Magnetic properties of Sr_0.5Ba_0.5Ho _xFe_12−xO₁₉ (x ≤ 0.10) nanohexaferrites

The Ho³⁺ ion-substituted SrBa nanohexaferrites, Sr_0.5Ba_0.5Fe_12−xHo _xO₁₉ (x ≤ 0.1) (Ho → SrBa NHFs), were fabricated via ultrasonic-assisted sol–gel route. The hexagonal structure and plate-like morphology were confirmed by X-ray powder pattern (XRD), transmittance electron microscope (TEM), scanning electron microscope (SEM), and high-resolution transmittance electron microscope (HR-TEM). It has been found out that the crystallite size (D _XRD) was within 45–74 nm range. Both elemental mapping and energy-dispersive X-ray spectroscopy (EDX) analyses provided the purity and expected chemical composition of the products. Using the M–H hysteresis loops (at 300 and 10 K), magnetic properties of Ho → SrBa NHFs are analyzed. Upon a sudden fall at x = 0.02, as soon as Ho ions are added, the magnetization is improved with the Ho substitution and prominently enhanced with decreasing temperature. However, the coercivity H _c is decreased with the Ho substitution. The Ho-doped samples represent a magnetically soft material at both high and low temperatures, which is also confirmed through M–T measurements. Their squareness ratio is found to be less than 0.5, suggesting the generation of multi-magnetic domains. The unsubstituted HFs and the samples with x = 0.02 and 0.06 show usual M–T variations with typical field-cooling (FC) and zero-field-cooling (ZFC) branches. However, the other samples (with x = 0.04, 0.08, and 0.10) exhibit unusual behavior with anomalies in their M–T plots. From ⁵⁷Fe Mössbauer spectroscopy data, the variation in line width, isomer shift, quadrupole splitting, and hyperfine magnetic field values on Ho³⁺ substitution have been determined. Mössbauer spectra show that Ho³⁺ ions located at generally octahedral 12 k site. The s-electron density of around Fe³⁺ ions has not influenced doped content.