We present two fiberized vector magnetic-field sensors, based on nitrogen-vacancy (NV) centers in diamond. The sensors feature sub-nT/Hz magnetic sensitivity. We use commercially available components to construct sensors with a small sensor size, high photon collection, and minimal sensor-sample distance. Both sensors are located at the end of optical fibres with the sensor-head freely accessible and robust under movement. These features make them ideal for mapping magnetic fields with high sensitivity and spatial resolution (≤ mm). As a demonstration we use one of the sensors to map the vector magnetic field inside the bore of a ≥100 mT Halbach array. The vector field sensing protocol translates microwave spectroscopy data addressing all diamonds axes and including double quantum transitions to a 3D magnetic field vector.

We study cross-relaxation features near zero magnetic field with ensembles of nitrogen-vacancy (NV) centers in diamond and examine their properties in samples with a range (0.9 ppm -16.0 ppm) of NV concentrations. The observed NV-NV cross-relaxation features between differently oriented NV centers in high ( 0.9 ppm)-NV-density samples hold promise for a variety of magnetometry applications where microwave fields (or any bias field) disturb the system under study. We theoretically determine the values of the bias magnetic fields corresponding to cross-relaxations between different axes and experimentally validate them. The behavior of zero-field cross-relaxation features as a function of temperature is also investigated. The sensitivity of magnetometry based on this method is determined to be in the 10 nT/ √ Hz range.

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