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[He_Intermag digest_2023_final.pdf](https://mdr.nims.go.jp/filesets/eac1d58c-17ac-46c8-8cdb-0c775469f0cd/download)

## Creator

ハ ツォン, ソン ジェユアン, [温 振超](https://orcid.org/0000-0001-7496-1339), 介川 裕章, [三浦 良雄](https://orcid.org/0000-0002-5605-5452), [大久保 忠勝](https://orcid.org/0000-0003-3548-1951), 能崎 幸雄, 三谷 誠司

## Rights

[Creative Commons BY Attribution 4.0 International](https://creativecommons.org/licenses/by/4.0/)

## Other metadata

[Nano artificial alloys in bulk-immiscible Ru-Cu system for spin-current generation](https://mdr.nims.go.jp/datasets/cae16106-dc36-4a68-b68f-e90c91b2675e)

## Fulltext

Nano artificial alloys in bulk-immiscible Ru-Cu system for  spin-current generation  Cong He1, Jieyuan Song1,2, Zhenchao Wen1, Hiroaki Sukegawa1, Yoshio Miura1, Tadakatsu Ohkubo1, Yukio Nozaki3, and Seiji Mitani1,2  1 National Institute for Materials Science (NIMS), Tsukuba 305-0047, Japan, HE.Cong@nims.go.jp 2Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8577, Japan  3Department of Physics, Keio University, Yokohama 223-8522, Japan  Spin current is a flow of spin angular momentum and is considered as an ideal information carrier for spintronic devices due to the merit of low power consumption. However, an efficient conversion between charge and spin current usually depends on the strong spin-orbit interaction (SOI) in the heavy 5d metals such as Pt and Ta. Here we report a remarkable enhancement in the spin transport property in a negligible SOI system of Cu50Ru50. Although Cu-Ru system is bulk-immiscible, the Cu50Ru50 film, prepared by the alternate monoatomic layer deposition method, is a nano artificial alloy and the atomic-scale intermixing has been directly confirmed by atomic-resolution energy-dispersive X-ray spectroscopy. Compared with pure Cu and Ru metals, we find 100 times enhancement in unidirectional spin Hall magnetoresistance detected in this nano artificial alloy, which is comparable to values in Pt and Ta. Our findings are expected to be applicable to other bulk-immiscible metallic systems with a weak SOI, which paves a way for designing novel alloy films with desirable properties and functionalities.   Index Terms— Immiscible Cu-Ru, spin current, thin film, USMR effect.  I. INTRODUCTION HARGE to spin current interconversion is a critical element for nanoscale spintronic devices such as a spin-orbit torque magnetic random-access memory (SOT-MRAM). How to efficiently generate a large spin current has received considerable attention in recent years. It has been reported that heavy 5d metals, such as Pt and Ta, are commonly utilized to enhance the charge-to-spin conversion due to their strong spin-orbit interaction (SOI) [1]-[2]. Materials that consist of light metals generally have a very weak SOI and are not considered as efficient spin current generators, except for the case of surface-oxidized copper film [3]-[4]. Therefore, investigations on efficient spin current generation using low-cost and weak SOI elements will be of great significance for potential spintronic applications. In addition, previous studies [5]-[7] suggested that the spin Hall effect could be significantly improved by alloying 5d elements such as Ir in Cu1-xPtx and Pt in Cu1-xPtx, and the maximum value was achieved for x ≈ 0.5 [6]-[7], which is also the reason why we adopted the compositional concentration of 50% in our study. In this digest, we report a remarkable enhancement in the spin transport property in the negligible SOI system of Cu50Ru50 film. Our atomic-resolution scanning transmission electron microscopy (STEM) observations indicate that the Cu50Ru50 film is a homogeneous nano artificial alloy although the Cu-Ru system is bulk-immiscible in the equilibrium state. The unidirectional spin Hall magnetoresistance (USMR) effect in this nano alloy is two orders of magnitude higher than pure Cu and pure Ru samples. II. EXPERIMENTAL METHODS Thin films were prepared by the sputtering method on single-crystal (0001) sapphire substrates. The substrate was annealed at a high temperature of ~1000 °C for 1 hour in a muffle furnace before the deposition process. A stack of a nano artificial alloy film is shown in Fig. 1a. Cu, Ru, (Ru + Cu) and ferromagnetic Ni81Fe19 (NiFe) layers were all deposited at 20 °C and followed by a post-annealing process at 300 °C for 20 min. The alternating monoatomic layer deposition (AMLD) method [8] was applied here to prepare (Ru + Cu)-mixed layer. The Ru/Cu alternating monolayer deposition repeated for 25 times. 2-nm MgO was deposited to protect functional layers of the film.   Microstructure of the films was investigated using X-ray diffraction (XRD) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). STEM samples were fabricated by the focused ion beam (FIB) milling technique on a dual-beam FEI Helios G4 UX microscope. HAADF-STEM observations, energy-dispersive X-ray spectroscopy (EDS) mapping and nano beam electron diffraction (NBED) were conducted using a Cs-corrected FEI Titan G2 80–200 ChemiSTEM with a Super X-ray EDS detector. Atomic-resolution EDS maps were collected and drift corrected in Bruker Esprit analysis system.  