Magnetic phase of the Fe-containing inclusions in synthetic diamond grits

Abstract

Synthetic diamond grits used in cutting and polishing tools are produced in the high-temperature high-pressure process in which Fe/Ni or Fe/Co alloys are used as solvent-catalysts. When the grits crystallize some of the solvent-catalyst metal is trapped as inclusions in the crystals. The size of the inclusions play an important role in the durability of the grits. In order to obtain information on the size and composition of the inclusions in MDAS ® 1 1 ®De Beers Industrial Diamonds (Pty.) Ltd. synthetic diamond grits we have performed magnetization measurements as well as elemental analysis of the inclusions in MDAS grits of US mesh 400–500 (mean size, d=30–38 μm) and mesh 200–230 ( d=63–75 μm). Temperature- and field-dependent magnetization measurements were made with a SQUID magnetometer. The ZFC/FC behaviour of both sets of grits supported the conclusion that the Curie temperature of the Fe-containing inclusions was above room temperature. Hysteresis was observed in the static field dependence of the magnetization, with coercive fields H c of 40 and 70 Oe for the inclusions in the 63–75 and 30–38 μm grits, respectively. Within the temperature range of our measurements (10–300 K) the normalized magnetization curves did not scale as H appl/ T, which would be expected for superparamagnetic, single domain particles. The magnetization results when combined with our recent Mössbauer spectroscopy of the same set of grits lead to the conclusion that the collapse of magnetic order in the inclusions is not due to the onset of superparamagnetic behaviour, but to the inclusions containing several different Fe/Ni (and Fe/Co) phases. This is supported by elemental analysis of the inclusions which show that α-Fe and several Fe–Ni (and Fe–Co) phases are present with concentrations ranging over at least two orders of magnitude.