Mössbauer investigations of MDAS diamond powder

Metal inclusions in synthetic MDAS diamond grits were investigated by Mössbauer spectroscopy as a function of the grit size. The larger grit sizes show a nicely developed sextet, which collapses to a paramagnetic singlet with decreasing grit size. This superparamagnetic behavior shows that the metal inclusions are of the order of 10 nm in size, and become larger with the grit size. One grit which was showing the collapse was further investigated from 300 K down to 4 K. Part of the paramagnetic singlet unfolds to a sextet with B_hf ∼33T, while another part unfolds with a narrower hyperfine magnetic field of ∼4T and is associated with the paramagnetic phase that coexists with a ferromagnetic phase in Invar at higher temperatures. A singlet with an isomer shift of about 1.1 mm/s is associated with similar findings in catalysts with iron particles supported on carbonaceous materials.