Superspin glass dynamics and memory effects in FeCo nanoparticle systems

The controlled tuning of the volume fraction in magnetic nanoparticle (NP) assemblies provides an effective route to stabilize the interaction-driven glassy magnetic dynamics. We demonstrate that the low-volume-fraction FeCo NP assemblies exhibit clear signatures of a superspin-glass-like state, including characteristic aging, and memory effects, arising from collective freezing mediated by long-range dipolar interactions. An enhancement of low-temperature coercivity in the low-volume fraction NPs system suggests the emergence of interaction-induced effective anisotropy. The present work establishes NPs' volume fraction as a scalable control parameter for engineering collective magnetic states and demonstrates a versatile strategy for designing NP-based magnetic materials with dynamic response.