The influence of Ho doping on the morphology and magnetic properties of M-type strontium ferrite with different Fe/Sr

Ho-doped strontium ferrite powder samples with different Fe/Sr ratio were successfully prepared by sol-gel auto-combustion method. Different Fe/Sr ratios (Sr1-xHoxFe12O19, Sr1-x/2HoxFe12-x/2O19, SrHoxFe12-xO19, Sr1+xHoxFe12-xO19, Sr1+xHoxFe12-2xO19) were designed to study the influence of Ho doping on the morphology and magnetic properties of M-type strontium ferrite. XRD analysis revealed a relative change in the content of the secondary phases Fe2O3, HoFeO3, and SrFeO3-x in Ho-doped samples as the Fe/Sr ratio varies. The excess of Sr and Fe in the internal environment resulted in such changes in the phase. SEM analysis confirmed that Ho doping led to an increase in grain size. Different doping methods could lead to significant difference in the size of crystal grain. XPS results indicated that the secondary phases caused by Ho doping led to a change in the oxidation state of the iron element. It was inferred from FTIR and Raman spectra that Ho doping into the lattice might occupy both 2b and 4f1 sites. The VSM showed that under the SrHoxFe12-xO19 doping mode (x = 0.05–0.2), the obtained samples showed a decrease in coercivity (Hc) with increasing Ho doping level (from 5.26 to 4.78 kOe), and the saturation magnetization (Ms) also decreased (from 76.73 to 73.28 emu/g). As the Fe/Sr ratio gradually increases, the saturation magnetization of the sample shows a trend of first increasing and then decreasing, while the coercivity shows a trend of first decreasing and then increasing. The intermediate ionic radius of Ho3+ (0.89 Å) among the rare earth elements results in its unique occupancy behavior, which also influences the formation of different phases, resulting in corresponding changes in coercivity and saturation magnetization.