Guidelines for interpreting microfocused Brillouin light scattering spectra

Abstract

We present an analysis of the influence of spin wave dispersion relations and profiles on microfocused Brillouin Light Scattering spectra. Three archetypal magnetic materials are reported: a 51-nm thick Bi-substituted YIG, a 25-nm thick Heusler compound and 50-nm thick CoFeB alloy. These samples were chosen because they exhibit strongly contrasting spectral features -peak frequencies, linewidth, skewness. The shapes of these spectral features reflect the underlying spin wave dispersion relations and the thickness profile of the related spin wave modes. While analytical expressions of the dispersion relations provide a satisfactory description of the spectra if the modes are in separate frequency domains, the exact dispersion relations and the exact mode profiles are required for a correct description of the spectra not only when mode hybridization is present in the range or near the range of frequencies and wavevector accessed by the experiment. Our examples of microfocused BLS spectra are handy references that can be used as interpretation guidelines for BLS spectra recorded on a broader range of materials.

Figures (6)

• Figure 1: Experimental (black dots) and calculated (solid lines) $\mu$-BLS spectra measured on (a): a 51-nm-thick BiYIG film, (b): a 20-nm-thick $\text{Co}_2\text{Mn}\text{Al}$ film, and (c): a 50-nm-thick CoFeB film. The $p = 0$ peak is highlighted in green and the $p=1$ in blue.
• Figure 2: TetraX simulations of spin wave properties. Top row: Dispersion relations in Damon-Eshbach geometry (dotted lines) and backward volume (solid lines) geometry for the (a) BiYIG (b) $\text{Co}_2\text{Mn}\text{Al}$ and (c) CoFeB sample. Bottom row: Numerically obtained thickness profiles of the spin waves at a $k = 0 ~\text{rad}/\mu\text{m}$ (solid lines) and at $k = 10 ~\text{rad}/\mu\text{m}$ (dashed lines) for the (d) BiYIG (e) $\text{Co}_2\text{Mn}\text{Al}$ (f) CoFeB sample. The green curves indicate the modes contributing to the $p=0$ peak, while the blue curves indicate the modes contributing to the $p = 1$ peak. The $m_0$ is for static magnetization.
• Figure 3: Comparative influence of film thickness on the shapes of $\mu$-BLS anti-Stokes spectra. Top row: for 25-nm-thick films of (a) BiYIG, (b) $\text{Co}_2\text{Mn}\text{Al}$ and (c) CoFeB. Bottom row: for 100-nm-thick films of (d) BiYIG, (e) $\text{Co}_2\text{Mn}\text{Al}$ and (f) CoFeB.
• Figure 4: Spin wave dispersion relation for all propagation directions for in-plane magnetized 25-nm-thick Heusler film (a) and 100-nm-thick Heusler film.
• Figure 5: Comparative contributions of the spin waves of selected wavevector orientations. (a): Dispersion relation in the Damon-Eshbach (dot line) and Backward Volume (bold line) geometry. (b): Partial $\mu$-BLS anti-Stokes spectra obtained if the only spin waves populated in the material would be the backward waves (solid line) and the Damon-Eshbach (dashed line). The contributions to the $p = 0$ peak are in green while in blue for the $p = 1$ peak.