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, Ho1.5La0.5CoMnO6, Ho1.2Er0.8CoMnO6, Ho1.3Sm0.7CoMnO6, 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 Ho1.3Sm0.7CoMnO6 showed the highest field-dependent magnetization values at 10 K, whereas Ho1.2Er0.8CoMnO6 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 Ho1.5La0.5CoMnO6, Ho1.2Er0.8CoMnO6, Ho1.3Sm0.7CoMnO6, and Ho2CoMnO6, respectively. At the same photon energy and a thickness of 1.0 cm, the corresponding TF values were 0.0896, 0.0858, 0.0882, and 0.0860. Thus, Ho1.2Er0.8CoMnO6 and Ho2CoMnO6 exhibited the lowest HVL and TF values among the investigated compositions. The shielding calculations identified Ho1.2Er0.8CoMnO6 and Ho2CoMnO6 as the most effective attenuators, with lower HVL and TF values across the investigated energy range. These results show that rare-earth substitution enables compositional tuning of magnetic and theoretical gamma-ray attenuation behaviour, with the observed trends reflecting the combined influence of rare-earth ionic size, magnetic character, substitution level, and valence-state complexity.