Unraveling Magnetization Reversal in High Entropy Alloy Nanoparticles through MOKE-FORC Mapping

High-entropy alloys (HEAs) offer a vast compositional design space for tuning magnetization reversal through compositional complexity and nanoscale confinement. We fabricate FeNiCrCoCu nanoparticles by pulsed-laser deposition followed by nanosecond laser-induced dewetting, forming uniform arrays with diameters of ~30–125 nm. SEM and TEM imaging confirm structural uniformity and crystallinity across the series.  Longitudinal MOKE measurements reveal a size-driven evolution from square, low-coercivity loops (Hc ≈ 42 Oe, Mr/Ms ≈ 0.45) at 30 nm to higher coercivity and the onset of lobes at 50 nm (Hc ≈ 55 Oe), followed by strongly slanted loops with suppressed remanence at 90–100 nm (Mr/Ms ≈ 0.04–0.06) and a sharp transition to a high-coercivity multidomain state at 125 nm (Hc ≈ 332 Oe, Mr/Ms ≈ 0.68). FORC analysis resolves this evolution in detail, from weak irreversible peaks at small sizes to butterfly features at 90 nm, a tilted “wishbone” ridge at 100 nm, and a pronounced coercivity peak near 400 Oe at 125 nm. These results map a clear trajectory from single-domain to vortex-mediated to multidomain reversal and demonstrate that MOKE–FORC is a powerful probe of size-dependent switching mechanisms and interaction fields in high-entropy magnetic nanoparticles.