Direct Imaging of Resonant Phonon-Magnon Coupling

Detection of phonons is critical for research on the interconversion between resonantly coupled magnons and phonons. Here we report the direct visualization of the resonant coupling of magnons and phonons by microfocused Brillouin light scattering on the $\mathrm{Ni}/{\mathrm{Li}\mathrm{Nb}\mathrm{O}}_3$ hybrid heterostructure. The static patterns of surface acoustic wave phonons, originating from the interference between the incident wave ${\psi }_0(A_0,\mathbf{k},{\phi }_0)$ and reflected wave ${\psi }_1(A_1,-\mathbf{k},{\phi }_1)$, can be modulated by magnetic field due to the magnon-phonon coupling. By analyzing the information of phonons obtained from Brillouin spectroscopy, the properties of the magnon system (Ni film), e.g., ferromagnetic resonance field and resonance linewidth, can be determined. The results provide spatially resolved information about phonon manipulation and detection in a coupled magnon-phonon system.

Figure 1

(a) Optical image of the $\mathrm{Ni}/{\mathrm{Li}\mathrm{Nb}\mathrm{O}}_3$ hybrid device with a 50-nm-thick $\mathrm{Ni}$ rectangular film. The green cone represents the BLS laser beam, and the red dot represents the position of the laser spot. SAW ψ₀ is excited using the left-hand IDT and the counter-propagating ψ₁ is generated via reflection from the right-hand unconnected IDT. A dc magnetic field H is applied at an angle θ = 30° with respect to the wave vector k_SAW of ψ₀. (b) Magnetic field H dependence of the BLS signal when the laser spot is located on one of the IDT fingers. (c) Optical image of the IDT fingers. The green rectangular region represents the area for the spatially resolved BLS measurements. The reference image is used for active stabilization of the position of the sample with respect to the laser focus. The magnetic field H and the wave vector k_SAW are along the y′ and x directions in the depicted coordinate systems, respectively. θ is fixed at θ = 30°. (d) Spatially resolved image of SAW phonons under a dc magnetic field H = 600 Oe.

Figure 2

Magnetic field H dependence of the BLS intensity when the laser spot is located (a) on the ${\mathrm{Li}\mathrm{Nb}\mathrm{O}}_3$ substrate and (b) on the $\mathrm{Ni}$ film. (c),(d) Frequency distribution for (a) and (b), respectively. (e),(f) Magnetic field H dependence of the averaged BLS intensity for (a) and (b), respectively. The magnetic field H and wave vector k_SAW are along y′ and x directions in the coordinate system, respectively, and θ is fixed at θ = 30°, as shown in Fig. 1.

Figure 3

Magnetic field H evolution of BLS spatially resolved images when the laser spot is located on the $\mathrm{Ni}$ film: (a) 600 Oe; (b) 510 Oe; (c) 370 Oe; (d) 310 Oe; (e) 200 Oe; (f) 160 Oe; (g) 90 Oe; (h) 45 Oe; (i) 0 Oe. The magnetic field H and wave vector k_SAW are along the y′ and x directions in the depicted coordinate systems, respectively. θ is fixed at θ = 30°.

Figure 4

(a) BLS intensity evolution of Fig. 3 at H = 200 Oe along the x′ direction. The solid line is a fit to Eq. (1). (b) H dependence of I_max/I_min in Fig. 3. The solid line is a linear fit. (c) Averaged spatial resolution image at different H in Fig. 3. (d) Field H dependence of the normalized BLS intensity for the two measurements. The symbols are experimental data, and the solid line is a fit to theory using Eq. (2).