  The elliptic flow of muons from the decay of charm and bottom hadrons is
measured in $pp$ collisions at $\sqrt{s}=13$ TeV using a data sample with an
integrated luminosity of 150 pb$^{-1}$ recorded by the ATLAS detector at the
LHC. The muons from heavy-flavor decay are separated from light-hadron decay
muons using momentum imbalance between the tracking and muon spectrometers. The
heavy-flavor decay muons are further separated into those from charm decay and
those from bottom decay using the distance-of-closest-approach to the collision
vertex. The measurement is performed for muons in the transverse momentum range
4-7 GeV and pseudorapidity range $|\eta|<2.4$. A significant non-zero elliptic
anisotropy coefficient $v_{2}$ is observed for muons from charm decays, while
the $v_{2}$ value for muons from bottom decays is consistent with zero within
uncertainties.
