Doping effect of Ho on the structural, optical, ac electrical transport and magnetic properties of Mg-Zn ferrites...

Mg-Zn ferrites nanoparticles having the formula Mg_0.4Zn_0.6Ho _x Fe_2-x O₄ (x = 0.00, 0.02, 0.04, 0.06 and 0.08) were synthesized using a hydrothermal method to investigate the effect of Ho substitution on their structural, ac electrical transport, optical and magnetic properties in this study. X-ray diffraction (XRD) analysis confirmed the formation of a cubic spinel structure for x ≤ 0.06, while a weak Ho₂O₃ secondary phase was detected at x = 0.08, suggesting the solubility limit of Ho ions within the spinel lattice. The experimental lattice parameters increased from 8.421 to 8.441 Å up to x = 0.06, while the crystallite size decreased with increasing Ho content. Transmission electron microscopy (TEM) analysis revealed that the average particle size decreases from 28.38 ± 0.47 to 20.66 ± 0.39 nm for x ≤ 0.06. The FTIR spectra indicated the characteristic metal-oxygen vibrational bands associated with the tetrahedral and octahedral sites. The tetrahedral band remained nearly unchanged at 532 cm^-1, while the octahedral band shifted toward lower wavenumbers as the Ho content increases from x = 0.00 to x = 0.06, suggesting the preferential occupancy of Ho ions at the octahedral sites. Optical analysis demonstrated a reduction in the optical band gap energy from 1.62 to 1.47 eV for x ≤ 0.06, indicating enhanced visible-light absorption. An enhancement of coercivity (H _c) is clearly observed and it increses from 33.9 (for x = 0.00) to 134.4 Oe (for x = 0.06). On the other hand, the magnetocrystalline anisotropy constant (K _a) rapidly increased and it shows a higher value ∼3032 erg cm^-3 for x = 0.06, while the value of K _a is 897 erg cm^-3 for undoped sample. The significant increase of K _a along with H _c (for x = 0.06) indicates the enhancement of magnetic stability, which is promising applications in permanent magnets, magnetic recording media and high-density data storage devices. Frequency dependent dielectric constant (ε') measurement has been displayed that an enhancement of ε' is clearly evidenced on Ho doping and it is found to larger at x = 0.06. The inclusion of Ho in Mg-Zn ferrites shows lower value of dielectric losses, which could make them suitable in high-frequency operations. It is also seen that AC electrical conductivity reaches its maximum value for x = 0.06, which may be attributed to the enhancement of densification. Electrical modulus analysis shows a shift of the relaxation peak toward higher frequencies, suggesting a reduction in relaxation time. It is inferred that Mg_0.4Zn_0.6Ho_0.06Fe_1.94O₄ exhibits the most optimized magnetic performance, characterized by the larger coercivity and enhanced magnetic anisotropy. In addition, its reduced optical band gap and favorable electrical response further extend its potential toward multifunctional optoelectronic and electromagnetic applications.