High-Field and Light-Dependent Magnetization Provides Evidence for Coexisting Granular Superconductivity and Superparamagnetism in Nitrogen-Doped Diamond
We investigated the field-, temperature-, and time-dependent magnetic moment of a nitrogen-doped type Ib diamond in darkness and under monochromatic light with a low laser intensity of about 160 $μ$W cm$^{-2}$. In darkness, below approximately 25 K, the magnetic moment exhibits unconventional hysteresis loops that can be explained by the coexistence of superparamagnetism and superconductivity in certain regions of the sample. Under illumination, this hysteresis largely disappears, leaving mainly the superparamagnetic contribution. Temperature-dependent measurements reveal an irreversibility below 25 K in darkness, which vanishes under illumination. Similarly, at 0.5 T and below 25 K, the time-dependent magnetization shows flux-creep-like behavior that disappears when the sample is illuminated. All light-induced changes are fully reversible: the magnetic properties measured in darkness are restored after the sample is heated above roughly 50 K. Removing the superparamagnetic contribution from magnetization measurements up to $\pm 14$ T reveals clear characteristics of a granular superconductor, namely, a well-defined penetration field $H_p \approx 2.6$ T and a critical field $H_{c2} \approx 10.6$ T. Based on theoretical considerations, we attribute the observed phenomena to the combined effects of internal stress and local variations in nitrogen concentration within localized regions that can be regarded as grains. This interpretation is supported by Raman and birefringence spectroscopy measurements on the same sample.