  Pressure-induced magnetic phase transition is attracting interest due to its
ability to detect superconducting behaviour at high pressures in diamond anvil
cells. However, detection of the local sample magnetic properties is a great
challenge due to the small sample chamber volume. Recently, optically detected
magnetic resonance (ODMR) of nitrogen vacancy (NV) centers in diamond have been
used for in-situ pressure-induced phase transition detection. However, owing to
their four orientation axes and temperature-dependent zero-field-splitting,
interpreting the observed ODMR spectra of NV centers remain challenging. Here,
we study the optical and spin properties of 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 Nd2Fe14B at about 7 GPa and map the critical temperature-pressure
phase diagram of the superconductor YBa2Cu3O6.6. These results highlight the
potential of silicon vacancy-based quantum sensors for in-situ magnetic
detection at high pressures.
