Effect of Bi³⁺ ions substitution on the structure, morphology, and magnetic properties of Co–Ni spinel ferrite nanofibers

In this study, various Co_0.5Ni_0.5Bi_xFe_2-xO₄ (x ≤ 0.10) spinel ferrite nanofibers (CoNiBi SFNFs) have been synthesized by electrospinning technique. The structure, morphological, and magnetic features of CoNiBi SFNFs was investigated. XRD analysis demonstrated the formation of single-phase cubic spinel structure for all samples. The cell parameter is increasing with the increase of Bi³⁺ content. The cubic morphology of all products was also verified by SEM and TEM microscopies. Mössbauer analysis was used to determine the hyperfine parameters by fitting room temperature mosspectra. The Bi³⁺ ions occupy mainly the B site. Vibrating sample magnetometry (VSM) was applied to get magnetic data for CoNiBi SFNFs at 300 and 10 K. The characteristics of the recorded magnetic hysteresis curves revealed the existence of just ferrimagnetic phases for all fabricated spinel nanofiber samples at both temperatures. The remanent magnetization M_r, saturation magnetization M_S, magnetic moment per formula n_B, coercivity H_c have been determined. Bi³⁺ ion substitution commonly diminishes the magnetic data except the content of x = 0.04. The maximum and minimum magnitude of magnetic parameters belong to Co_0.5Ni_0.5Bi_0.04Fe_1.96O₄ and Co_0.5Ni_0.5Bi_0.10Fe_1.90O₄ SFNFs, respectively. Pristine Co_0.5Ni_0.5Fe₂O₄ and Bi³⁺ ion doped samples exhibit soft magnetic at 300 K and hard magnetic nature at 10K conditions. Squareness ratio (SQR = M_r/M_s) provided extra information about the domain structure of fabricated nanofiber samples. At 10K conditions, SQR of the sample Co_0.5Ni_0.5Bi_0.04Fe_1.96O₄ is almost equal to 0.5 and this critical value is assigned to the formation of single domains with uniaxial symmetry for this nanofiber. This one only and all others have SQRs that are much lower (0.073–0.366) at 300K or higher (0.620–0.781) at 10K. Those ranges specify the multi-domain wall structure for samples.