**2. Experimental details**

The tri-layer film stack (Ta/FeGaB/Ta) were deposited with DC & RF co-sputtering targets with base pressure < 1.0 × 10−7 Torr at room temperature onto fresh Si substrates. Fe80Ga20 and B targets are employed for FeGaB thin films. Here, the first Ta layer was employed as a buffer layer 25 nm, with different thickness (15, 25, 50, and 75 nm) of FeGaB film and 5 nm thick Ta film were deposited as the magnetic layer and the capping layer, respectively. The composition of the FeGaB film was characterized by using XPS and found as ~70, ~18, and ~12 at.% of Fe, Ga, and B, respectively. The surface morphologies images of samples were captured by Oxford Asylum MFP-3D AFM. The static magnetic properties of these samples were performed by employing Quantum Design® SQUID MPMS at room temperature. The dynamical magnetic properties studied using NanOsc Instruments Cryo-FMR in the VersaLab system with temperature variation 100–300 K and exciting frequency from 2 to 20 GHz.
