We report on stable colloidal aqueous suspensions of magnetic nanostructures made of copper, nickel, and zinc ferrites. These magnetic fluids could represent a new alternative for biological applications. The basic steps of the nanoparticles synthesis, their chemical surface treatment, and their peptization in a stable colloidal sol are given. Their chemical composition is carefully checked, and X-ray diffraction patterns provide both their mean size and a structural characterization. Magnetization results obtained at 300 K are presented and discussed.

We analyze potentiometric and conductimetric measurements simultaneously performed on Electric Double-Layer Magnetic Fluid based on cobalt ferrite nanoparticles, in order to obtain the pHdependence of the particle surface charge density. We propose a mechanism for the charging of the particle surface. This model considers the ferrofluid solution as a mixture of strong and weak diprotic acids. We show how an exact analytical treatment involving proton transfer between the particle surface and the bulk solution allows the construction of a speciation diagram of the charged superficial sites. The saturation value of the superficial density of charge is found to be equal to 0.326 ± 0.065 C m −2 .

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Magnetic resonance is used to study magnetic dipole particle–particle interaction in ionic water-based iron-manganese magnetic fluids. A set of six samples having particle concentration running from 1.2×1016 to 6.3×1016 particles/cm3 were frozen below room temperature and analyzed in the range of 100–250 K. Average values of magnetic particle–particle interaction energy were obtained from the temperature dependence of the resonance linewidth broadening. At 1.2×1016 particles/cm3 magnetic particle–particle interaction energy is found to be of the order of 1.2 meV. However, at 6.3×1016 particles/cm3 magnetic particle–particle interaction energy goes to 32 meV. The enhancement of the magnetic particle–particle interaction energy far beyond the linearity is associated to cluster structuration. A one-dimensional model for cluster structuration is presented.