) Experimental XAS and XMCD spectra of Fe element for the n-YIG film samples.The YIG films are annealed at T a (= 580 °C, 600 °C, 650 °C, 800 °C).The red and black lines show the positive helicity (+) and the negative helicity (−), respectively.The iridescent lines present a different spectrum ((+)-(−)).The inset presents the amplified display for the Fe_L 2 edge at T a = 580 °C.Data are offset for clarity.All measurements are performed at Room temperature.