Evidence of Individual Superspin Relaxation in Diluted Fe3O4/Hexane Ferrofluids

We used dc magnetization and ac susceptibility to investigate the magnetic relaxation of ferrofluids made of 8 nm average-diameter Fe3O4 nanoparticles dispersed in hexane. Samples of different concentrations ( delta) spanning two orders of magnitude ranging from 0.66 to 0.005 mg (Fe3O4)/mL (hexane) were used to vary the interparticle interaction strength. Our data reveal a critical concentration, delta(c) = 0.02 mg/mL, below which the ferrofluid behaves like an ideal nanoparticle ensemble where the superspins relax individually according to a Neel-Brown activation law iota(T) = iota(0)exp ( E-B /k(B)T ) with a characteristic time iota(0) similar to 10(-9) s. That is further confirmed by the observed invariance of the relative peak temperature variation per frequency decade Delta = Delta T/T center dot Delta log(f), which stays constant at similar to 0.185 when delta < delta(c). At higher concentrations, between 0.02 and 0.66 mg/mL, we found that D exhibits a monotonic increase with the inverse concentration, 1 /delta, and the collective superspin dynamics is described by a Vogel-Fulcher law, iota(T) = iota(0)exp[ E-B /k(B)(T-T-0) i. Within this regime, the dipolar interaction strength parameter T-0 increases from T-0 = 0 K at delta(c) = 0.02 mg/mL to T-0 = 14.7 K at delta = 0.66 mg/mL.