Direct observation of coherent magnons with suboptical wavelengths in a single-crystalline ferrimagnetic insulator

Spin-wave dynamics were studied in an extended thin film of single-crystalline yttrium iron garnet using time-resolved scanning transmission x-ray microscopy. A combination of mechanical grinding and focused ion beam milling has been utilized to achieve a soft x-ray transparent thickness of the underlying bulk gadolinium gallium garnet substrate. Damon-Eshbach type spin waves down to about 100 nm wavelength have been directly imaged in real space for varying frequencies and external magnetic fields. The dispersion relation extracted from the experimental data agreed well with theoretical predictions. A significant influence of the ion milling process on the local magnetic properties was not detected.

Figure 1

(a) Optical micrograph of the 185 nm YIG film sample after substrate thinning. (i) Framing ceramic ring (aluminium oxide). (ii) YIG film on the remaining $20\mu \mathrm{m}$ thick GGG substrate. (iii) Copper stripline antennas (2 $\mu \mathrm{m}$ width in the center region). (b) SEM image of a FIB-milled x-ray window with stripline antenna visible on the other side.

Figure 2

Upper part: schematics of the sample architecture and measurement setup. Circularly polarized x rays are focused onto the sample region of the ion milled x-ray window in the GGG substrate. The x-ray transmission intensity is measured by a detector with an avalanche photodiode. Lower part: TR-STXM images of Damon-Eshbach geometry spin waves in the 185 nm thick YIG-film sample at an external magnetic bias field of ${\mu }_0{H}_{\mathrm{ext}}=18$ mT and a frequency of $f=2.5$ GHz. (a) Direct x-ray intensity image of the sample's transmission window. The central dark horizontal stripe is the antenna. (b) Single time frame of the time-resolved measurement with dynamical normalization that emphasizes the spin waves over the static background. Grayscale values represent the changes of the out-of-plane magnetization component. (c) Result of time-domain Fourier analysis, showing amplitude and phase of the waves in HSV (hue-saturation-value) color space (color code above the image).

Figure 3

Measured wave vectors and wavelengths of stripline excited spin waves in Damon-Eshbach configuration (see schematic in the upper right corner) at varying magnetic bias fields at a constant frequency $f=5.5$ GHz in YIG films of thickness $d=185$ nm (blue triangles) and $d=80$ nm (red squares). The lines of corresponding color represent the theoretically predicted values according to Eq. (1).

Figure 4

Upper part: schematics of the 80 nm YIG film sample with the 2 $\mu \mathrm{m}$ diameter Permalloy disk beneath the antenna. (a) Micromagnetic simulation of the in-plane magnetization in YIG below a Permalloy disk. (b) Corresponding STXM measurement of the in-plane magnetization in the sample at the iron $L_3$ edge. Grayscale intensity indicates the horizontal in-plane magnetization component. (c) X-ray transmission image of the region around the disk (black area). The antenna is visible as dark gray areas surrounding the disk. (d),(e) Examples of spin wave measurements in the sector indicated by the red dotted rectangle in (c) displayed in HSV color-space representation [color code next to (b)]. The yellow dotted line marks the disk's edge. Measurements were taken at $f=4.8$ GHz and $f=6.3$ GHz, respectively, at a field of ${\mu }_0{H}_{\mathrm{ext}}=5$ mT. (f),(g) HSV-color images of measurements in the Permalloy disk covered area at ${\mu }_0{H}_{\mathrm{ext}}=5$ mT and $f=4.3$ GHz at two different x-ray energies (iron edge for YIG $+$ Permalloy; nickel edge for Permalloy only).

Figure 5

Measured dispersion relation (squares) of spin waves in the YIG film emitted from the bottom edge of the disk at ${\mu }_0{H}_{\mathrm{ext}}=5$ mT. The line graph depicts the theoretical predictions according to Eq. (1). The schematic on the upper left depicts the measurement geometry and area (purple) on the sample.