Energy barrier distribution of a noninteracting nano-sized magnetic particle system

AC-susceptibility and magnetic relaxation data are analysed to determine the distribution of energy barriers (E-b) of an assembly of small magnetic particles. The assembly, a dilute frozen ferrofluid, consists of spherical nano-sized maghemite (gamma-Fe2O3) particles with a median diameter of 70 Angstrom. Assuming a direct proportionality between E-b and particle volume, the volume distribution is found to be described by a normalised gamma distribution. The system is not adequately described by the commonly adopted log-normal distribution function, which comparably should yield an excess amount of large particles sizes. From the analyses, accurate values of the Arrhenius law prefactor (tau(0) = 4 x 10(-10) s) and the magnetic anisotropy constant are derived.