  The fine control of magnetism and electronic structure is crucial since the
interplay between magnetism and band topology can lead to various novel
magnetic topological states including axion insulators, magnetic Weyl
semimetals and Chern insulators etc. Through crystal growth, transport,
thermodynamic, neutron diffraction measurements, we show that with Sb-doping,
the newly-discovered intrinsic antiferromagnetic topological insulator MnBi4Te7
evolves from antiferro-magnetic to ferromagnetic and then ferrimagnetic. We
attribute this to the formation of Mn(Bi,Sb) antisites upon doping, which
result in additional Mn sublattices that modify the delicate interlayer
magnetic interactions and cause the dominant Mn sublattice to go from
antiferromagnetic to ferro-magnetic. We further investigate the effect of
antisites on the band topology using the first-principles calculations. Without
considering antisites, the series evolves from antiferromagnetic topological
insulator (x = 0) to ferromagnetic axion insulators. In the exaggerated case of
16.7% of periodic antisites, the band topology is modified and type-I magnetic
Weyl semimetal phase can be realized at intermediate dopings. Therefore, this
doping series provides a fruitful platform with continuously tunable magnetism
and topology for investigating emergent phenomena, including quantum anomalous
Hall effect, Fermi arc states, etc.
