Enhancement of Acoustic Spin Pumping by Acoustic Distributed Bragg Reflector Cavity

Abstract

Surface acoustic waves (SAWs) in the GHz frequency range passing through a ferromagnetic layer can excite magnon–phonon dynamics, i.e., a precessional magnetization motion mediated by the magnetoelastic effect [1-3]. This process is known as acoustic ferromagnetic resonance (A-FMR) [4-7]. The A-FMR driven by SAWs can generate spin currents diffusing into adjacent non-magnetic metal layers via the spin pumping effect [8]. This coupled magnon–phonon dynamics can thus be used as a spin current generation method [9] named as acoustic spin pumping (ASP) [10,11]. The generated spin currents can be detected by the inverse spin Hall effect (ISHE) [9,12], or the inverse Edelstein effect (IEE) [6,13].In SAW driven A-FMR devices, interdigital transducers (IDT) are used for the generation and detection of SAWs [2–7,9,14]. Since theSAWs propagate on both sides of the IDT, at least half of the phonon energy is lost. To reduce the loss and enhance the spin current generation via ASP, we employ acoustic cavity structures. An acoustic cavity (resonator) consists of a pair of acoustic wave reflector gratings, analogous to the distributed Bragg reflector for light [15]. In this presentation, we demonstrate the enhancement of A-FMR and ASP in the presence of acoustic cavities.By applying an external magnetic field, SAWs passing a Ni film induce A-FMR. When the A-FMR occurs, SAW power is attenuated due to the energy conservation, thus the SAW power absorption PSAW is proportional to the induced A-FMR intensity. The measured SAW power absorption in Fig.1 shows the A-FMR signal fitted with a Lorentzian curve. We confirmed an enhancement of 2.04 ± 0.02 times A-FMR on the sample with acoustic cavity.The A-FMR in the Ni layer generates spin current into the Cu layer by the ASP. The generated spin current is converted to a charge current at the interface between Cu and Bi2O3 via IEE. We detect the generated electric voltage via IEE at the maximum A-FMR field angles φ = 45° and 225°, as shown in Fig.2. We define ΔVASP as the amplitude of the symmetric Lorentzian fitting of DV. The detected electric voltage from the ASP is caused by the charge current, derived from an electric field E induced by the IEE, which is proportional to the flow direction of the spin current density Js and its spin polarization σ; E ∝ Js×σ [16]. The converted charge current density Jc is described as Jc = λIEEJs, where λIEE is the IEE length [17]. Since the Ni/Cu/Bi2O3 trilayers of all samples are fabricated at the same time and the applied external magnetic field angle is the same, thus, ΔVASP ∝ Js. The enhancement factor of ΔVASP is 2.96 ± 0.02 at the low-power range, and 1.6 ± 0.7 at the high-power range. However, the power dependence of ΔVASP has non-linear behavior in the range from 25 mW to 126 mW. This non-linearity in the power dependence is not fully understood. Since this behavior is similar to the case of FMR experiments at the high input radio-frequency (rf) power [21,22] we assume it is from the saturation of magnetic precessional cone angle [23] or multi-magnon scattering [24,25]. We observe a more significant enhancement factor of ASP in the low-power range than the enhancement of A-FMR. In contrast, we find a similar enhancement factor of ASP in the high-power range with that enhancement of A-FMR. The enhancement factor in the high-power range is well described by the multi-magnon scattering in Ref. [26]. However, as far as the author knowledge goes the origin of the higher enhancement factor in the low-power range has not been observed and further understanding is required.In summary, we have demonstrated the enhancement of A-FMRand the spin current generation by using acoustic cavities. Enhancement of 2.04 ± 0.02 times of A-FMR and enhancements of spin current generation from 1.6 ± 0.7 (at a high input rf power) to 2.96 ± 0.02 (at a low input rf power) times were achieved. All the measurements in the present study were carried out at room temperature. At lower temperatures, the SAW confinement can be strengthened by minimizing the interaction with thermal phonons. Minimization of phonon energy losses by further engineering of the acoustic cavities, as well as minimization of magnon energy losses by appropriate selection of materials may lead to magnon–phonon studies in the strong coupling regime. **