Magnetic Properties of Ferrofluid with Cobalt Ferrite Particles

Abstract

A ferrofluid is a stable colloidal suspension of nano-sized magnetic particles dispersed in a carrier liquid. At sufficiently low temperatures, frozen ferrofluids exhibit a spin-glass type disordered structure with the frustration caused by the competing dipole—dipole interactions of randomly oriented grains [1]. At elevated temperatures the magnetic behaviour of ferrofluids is dominated by fast superparamagnetic fluctuations of the particles magnetic moments [2-4]. It is known that the magnetic properties of small particles differ from those of the corresponding bulk materials. Besides the relaxation effects, the most important factors responsible for these differences are: (i) the symmetry restriction at the particle surface, and (ii) high chemical reactivity of the surface atoms. In a ferrofluid the magnetic properties of an assembly of ultrafine particles can be changed and controlled by the surfactant used to coat the particles; the surfactant can prevent intergrain interactions but can also considerably enhance the non-collinear spin structures in the surface layer [2]. The present work was motivated by the desire to study these various surface effects. The cobalt ferrite particles were chosen since they are available in a powder form either as particles covered by the surfactant molecules or in a form of ferrofluid in which the particles are in a non-magnetic liquid. Moreover, the structure and magnetic properties of the bulk CοFe2O4 crystals are well known