Dynamic susceptibility evidence of surface spin freezing in ultrafineNiFe2O4 nanoparticles

Abstract

We investigated the dynamic behavior of ultrafineNiFe2O4 nanoparticles(average size ⟨D⟩ = 3.5 nm) that exhibit anomalous low temperature magnetic properties suchas low saturation magnetization and high-field irreversibility in bothM(H) and ZFC–FC processes. Besides the expected blocking of the superspin, observed atT1≈45 K, the system undergoes a magnetic transition atT2≈6 K. For the latter, frequency- and temperature-resolved dynamic susceptibility data revealcharacteristics that are unambiguously related to collective spin freezing: the relativevariation (per frequency decade) of the in-phase susceptibility peak temperature is∼0.025, critical dynamics analysis yields an exponentzν = 9.6 and a zero-field freezingtemperature TF = 5.8 K, and,in a magnetic field, TF(H) is excellently described by the de Almeida–Thouless line . Moreover, out-of-phase susceptibility versus temperature datasets collectedat different frequencies collapse on a universal dynamic scaling curve. All theseobservations indicate the existence of a spin-glass-like surface layer that surrounds thesuperparamagnetic core and undergoes a transition to a frozen state upon cooling below5.8 K.