Quantum Sensing using NV Microscope*

The nitrogen-vacancy (NV) center is a point defect in diamond, where a nitrogen atom replaces a carbon atom near a vacancy. These NV centers have electron spin states that are highly sensitive to external magnetic fields, electric fields, and temperature, making them ideal for quantum sensing. In an NV-based magnetometer, the NV spin state is optically initialized with a green laser and read out via photoluminescence (PL), while a tunable microwave source drives transitions between spin states. The local magnetic field is inferred from the splitting of electron spin resonance (ESR) dips, which is proportional to the field strength.

In a scanning NV microscope, the conventional AFM probe is replaced by a diamond probe with an NV center at its tip, enabling high-resolution nanoscale magnetic field imaging. Advanced techniques like T₁​ relaxometry can also be used, where the NV spin relaxation time (T₁​) provides contrast for detecting local magnetic noise and temperature variations, broadening the microscope's range of applications.

In this talk, I will introduce the scanning NV microscope and demonstrate how I use it to study the magnetic properties of nanomaterials, including FeSe flakes, CoFeB nanomagnets, and Sr₂FeReO₆. In the CoFeB nanomagnet system, we detected spontaneous magnetic moment switching by monitoring time-dependent NV fluorescence. The size-dependent behavior was investigated, revealing decreased stability as the size scales down. We found that T₁ relaxometry and noise spectroscopy are powerful tools for probing this kind of probabilistic switching behavior. I will also introduce how we apply NV microscope to study phase transition in magnetic materials.