Effect of Nb³⁺ ion substitution on the magnetic properties of SrFe₁₂O₁₉ hexaferrites

The crystal structure and magnetic properties of SrNb_xFe_12–xO₁₉ (0.00 ≤ x ≤ 0.08) nanohexaferrites (NHFs) fabricated using a sol–gel technique is presented in this study. The X-ray powder diffractometry (XRD) and Infrared spectroscopy (FT-IR) confirmed the formation of M-type hexaferrite phase. The analyses of magnetization versus applied magnetic field, M(H), were performed at room (300 K; RT) and low (10 K) temperatures. The Bohr magneton number (n_B), saturation (M_s) and remanent (M_r) magnetization values increase slightly with increasing Nb³⁺ content. The room-temperature values of the magnetic parameters M_r = 31.41–33.28 emu/g, M_s = 57.10–60.14 emu/g and coercivity (H_c) between 4274 and 4540 Oe, at 10 K, magnetization data were detected that are much higher with respect to RT values: M_r = 45.96–51.06 emu/g, M_s = 94.42–95.99 emu/g. The magnetic results indicate that the samples are magnetically hard materials at both considered temperatures. The squareness ratio (SQR) is found to be around 0.50, implying single-domain NPs with uniaxial anisotropy for pristine and substituted samples. With exception, the x = 0.0 sample indicated the formation of multi-domain structure with uniaxial anisotropy at 10 K. Field cooling (FC) susceptibility measurements were applied in temperature range of 5–350 K for pristine sample and samples that contained some Nb³⁺ ions. The analyses of dc susceptibility data also proved that Nb³⁺ ion substitution increases the magnetization and, additionally, allows for an easier alignment of the magnetic domains. The obtained magnetic results were investigated deeply with relation to structural and microstructural properties. The observed remanent magnetization (M_r) and coercivity (H_c) render the products are useful for permanent magnets and high-density recording media.