  We investigated the evolution of ferromagnetism in layered Fe$_3$GeTe$_2$
flakes under different pressures and temperatures using in situ magnetic
circular dichroism (MCD) spectroscopy. We found that the rectangle shape of
hysteretic loop under an out-of-plane magnetic field sweep can sustain below 7
GPa. Above that pressure, an intermediate state appears at low temperature
region signaled by an 8-shaped skew hysteretic loop. Meanwhile, the coercive
field and Curie temperature decrease with increasing pressures, implying the
decrease of the exchange interaction and the magneto-crystalline anisotropy
under pressures. The intermediate phase has a labyrinthine domain structure,
which is attributed to the increase of ratio of exchange interaction to
magneto-crystalline anisotropy based on Jagla's theory. Moreover, our
calculation results reveal a weak structural transition around 6 GPa, which
leads to a drop of the magnetic momentum of Fe ions.
