ENERGY BARRIER DISTRIBUTION OF A NONINTERACTING NANO-SIZED MAGNETIC PARTICLE SYSTEM

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