## 2 Materials and Methods  
To study the magnetic properties, single-crystal natural diamond grains of IIa type with an average \"diameter\" of \(\approx\)0.5 mm, irradiated with fast reactor neutrons with fluences close to the critical damage level of diamond, were selected according to their Raman data. At the critical level of radiation damage, diamonds are irreversibly transformed into a graphite-like material during subsequent high-temperature annealing. Diamonds irradiated with fast neutrons are better suited for studying the radiation-induced transformations of the crystal structure than diamonds after ion implantation. The interaction cross-section of neutrons with carbon atoms is small; therefore, the radiation damage due to these interactions is uniformly distributed over the whole crystal volume. As reference samples, we used natural and CVD diamonds, both irradiated with fast neutrons and implanted with helium ions.  
Irradiation with fast neutrons was carried out in the nuclear reactor of the National Research Center \"Kurchatov Institute\" [52] and in the IVV-2M research nuclear reactor of the Institute of Metal Physics of Ural Branch of Russian Academy of Sciences [48], while implantation with helium ions was performed at the \"High Voltage Engineering Europa B.V.\" ion accelerator. Raman spectra were measured under laboratory conditions on Horiba Jobin Yvon Lab-RAM HR (Lille, France) and Nanofinder HE (Louis TII-Tokyo Instruments, Tokyo, Japan) spectrometers with excitation at 473 and 532 nm, respectively. The resolution of these Raman spectrometers was \(\Delta\
u_{\mathrm{R}}=\) 0.5-1.0 cm\({}^{-1}\). The samples were annealed in an oven with graphite walls, in a vacuum of 10\({}^{-5}\) Torr, at temperatures from 200 to 1700 \({}^{\circ}\)C (for 60 min at fixed temperature). After the heat treatment, the samples were etched in a hot H\({}_{2}\)SO\({}_{4}\) + K\({}_{2}\)Cr\({}_{2}\)O\({}_{7}\) solution at \(\approx\)180 \({}^{\circ}\)C, to remove graphite that might be formed on external surfaces. Cleaning of the virgin and irradiated grains of natural diamond (113 pcs of each type, average grain diameter of \(\approx\)0.5 mm) from possible magnetic contamination on the surface was carried out by etching in hot acids, as well as in a quartz test tube (successively filled with alcohol and distilled water) placed in an ultrasonic bath PSB-1335-05 \"Ecoton\" filled with water. The ultrasound frequency was 35 kHz, and the power of the ultrasound generator was 50 W. The processing time of virgin and irradiated diamond grains in each liquid under laboratory conditions was 10 min. Recommendations [51; 53] were taken into account to exclude possible sources of magnetic contamination of diamond grains.  
Magnetic measurements in the range of magnetic field \(B\) from \(-\)8 to 8 T were carried out on a High Field Cryogen Free Measurement System with a Vibrating Sample Magnetometer (Cryogenic Ltd., London, UK) in the temperature range from 2 to 300 K. The magnetic field \(B\) was swept at a rate of up to 0.1 T/min. The samples were placed in a thin-walled hard gelatin ampoule and sealed with dry cotton to fix their position during the measurements. The ampoule with samples was fixed in a thin-walled polypropylene tube mounted at the end of the vibration head rod. The rod vibration frequency was 21 Hz. The oscillation amplitude was in the range of 1-2 mm.