Spin Waves and Spin Pumping Driven by Cavity Confined Bulk Hypersonic Waves

Abstract

In recent years, the magnon-phonon coupling in magnetostrictive materials has gained renewed interest due to emerging spintronic and magnonic applications, such as low power consumption microwave devices. Linear and parametric acoustically driven spin waves (ADSW) were studied in various hybrid magnon-phonon structures containing piezoelectric and ferro (ferri) magnetic layers [1-4]. Previously [5-7], we have demonstrated the piezoelectric excitation of linear and parametric ADSW in the gigahertz frequency range and their detection by the combination of spin pumping and inverse spin Hall effect (ISHE) in the spintronic hybrid High overtone Bulk Acoustic wave Resonator (HBAR) with the structure ZnO – YIG– GGG–YIG – Pt (Fig.1a).Here, the features of phonon-magnon interconversion in the phononic cavity mentioned above are studied in detail. The pump frequency fp , power P, and magnetic field H dependences of the ISHE voltage UISHE as well as resonance frequencies fn are experimentally and theoretically studied. As a result of magnetoelastic interaction, bulk acoustic waves (BAW) cavity modes excite magnons in the YIG films either at frequencies fn or at half resonance frequencies (when the threshold power is exceeded). When specifying a certain frequency range, depending on the value of the applied magnetic field, either direct excitation of linear ADSWs or parametric excitation (in low magnetic fields) is observed.In the first case the important features of ADSW and acoustic spin pumping are determined by double resonance: the magnetoelastic (MER) in YIG films and BAW resonance of the entire resonator structure. Electrical detection of ADSWs occurs while simultaneously measuring the electrical response of the ZnO transducer and the dc voltage signal on the Pt stripe. The dependences of the resonance frequencies fn and UISHE on the field H and the frequency f are correlated with each other (Fig. 1b). The fitting of the theoretical and experimental dependences allows us to determine a number of magnetic and magnetoelastic parameters of YIG and field dependencies of the resonance frequencies: magnetoelastic fMER (H) and ferromagnetic fFMR(H). For the frequency range around 2.4 GHz, the magnetic field corresponding to the ferromagnetic resonance is HFMR ≈ 534Oe.In the second case when the field decreases below HFMR, the HBAR spectrum fn is practically independent of the magnetic field, but the clearly detectable voltage UISHE (f, H) is significantly field dependent. The signal from the parametric ADSW was observed when: (i) the pump frequency coincided with one of the HBAR resonant frequencies, (ii) half of the pump frequency coincided with one of the spin wave frequencies for a given magnetic field (see Fig.2a), and (iii) the pump power exceeded a threshold, depending on the field (see Fig 2b). It can be seen from Fig. 2 the field of the maximum voltage is close to the critical field Hc = 184 Oe, at which fFMR(Hc) = fp/2. It indicates that UISHE(Hc) originates from the detection of parametric ADSW with wave number q = 0. The voltage signal is observed in the field range below 432 Oe. This field correlates with the value Hc1 = fp /(2γ) in Fig. 2a. Above this field, the excitation of any parametric spin wave is impossible; therefore, the voltage signal is suppressed. But at the higher fields, the linear excitation of ADSW takes place at the HFMR, at which fFMR(HFMR) = fp. In Fig. 2, the field Hc2= Hc1/2=fp/(4γ) corresponds to the upper limit on H for the possible decays of parametric magnons with the frequency fp/ 2 into two secondary parametric ones at a frequency fp/4. Measurements with a smaller field step show that, in some field ranges, the UISHE (H) dependence has additional features, which can be explained by parametric excitation of various groups of magnons. For example, in a field near Hc3 = 100 Oe, the nonmonotonic behavior of the UISHE (H) dependence is apparently due to the magnon confluence process, for which the condition fFMR (Hc3) = fp /3 is satisfied [8].For certain ranges of fields and powers, the data of Fig. 2b can be fitted as UISHE (P, H) ~ (P-Pth (H))1/2, where the fitting parameter Pth (H) ≈ 0.4 mW can be interpreted as a threshold power. We explain the low threshold obtained by the high efficiency of electric power transmission into the acoustic pump confined in the phononic cavity - high-Q hybrid HBAR.This work was carried out in the framework of the State task 0030-2019-0013 "Spintronics" and with partial support of the RFBR (Project No. 20-07-01075). **