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Introduction to βNMR

Nuclear magnetic resonance (NMR) and related nuclear methods are widely used in condensed matter physics. The magnetic moment of a nucleus acts as a sensitive probe of the local magnetic and electronic environment. All forms of magnetic resonance require generation of nuclear spin polarization out of equilibrium followed by a detection of how that polarization evolves in time. The spin precession rate or Larmor frequency is a measure of the local magnetic field at the nucleus, whereas the spin relaxation rate is determined by spin dynamics near the Larmor frequency. However, there are also significant differences which influence the specific applications. For example, in conventional magnetic resonance a relatively small nuclear polarization is generated by applying a large magnetic field after which it is tilted with a small RF magnetic field. An inductive pickup coil is used to detect the resulting precession of the nuclear magnetization. Typically one needs about 10¹⁸ nuclear spins to generate a good NMR signal with stable nuclei. Consequently conventional NMR is mostly a bulk probe of matter. On the other hand, in related nuclear methods such as muon spin rotation (μSR) or β-detected NMR (β-NMR) a beam of highly polarized radioactive nuclei (or muons) is generated and then implanted into the material. The polarization tends to be much higher – between 10% and 100%. Most importantly, the time evolution of the spin polarization is monitored through the anisotropic decay properties of the nucleus or muon which requires about 10 orders of magnitude fewer spins. For this reason nuclear methods are well suited to studies of dilute impurities, small structures or interfaces where there are few nuclear spins.

The principles of β-NMR are almost identical to μSR. As in μSR the nuclear detection method allows experiments to be performed in any magnetic field including zero field. Condensed matter applications generally require a high signal to noise which means high polarization and high rate. So far this has been much easier to achieve in the case of muons compared to radioactive nuclei. However, at radioactive ion beam facilities such as ISOLDE and ISAC it is possible to generate intense (>10⁸/s) highly polarized (80%) beams of low energy radioactive nuclei. Under these circumstances one can realize a significant enhancement in the signal to noise in β-NMR . Furthermore one has the added possibility to control the depth of implantation on an interesting length scale (6–400 nm).