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(a). XRD pattern of the powder sample indexed with the (hkl) reflections of the cubic spinel; (b) TEM and (c) HRTEM images of the CoFe2O4 nanoparticles. Electron diffraction pattern in the inset of Fig. 1(b) was indexed according to the spinel structure; (d) DSC curve for the liquid sample in the temperature range 150–240 K.
Magnetic field dependence of magnetization for the liquid sample at T = 5 and 300 K. The dashed curve drawn through the T = 5 K hysteresis loop is a guide to the eye. The solid curve is the fit of the sum of Langevin functions weighted by log-normal distribution of particle sizes to the experimental data at 300 K. The inset represents resulting distribution function with σ = 0.27 and D av = 6.0 nm.
Temperature dependence of (a) the real part χ′ and (b) the imaginary part χ″ of the ac magnetic susceptibility for the liquid sample at frequencies in the range f = 1–1500 Hz. The inset in Fig. 3(a) shows the best fit of the VF law to the experimental T m1(f) and T m2(f) data.
Frequency dependence of the imaginary part of the ac susceptibility at T = 148 K. The solid curve is the fit of imaginary part of the ac susceptibility, Eq. (1) , to the experimental data in the frequency range f = 1–500 Hz (1.84 ≤ ln ω ≤ 8.05). Inset: Fit of the real part of Eq. (1) to the experimental χ′(ln ω) data at T = 148 K.
Calculated ln(1/τ) vs T dependencies for selected pairs of values (D,T 0), where particle diameter and interaction parameter are shown in the range (a) D = 6.3–5.3 nm, T 0 = 25–73 K, (b) D = 5.8–5.2 nm, T 0 = 1–40 K. Full symbols with error bars represent the experimental values: (a) ln(1/τc I) obtained as fitting parameters from the lower frequency peak and (b) ln(1/τc II) estimated from the measured susceptibilities at f ≥ 1000 Hz.
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