Structural, Magnetic and Microwave Properties of Gadolinium-Substituted Ca-Ba M-Type Hexagonal Ferrites

This paper presents a study on attempting to substitute Gd into a Ca-Ba M-type hexaferrite (Ca0.5Ba0.5GdxFe12−xO19, where x = 0, 0.05, 0.1, 0.15, 0.2, 0.25) using the sol–gel method. The structural, magnetic and microwave properties of the resultant material were investigated. Since Gd has a much larger ionic radius than Fe, substitution is not straightforward as revealed by the structural analysis. X-ray diffraction (XRD) patterns revealed some substitution into the M-type phase due to a changing lattice parameter; however, significant quantities of additional phases of hematite (α-Fe2O3) and gadolinium orthoferrite started to form at concentrations of up to Gd = 0.10 and at higher Gd concentrations, respectively. High-resolution transmission electron microscopy of selected compositions showed d-spacings corresponding to the three phases observed in XRD, confirming the incomplete substitution of Gd. Scanning electron microscopy showed platelet-shaped grains. Vibrating sample magnetometry at room temperature showed varied results owing to a myriad of interactions from the hexaferrite and secondary phases. The microwave complex permittivity and permeability in the frequency range of 2–11 GHz showed little frequency dependence with nominal values for the complex permeability. All the compositions exhibit low magnetic losses with frequency except Gd = 0.10. Such type of materials can be used for microwave devices in the low-GHz range and as well as permanent magnets.