What are NV centers?

NV centers are fluorescent, paramagnetic defects in diamond. Their fluorescence depends on their spin state, which allows us to use the fluorescence to quantitatively map magnetic fields by monitoring the Zeeman shift of the spin states. Rather uniquely, this works at room temperature, and we can utilize single NV defects to build up images with incredible spatial resolution and sensitivity.

What can the NV center do?

The NV center is an excellent magnetometer: it can measure even very small magnetic fields (both static and time-varying) with very high spatial resolution. The sensitivity of our standard experiments (DC field imaging) is typically $2 \mu T/\sqrt{Hz}$, though this can vary center-to-center, and we can resolve features on the tens-of-nanometer scale.

If you can formulate your scientific question in the form: “what’s the magnetic field at x”, NV centers are a great tool. They won’t however, formulate the question for you.

How does scanning NV magnetometry differ from typical NV experiments?

Incorporating the NV center at the end of a diamond AFM tip allows very high spatial resolution images (beyond the optical diffraction limit) to be generated. However, there are a few special considerations:

• Imaging is time-consuming. A typical T1 measurement might take ~1 hour for a single NV center (which isn’t so bad!), but a 100x100 pixel T1 image would take a week to acquire. There is a tradeoff between image size/resolution and experimental complexity. The most common type of experiment is CW ODMR, where that same image can be acquired in a few hours.
• Scanning is non-contact (ideally!). The system operates with a fly-height between the surface and the tip; typically this results in (at least) 60nm between the NV center and the sample. This is fine for imaging stray fields from magnetic materials, but is too far for some typical NV applications (e.g., detection of nuclear spins).
• Spatial resolution is determined by fly-height, not the diffraction limit. The spatial resolution function is determined by how far from the surface the tip is; since the tips are known to have only a single NV center, our optical diffraction limit is not what determines our spatial resolution. One example of this is the imaging of the cycloid in BiFeO3 - the 60nm period of this magnetization can be readily resolved.

A note on M vs B:

In many materials we are interested in understanding how unpaired spins couple together and form ordered states - the magnetism of the system. This is the magnetism, M, of the system, and is a very powerful value to quantify in many cases.

The NV center measures the magnetic field, B, at a point in space. This is related to the magnetism, and for a magnetism, we can calculate the stray magnetic field we expect. However, we cannot make the reverse transformation - if we know B, it is not generally possible to state M**.** There are infinitely many M which can give rise to the same pattern of magnetic fields. This is not a limitation of the NV center - this is a fundamental problem, common to all techniques which measure B.

However, if we know some additional information about M, we can constrain the solutions and make some progress. For example, if we know M always lies in an out-of-plane direction (perhaps from some VSM measurements), then we can calculate the magnetization map which gives rise to our NV center data. For more details on this reconstruction approach, check out the reference below:

Improved Current Density and Magnetization Reconstruction Through Vector Magnetic Field Measurements