Temperature and field dependent Mossbauer studies of the metallic inclusions in synthetic MDAS diamond grits

Mossbauer spectroscopy of the metallic inclusions in diamond grits produced in high-temperature high-pressure synthesis have revealed varying but interesting results. The MDAS(1) grits synthesized with Fe/Ni solvent-catalysts show Mossbauer spectra that vary with grit size, with the ferromagnetic order observed in the larger grits collapsing as one went to smaller grit sizes. Two sets of De Beers MDAS diamond grits of US mesh size 400-500 (d=30-38 mum) and 200-230 (d=63-75 mum) were selected for temperature- and field-dependent investigations. Transmission Mossbauer measurements were made at temperatures of 300 K and 80 K, in zero field and in an external field of 0.60 T, on the metallic inclusions in these grits. The Mossbauer spectra of the inclusions are rather complex, reflecting the contributions of several different magnetic phases. Our results show that the temperature variation of the Mossbauer spectra is not due to superparamagnetic relaxation of ferromagnetic inclusions but rather to magnetic ordering temperatures of the order of room temperature. Based on the spectral lineshapes and elemental analyses, we suggest the inclusions in the 63-75 mum grits contain iron mainly in Fe-Ni alloys with composition in the vicinity of 70 at.% Fe, and those in the 30-38 mum grits contain, in addition to Fe-Ni alloys, a phase with hyperfine parameters close to those of a Fe-Co alloy.