Estudo das propriedades estruturais e magnéticas do sistema cerâmico Ba(0,9)La(0,1)Fe(12-y)Al(y)O(19)

Abstract

Ferrites are one of the most widely used materials in the world. Among them, M-type hexaferrites are widely researched for their excellent magnetic properties. In this work, the structural and magnetic properties of the hexaferrite ceramic system Ba0.9La0.1Fe12−yAlyO19 with content of the cation Al3+ were studied of y = 0.0; 0.1; 0.3; 0.5; 0.7; 1.0 and 1.3. The samples were obtained by solid state reaction synthesis and sintered by the conventional ceramic method. X-ray diffraction, Raman spectroscopy and magnetometry measurements of the vibrating sample were performed at room temperature. First, the influence of the dopant La3+ on the structural and magnetic properties of the system Ba1−xLaxFe12O19 with the cation composition La3+ of x = 0.1 and 0.6, below and above its solubility limit. The evolution of the structural and magnetic properties with the content of the cation Al3+ in the ceramic system Ba0.9La0.1Fe12−yAlyO19 with y = 0.0 to 1.3 keeping the concentration of La3+ fixed at x=0.1. The Rietveld refinement method was used to determine the structural properties and the saturation approximation law to determine the saturation magnetization. The qualitative and quantitative phase analysis showed the appearance of a major phase of barium hexaferrite, with hexagonal structure and space group P63/mmc and a minor secondary phase of hematite (α-Fe2O3) with rhombohedral structure and space group R − 3c in all samples, except for the sample with 0.6 content of La3+. In this sample, in addition to the majority phase that corresponded to hematite and the minority phase of hexaferrite, a third phase appeared, identified as an orthoferrite phase (LaFeO3). The crystal lattice parameters a and c decreased with increasing content of the cation La3+. These parameters presented a minimum value in the composition y =0.7 for the system co-doped with La3+ and Al3+. The model followed in the Rietveld refinement adopted assumes that the substitution of the cation Ba2+ by the cation La3+ occurs in the 2d sites and for the ion Al3+ at the sites 4fIV , 4fV I and 12k in the hexaferrite structure, it was adequate, according to the refinement parameters obtained. Variations in the geometric parameters of the oxygen polyhedra (average bond length, and distortion index) were observed at the Ba(2d) site, and at the Fe(2b, and 12k) sites when the Ba2+ cation was replaced by the cation La3+ and at the sites Fe(4fIV , 4fV I , 12k) when replacing the cation Fe3+ by the cation Al3+. The dynamics of the structure, studied through the Raman spectra, showed all the peaks referring to the Fe-O bonds in the different crystallographic sites of the hexaferrite. New peaks were observed by the presence of the hematite phase in the sample with x = 0.6. The widenings and shifts in the peaks were attributed to the presence of dopants content in the hexaferrite sites. The magnetic measurements showed that there was not a high decrease in saturation magnetization for the 0.1 concentration of La3+, however, for the 0.6 concentration there was a sharp drop and an increase in the coercive field. The saturation magnetization reached a minimum value in the composition y = 0.7 and the coercive field increased until reaching the maximum value at y = 1.0 of the ion Al3+ for the system Ba0.9La0.1Fe12−yAlyO19. The magnetocrystalline anisotropy increased for the compositions from 0.1 to 0.3 and remained constant for values above this composition.