Compositional control of structural, magnetic, and gamma-ray attenuation properties in rare-earth substituted Ho-based double perovskite oxides synthesized via mechanical alloying and oxidation

Rare-earth substitution provides a route for tailoring the structural, magnetic, and gamma-ray attenuation behaviour of Ho-based Co-Mn double perovskites; however, the combined effects of rare-earth ionic size, magnetic character, and substitution level remain insufficiently understood. In this study, Ho_1.5La_0.5CoMnO₆, Ho_1.2Er_0.8CoMnO₆, Ho_1.3Sm_0.7CoMnO₆, and Ho₂CoMnO₆ were synthesized through a two-step route involving mechanical alloying to obtain precursor powders, followed by annealing at 1000 °C in air to promote oxidation and double-perovskite phase formation. Their structural, thermal, surface-chemical, magnetic, and gamma-ray attenuation properties were examined by X-ray diffraction, thermal analysis, X-ray photoelectron spectroscopy, SEM-EDS, vibrating-sample magnetometry, and theoretical shielding calculations based on half-value layer (HVL) and transmission factor (TF) parameters. The XRD patterns were consistent with predominantly monoclinic P2₁/n type double-perovskite structures, while weak additional reflections suggested possible minor secondary phases. SEM-EDS showed homogeneous elemental distributions, and XPS analysis indicated Co²⁺ dominated and mixed Mn-valence characteristics in the spectra where reliable component analysis was possible. All compositions exhibited a low-temperature ferromagnetic-like field response but did not reach magnetic saturation at 3 T. Ho₂CoMnO₆ and Ho_1.3Sm_0.7CoMnO₆ showed the highest field-dependent magnetization values at 10 K, whereas Ho_1.2Er_0.8CoMnO₆ displayed the lowest response. The theoretical gamma-ray attenuation calculations showed clear composition-dependent differences. At 0.662 MeV, the HVL values were 1.0650, 0.9972, 1.0438, and 0.9967 cm for Ho_1.5La_0.5CoMnO₆, Ho_1.2Er_0.8CoMnO₆, Ho_1.3Sm_0.7