Magnetoelastic excitation of single nanomagnets for optical measurement of intrinsic Gilbert damping

We report an alternative all-optical technique to drive and probe the spin dynamics of single nanomagnets. Optically generated surface acoustic waves (SAWs) drive the magnetization precession in nanomagnets via magnetoelastic (MEL) coupling. We investigate the field-swept dynamics of isolated Ni nanomagnets at various SAW frequencies and show that this method can be used to accurately determine the intrinsic Gilbert damping of nanostructured magnetic materials. This technique opens a new avenue for studying the spin dynamics of nanoscale devices using nonthermal (“cold”) excitation, enabling direct observation of the MEL driven dynamics.

Figure 1

(a) Schematic plot of all-optical “cold” magnetization excitation in single Ni nanomagnet (b) SEM image of a Ni nanomagnet embedded between an Al nanowire array ($\mathrm{pitch}=300\mathrm{nm}$).

Figure 2

TR-MOKE time traces of the optically (OPT) excited (a) Ni film and (b) isolated nanomagnet (NM), and (c), (d) the corresponding field-dependent Fourier spectra of the film and NM, respectively, measured with a fixed field angle ${\theta }_H={30}^{\mathrm{o}}$. (e) TR-MOKE trace of the MEL driven nanomagnet ($H_{\mathrm{app}}=3.7\mathrm{kOe}$), and (f) nonmagnetic signal of the acoustically modulated nanostructure. The illustrations in panels (a), (b) and (e), (f) indicate the pump-probe configurations used and the applied field geometry, respectively. The field-dependent Fourier spectra of the acoustically driven NM shown in (g), which reveals strong MEL enhancement when the magnetic and nonmagnetic modes are degenerate. SAWs are identified by monitoring the transient reflectivity, shown in (h), and therefore do not depend on the applied field.

Figure 3

Field dependence of the effective damping of the Ni film and single nanomagnet (NM) measured using conventional TR-MOKE (closed black circles and open pink squares, respectively). For comparison, the damping measured using the MEL approach is included (blue triangles). The effective damping of the film was fit using Eq. (1) to estimate the intrinsic damping (0.03) (red line).

Figure 4

Field dependence of the normalized complex Fourier spectra (imaginary Fourier component–circles; real Fourier component–squares) of the MEL driven dynamics at (a)–(d) four distinct SAW frequencies, and (e)–(h) four distinct applied field geometries using a single sample ($\mathrm{pitch}=400\mathrm{nm}$). (i) Pinning width determined by fitting both the real and imaginary Fourier spectra from panels (a)–(d) plotted against $f_{\mathrm{SAW}}$, including the fit to Eq. (3) (red dashed line) used to estimate the damping, and (j) summary of $\mathrm{\Delta }{H}_p$ from panels (a), (e)–(h) plotted against ${\theta }_H$. The data exhibits no significant variation of the pinning width as a function of the applied field angle, which supports the interpretation that the relationship between $\alpha$ and $\mathrm{\Delta }{H}_p$ for the nanomagnet is not complicated by extrinsic mechanisms.