  Topological Hall effect (THE) is a hallmark of scalar spin chirality, which
is found in static skyrmion lattices. Recent theoretical works have shown that
scalar spin chirality could also emerge dynamically from thermal spin
fluctuations. Evidence of such a mechanism was found in the kagome magnet
YMn6Sn6 where fluctuations arise from frustrated exchange interactions between
Mn kagome layers. In YMn6Sn6, the rare-earth ion Y3+ is non-magnetic. When it
is replaced by a magnetic ion (Gd3+-Ho3+), the intrinsically antiferromagnetic
Mn-Mn interlayer coupling is overwhelmed by the indirect ferromagnetic Mn-R-Mn
one, relieving frustration. This generates interesting anomalous Hall
conductivity, but not THE. Here we show that Er lies in an intermediate regime
where direct and indirect interactions closely compete, so that ErMn6Sn6 can
switch from one regime to the other by temperature, i.e., from a collinear
ferrimagnetic ground state to a spiral antiferromagnet at 78 K. The AFM phase
forms a dome in the temperature-field phase diagram. Close to the boundary of
this dome, we find a sizable fluctuations-driven THE, thus underscoring the
universality of this chiral fluctuation mechanism for generating non-zero
scalar spin chirality.
