FROM PURE SUPERPARAMAGNETIC REGIME TO GLASS COLLECTIVE STATE OF MAGNETIC-MOMENTS IN GAMMA-FE2O3 NANOPARTICLE ASSEMBLIES

Studies of the frequency dependence of the temperature of the AC susce ptibility peak, of the thermal variation of the nonlinear DC susceptib ility, and of ageing effects on the magnetization relaxation in gamma- Fe2O3 4.7 nm nanoparticle assemblies with interparticle interactions o f varying strength, give evidence of three magnetic regimes: pure supe rparamagnetic, superparamagnetic modified by the interactions, and col lective. The properties of the latter regime, called glass collective state, are close to those of a canonical spin glass. (C) 1998 Elsevier Science B.V. All rights reserved.