On the Use of 31Mg for β-Detected NMR Studies of Solids

It has long been desirable to develop a spin-1/2 nucleus as a probe for β-detected NMR studies of solid materials. As a pure magnetic probe, it would greatly compliment our most extensively used nucleus ⁸Li (spin-2), which is sensitive to both dipolar and quadrupolar interactions. Following the recent successful polarization of spin-1/2 ³¹Mg at TRIUMF, we report here on its use to study solids where host magnesium atoms occupy lattice sites with cubic symmetry — a situation most directly comparable to (stable) ²⁵Mg NMR. We show preliminary results in: rocksalt MgO; spinel MgAl₂O₄; and antifluorite Mg₂Si, which is an interesting semiconducting thermoelectric. Deviations from the results with conventional ²⁵Mg NMR are attributed to the systematics associated with the β-NMR technique. The ³¹Mg decay chain is found to complicate the extraction of spin-lattice relaxation rates due to a time-dependent background signal introduced by its progeny. The prospect of further use of ³¹Mg as a β-NMR probe for the study of condensed matter is good.