  In this article, the thermomagnetic properties of a system of Ga-substituted
barium hexaferrite nanoparticles ( ) prepared by ball milling were
investigated. The thermomagnetic curves for the samples with x ranging from 0.0
to 1.0 exhibited sharp peaks with high magnetization just below TC (Hopkinson
peaks). The height of the peak for our samples was similar or larger than
previously observed or calculated values. Theoretical treatment of the
experimental data demonstrated that the peaks are due to the effect of
superparamagnetic relaxations of the magnetic particle. This effect was
confirmed by hysteresis measurements at, and just below the temperature at
which the peak occurred. Consequently, the particle diameters were calculated
from the experimental data using a theoretical model based on the
superparamagnetic behavior of a system of uniaxial, randomly oriented, single
domain, non-interacting particles. The calculated diameters of 11 - 26 nm are
less than the physical diameters determined from TEM measurements. The factors
responsible for the low calculated values are discussed.
