Change in the short range order in the Al - γ-MnOOH mixed system by mechanical stressing

The process of complex formation between Al and γ-MnOOH by milling was studied by ²⁷Al-NMR, ESR and SQUID. With the progress of milling, partial amorphization took place with a swift loss of Al crystallinity. Nano-scale homogeneity increased rapidly from 10% to 70% in the first 3h of milling and approached 90% after milling for 30h. The ESR signal with g=2.009±0.003 and the hyperfine structure parameter, |A|, 81.4×10^-4cm^-1, appeared and increased with milling time, indicating partial and gradual change in the ionic Mn state from Mn(III) to Mn(II). At the same time, the magnetization increased, at temperatures lower than 50K. This is also attributed to the yield of the paramagnetic Mn(II) ions, i.e., and the evolution of paramagnetism from the temperature insensitive helimagnetism of the intact γ-MnOOH. A new peak at around 0 ppm appeared in ²⁷Al-NMR spectra after milling a mixture, indicating the partial oxidation of Al. From the observations, we conclude that O^2- and/or OH^- ions transfer from γ-MnOOH to the vicinity of Al atoms, resulting in a redox reaction during milling. A linear relationship with negative inclination was obtained between the Knight shift (K) of ²⁷Al-NMR and molar susceptibility (χ) for a milled mixture characterized in the temperature range of 10-290K. This fact suggests that Mn atoms are close to Al atoms and induce the hyperfine field at Al nuclei.