  Nanographene-based magnetism at interfaces offers an avenue to designer
quantum materials towards novel phases of matter and atomic-scale applications.
Key to spintronics applications at the nanoscale is bistable spin-crossover
which however remains to be demonstrated in nanographenes. Here we show that
antiaromatic 1,4-disubstituted pyrazine-embedded nanographene derivatives,
which promote magnetism through oxidation to a non-aromatic radical are
prototypical models for the study of carbon-based thermal spin-crossover.
Scanning tunneling spectroscopy studies reveal symmetric spin excitation
signals which evolve at Tc to a zero-energy peak, and are assigned to the
transition of a S = 3/2 high-spin to a S = 1/2 low-spin state by density
functional theory. At temperatures below and close to the spin-crossover Tc,
the high-spin S= 3/2 excitations evidence pronouncedly different
temperature-dependent excitation energies corresponding to a zero-field
splitting in the Hubbard-Kanamori Hamiltonian. The discovery of thermal spin
crossover and temperature-dependent zero-field splitting in carbon
nanomaterials promises to accelerate quantum information, spintronics and
thermometry at the atomic scale.
