  Nitrogen vacancy (NV) centres in diamond are attractive as quantum sensors
owing to their superb coherence under ambient conditions. However, the NV
centre spin resonances are relatively insensitive to some important parameters
such as temperature. Here we design and experimentally demonstrate a hybrid
nano-thermometer composed of NV centres and a magnetic nanoparticle (MNP), in
which the temperature sensitivity is enhanced by the critical magnetization of
the MNP near the ferromagnetic-paramagnetic transition temperature. The
temperature susceptibility of the NV center spin resonance reached 14 MHz/K,
enhanced from the value without the MNP by two orders of magnitude. The
sensitivity of a hybrid nano-thermometer composed of a Cu_{1-x}Ni_{x} MNP and a
nanodiamond was measured to be 11 mK/Hz^{1/2} under ambient conditions. With
such high-sensitivity, we monitored nanometer-scale temperature variation of
0.3 degree with a time resolution of 60 msec. This hybrid nano-thermometer
provides a novel approach to studying a broad range of thermal processes at
nanoscales such as nano-plasmonics, sub-cellular heat-stimulated processes,
thermodynamics of nanostructures, and thermal remanent magnetization of
nanoparticles.
