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Evidence of Individual Superspin Relaxation in Diluted FeO/Hexane Ferrofluids

Materials, ISSN: 1996-1944, Vol: 16, Issue: 13
2023
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We used dc magnetization and ac susceptibility to investigate the magnetic relaxation of ferrofluids made of 8 nm average-diameter FeO nanoparticles dispersed in hexane. Samples of different concentrations (δ) spanning two orders of magnitude ranging from 0.66 to 0.005 mg (FeO)/mL (hexane) were used to vary the interparticle interaction strength. Our data reveal a critical concentration, δ = 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 presented.) with a characteristic time τ ~10 s. That is further confirmed by the observed invariance of the relative peak temperature variation per frequency decade (Formula presented.), which stays constant at ~0.185 when δ < δ. At higher concentrations, between 0.02 and 0.66 mg/mL, we found that Δ exhibits a monotonic increase with the inverse concentration, (Formula presented.), and the collective superspin dynamics is described by a Vogel–Fulcher law, (Formula presented.). Within this regime, the dipolar interaction strength parameter T increases from T = 0 K at δ = 0.02 mg/mL to T = 14.7 K at δ = 0.66 mg/mL.

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