Insights into the structural, chemical and magnetic evolution of HPMS NdFeB powders milled in air

ABSTRACT Recycling NdFeB permanent magnets is essential to meet the rapidly growing demand for rare-earth materials while reducing the environmental impact of primary extraction. Hydrogen Processing of Magnetic Scrap (HPMS) is a highly efficient short-loop route for producing recycled NdFeB powders, yet the high reactivity of these materials makes their handling and post-processing particularly challenging. In this work, we provide a comprehensive, multi-technique characterization of HPMS-derived powders subjected to high-energy milling under air atmosphere, conditions that reflect potential industrial scenarios for recycling. Planetary and impact milling routes in air were evaluated as potential alternatives to inert-atmosphere jet milling, enabling particle-size reduction down to 1 μm. X-ray diffraction with Rietveld refinement, Raman microscopy, X-ray absorption spectroscopy, Mössbauer spectroscopy, and magnetic measurements are used to demonstrate how distinct milling mechanisms drive different oxidation pathways, amorphization levels, and secondary-phase formation. These techniques also reveal the coupled hydride/oxidation/amorphization mechanism, which would be difficult to identify with a single-technique study. The results show that while particle refinement is indeed possible in air, oxidation of Nd-rich grain boundary phases and partial decomposition of the Nd 2 Fe 14 B matrix ultimately limit magnetic performance. This study therefore reveals the interplay between hydride stability, oxidation, and structural disorder during air-based milling of HPMS powders.