In this research, nano-sized powders of Zr–Mn substituted strontium hexaferrite (Sr(Zr,Mn)$_x$Fe$_{12−2x}$O$_{19}$ ($x = 0, 2, 2.5, 3$)) were synthesized by sol–gel auto-combustion route using subsequent heat treatment. The samples were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscope (FESEM), transmission electron microscopy (TEM), Mössbauer spectroscopyand vibration sample magnetometer (VSM) techniques. XRD and Mössbauer spectroscopy results revealed formation of Sr(Zr,Mn)$_x$Fe$_{12−2x}$O$_{19}$ accompanied with Mn$_{\delta}$Fe$_{2−\delta}$O$_4$ lateral phase in the samples. Also, FTIR and XRDresults demonstrated presence of SrO impurity phase. FESEM micrographs show particle size reduction and presence of two distinct powder morphologies with different brightness levels with Zr$^{4+}$ and Mn$^{2+}$ substitutions which approves existence of lateral phases in the substituted samples. TEM micrographs show nanometric particles with sizes smaller than 100nm with high crystallinity. Mössbauer results showed that at low level of substitution, Zr$^{4+}$ ions prefer to occupy both 4f1 and 2b however, at higher level of substitution, they prefer exclusively 4f1 site. While, Mn$^{2+}$ ions distributed approximately equally between 12k and 2a sites. The presence of nonmagnetic Zr$^{4+}$ cation leads to decrease in exchange interaction, especially at 12k and 2a sites. VSM results showed decrement of coercivity force (${}_i$H$_c$) from 5593.60 to 3282.46 Oe and maximum magnetization from 62.60 to 46.15 emu g$^{−1}$, respectively, by increment of Zr–Mn substitution values. Variations in maximum magnetization magnitude have been explained on the basis of occupation of the substituted cations at different iron sites.

Single-phase La-substituted bismuth ferrite (Bi$_{1−x}$La$_x$FeO$_3$) nanoparticles have been synthesized by thermal decomposition of a glyoxylate precursor. The crystal structure transition of BiFeO3 from the rhombohedral(R3c) to the cubic $Pm\bar{3}m$ structure by La addition was confirmed by X-ray diffraction and infrared spectrometry methods. Furthermore, the Bi$_{1−x}$La$_x$FeO$_3$ nanoparticles showed a weak ferrimagnetism behaviour, while the magnetization increased from 0.18 to 0.48 emu g$^{−1}$ with La substitution. The Bi$_{1−x}$La$_x$FeO$_3$ nanoparticles exhibitedstrong absorption in the visible region with the optical band gap calculated from Tauc’s plot in the range of 2.19–2.15 eV. Furthermore, the effects of La substitution on the photodegradation of the methylene blue (MB) undervisible light were also studied. The photodegradation of MB dye was enhanced from 64 to $\sim$99% with increasing La substitution from $x = 0$ to 0.1 and then decreased to 8% for $x = 0.15$.