Generation and Imaging of Magnetoacoustic Waves over Millimeter Distances

Using hybrid piezoelectric-magnetic systems we have generated large amplitude magnetization waves mediated by magnetoelasticity with up to 25 degrees variation in the magnetization orientation. We present direct imaging and quantification of both standing and propagating acoustomagnetic waves with different wavelengths, over large distances up to several millimeters in a nickel thin film.

Figure 1

(a) Schematic setup of the hybrid devices with ferromagnetic $U$-shaped structures (nickel) on a piezoelectric substrate (${\mathrm{LiNbO}}_3$), The structures are deposited on the acoustic path between two IDTs separated by $d=6\mathrm{mm}$. Additional nonmagnetic Copper structures are also included in the hybrid devices. The x rays illuminate the sample with short pulses of 20 ps and a frequency of 500 MHz. The interdigital transducer, IDT1, is excited with an ac electric signal that generates a surface acoustic wave propagating through the ${\mathrm{LiNbO}}_3$. An image at the center of the acoustic path taken with a PEEM microscope is shown as an inset. Vertical stripes result from the piezoelectric voltage associated to the SAWs. (b) Schematic representation of the magnetization oscillation as a function of the SAW strain phases. (c) Cartoon of the in-plane strain caused by the SAWs in the piezoelectric (in green color scale) and magnetic modulation in the ferromagnet (in orange-cyan color scale).

Figure 2

(a) XMCD image with a field of view of $50\times 50\mu {\mathrm{m}}^2$ showing both the ferromagnetic (Ni) and the piezoelectric (${\mathrm{LiNbO}}_3$) materials in the presence of a SAW—stripes indicate the presence of a magnetic wave. (b) Image of the same location and conditions as in (a) obtained from the substraction of two PEEM images with opposite phases—stripes in the ferromagnet correspond to a magnetic wave and stripes in the piezoelectric correspond to the electric component of the piezoelectric wave. (c) Profiles of the magnetoacoustic waves in the ferromagnet from XMCD images taken at different SAW amplitudes. Each amplitude is used to calculate an associated strain value from the photoelectron shift of the local surface potential in the piezoelectric material. (d) Dependence of the magnetoacoustic waves with the SAW amplitude. (e)–(h) XMCD images of magnetoacoustic waves of frequencies 500, 375, 250, and 125 MHz having wavelengths of 8, 12, 16, and $32\mu \mathrm{m}$.

Figure 3

[(a)–(c)] XMCD image of a magnetoacoustic wave under different applied magnetic fields. The image includes the ferromagnetic (Ni) film and the piezoelectric (${\mathrm{LiNbO}}_3$) substrate together with a nonmagnetic metallic structure (Cu). The arrows indicate the magnetization direction within the ferromagnetic film. (d) Horizontal cuts from (a) and (c) and from an additional field of 9.5 mT (image not shown). Schematic arrows indicate the magnetization direction at different wave points. (e) Magnetoacoustic wave amplitude as a function of the external applied field.

Figure 4

Time evolution of a magnetoacoustic wave in a ferromagnetic material (Ni) for a propagating wave (a) and for a standing wave (b). The waves are created with two IDT facing each other and separated 6 mm. A small detuning between the wave and the synchrotron frequencies combined with a subtraction of opposite phases allows for the visualization of the magnetoacoustic waves. In (a) the maxima and minima are seen to shift position with time, while in (b) the positions are fixed but the amplitude oscillates periodically.