  The weak antilocalization (WAL) effect is known as a quantum correction to
the classical conductivity, which never appeared in two-dimensional magnets. In
this work, we reported the observation of a WAL effect in the van der Waals
ferromagnet Fe5-xGeTe2 with a Curie temperature Tc ~ 270 K, which can even
reach as high as ~ 120 K. The WAL effect could be well described by the
Hikami-Larkin-Nagaoka and Maekawa-Fukuyama theories in the presence of strong
spin-orbit coupling (SOC). Moreover, A crossover from a peak to dip behavior
around 60 K in both the magnetoresistance and magnetoconductance was observed,
which could be ascribed to a rare example of temperature driven Lifshitz
transition as indicated by the angle-resolved photoemission spectroscopy
measurements and first principles calculations. The reflective magnetic
circular dichroism measurements indicate a possible spin reorientation that
kills the WAL effect above 120 K. Our findings present a rare example of WAL
effect in two-dimensional ferromagnet and also a magnetotransport fingerprint
of the strong SOC in Fe5-xGeTe2. The results would be instructive for
understanding the interaction Hamiltonian for such high Tc itinerant
ferromagnetism as well as be helpful for the design of next-generation room
temperature spintronic or twistronic devices.
