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

The magnetic properties of Fe^II 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 <i>T</i>₁ and the decoherence time <i>T</i>₂ to find that the complexes are paramagnetic in the investigated temperature range from 20 to 80 °C. We quantitatively describe the <i>T</i>₁ time by a model considering the fluctuating magnetic field of the Fe^II ions. While we see signatures of a local change in spin state in the <i>T</i>₁ relaxometry data, apparent structural changes in the SCO material dominate the local magnetic environment of the NV centers. The results for the <i>T</i>₂ time contrast the findings of the <i>T</i>₁ times for the SCO complexes, which we attribute to different NV detection sensitivities toward Fe^II and Fe^III 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.