Size-dependent magnetic responsiveness of magnetite nanoparticles synthesised by co-precipitation and solvothermal methods

The dependence of the magnetic responsiveness on magnetite nanoparticle size has been studied. Monodisperse magnetite nanoparticles of 10 nm diameter were prepared by an ultrasonically enhanced co-precipitation procedure. A carboxyl-functionalised solvothermal approach was applied to synthesise magnetite nanoparticles with an average size of 30 nm. The particle sizes and their distribution have been determined by analysing TEM images and considering nanoparticle formation mechanisms. The magnetic characterisation revealed an inverse dependence between the magnetite nanoparticle size and its ability to respond to external magnetic fields, which was explained by the decrease of magnetic dipoles inside the tailing-away crystal of the magnetite nanoparticles. Negligible hysteresis with a small value of 5 Oe was found for the 10 nm nanoparticles, while the larger value of 80 Oe was determined for the 30 nm nanoparticles.