Unconventional Magnetization below 25 K in Nitrogen-doped Diamond provides hints for the existence of Superconductivity and Superparamagnetism

Abstract

The magnetization of nitrogen-doped single crystalline diamond bulk samples shows unconventional field and temperature hysteresis loops at T 25 K. The results suggest the existence of superparamagnetic and superconducting regions in samples with nitrogen concentration <200 ppm. Both phases vanish at temperatures above 25 K where the samples show diamagnetic behavior similar to undoped diamond. The observation of superparamagnetism and superconductivity is attributed to the nitrogen doping and to the existence of defective regions. From particle-induced X-ray emission with ppm resolution we rule out that the main observations below 25 K are due to magnetic impurities. We investigated also the magnetic properties of ferromagnetic/high-temperature superconducting oxide bilayers. The magnetization results obtained from those bilayers show remarkable similarities to the ones in nitrogen-doped diamond.

Weak Electron Irradiation Suppresses the Anomalous Magnetization of N‐Doped Diamond Crystals

“…Measurements at other field directions were also done to check for the existence of any magnetic anisotropy. The magnetic moment did not show any anisotropy within error, in agreement with the reported data in the study by Barzola-Quiquia et al [26] We note that the high sample-temperature stabilized during irradiation is not the reason for the changes we discuss in the following text. To prove this, we show in Figure 1 the magnetization field hysteresis loop at 2 K of sample 4 before and after annealing at 1200 C in vacuum for 4 h. The field loops indicate that the main field hysteresis does not change significantly after annealing.…”

“…The origin (a) can be ruled out because the measured behavior does not depend on the direction of the applied field with respect to the main axes of the crystals nor on their shape. [26] This independence has been confirmed once more in the diamond crystals studied in this work.…”

“…Measured samples with their main characteristics and treatments. ð⋆Þ: These samples were previously characterized in the study by Barzola-Quiquia et al [26] The concentration of PM centers from SQUID measurements was obtained from the linear slope of the magnetic susceptibility versus 1=T assuming J ¼ 1=2 and g J ¼ 2 and has an experimental error ≲ AE 7 ppm. The error of the average C center concentration obtained from EPR is % 15%.…”

“…In a first published study of similar diamond crystals, a correlation between the magnitude of the anomalous magnetization below 30 K and the C centers was found. [26] Therefore, we characterized the concentration and the space distribution of this magnetic defect. The characterization of this centers in diamond using EPR spectra obtained at room temperature of samples 1 (asreceived) and 2 (after irradiation), see Table 1, for B 0 k [111] showing the signals from: a) C centers and b) from NV À centers.…”

“…The relatively low average N content and further details of the measured diamond samples suggest the existence of a granular structure in the concentration of nitrogen. [26] To facilitate the comparison of the present results with those in the study by Barzola-Quiquia et al, [26] we give a summary of the main results obtained from the measurements of eight different nitrogen-doped crystals: a) All samples with nitrogen concentration below 120 ppm show unconventional magnetic moment behavior in the field hysteresis and temperature dependence below % 30 K. As example, we show in Figure 1, the field hysteresis of one of the measured samples in this study, before and after annealing. Above % 30 K all samples behave as a typical undoped diamagnetic diamond.…”