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The response of a typical ferrofluid (FF) lies in its explicit property of chain formation of magnetic nanoparticles. The most significant magneto-optic (MO) and magneto-viscous (MV) effects of FF are attributed to chaining effect. In the present research, an effort was made to analytically justify the dependence of the structure evolution of FFs on different measurable parameters involved in MO and MV effects. The problem is treated with the help of dimensional analysis and an empirical relation is formulated relating the equilibrium chain length with Verdet coefficient (constant), particle diameter, viscosity of the carrier fluid, particle density, magnetization and shear rate. The formulated relation of chain length is supported by error analysis to yield the uncertainty in the result. The maximum uncertainty in four sets of data is found as ∼0.75. [ABSTRACT FROM AUTHOR]

SQUID magnetometry and electron magnetic resonance (EMR) spectroscopy at 9.28 GHz were employed to probe magnetism and surface spin disorder in Fe3 O4 nanoparticles. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis showed magnetite particles with size ≃8 nm. Magnetization (M) versus temperature studies yielded blocking temperature TB ≃ 170 K in H = 50 Oe. For T > TB, M scales approximately as H/T with μP ≃ 5400 μB as the magnetic moment/particle. At 5 K, the magnitudes of hysteresis loop parameters viz. coercivity Hc ≃ 37 Oe and saturation magnetization M8 = 62 emu/g are considerably smaller than the reported values for larger 150 nm Fe3 O4 particles. In EMR studies, a single symmetric line at g ≃ 2.11 with linewidth ΔH = 850 Oe is observed at 300 K with the g-value and ΔH increasing with decreasing T. These results are interpreted in terms of a disordered surface spin layer of 0.5 nm thickness. [ABSTRACT FROM AUTHOR]

In this paper, we compare the magnetic relaxation of DNA-coated γFe2 O3 with that of parent 7-nm γFe2 O3 particles by measuring the temperature dependence of AC susceptibilities (χ′ and χ″) at frequency f = 0.1, 1, 10, 100, and 1000 Hz and electron magnetic resonance (EMR) at 9.28 GHz. Changes in the blocking temperature TB (where χ′ peaks) with frequency are analyzed using the relation: TB = Ta + [T0/ln(f0/f)] derived from Vogel-Fulcher relaxation, where Ta(T0) measures the effective energy barrier (interparticle interaction). The fit to the data yields f0 = 1.3 × 1016 Hz, Ta = 2760 K(2960 K), and T0 = 40 K(20 K) for the uncoated (DNA-coated) γFe2 O3. From the relative shift ΔTm of Tm (the temperature where χ″ peaks) with change Δ log f in frequency, the parameterΦ = ΔTm/(TmΔ log f) is determined to be 0.04(0.06) for the uncoated (coated) ΔFe2 O3. These results show only minimal changes in the magnetic dynamics of γFe2 O3 upon DNA coating. [ABSTRACT FROM AUTHOR]