Abstract
Cooperative effects in hybrid nanocomposites are an interesting field of research. The behavior of magnetic nanocomposites is the result of spatial arrangements of phases and their magnetic correlations through exchange and dipolar forces. Understanding these influences is an important task in order to produce magnetic composites with improved properties. In this work, we present a study on the structural and magnetic properties of nanocomposites made of barium hexaferrite (BaFe12O19) coated either with cobalt ferrite spinel (CoFe2O4) or core-shell CoFe2O4@CoFe2 nanoparticles, i.e. BaFe12O19/CoFe2O4 or BaFe12O19/CoFe2O4@CoFe2. The BaFe12O19 (BFO) phase was produced using the ionic coordination reaction technique and then coated with CoFe2O4 (CFO) nanoparticles synthesized by the polyol method. Several concentrations of the BFO and CFO phases were considered. The BFO/CFO samples were reduced in hydrogen gas, where partial reductions of cobalt ferrite give rise to the alloy CoFe2 (CF). Thus, we produced a new nanocomposite of the type BFO/CFO@CF. These samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), vibrating sample magnetometry (VSM) and Mössbauer spectroscopy (ME). The XRD and ME analyzes revealed the presence of the phases BFO and CFO, for the non-reduced nanocomposites, and for the partially reduced nanocomposites we found three phases BFO, CFO and CF. TEM images showed that these phases are structurally coupled. Isothermal remanent magnetization (IRM) and direct current remanent demagnetization (DCD) measurements were done and the analyzes through the Henkel and δm plots were performed, providing information about the nature of the interactions among the phases and their magnetic behavior. The samples BFO/CFO and BFO/CFO@CF with the lowest percentage of BFO (∼7 wt%) showed exchange coupling between the phases while the other samples exhibited predominantly magnetic dipolar interactions.
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