Al-doped BaFe12O19 hexaferrites: Dopant distribution in the crystal lattice and its effect on the magnetic order

Abstract

We hereby conducted studies on the distribution of aluminum ions in the crystal lattice of doped BaFe12-xAlxO19 hexaferrite nanoparticles (x = 0.5, 1.0, 1.5 at. %) and their effect on the magnetic order of the ferrite. 57Fe Mössbauer spectroscopy, X-ray absorption spectroscopy, X-ray diffraction and magnetic measurements were used to elucidate the incorporation of Al3+ into the barium hexaferrite (BaM) lattice. As a result, the positions of the dopant in the complex lattice with five non-equivalent positions of Fe3+ ions were discussed. The relationship between the percentage of aluminum as dopant ion, its location in the BaM lattice, and the changes in the magnetic order of the hexaferrite nanoparticles was quantified. For the different amount of Al3+, the iron ions are replaced in the octahedral crystallographic sites by aluminum ions. No such replacement occurs in the case of the tetrahedral and bi-pyramidal crystallographic sites. This preferential ionic substitution gives rise to changes in the magnetic order of the BaM that is expressed by changes in the values of the hyperfine fields and the Curie temperature (Tc), which decreased approximately 30 °C for every 0.5 at.% of aluminum per f.u. The mean value of the modulus of the exchange coupling constant was estimated as a function of the percentage of Al3+ from the measured Tc value.