Dynamic susceptibility evidence of surface spin freezing in ultrafine NiFe_2O_4 nanoparticles

摘要

We investigated the dynamic behavior of ultrafine NiFe_2O_4nanoparticles (average size(D) = 3.5 nm) that exhibit anomalous low temperature magnetic properties such as lowsaturation magnetization and high-field irreversibility in both M (H) and ZFC—FC processes.Besides the expected blocking of the superspin, observed at T1 ti 45 K, the system undergoes amagnetic transition at T2 ti 6 K. For the latter, frequency- and temperature-resolved dynamicsusceptibility data reveal characteristics that are unambiguously related to collective spinfreezing: the relative variation (per frequency decade) of the in-phase susceptibility peaktemperature is —0.025, critical dynamics analysis yields an exponent zv = 9.6 and a zero-fieldfreezing temperature TF = 5.8 K, and, in a magnetic field, TF(H) is excellently described bythe de Almeida—Thouless line δT_F= 1 —T_F(H)/T_F∝H~(2/3).Moreover, out-of-phasesusceptibility versus temperature datasets collected at different frequencies collapse on auniversal dynamic scaling curve. All these observations indicate the existence of aspin-glass-like surface layer that surrounds the superparamagnetic core and undergoes atransition to a frozen state upon cooling below 5.8 K.