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Pressure is a significant parameter to investigate the properties of matter. The phenomenon of pressure-induced magnetic phase transition has attracted great interest due to its ability to detect superconducting behaviour at high pressures in diamond anvil cell. However, detection of the local sample magnetic properties is a great challenge due to the small sample chamber volume. Recently, in situ pressure-induced phase transition has been detected using optically detected magnetic resonance (ODMR) method of the nitrogen vacancies (NV) centers in diamond. However, the NV centers with four orientation axes and temperature-dependent zero-field-splitting present some difficulty to interpret the observed ODMR spectra. Here, we investigate and characterize the optical and spin properties of the implanted silicon vacancy defects in 4H-SiC, which is single-axis and temperature-independent zero-field-splitting. Using this technique, we observe the magnetic phase transition of a magnetic Nd2Fe14B sample at about 7 GPa. Finally, the critical temperature-pressure phase diagram of the superconductor YBa2Cu3O6.66 is mapped and refined. These experiments pave the way for the silicon vacancy-based quantum sensor being used in situ magnetic detection at high pressures.

Superconducting MgB2–MgO composite with about 75% mole concentration of MgO was synthesized in situ by a single-replacement reaction. The resistance versus temperature curve shows that the composite has a high superconducting transition temperature to 38.0 K and metallic transport behavior with low resistivity and highly residual resistance ratio comparable to a pure MgB2 sample. These characteristics are explained by using the statistical percolation model and a conductivity expression with temperature for the metal–insulator MgB2 composite is given in this paper. The results indicate that MgB2 superconductor can tolerance a high content of insulating contamination and this metal–insulator MgB2–MgO composite can be utilized for the superconducting fault current limiter.