Structural, magnetic, and radiation shielding properties of Ho3+–substituted Ni–Zn–Co nanoferrite

<title>Abstract</title> <p> This work reports on the synthesis, structural, magnetic, and radiation shielding properties of Ni _0.4 Zn _0.3 Co _0.3 Ho _x Fe _2−x O ₄ (x = 0.00–0.10; step 0.02) nanoferrite prepared via a citrate combustion method. X-ray diffraction analysis confirmed the formation of a single-phase cubic spinel structure across the entire compositional series. As the Ho ³⁺ content increases, the lattice parameter exhibits a non-linear expansion from 8.3747 Å (x = 0.00) to 8.3802 Å (x = 0.10) driven by competing bond strengths and ionic size disparities. Furthermore, the crystallite size systematically decreases from 60.28 to 45.91 nm. Magnetic characterization reveals a systematic enhancement in both saturation magnetization and coercivity with Ho ³⁺ incorporation. This enhancement is attributed to strengthened octahedral–tetrahedral superexchange interactions and elevated magnetocrystalline anisotropy. The net magnetic moment increased from 0.40 to 0.48 µ _B , shifting the characteristic operating frequency from 11.04 to 14.41 GHz which validating the material's suitability for high-frequency Ku-band electronic applications. Concurrently, the ionizing radiation attenuation capabilities improved significantly with increasing Ho ³⁺ -content. At 0.8 MeV, the equivalent atomic number increased from 24.2 to 33.5. Furthermore, at a photon energy of 1 MeV, the exposure buildup factor decreased from 6.8 to 4.8, while the energy absorption buildup factor decreased from 9 to 8. The Ni _0.4 Zn _0.3 Co _0.3 Ho _0.10 Fe _1.90 O ₄ exhibits superior shielding efficiency and the lowest buildup factors across the entire energy range (0.015–15 MeV). The obtained findings demonstrate that Ho ³⁺ substitution improve the magnetic response and radiation attenuation ability of Ni _0.4 Zn _0.3 Co _0.3 Ho _x Fe _2−x O ₄ for high-frequency and radiation shielding applications. </p>