INTRODUCTIONFerro spinel compounds possess high permeability so that they can be used in electronics [1][2][3], in microwave and high density information storage devices [4], as ferrofluids [5] and magnetic drug delivery materials [6], etc. Spinel ferrites, possessing cubically close packed array of oxygen anions with the metallic ions occupy two crystallographically different sites, i.e. octahedral [B] and tetrahedral (A) site. Three kind of magnetic interactions are possible, between metallic ions, through the intermediate O ions, by super-exchange mechanism, namely, A-A interaction, B-B interaction and A-B interaction. It has been established experimentally that these interaction energies are negative and hence induce an anti-parallel orientation.Due to the specific electronic configurations of cations and the types of super-exchange interactions among them, the magnetic properties of ferrites are strongly dependent on the occupancy and exchange of cations in the two sites. In spinel ferrites, direct interactions are negligible, due to the large distance between cations. Super-exchange interaction occurs between two metal cations through the existence of a bridging oxygen ion. It involves the temporary transfer of one oxygen 2p electron to a neighboring metal ion. The A-B interaction is the strongest super-exchange interaction in the spinel structure, followed Cobalt ferrite, nickel ferrite and zinc ferrite spinel oxides are synthesized by low temperature hydrothermal method using ethylenediamine tetracetic acid (EDTA) as complexing agent. FTIR and XRD studies show the formation of pure and single spinel phase. The average crystalline size was determined from X-ray diffraction line broadening using Scherrer equation. FE-SEM studies revealed that all the synthesized ferrites are having nearly octahedron crystals with an average particle size of 20 to 40 nm. Magnetic behavior of cobalt ferrite, nickel ferrite and zinc ferrite spinels studied by vibrating sample magnetometer at room temperature shows ferromagnetic behaviour of cobalt ferrite, super paramagnetic nature of nickel ferrite and non-magnetic behaviour of zinc ferrite. Zeta potential measured at various pH conditions shows that cobalt ferrite and nickel ferrite can be used for the preparation of magnetic nanofluids with stable colloidal dispersion at 4.5 > pH < 7.5 and 3.5 > pH < 9.5, respectively.

Cobalt ferrite nanoparticles are prepared by low temperature hydrothermal synthesis using EDTA as template. IR, XRD and FESEM analysis indicate formation of single spinel phase cobalt ferrite particles having average particle size of 26 nm. VSM analysis of cobalt ferrite show hysteresis loop with a moderate coercivity of 580 Oe and saturation magnetization of 60 emu/g. Hyperthermia analysis under various AC magnetic field were carried out for magnetic nanofluids prepared by dispersing various amounts of the synthesized cobalt ferrite nanoparticle in water. The specific absorption rate (SAR) values are found to be comparatively higher for magnetic nanofluids containing cobalt ferrite up to 0.6 wt. % and hence the optimum saturation temperature of 43 °C required for the hyperthermia treatment of cancer cells is achieved by the application of lower magnetic field. Ultrasonic investigation on the prepared nanofluids show no particle-particle interaction up to the concentration of 0.6 wt. % and beyond which agglomeration of particles occur results in the formation of small clusters in the magnetic nanofluids. Presence of clusters reduces SAR values and higher magnetic field is required to attain the optimum saturation temperature of 43 °C.

Superparamagnetic nickel ferrite (NiFe2O4) nanoparticles were synthesized by low temperature hydrothermal method using EDTA as templating agent. The synthesized nanoparticles were characterized by FTIR, XRD, FESEM and TEM analysis. The XRD analysis shows the presence of cubic spinel phase with crystallite size of 26.42 nm. The morphology analyzed by FESEM and TEM techniques indicate the presence of cubical shape particles having average particle size of 28.44 nm with no agglomeration. The VSM analysis gives “S” shape curve with zero coercivity and saturation magnetization (Ms) of 30 emu/g indicating the presence of small magnetic particles exhibiting the superparamagnetic behaviour. Nickel ferrite nanofluids were prepared by mixing carrier fluid water with varying amounts of nickel ferrite nanopowder. Zeta potential measurements of dilute solution of nickel ferrite show the formation of stable nickel ferrite nanofluids. Hyperthermia and ultrasonic studies were carried out on various concentrations of nickel ferrite nanofluids. The SAR values and ultrasonic parameters were calculated. Less concentrated nickel ferrite nanofluids quickly attain threshold hyperthermia temperature of 43 °C when exposed to lower applied AC magnetic field of range 5 mT to 20 mT. Ultrasonic studies show at lower concentrations nickel nanoparticles interact with carrier fluid water through cohesive forces mainly hydrogen bonding and keep the particles in an isolated state there by continue to exhibit superparamagnetic characteristics. At higher concentrations, particle-particle interaction predominantly occurs leading to increase in particle size beyond 30 nm, so that the magnetic characteristics of nickel ferrite nanoparticles changes from superparamagnetic to ferromagnetic nature. At low concentrations induction heating occurs through Neel and Brownian relaxation mechanism whereas at high concentrations hysteresis loss mechanism operate to attain the threshold temperature of 43 ºC.