Superparamagnetic particles with pronounced anisotropy suspended in a fluid: dynamic susceptibility of magnetized cobalt ferrite nanosuspension

Abstract

The theory of linear magnetic response for the case of uniaxially anisotropic superparamagnetic nanoparticles suspended in a fluid is developed for the situations where, along with the probing field, a stationary bias field is present. The built up description allows for both mechanisms of magnetic relaxation available to the particles: internal (relaxation of the magnetic moment inside the particle) and external (relaxation together with the particle body due to its Brownian orientational diffusion in a fluid). In this framework, the dynamic magnetic susceptibility of a nanosuspension (magnetic fluid) is considered in the high-dilution limit, i.e., neglecting the interparticle interactions. It is proven that the absorption spectrum (the frequency dependence of the out-of-phase component of dynamic susceptibility) in a general case has at least two well-distinctive maxima. The bias field affects the
positions and heights of both these maxima but leave them well resolved. Moreover, in the presence of bias under certain conditions the spectrum acquires a third maximum. The developed theory is used for interpretation of the magnetodynamic measurements on a nanosuspension of cobalt ferrite. A minimally sufficient account for polydispersity of the real sample–three fractions–is shown to provide a fairly good description of the measured spectrum in a wide frequency range.