Optical probing of magnons and phonons in Ni80Fe20 nanodot arrays

Abstract

Control of collective spin excitations by static or dynamic strain is an emerging phenomenon that requires in-depth understanding for design of future spin-wave-regulated devices. Here, we explore mutually interacting spin waves and acoustic wave modes in addition to few non-interactive modes through all optical excitation in ordered arrays of Ni80Fe20 nanomagnets. The acoustic wave originated from elastic deformation resonantly couple to the spin wave via magnetoelastic effect at their overlapping frequency. We demonstrate that the choice of the lattice type in which the magnetic nanodots are arranged is crucial for the observation of the magnetoelastic interaction. Therefore, the study shows that the simultaneous existence of elastic wave and spin wave offer ingeneously advantageous features to pave the way of energy-efficient magnetoacoustic devices.

Figures (4)

• Figure 1: Scanning electron micrographs of Py nanodots forming (a) hexagonal and (b) square lattice. For both samples, the size of the nanodots $d=200$ nm, is fixed, and the spacing between them $s$ is selected to set the lattice constant, $a =$ 350 nm. (c) Schematic illustration of measurement geometry used in TR-MOKE magnetometry. (d) The geometry of the computational domain used in finite element method calculations of ME modes in square lattice of Py dots with slanted side faces (light gray) on $\rm Si/SiO_2$ substrate.
• Figure 2: Background subtracted time-resolved (a) reflectivity and (c) Kerr rotation trace indicating acoustic (SAW) and magnetization (SW) dynamics, respectively, detected for hexagonal lattice of square Py dots on SiO$_2$ substrate (see Fig. \ref{['fig:structure']}(a)) at magnetic field of $\mu_0 H_{\rm ext}$ = 185 mT. (b) and (d) are FFT power spectra of the time-resolved reflectivity and Kerr rotation, respectively. The peaks indicate the frequencies of the excited SW and SAW modes, respectively.
• Figure 3: The measured (a-d) and simulated (e-h) field-dependent spectra of SAW (a,c,e,g) and SW (b,d,f,h), performed for hexagonal (a,b,e,f) and square (c,d,g,h) lattice of Py dots on ${\rm Si/SO_2}$ substrate. The simulated profiles (i) of SAW and SW allows to relate the lines in the SAW spectrum E1-E4 to Rayleigh ($\rm R_x, R_y$) and Sezawa ($\rm S_x, S_y$) SAW and the peaks in SW spectrum M3, M4 to egde mode ($\rm M_e$) and fundamental mode ($\rm M_f$) -- M2 refers to the field-independent mode. The phononic profiles shown in (i) are the displacements along the $x-$ ($\rm R_x, S_x$) or $y-$ ($\rm R_y, S_y$) direction, and the magnon profiles show the $z-$component of magnetization. The standing SAW modes, visible at $k=0$, originate from the higher band folded into the center of the 1$^{\rm st}$ Brillouin zone. The orientation of nodal lines of SAWs on the surface indicates $x-$direction (R$_x$, S$_x$) or $y-$direction (R$_y$, S$_y$), respectively. The profiles for SW modes show $z-$component of magnetization.
• Figure 4: The spatial profile of the $y-$component of the ME field for Rayleigh ($\rm R_x, R_y$) and Sezawa ($\rm S_x, S_y$) SAW at the upper face of the Py dot, for the case of (a) hexagonal and (b) square lattice. Note that the non-zero spatial average is only observed for the Sezawa SAW in the square lattice of dots.