The implantation of diamonds with helium ions has become a common method to create hundreds-nanometers-thick near-surface layers of NV centers for high-sensitivity sensing and imaging applications; however, optimal implantation dose and annealing temperature are still a matter of discussion. In this study, we irradiated HPHT diamonds with an initial nitrogen concentration of 100 ppm using different implantation doses of helium ions to create 200-nm thick NV layers. We compare a previously considered optimal implantation dose of ∼${10}^{12}$ He${}^{+}/$cm${}^2$ to double and triple doses by measuring fluorescence intensity, contrast, and linewidth of magnetic resonances, as well as longitudinal and transversal relaxation times $T_1$ and $T_2$. From these direct measurements, we also estimate concentrations of P1 and NV centers. In addition, we compare the three diamond samples that underwent three consequent annealing steps to quantify the impact of processing at 1100 ${}^{°}$C, which follows initial annealing at 800 ${}^{°}$C. By tripling the implantation dose, we have increased the magnetic sensitivity of our sensors by $28\pm 5$%. By projecting our results to higher implantation doses, we demonstrate that it is possible to achieve a further improvement of up to 70%. At the same time, additional annealing steps at 1100 ${}^{°}$C improve the sensitivity only by 6.6 ± 2.7%.

In this work, we present a novel, compact, power efficient and variable magnetic field source design for magnetic field imaging microscopy. The device is based on a pair of diametrically magnetized permanent magnet cylinders with electro-mechanical rotation control and ferrite flux homogenizers. A Hall probe and NV centers in diamond are used to demonstrate a proof of concept of a proposed magnetic field setup and to characterize the homogeneity of the produced magnetic field on a micrometer scale. Numerical simulation results are compared with experimental results showing good agreement of the distribution of the magnetic field in the setup. As a result, a magnetic field source with a tunable field amplitude in the range from 1 mT to 222 mT is demonstrated, achieving a magnetic field homogeneity of 2 ppm/μm or 0.5 μT/μm at 222 mT in a 25 × 25 μm field of view.