Holmium oxide-induced enhancement of flux pinning and superconducting properties in bulk MgB2

This study investigates the influence of holmium oxide (Ho2O3), a heavy rare-earth oxide, on the superconducting properties of MgB2. Bulk MgB2 samples containing 0–5 wt. % of Ho2O3 were synthesized via in situ solid-state reaction using magnesium and nanosized boron powders under an Ar atmosphere at 775 °C for 3 h. Prior to sample preparation, commercial Ho2O3 powder was subjected to ball milling, resulting in a reduction of the particle size from approximately 1.2 μm to 253 nm. Microstructural characterization confirmed the effective refinement of Ho2O3 particles and revealed a phase transformation from a cubic to a monoclinic crystal structure with increasing milling duration. X-ray diffraction results identified MgB2 as a primary phase in all samples. Most of the added Ho2O3 remained unreacted, while a small fraction reacted with boron to form minor HoB4 secondary phases. Magnetization measurements showed that both pure and Ho2O3 added samples exhibited a superconducting transition temperature (Tc) of approximately 37.5 K, with a sharp transition width of less than 0.7 K, indicating that Ho2O3 addition had a negligible effect on Tc. The critical current density (Jc) measured at 20 K under self-field conditions increased from 491.68 kA cm−2 for the undoped sample to 536 kA cm−2 for the sample containing 0.10 wt. % Ho2O3. At self-field, 15 K, the Jc reaches 664.54 kA cm−2 for optimum-doped MgB2 sample. This enhancement is attributed to improved flux pinning arising from the presence of finely dispersed secondary phases. These results demonstrate that effective Ho2O3 particle refinement through ball milling, together with the optimization of Ho2O3 addition, plays a crucial role in enhancing the superconducting performance of bulk MgB2 while preserving its superconducting transition characteristics.