Fig. 4

a Background subtraction in EPR signal of DND and b estimations of the double-integrated intensities of the g = 4.27 line for a DND sample and reference fluorescent Ib HPHT synthetic diamond. Panel a: as-registered first derivative EPR signal of the DND in the ± 15 mT half magnetic field range (curve 1, blue); the same, but integrated EPR signal in the same ± 15 mT range of magnetic field (curve 2); the same first derivative EPR signal of DND, but after subtraction of the broad parasitic EPR signal from remaining non-removable iron-containing paramagnetic complexes shown by red contour of Lorentzian shape in the upper curve (curve 3, blue). Panel b: Estimation of the double integrated intensity of the g = 4.27 line for a fluorescent Ib HPHT diamond having NV⁻ (upper curve, shaded area) and the DND sample (bottom curve, shadow area). The bottom spectrum in panel b consists of two contours of Lorentzian shape, one of which centred at lower magnetic field (≈ 150.932 mT), is assigned to the NV⁻ centres of DND (the area below this contour in red is highlighted). Microwave frequency: ν = 9.0785 GHz