  Superconductivity and long-range ferromagnetism hardly coexist in a uniform
manner. The counter-example has been observed, in uranium-based superconductors
for instance, with a coexisting temperature limited to about 1 K. Here, we
report the coexistence of high temperature superconductivity and itinerant
ferromagnetism in lithium intercalated FeSe flakes. In superconducting samples
with transition temperature around 40 K, we observe the anomalous Hall effect
with a hysteresis loop in transverse resistivity and a butterfly-like pattern
of magneto-resistance. Intriguingly, such ferromagnetism persists down to a
temperature at which the zero-field resistance fully vanishes. Furthermore, the
superconductivity is enhanced under an in-plane magnetic field, suggestive of
the participation of spin-polarized Cooper pairs. The surprising finding
underscores a uniform coexistence of the two antagonistic phenomena on a
record-high energy scale.
