Abstract
We investigated the effect of Ni substitution on oleic acid coated Fe3O4 magnetic nanoparticles of 10–13 nm size dispersed in kerosene medium with enhanced colloidal stability. Study of molecular dynamics and coating of oleic acid to ferrite NixFe3−xO4 (0 ≤ x ≤ 0.7) nanoparticles was confirmed by Fourier transform infrared spectroscopy (FTIR) measurements. The increase of Zeta potential (ζ) up on Ni concentration confirms the stability of Ni substituted Fe3O4 magnetic nano fluid. Magnetization measurements (M-vs-H) reveals the substitution of Ni2+ suppresses the magnetic moment thereby decrease of saturation magnetization while increase in Ni concentration from x = 0–0.7 of Fe3O4 nano magnetic fluid. Moreover, field dependent elastic (G′) and viscous (G′′) modulus recorded at varying strain rate confirms a cross-over from viscoelastic-to-viscous behavior of Ni substituted Fe3O4 nanofluid. Further, the induced spin-disorder upon Ni concentration in corroboration with microwave resonant absorption (EPR) measurements through the increase of resonance field and narrowing of peak-to-peak linewidth indicating the suppression of magnetic dipole interactions and enhancement of super-exchange-interactions (Fe3+-O-Ni2+/Fe2+-O-Ni2+) . Moreover, the dominance of spin-spin interactions over the spin-lattice interactions was determined by EPR relaxation-time-scales: spin-spin relaxation (τ2) increases and spin-lattice relaxation (τ1) decreases, which is in conjunction with lande-g-factor while increasing Ni concentration. The present results are useful for knowing the effect of Ni concentration on magnetic and viscoelastic properties of NixFe3−xO4 (0 ≤ x ≤ 0.7) magnetic nanoparticles. Also, it offers a chance to create such types of stable nano magnetic fluids that may be actively controlled by outside magnetic fields, which results in heat sink damping materials in sealed devices and biomedical applications, etc.
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