  The discovery of ferromagnetism in two-dimensional (2D) monolayers has
stimulated growing research interest in both spintronics and material science.
However, these 2D ferromagnetic layers are mainly prepared through an
incompatible approach for large-scale fabrication and integration, and
moreover, a fundamental question whether the observed ferromagnetism actually
correlates with the 2D crystalline order has not been explored. Here, we choose
a typical 2D ferromagnetic material, Fe3GeTe2, to address these two issues by
investigating its ferromagnetism in an amorphous state. We have fabricated
nanometer-thick amorphous Fe3GeTe2 films approaching the monolayer thickness
limit of crystallized Fe3GeTe2 (0.8 nm) through magnetron sputtering. Compared
to crystallized Fe3GeTe2, we found that the basic ferromagnetic attributes,
such as the Curie temperature that directly reflects magnetic exchange
interactions and local anisotropic energy, do not change significantly in the
amorphous states. This is attributed to that the short-range atomic order, as
confirmed by valence state analysis, is almost the same for both phases. The
persistence of ferromagnetism in the ultrathin amorphous counterpart has also
been confirmed through magnetoresistance measurements, where two unconventional
switching dips arising from electrical transport within domain walls are
clearly observed in the amorphous Fe3GeTe2 single layer. These results indicate
that the long-range ferromagnetic order of crystallized Fe3GeTe2 may not
correlate to the 2D crystalline order and the corresponding ferromagnetic
attributes can be utilized in an amorphous state which suits large-scale
fabrication in a semiconductor technology-compatible manner for spintronics
applications.
