PHYSICAL REVIEW B 102, 174413 (2020) Highly nonlinear frequency-dependent spin-wave resonance excited via spin-vorticity coupling Yuki Kurimune, 1 Mamoru Matsuo , 2, 3, 4, 5 Sadamichi Maekawa, 4, 2 and Yukio Nozaki 1, 6 , * 1 Department of Physics, Keio University, Yokohama 223-8522, Japan 2 Kavli Institute for Theoretical Sciences, University of Chinese Academy of Sciences, No. 3, Nanyitiao, Zhongguancun, Haidian District, Beijing 100190, China 3 CAS Center for Excellence in Topological Quantum Computation, University of Chinese Academy of Sciences, Beijing 100190, China 4 RIKEN Center for Emergent Matter Science (CEMS), Wako, Saitama 351-0198, Japan 5 Advanced Science Research Center, Japan Atomic Energy Agency, Tokai 319-1195, Japan 6 Center for Spintronics Research Network, Keio University, Yokohama 223-8522, Japan (Received 24 February 2020; revised 30 September 2020; accepted 14 October 2020; published 9 November 2020) A nonuniform vorticity of lattice deformation in a surface acoustic wave (SAW) can generate a spin current (SC) in nonmagnetic metals via spin-vorticity coupling (SVC). We demonstrated a strong enhancement of SVC- derived SC generated in Cu and Pt films with increasing the frequency of the SAW by observing the spin-wave resonance (SWR) in an adjacent NiFe film. The comparative amplitudes and high-order frequency variations of SWR in NiFe/Cu and NiFe/Pt bilayers imply that the amplitude of the SC generated via SVC in a SAW is robust against the strength of spin-orbit interaction in nonmagnetic metals. DOI: 10.1103/PhysRevB.102.174413 I. INTRODUCTION A nonconservative flow of spin angular momentum, i.e., spin current (SC), is necessary to control the magnetization of nanoscaled ferromagnets in spintronic devices such as mag- netic random access memory [1–5] and spin auto-oscillators [6–8]. A variety of methods to produce a SC and the conse- quent torque on magnetization have been developed, e.g., the spin Seebeck effect [9], nonlocal spin injection from ferro- magnetic conductors [10], the spin pumping effect [11–14], the spin Hall effect (SHE) [15–17], and the Rashba-Edelstein effect [18,19]. All of these methods utilize ferromagnetic materials and/or nonmagnetic heavy metals with strong spin- orbit interaction (SOI), which plays an important role in SC generation via SHE. The necessity of particular materials to produce a SC reduces a degree of freedom in the material choice of spintronic devices. Recently, an alternative method to produce a SC from a macroscopic rotational motion was theoretically proposed [20] followed by experimental demonstrations using a tur- bulent flow of liquid metal [21], a surface acoustic wave (SAW) in a Cu film [22], and a gradient in the electrical mobility in a surface-oxidized Cu film [23]. A nonuniform vorticity of the velocity field in the macroscopic rotation leads to a nonuniform spin accumulation via spin-vorticity cou- pling (SVC) [24–28], which enables a universal conversion between macroscopic and microscopic spin angular momen- tum according to an angular momentum conservation law. Kobayashi et al . demonstrated a spin-wave resonance (SWR) by injecting a SAW into a NiFe/Cu bilayer deposited on a LiNbO 3 substrate. The schematic principle of the SAW- * Corresponding author: nozaki@phys.keio.ac.jp driving SWR is shown in Fig. 1(a). The magnitude of vorticity of the lattice deformation in the SAW exponentially decays in the depth direction. The gradient of vorticity in the Cu layer leads to a gradient of spin accumulation via SVC which generates an alternating SC. When the SC is injected to the NiFe fabricated adjacent to Cu, a spin wave whose wave number is consistent with that of the SAW can be resonantly excited. In this article, we experimentally demonstrated a strong enhancement of SC generation with increasing the frequency of SAW by measuring the SWR in a NiFe film attached to a Cu, Ti (weak SOI), or Pt (strong SOI) film as a SC-generating material. For comparison, the frequency dependencies of the SWR caused by the Barnett [29–32] and the magnetoelastic (ME) [33,34] effects in ferromagnetic metals (FMs) were also measured using NiFe and Ni single films, respectively, because these effects also affected the frequency dependencies of SWR in NiFe/nonmagnetic metal (NM) bilayers. We found that the SVC can generate the SC in the Cu comparable to the Pt although the strength of SOI is totally different. Moreover, from the order of the nonlinear frequency dependence of SC generation, we determined the primary contribution of SVC in the SC generation using SAW. As described in Sec. II, the theory predicts that the SVC can create spin accumulation in conductive materials in two different processes. These find- ings are significant to understand the microscopic mechanism of the SVC and the material dependence. The reminder of this paper is organized as follows. Section II briefly summarizes the theory for SC generation via SVC together with the frequency-dependent expression of the SC which plays a significant role for quantitative under- standing of the SVC-related SC generation. Sections III and IV describe the experimental setup and the measured SWR excited in the NiFe attached to nonmagnetic materials by applying the Rayleigh-type SAW (R-SAW), respectively. The 2469-9950/2020/102(17)/174413(7) 174413-1 ©2020 American Physical Society