Fônons em hexaferritas Ba2Me2Fe12O22 (Me= Co, Mg e Zn) e BaFe12O19

Abstract

In this study we investigated the hexaferrites type Y Ba2(Mg1-xCox)2Fe12O22 (x = 0; 0.2; 0.4; 0.6; 0.8 and 1), Ba2Zn2Fe12O22 (Zn2Y) and type M BaFe12O19 (BaM). The experimental techniques used were the X-ray diffraction, XANES (X-ray Absorption Near Edge Structure), XPS (X-ray Photoelectron Spectroscopy), Specular Reflectance IR Spectroscopy, Raman Spectroscopy, Magnetometry and Impedance Spectroscopy. The investigated samples were synthesized by ceramic conventional method. The results of X – ray diffraction confirmed to obtain the desired phase with a small amount of subphases, except for BaM did not show traces of impurities or subphases. XANES and XPS measurements indicated a predominance of 3+ valence for Fe ion and the presence of the 2+ and 3+ oxidation states to Co ions in the samples containing cobalt. As for measures of Raman spectroscopy and reflectance speculate at room temperature it was possible to classify the main bands, qualitatively, making a comparison of the spectra measured with ferrites spectra reported in the literature. From this analysis it was possible associate the high-frequency bands (560-700 cm -1) and low frequency bands (<130 cm-1) with vibration tetrahedra (Mg, Co and Zn)O4 and Ba-O, respectively. As for the system Ba2(Mg1-xCox)2Fe12O22, the specular reflectance spectra and X-ray diffraction patterns showed no appreciable change with magnesium substitution by cobalt, showing no change of space group in the samples due to this substitution. Analysis of the Raman spectra of this system showed that the bands shift to low frequency regions as it grows cobalt content in the structure and new bands at 623 cm-1 and 459 cm-1 appear, which are due to vibration Co-O bonds. Anomalies were observed in the phonon parameters (frequency, width and intensity) in Raman spectroscopy measurements at high and low temperatures in Ba2Mg2Fe12O22 (Mg2Y) samples, Ba2Zn2Fe12O22 (Zn2Y) and BaFe12O19 (BaM) at temperatures where they exhibit magnetic phase transitions. With the aid of additional magnetic measurements, we analyze these results in terms of the spin-phonon coupling. Furthermore, the observation of spin-phonon coupling in the low temperature on Mg2Y suggested magnetostriction as magnetoelectricity mechanism for this material. The electrical measurements on Mg2Y showed that the relaxation process is due to conductive mechanisms being thermally activated and showed grain and grain boundary contributions to the impedance of the sample.