A standard DC 4-probe method was used to measure the resistivity of the films. Films were patterned into Hall bars to detect the USMR effect. USMR measurements were performed by injecting an AC current at 137 Hz in the longitudinal direction of Hall bar. Lock-in amplifiers were used to record the first and second-harmonic signal of longitudinal resistance. An external magnetic field (H) was applied along the transverse direction (y-axis) of the Hall bar (see the inset in Fig. 1b). III. RESULTS AND DISCUSSION The Cu50Ru50/NiFe/MgO film was sputtered on the (0001) plane of sapphire substrate according to the AMLD preparation method where the thickness of the individual layer is ~0.2 nm in the present study, which is nearly same as the interplanar distance of close-packed fcc-Cu(111) and hcp-Ru(0002) planes. Sapphire (0001) substrates are used here to selectively deposit C Cu(111) and Ru(0002) atomic planes due to their small lattice mismatch (–1.7% between Ru(0002) and sapphire(0002),  and –5.4% between Ru(0002) and Cu(111)). The low-magnification HAADF-STEM image in Fig. 1a shows the experimentally obtained microstructure. Interestingly, the image contrast in the (Ru + Cu) mixed layer with a total thickness of ~10 nm seems rather homogeneous and is brighter than that of the NiFe layer. Elemental distribution maps of Cu and Ru in Fig. 1a also reveal that Cu and Ru atoms are uniformly distributed in the (Ru + Cu) mixed layer, which indicates that significant intermixing occurs. An enlarged HAADF image of the (Ru + Cu) mixed layer suggests that the mixed layer has an fcc structure, with a viewing direction of [110]fcc. The fcc lattice parameter is measured to be a = 3.730Å according to the drift-corrected atomic HAADF images taken along different zone axes. It is noted that the brightness of each atomic column in the atomic HAADF-STEM image keeps nearly the same, indicating that Cu and Ru atoms are nearly fully mixed rather     Fig. 1. (a) Microstructural and compositional analysis the nano artificial Cu50Ru50 alloy: low magnification HAADF-STEM image and related EDS maps. (b) USMR measurements of Cu50Ru50/NiFe sample showing the second harmonical longitudinal Hall resistance Rxx2ω  as a function of the external in-plane magnetic field along the y axis (Hy) at Jc = 1×107 A/cm2. The inset image is a schematic diagram of Rxx2ω measurement. than forming the predicted multilayered structure, since the contrast of the HAADF image is nearly in direct proportion to the square of an atomic number [9]. Our quantified EDS elemental line profiles across the interfaces further show that the composition ratio between Ru and Cu atoms in the nano artificial alloy is close to 1:1, which is consistent with the designed value according to the preparation method. Atomic-resolution EDS maps of Cu and Ru are also obtained and clearly demonstrate that each atomic column in the typical fcc-structured stacking sequence of ABCABC is both Cu-rich and Ru-rich, indicating the formation of an fcc random solid solution. Therefore, we conclude that, although the Cu-Ru system is bulk-immiscible in the equilibrium state and such a RuCu nanoalloy should never exist due to the positive mixing enthalpy between Cu and Ru elements, a nano artificial Cu50Ru50 alloy is successfully prepared using our AMLD method. The origin of the formation of the nano artificial alloy is inferred to be related to the strain at each monoatomic layer during deposition. We next used the USMR measurement [3], [10] to detect the charge-spin conversion effect in this nano artificial alloy. We also prepare Cu (10 nm)/NiFe and Ru (10 nm)/NiFe samples as references. Figure 1b shows the USMR signal detected in the Cu50Ru50/NiFe sample at a charge current density Jc of 1×107 A/cm2. The USMR signal Rxx(USMR)2ω   is obtained after subtracting a Joule heating effect  Rxx(∇T)2ω   from the saturated second harmonical longitudinal Hall resistance Rxx2ω , i.e.,  Rxx(USMR)2ω   = Rxx2ω  – Rxx(∇T)2ω   [3], [10], and calculated to be 0.002 mΩ, 0.008 mΩ and 1.010 mΩ for Cu/NiFe, Ru/NiFe, and Cu50Ru50/NiFe samples, respectively. It is interesting to find that the USMR signal detected in Cu50Ru50/NiFe is two orders of magnitude larger than that in Cu/NiFe or Ru/NiFe sample, which is even comparable to the value measured in strong SOI element-containing films such as Pt/Co and Ta/Co. A similar phenomenon may occur in other bulk-immiscible metallic films with a weak SOI, which might lead to novel design strategies for future spintronic devices. ACKNOWLEDGEMENTS This work was partially supported by JST CREST (Grant No. JPMJCR19J4), and the JSPS KAKENHI Grant No. 20K04569. REFERENCES [1] L. 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Shima et al., “Low-temperature fabrication of L10 ordered FePt alloy by alternate monatomic layer deposition,” Appl. Phys. Lett., vol. 80, pp. 288-290, Jan. 2002. [9] C. He et al., “Unusual solute segregation phenomenon in coherent twin boundaries,” Nat. Commun., vol. 12, pp. 722, Feb. 2021. [10] C. O. Avci et al., “Unidirectional spin Hall magnetoresistance in ferromagnet/normal metal bilayers,” Nat. Phys., vol. 11, pp. 570-575, July 2015.