Hopkinson peak and superparamagnetic effects in BaFe12-xGaxO19 nanoparticles

In this article, the thermomagnetic properties of a system of Ga-substituted barium hexaferrite nanoparticles (BaFe12-x Gax O19 ) 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 T C (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.