Estimating the contribution of Brownian and Néel relaxation in a magnetic fluid through dynamic magnetic susceptibility measurements

Abstract We demonstrate how dynamic magnetic susceptibility measurements (DMS) can be used to estimate the relative contributions of Brownian and Neel relaxation to the dynamic magnetic response of a magnetic fluid, a suspension of magnetic nanoparticles. The method applies to suspensions with particles that respond through Brownian or Neel relaxation and for which the characteristic Brownian and Neel relaxation times are widely separated. First, we illustrate this using magnetic fluids consisting of mixtures of particles that relax solely by the Brownian or Neel mechanisms. Then, it is shown how the same approach can be applied to estimate the relative contributions of Brownian and Neel relaxation in a suspension consisting of particles obtained from a single synthesis and whose size distribution straddles the transition from Neel to Brownian relaxation.