Electronic Spin Relaxation and Clustering in High Pressure High Temperature Synthesized Microcrystalline Diamond...

The negatively charged Nitrogen-vacancy (NV^-) color center in diamond is widely studied because of numerous applications of this unique quantum system in sensing and quantum information sciences. While substitutional nitrogen is required to form NV^- centers, it also yields other paramagnetic defects - primarily the neutrally charged substitutional nitrogen centers (P1) - that decrease NV^- spin coherence which in turn degrades performance in applications. Herein, we investigate high-pressure high-temperature synthesized diamond microparticles (ca. 140-185 μm) having lower - ranging from 3 to 38 ppm - than the typical nitrogen content of type 1b diamond (ca. 100 ppm and higher) typically used for production of fluorescent diamond particles with NV^- centers. A suite of electron paramagnetic resonance, optically detected magnetic resonance, and nuclear magnetic resonance methods are used to characterize spin properties of P1 and NV^- centers in the particles. Upon decreasing the nitrogen content from 29 ppm to 3 ppm, the ensemble NV^- $T_2$ relaxation time increased by about 3-fold as measured directly in the Hahn Echo experiment at 1.2 Tesla. Analysis of electronic relaxation of P1 centers revealed the existence of at least two distinct populations of P1 centers, consisting of fast and slower relaxing spins. Even with <10 ppm nitrogen contents, the analysis indicated a highly heterogenous distribution of P1 centers, suggesting the possibility of P1 spin clustering in even at low concentrations. The combined data demonstrate that the particles prepared from HPHT diamond with a low nitrogen content offer improved spin properties that are beneficial for NV^- sensing applications.