## 3 Results and Discussion
### Magnetic Properties of Diamonds Irradiated with Fast Neutrons  
Figure 5 shows the results of measurements of the magnetic moment \(M\) of natural diamond grains irradiated with fast neutrons and selected by Raman spectroscopy (113 pcs, average grain diameter of \(\approx\)0.5 mm) as a function of magnetic field \(B\) in the temperature range from 2 to 300 K. Virgin (unirradiated) diamond grains (also 113 pcs, average grain diameter of \(\approx\)0.5 mm) were diamagnetic in the temperature range from 2 to 300 K with the magnetic susceptibility \(\chi_{\rm diff}\approx-1\times 10^{-5}\) SI units. According to quantum chemical calculations [83], the magnetic susceptibility of unirradiated natural diamond (in SI units [84; 85]) is equal \(\chi_{\rm SI}=4\pi\chi_{\rm cgs}=-(1.9-2.1)\times 10^{-5}\). Handbook [86] gives the value of the specific magnetic susceptibility of natural diamond for \(T=100\)-293 K as \(\chi_{\rm p}=-6.2\times 10^{-6}\) cm\({}^{3}\)/g; considering the density of unirradiated diamond grains \(\rho_{\rm vir}=3.515\) g/cm\({}^{3}\)[87], we have (in SI units) \(\chi=\rho_{\rm vir}\chi_{\rm\rho}=-2.2\times 10^{-5}\). Note that, in the virgin (not irradiated with neutrons) grains of natural diamond, the CW-ESR signal was not observed at room temperature [88]. It was shown there that irradiation of natural diamond grains with neutrons (with fluence \(F\approx 1\times 10^{21}\) cm\({}^{-2}\) above critical level of radiation damage) led to the appearance of an intense CW-ESR signal from metastable paramagnetic centers with paramagnetic relaxation times \(>\)10\({}^{-5}\) s. The possible reason for this signal appearance in irradiated diamond is the metastable non-compensated spins located on the inner surface of nanopores (clusters of vacancies in diamond crystal matrix). According to [89], the energy of the exchange interaction of uncompensated spins is \(J=2k_{\rm B}\Theta/z\), where \(k_{\rm B}\) is the Boltzmann constant. For the number of geometric neighbors in a random system \(z\approx 15.47\)[90] and the Curie-Weiss temperature \(\Theta=150\) K (at which the magnetic moment of irradiated diamond is zero; see Figure 5), we have \(J\approx 1.7\) meV. Note that the observed Curie-Weiss temperature \(\Theta\) is close to the temperature of \(\approx\)145 K, observed for carbon nanofoam (see Figure 3 in [13]). Figure 5: Magnetic moment \(M\) of natural diamond grains (113 pcs) irradiated with fast reactor neutrons (\(F=5\times 10^{20}\) cm\({}^{-2}\)). The measurement absolute temperatures (\(T\) from 2 to 300 K) are shown in the figure. Figure 6 shows the dependences of the differential magnetic susceptibility \(\chi_{\rm diff}\) on \(B\) (calculated from the data in Figure 5 using the formula \(\chi_{\rm diff}(B)=(\mu_{0}/V){\rm d}M/{\rm d}B\), where \(\mu_{0}=1.25\)\(\mu\)H/m is the magnetic constant) for the volume \(V\approx 7.4\) mm\({}^{3}\) of 113 diamond grains irradiated with fast neutrons. It can be seen that the differential magnetic susceptibility of irradiated diamond grains at the temperature of 2 K in the limit of zero external magnetic field (\(B\approx 0\)) is \(\chi_{\rm diff}(B\approx 0)\approx 1.8\times 10^{-3}\) SI units. This value almost coincides with the experiment [48], where the SQUID measurements of neutron irradiated polycrystalline CVD diamond films without metallic impurities for \(F=5\times 10^{20}\) cm\({}^{-2}\) at the temperature \(T=2\) K show \(\chi\approx 1.7\times 10^{-3}\) SI units. CVD polycrystalline diamonds are grown on foreign substrates (typically silicon) with a microwave plasma CVD system using methane-hydrogen mixtures. Such films contained rather high concentration (\(\sim\)10\({}^{20}\) cm\({}^{-3}\)) of bounded hydrogen (CH\({}_{x}\) groups) located at the inter-crystalline boundaries. It was shown earlier [91] that neutrons act on grain boundaries in diamond dislodging hydrogen into the grain bulk for distances of the order of a few micrometers. At the same time, numerous defects are formed, including those with uncompensated spin.