  The effects of spin-orbit coupling and symmetry breaking at the interface
between a ferromagnet and heavy metal are particularly important for spin-based
information storage and computation. Recent discoveries suggest they can create
chiral spin structures (e.g. skyrmions), which have often been identified
through the appearance of the bump/dip features of Hall signals, the so-called
topological Hall effect (THE). In this work, however, we have present an
unusual anomalous Hall effect (UAHE) in MnxGa/Pt bilayers and demonstrated that
the features extremely similar to THE can be generated without involving any
chiral spin structures. The low temperature magnetic force microscopy has been
used to explore the magnetic field-dependent behavior of spin structures, and
the UAHE as a function of magnetic field does not peak near the maximal density
of magnetic bubbles. The results unambiguously evidence that the UAHE in
MnxGa/Pt bilayers shows no correlation with chiral spin structures but is
driven by the modified interfacial properties. The bump/dip features of Hall
signals cannot be taken as an unambiguous signature for the emergence of chiral
spin structures, and a wealth of underlying and interesting physics need
explored.
