Abstract
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.
Highlights
Barium hexaferrite BaFe12O19 (BaM) possess interesting properties such as large saturation magnetization, high coercivity, high Curie temperature, large uniaxial magnetic anisotropy and chemical stability
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 average particle size for the pure sample is (42 ± 13) nm, and for the sample with x = 1.0 is (41 ± 13) nm. These values indicate that the synthesized powders consist of single domain magnetic nanoparticles with a relatively narrow particle size distribution
Summary
Barium hexaferrite BaFe12O19 (BaM) possess interesting properties such as large saturation magnetization, high coercivity, high Curie temperature, large uniaxial magnetic anisotropy and chemical stability. These materials have been investigated due to their importance for both fundamental research and technological applications in permanent magnets, high-density magnetic recording, magneto-optics and microwave devices [1,2,3,4,5,6]. Using a completely different approach, the origin of the peak was explained by the superparamagnetic behavior of magnetic particles which do not exhibit a peak as a result of the variations of the saturation magnetization and anisotropy field of the material [23]. The Hopkinson peak height was analyzed in terms of the superparamagnetic relaxation processes of the particles and compared with the experimental data to arrive at a conclusion concerning the mechanism responsible for the peak, and the particle size distribution of the synthesized powders
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