Magnetic resonance in gold-iron alloys near the percolation limit

The interplay between dilution and frustration in metallic ferromagnets is an important and complex problem. We give a complete description, both qualitative and quantitative, of the line shift and linewidth obtained by ferromagnetic resonance (FMR) in a dilute ${\mathrm{Au}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Fe}}_{\mathrm{x}}$ ferromagnet and, due to the large fields applied for resonance, we observe effects specifically associated with the proximity to the percolation threshold. We emphasize the importance of finite clusters in the physical behavior of these systems. From the FMR parameter we are able to deduce the dynamics and size distribution, the latter being that predicted by percolation theory. At low temperatures, the ${\mathrm{Au}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Fe}}_{\mathrm{x}}$ dilute ferromagnets behave similarly to glasses, metastability arising in this case from competing interactions.