Impact of Gd substitution on the structure, hyperfine interactions, and magnetic properties of Sr hexaferrites

Abstract Gd3+-substituted nano-sized Sr-hexaferrite, SrGdxFe12-xO19 (0.00 ≤ x ≤ 0.05) NHFs (SrGd NHFs), were fabricated by a one-pot sol-gel combustion approach assisted by citric acid. The crystal structures, surface morphologies, magnetic properties, and hyperfine interactions of the calcined samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDX), high-resolution TEM (HR-TEM), vibrating sample magnetometry (VSM), and Mossbauer spectrometry. The oxidation states and chemical compositions of the NHFs were evaluated using XPS. The single-phase crystalline structures of all products were confirmed by XRD analysis. The average crystallite sizes determined from the peaks of the highest intensity using the Scherrer equation were between 38 and 50 nm. Using 57Fe Mossbauer spectroscopy data, the quadrupole splitting, hyperfine magnetic field, isomer shift, and line width variation of the products were determined. The Mossbauer spectra showed that octahedral (Oh) 12k and 2a sites were preferentially occupied by Gd3+ ions. The Gd3+ content was found to have no influence on the electron density around the Fe3+ ions at any crystallographic site. The magnetization (M) versus magnetic field (H) data were collected at room (300 K) and ultra-low (10 K) temperatures. Neither the zero-field cooling (ZFC) nor field cooling (FC) mode plots of magnetization vs. temperature revealed any blocking temperature peaks. Both the M-H curves and FC-ZFC magnetization revealed ferrimagnetic behavior of the products. Furthermore, the FC-ZFC magnetization exhibited super-spin glass behavior at low temperatures. Reductions in the coercive field (Hc), saturation magnetization (Ms), and remanence (Mr) magnetization were achieved with Gd substitution of up to 0.03 into the hexaferrite system; however, with further increase in Gd content, these values started to increase. The calculated squareness ratio (Mr/Ms) revealed that all of the NHF samples were composed of single magnetic domain particles with uniaxial anisotropy.