Magnetic dynamics of Zn57Fe2O4 nanoparticles dispersed in a ZnO matrix

The magnetic properties of diluted ZnFe2O4 nanoparticles with nearly normal spinel structure, dispersed in a nonmagnetic ZnO matrix, are presented. A magnetic transition to an ordered state is observed at about 19 K, from Mossbauer and magnetic measurements. Detailed characterization of the frequency and field dependence shows that, below the magnetic transition, the system displays several features that suggest a collective freezing of ferromagnetic particles. Although single-domain particles are inferred from structural data, a dynamic study of this transition hints toward a spin-glass-like process, with T-g = 17.0(4) K and critical parameters zv = 20 +/- 2, much higher than those of canonical spin glasses. Some facets of these large critical parameters in fine-particle systems are discussed.