Probing the Fluctuating Magnetic Field of Fe-Triazole Spin-Crossover Thin Layers with Nitrogen-Vacancy Centers in Diamond

The magnetic properties of FeII spin-crossover (SCO) complexes can be changed upon temperature variation, often exhibiting thermal hysteresis. Particularly interesting for magnetic-memory applications are thin layers of SCO complexes, where practical magnetic probing techniques are required. While conventional magnetometry on SCO complexes employs cryogenic temperatures, nitrogen-vacancy (NV) centers are quantum magnetometers that can operate at room temperature with high spatial resolution and magnetic-field sensitivity. In this work, we apply thin layers of Fe-triazole SCO complexes onto a single-crystal diamond with shallow NV centers and probe the fluctuating magnetic field. We combine a wide-field technique with temperature-dependent measurements of the NV centers' longitudinal spin-relaxation time T 1 and the decoherence time T 2 to find that the complexes are paramagnetic in the investigated temperature range from 20 to 80 degrees C. We quantitatively describe the T 1 time by a model considering the fluctuating magnetic field of the FeII ions. While we see signatures of a local change in spin state in the T 1 relaxometry data, apparent structural changes in the SCO material dominate the local magnetic environment of the NV centers. The results for the T 2 time contrast the findings of the T 1 times for the SCO complexes, which we attribute to different NV detection sensitivities toward FeII and FeIII of the protocols. Our results on the magnetic properties of SCO materials highlight the capabilities of the NV center as a susceptible sensor for fluctuating magnetic fields. At the same time, spin switching of the complexes cannot be observed.