  Destructive interference of electron hopping on the frustrated kagome lattice
generates Dirac nodes, saddle points, and flat bands in the electronic
structure. The latter provides the narrow bands and a peak in the density of
states that can generate correlated electron behavior when the Fermi level lies
within them. In the kagome metal CoSn, this alignment is not realized, and the
compound is a Pauli paramagnet. Here we show that replacing part of the tin
with indium (CoSn1-xInx) moves the Fermi energy into the flat band region, with
support from band structure calculations, heat capacity measurements, and angle
resolved photoemission spectroscopy. The associated instability results in the
emergence of itinerant antiferromagnetism with a Neel temperature up to 30K.
Long range magnetic order is confirmed by neutron diffraction measurements,
which indicate an ordered magnetic moment of 0.1-0.2 Bohr magnetons per Co (for
x = 0.4). Thus, CoSn1-xInx provides a rare example of an itinerant
antiferromagnet with a small ordered moment. This work provides clear evidence
that flat bands arising from frustrated lattices in bulk crystals represent a
viable route to new physics, evidenced here by the emergence of magnetic order
upon introducing a non-magnetic dopant into a non-magnetic kagome metal.
