plane saturation and volume magnetization measurements. This value is in good agreement with the value shown above extracted from fitting the FMR data. 

A comparison of the VNA FMR data with results from cavity FMR measurements taken at 9.47 GHz is shown in Fig. 4 (Panels a-c). One sees that the SSWM’s are clearly visible in the broadband FMR but appear very weak in the cavity FMR experiment. We expect that the unpinned SSWM should not produce a strong response in the cavity due to the high symmetry of the mode, resulting in a low overlap integral with the highly homogeneous driving field. Nevertheless, some signature of this mode is apparent in all the films from this series. This suggests that a weak surface pinning may be present and must be asymmetric as well; i.e., larger at one film surface than the other. 

As a check to confirm that the smaller peak seen in the broadband FMR is in fact the lowest frequency odd symmetry SSWM, a Brillouin light scattering (BLS) study of the 60nm thick single-layer film was performed. In thermal BLS SSWM’s are excited through thermal fluctuations and can therefore appear regardless of mode symmetry [18]. A BLS intensity spectrum measured at the angle of light incidence of 5 degree from the normal to the film is shown in Panel 3(d). We found that the frequency position of the first BLS peak above the fundamental (F) dipole mode for a given field and a given magnon wavenumber is consistent with the field position of the lower field peak detected in the broadband FMR. This unambiguously identifies the smaller peak as the first odd symmetry SSWM (SSWM1 in the panel). Furthermore, the BLS data indicate the presence of a dipole gap [19] in the spin wave spectrum, where the Damon-Eschbach mode repels the first dipole exchange branch [20]. Without pinning, the modes are orthogonal and should cross, and the existence of a gap indicates some small degree of asymmetric pinning at the film surfaces that modifies the symmetry of the modes. Full details of the BLS study will be published elsewhere. 

Lastly, we comment on eddy current effects on the transducer FMR response. Amplitudes for absorption are well described by the theory of Kostylev [14]. In particular, it is shown that large amplitude SSWM peaks can be observed from metallic films without surface magnetisation pinning provided that an asymmetric eddy-current contribution to the total microwave magnetic field exists. This can in fact be realized simply by stripline transducers and used to detect SSWM resonances that are weak or not visible in conventional cavity FMR. This eddy current theory also describes, as experimentally observed, that the SSWM amplitudes in single-layer films are practically independent of driving frequency. 

