Evidence of Individual Superspin Relaxation in Diluted Fe(3)O(4)/Hexane Ferrofluids

We used dc magnetization and ac susceptibility to investigate the magnetic relaxation of ferrofluids made of 8 nm average-diameter Fe(3)O(4) nanoparticles dispersed in hexane. Samples of different concentrations (δ) spanning two orders of magnitude ranging from 0.66 to 0.005 mg (Fe(3)O(4))/mL (hexane) were used to vary the interparticle interaction strength. Our data reveal a critical concentration, δ(c) = 0.02 mg/mL, below which the ferrofluid behaves like an ideal nanoparticle ensemble where the superspins relax individually according to a Néel–Brown activation law [Formula: see text] with a characteristic time τ(o) ~10(−9) s. That is further confirmed by the observed invariance of the relative peak temperature variation per frequency decade [Formula: see text] , which stays constant at ~0.185 when δ < δ(c). At higher concentrations, between 0.02 and 0.66 mg/mL, we found that Δ exhibits a monotonic increase with the inverse concentration, [Formula: see text] , and the collective superspin dynamics is described by a Vogel–Fulcher law, [Formula: see text]. Within this regime, the dipolar interaction strength parameter T(0) increases from T(0) = 0 K at δ(c) = 0.02 mg/mL to T(0) = 14.7 K at δ = 0.66 mg/mL.