it's not actually a halo, but the angle of the top of the left wing of Victory coupled with the lighting makes it look like a halo.
with a different picture it looks less halo like.
Constantine I
A.D. 323-4
18x19mm 2.6g
CONSTANTINVS AVG; laureate head right.
SARMATIA DEVICTA; Victory advancing r., holding trophy, palm branch, spurning captive std. on ground to right.
In ex. PTR crescent
RIC VII Trier 435
