Estudo da influência da dopagem e do acoplamento de fases magnéticas sobre o (BH)max da BaFe12O19

Abstract

The M-type barium hexaferrite (BaFe12O19), an iron oxide with hexagonal geometry, is a material that play an important technological role in applications such as permanente magnets, microwave devices and magnetic recording. In this work, we study the influence of two experimental approaches used to improve the maximum energy product, (BH)max, of barium hexaferrite: doping with lanthanum (La) and cobalt (Co), and the coupling of magnetic phases. In the first one, we produced samples of barium hexaferrite, using the Ionic Coordination Reaction (ICR) method, doped with a fixed amount of La and a variable amount of cobalt, Ba0.7La0.3Fe12-xCoxO19, x = (0.0, 0.25, 0.50, 0.75 and 1.0). In the second, through the polyol process, we coated samples of pure and impurity-free barium hexaferrite (also obtained by the ICR method) with superparamagnetic nanoparticles of cobalt ferrite (CoFe2O4), obtaining coupled nanocomposites of BaFe12O19/CoFe2O4 with different proportions. Then, these samples were submitted to a reductive process in a hydrogen atmosphere, where there was a partial reduction of cobalt ferrite, giving rise to a new nanocomposite of the BaFe12O19/CoFe2O4@CoFe2 type. These samples were characterized by the use of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM), as a function of magnetic field and temperature and Mössbauer spectroscopy. In the first approach, XRD measurements showed the presence of a small amount of α-Fe2O3 in the doped samples. The SEM images revealed a morphology like nanorod, smaller than the critical limit for single-domain particles. Optimized magnetic properties were achieved for the sample with concentration of Co x = 0.25. Furthermore, this sample displayed the highest value of (BH)max of 6.98 kJ/m3 (0.877 MGOe), which corresponds to a significant improvement of 8% over the pure sample. For the second approach, XRD analyzis revealed the presence of two phases, BaFe12O19 and CoFe2O4, for the as prepared nanocomposites, and three phases, BaFe12O19, CoFe2O4 and CoFe2, for the nanocomposites after reduction. The TEM micrographs showed that the different phases are structurally coupled. Magnetization (Mr) and remanent demagnetization (Md) measurements were performed at room temperature. Through Mr and Md it was possible to make the Henkel and δm plots of the nanocomposites that provided information about the nature of the interactions between the phases and about the magnetic behavior. We detected a coupling of the exchange-spring type between the phases of the nanocomposites.