Hematite (α-Fe2O3) is an important ore for the Australian iron mining industry. 57Fe Nuclear Magnetic Resonance (NMR) of hematite is facilitated by the intrinsic hyperfine magnetic field present on the nuclei due to the canted antiferromagnetic (or weakly ferromagnetic) ordering of the substance, removing the need for an external DC magnetic field. Furthermore, the presence of the ferromagnetic NMR enhancement factor[1] amplifies both the Radio-Frequency (RF) field experienced by the 57Fe nucleus, and the signal received from the sample, making signal acquisition times extremely quick.
This first part of this work presents a full characterization of multiple hematite samples covering a wide range of enhancement factors. This includes determination of T1, T2 and the inhomogeneous linewidth of the samples, as well as the spin-echo response as a function of the B1 RF field. The input B1 field corresponding to maximum spin-echo magnitude varies by 2 orders of magnitude across the sample set, suggesting a similar variation in the enhancement factors of the samples.
The ferromagnetic enhancement factor varies wildly between samples due to differences in magnetic domain wall sizes from defects or the particle size of the sample. This makes quantitative NMR of hematite quite challenging. The second part of this work presents a novel method to calibrate for the unknown enhancement factor and thus allow for a quantitative NMR measurement of hematite. With a set of hematite samples of known grade, it is possible to derive an inverse proportionality between the input B1 field resulting in maximum spin echo magnitude and the spin-echo magnitude itself. A sample of unknown grade can thus be calibrated against this curve to determine the amount of hematite present in the sample.