  In ferromagnets, electric current generally induces transverse Hall voltage
in proportion to magnetization (anomalous Hall effect), and it is frequently
used for electrical readout of the up and down spin states. While these
properties are usually not expected in antiferromagnets, recent theoretical
studies predicted that non-coplanar antiferromagnetic order with finite scalar
spin chirality (i.e. solid angle spanned by neighboring spins) can often induce
large spontaneous Hall effect even without net magnetization or external
magnetic field. This phenomenon, i.e. spontaneous topological Hall effect, can
potentially be used for the efficient electrical readout of the
antiferromagnetic states, but its experimental verification has long been
elusive due to the lack of appropriate materials hosting such exotic magnetism.
Here, we report the discovery of all-in-all-out type non-coplanar
antiferromagnetic order in triangular lattice compounds CoTa3S6 and CoNb3S6, by
performing the detailed magnetic structure analysis based on polarized neutron
scattering experiments as well as systematic first-principles calculations.
These compounds are reported to host unconventionally large spontaneous Hall
effect despite their vanishingly small net magnetization, and our analysis
revealed that it can be well explained in terms of topological Hall effect,
which originates from the fictitious magnetic field associated with scalar spin
chirality in non-coplanar antiferromagnetic orders. The present results
indicate that the scalar spin chirality mechanism can offer a promising route
to realize giant spontaneous Hall response even in compensated
antiferromagnets, and highlight intercalated van der Waals magnets as an unique
quasi-two-dimensional material platform to enable various nontrivial manner of
electrical reading and possible writing of non-coplanar antiferromagnetic
domains.
