  Strong magnetic field induces at least two phase transitions in graphite
beyond the quantum limit where many-body effects are expected. We report on a
study using a state-of-the-art non-destructive magnet allowing to attain 90.5 T
at 1.4 K, which reveals a new field-induced phase and evidence that the
insulating state destroyed at 75 T is an excitonic condensate of electron-hole
pairs. By monitoring the angle dependence of in-plane and out-of-plane
magnetoresistance, we distinguish between the role of cyclotron and Zeeman
energies in driving various phase transitions. We find that, with the notable
exception of the transition field separating the two insulating states, the
threshold magnetic field for all other transitions display an exact cosine
angular dependence. Remarkably, the threshold field for the destruction of the
second insulator (phase B) is temperature-independent with no detectable
Landau-level crossing nearby. We conclude that the field-induced insulator
starts as a weak-coupling spin-density-wave, but ends as a strong-coupling
excitonic insulator of spin-polarized electron-hole pairs.
