  Hexagonal D019-type Mn3Z alloys that possess large anomalous and
topological-like Hall effects have attracted much attention due to their great
potential in the antiferromagnetic spintronic devices. Here, we report the
preparation of Mn3Ga film in both tetragonal and hexagonal phases with a tuned
Ta/Ru seed layer on the thermally oxidized Si substrate. A large coercivity
together with a large anomalous Hall resistivity is found in the Ta-only sample
with mixed tetragonal phase. By increasing the thickness of Ru layer, the
tetragonal phase gradually disappears and a relatively pure hexagonal phase is
obtained in the Ta(5)/Ru(30) buffered sample. Further magnetic and transport
measurements revealed that the anomalous Hall conductivity nearly vanishes in
the pure hexagonal sample, while an abnormal asymmetric hump structure emerges
in the low field region. The extracted additional Hall term is robust in a
large temperature range and presents a sign reversal above 200K. The abnormal
Hall properties are proposed to be closely related with the frustrated spin
structure of D019 Mn3Ga.
