Magnetic size growth in nanocrystalline copper ferrite

When nanocrystalline copper ferrite (average grain size D≈6 nm) is subjected to high-energy-milling in air over different periods up to 12 h, we observe both a progressive enhancement of the ferriteʹs magnetic response and a shifting of its superparamagnetic limit. These are revealed by the shift to higher values of the Mössbauer blocking temperature, the maximum of the zero-field cooled magnetization and the start of the irreversibility between the zero-field and field-cooled magnetization curves, while the saturation magnetization and the mean magnetic moment per particle increase. The X-ray diffraction data show that the spinel improves its crystallinity with the milling, by increasing the grain size up to ≈13 nm and, also, reducing its micro-strain level. After 10 h of milling the copper ferrite stabilizes in its cubic metastable phase.