Resonance frequency of the n = 2 mode as a function of Py thickness (black circles). The blue circles represent the frequency for the equivalent thickness of n = 3 and n = 4 modes detected in the 12-nm-thick Py (inset). The equivalent thickness is calculated as a corresponding thickness of n = 3 and n = 4 modes recalculated to the n = 2 mode. The red line is a fit to all data points, representing the universal 1/ d Py 2 behaviour for n = 2 modes in Py films with different thickness, and for different modes in the 12-nm-thick Py. Vertical error bars are extracted from the fitting errors in the analysis of the difference time delay scans using the decaying cosine function. If not otherwise shown, the error bar is smaller than the symbol size.
