We have studied the electron spin resonance (ESR) of a 0.59 carat synthetic diamond single crystal at room temperature. The crystal was grown on a "split-sphere" apparatus in the Fe-Ni-C system by the temperature gradient method. After high-temperature/high-pressure treatment of the diamond, it was observed that as the microwave power supplied to the sample increased from 70 µW to 70 mW in an H 102 cavity, the ESR signal from the P1 center (a nitrogen atom substituting for carbon at a lattice point of the diamond crystal: C-form nitrogen) is inverted. In the original diamond (before high-temperature/high-pressure treatment), no inversion of the ESR signal was observed.Key words: electron spin resonance, signal inversion, high-temperature/high-pressure treatment of diamond, nickel in diamond, P1 center.Introduction. Diamond crystals in practice are not considered as a promising working substance for masers (see, for example, the review articles in [1]). However, an inverted population of the spin levels (inverted ESR signal) for the paramagnetic center (nitrogen atom + carbon vacancy) in diamond crystals was observed more than 25 years ago [2] when they were illuminated with photon energies smaller than the bandgap width (see [3]).In [4], in Fig. 7 in the complex ESR spectrum of a type IIa synthetic diamond crystal implanted with phosphorus ions, inversion of three lines (signals) is quite pronounced (at T = 200 K and microwave power 20 mW). From this figure it can be determined that the low-field and high-field ESR lines are at a distance of 3.36 mT from the central line. Since the magnetic induction B is parallel to the [100] crystallographic direction, such a line splitting corresponds to the P1 center in diamond [5]. However, the authors of [4] did not take this into consideration. Furthermore, in the ESR spectrum of a natural diamond crystal shown in [6] in Fig. 1f, at room temperature inversion can be observed in the line for resonant absorption by electron spins.Earlier in [7], inversion of the ESR signal was observed at room temperature in synthetic diamond crystals (the induction vector B of the polarizing magnetic field was parallel to the [111] direction) that were grown on "splitsphere" type apparatus in the Fe-Ni-C system and subjected to high-temperature/high-pressure treatment on the same apparatus. The synthesis technology, the high-temperature/high-pressure treatment, and the optical properties of such diamonds are described in [8,9].This work was devoted to study of the inversion of the ESR signal from P1 centers in a synthetic diamond single crystal, grown in the Fe-Ni-C system at a pressure of 5.4-5.5 GPa and a temperature of 1750-1800 K for 65 h, and then subjected to high-temperature/high-pressure treatment (6.8 GPa, 2250 K, 12 h). In order to confirm the results of [7], we used an external spin label for orientation of the induction vector BN[100].The experiment. The ESR spectra were recorded under normal conditions (at room temperature, in the dark, in air) on a RadioPAN SE/X-2543 spectrome...
