We have studied in earlier chapters that spin-based techniques like neutron scattering, muon spin resonance spectroscopy, and nuclear magnetic resonance can give detailed information on the magnetic structure of a material down to the atomic scale. These techniques, however, cannot provide a real-space image of the magnetic structure and are not sensitive to samples having nanometre-scale volumes. On the other hand, techniques like magnetic force microscopy, scanning hall bars, and superconducting quantum interference devices (SQUIDs) enable real-space imaging of the magnetic fields in nanometre-scale samples. But they have a constraint of finite size, and also they act as perturbative probes working in a rather narrow temperature range. A relatively new technique of magnetometry based on the electron spin associated with the nitrogen-vacancy (NV) defect in diamond combines the powerful aspects of both these classes of experiments. A very impressive combination of capabilities has been demonstrated with NV magnetometry, which sets it apart from other magnetic sensing techniques. That includes room-temperature single-electron and nuclear spin sensitivity, spatial resolution on the nanometre scale, operation under a broad range of temperatures from ∽1 K to above room temperature, and magnetic fields ranging from zero to a few tesla, and most importantly it involves a non-perturbative operation [1]. Here we present a concise introduction to NV magnetometry. There are a few excellent review articles [2, 3] and tutorial article [4] on NV magnetometry, and readers are referred to those for a more detailed exposure to the subject.

Physics of the Nitrogen-Vacancy (NV) Centre in Diamond

Figure 12.1 presents an NV centre in the crystal lattice of a diamond. It is a point defect consisting of a substitutional nitrogen atom and a missing carbon atom in the neighborhood. The NV centres can have negative (NV−), positive (NV+), and neutral (NV0) charge states of which NV− is used for magnetometry [2]. An NV− centre has six electrons, five of which come from the dangling bonds of the three neighbouring carbon atoms and the nitrogen atom. The negative charge state arises from one extra electron captured from an electron donor. The NV axis is defined by the line connecting the nitrogen atom and the vacancy. There can be four NV alignments depending on the four possible positions of the nitrogen atom with respect to the vacancy.