  We calculated the magnetic anisotropy constant Ku of MnBi using a first
principles approach, and obtained a negative Ku in agreeable with experimental
results. Furthermore, we also found a band filling dependence indicating that a
slight decrease in the valence electron number will change Ku from negative to
positive. When some of the Bi is replaced with Sn to decrease the valence
electron number, the Ku value of MnBi1-xSnx drastically changes to a positive
value, Ku~2 MJ/m3, for x > 0.05.
