x-ray reflectometry. 

Note that the capping and the seed layer have the same thickness in Series 2, in contrast to the structures grown in Series 1. This gave Series 2 a symmetric combination with respect to transducer coupling to the film and substrate through the capping/seed layers. Comparison of the FMR data obtained on both series reveals no effect of change in the capping layer design. 

All bilayers of Series 1 consist of a Cobalt (Co) layer grown on top of a thicker Permalloy (Py) layer (”Si/Py/Co” geometry.) Thicknesses of each Co and Py layer are varied across the series. All bilayers of Series 2 consist of a 10 nm thick Co layer and are divided into two sub-series. The ”Si/Py/Co” sub-series has the Cobalt layer grown on top of the Py layer as Si/Ta/Py/Co/Ta, similar to the structures in Series 1. The ”Si/Co/Py” subseries has reversed ordering of Py and Co layers: Si/Ta/Co/Py/Ta. The total sample thickness for all samples in Series 1 and Series 2 is smaller than the microwave magnetic skin depth [16] over the frequency range 100 MHz - 20 GHz. Note that the microwave magnetic skin depth varies from 102 nm at 7.5 GHz to 111 nm at 18 GHz as calculated from the material parameters derived from the single layer reference Py films, and assuming 0.008 for the Gilbert damping parameter and of 4 . 5 · 10 6 Siemens/m for the conductivity of Py [17]. Whereas the classical skin depth actually decreases with increasing frequency, the magnetic skin depth instead increases slightly because of increased magnetic losses αω (see Eq.(2.9) in Ref.[16]). 

An Agilent N5230A PNA-L microwave vector network analyzer (VNA) was used to apply the microwave signal to the samples and to measure magnetic absorption. As a measure of the absorption we use the microwave scattering parameter S 21 [2]. The sample sits on top of the transducer with the magnetic layers facing it. To avoid direct electric contact the sample surface is separated from the transducer by a 15 µ m thick Teflon layer. The microwave frequency is held constant and the static magnetic field H is slowly increased. The raw data then appears as resonance curves in the form of S 21( H ) as a function of H . This process is repeated for a number of frequencies. We also measure S 21 for the transducer with no sample ( S 21$_{0}$( H )) to eliminate any field-dependent background signal from the results. The results presented below are Re ( S 21( H ) /S 21$_{0}$( H )). It is worth noting that the raw data | S 21( H ) | show the same qualitative behavior, so artefacts arising from the mathematical processing of data are not significant. 

Detailed broadband FMR measurements on the samples from Series 1 were taken using a 

