Magnetic criticality-enhanced nanodiamond-thermometer under ambient conditions*

Nanoscale temperature sensing is useful for research in physics, chemistry and life science. A nano-thermometer with high sensitivity will unveil many unknowns such as heating dissipation in nano-circuits, energy transfer in nano-scale chemical reaction, and temperature heterogeneities in living cells. Nitrogen vacancy (NV) centers in diamond have been demonstrated as room-temperature atomic quantum sensors due to their superb coherence properties. While NV center spins are sensitive to external magnetic field, they are relatively insensitive to temperature. Here, we designed and experimentally demonstrated a hybrid nanosensor composed of a fluorescence nanodiamond and a magnetic nanoparticle, in which the temperature sensitivity is enhanced by the critical magnetization of the magnetic nanoparticle near the ferromagnetic-paramagnetic phase transition. We experimentally realized a sensitivity of 11 mK/√Hz with NV centers in nanodiamond. The working range of this hybrid sensor can be designed from cryogenic temperature to 600 K by choosing materials with different critical temperatures.