Reduction and change of magnetic properties of Co(OH)₂ on milling with Al

The reduction process and change of the magnetic properties have been investigated for a mixture comprising Co(OH)₂ and metallic Al as a function of mechanical milling. Dehydration and apparent thermit reaction by milling are detected by thermal analyses (TGDTA). With increasing the milling time, the magnetization, M, becomes temperature-independent above 50K, whereas M at 5K rapidly decreases. At above 100K, M shows a maximum after milling for12h due to reduction to form metallic Co. Zero-field ⁵⁹Co-spin echo NMR signals are detected for the milled mixture. Marked negative Knight shift of ²⁷Al NMR by milling is induced by the polarization of 3d spins of neighboring Co atoms. At the later stage of milling partial re-oxidation of Co and compounding with aluminum takes place. The neighboring structure of cobalt atoms is widely distributed, as confirmed by the separate ⁵⁹Co NMR signals.