Structural, magnetic, electromagnetic, and hyperfine interactions of rare earth Ho³⁺ doped in mixed Co–Ni spinel ferrites

This study reports the hydrothermal synthesis of Ho³⁺ ion substituted Ni-Co (Ni_0.5Co_0.5Ho_xFe_2−xO₄ (x ≤ 0.05) nanospinel ferrites (Ho→NiCoFe) NSFs). Their structure, morphology, magnetic/microwave properties, and hyperfine interactions have been investigated via X-ray diffractometry (XRD), Scanning/Transmission electron microscopies (SEM/TEM), High resolution transmission electron microscopy (HR-TEM), Vibrating sample magnetometer (VSM), microwave absorption analyzer, and Moss spectroscopy techniques. The single-phase composition (purity of the products) and crystallinity of all products were verified by Energy X-ray Spectrophotometer (EDX) and XRD. The crystallite size (D) of the products is within the range of 14–24 nm. The magnetization curves were performed via a VSM device at ambient (300 K (RT)) and low (10 K), which indicate hard magnetic features with ferrimagnetic behavior for each of the as-prepared Ho→NiCoFe NSFs. The rise in Ho³⁺ substituting content causes a reduction in Ms, which is attributed to the deterioration of A-B exchange interactions and redistribution of cations among A and B sites. Contrarily, the coercivity increases with the rise in Ho³⁺ substituting content, which might be caused by the changes in cationic distribution at A and B sublattices, lattice distortions, reduction in crystallites/grains size, and rise in the number of grain boundaries. Other magnetic parameters were also determined, discussed, and correlated with structural and morphological changes. The Mössbauer spectra consisted of five magnetic sextets for all samples. The doublet formation was observed only x = 0.04 and 0.05. The doped ions substituted with Fe³⁺ ions at B site. Mössbauer spectroscopy was used for the calculation of cation distribution. Microwave features were examined within 12–33 GHz. S11-S21 parameters were used for periodicity scatterings of the actual/imaginary parts of permeability and permittivity. RL coefficient demonstrated intensive electromagnetic absorption with average level − 10…-15.9 dB that corresponded to the absorption of the natural media.