Spin-glass-like magnetic ordering in Zn substituted magnetite magnetic fluids

Electron spin resonance (ESR) spectra of magnetic fluids involving polydispersed Zn0.5Fe0. 5Fe2O4 (FZ5) and Zn0.7Fe0. 3Fe2O4 (FZ7) nanomagnetic particles are scanned from 4.2 to 300 K. The FZ7 fluid exhibits certain distinct features below 40 K which are different from FZ5 fluid. These include (i) an isotropic shift in resonance field in zero-field-cooled ESR study, (ii) deviation of resonance field from sin2 θ behavior (where θ is the angle between axis of the particle and field) in field cooled (FC) sample and (iii) abrupt increase in anisotropy field for FC sample. The results are analyzed in light of the core-shell model for nanomagnetic particles.