  In this article, we assign the $Z_c^\pm(3900)$ to be the diquark-antidiquark
type axialvector tetraquark state, study its magnetic moment with the QCD sum
rules in the external weak electromagnetic field by carrying out the operator
product expansion up to the vacuum condensates of dimension 8. We pay special
attention to matching the hadron side with the QCD side of the correlation
function to obtain solid duality, the routine can be applied to study other
electromagnetic properties of the exotic particles.
