  We report the detection of linearly polarized emission at 53 and 89 $\mu$m,
from the radio-loud active galactic nucleus (AGN) Cygnus A using HAWC+ onboard
SOFIA. We measure a highly polarized core of $11\pm3$% and $9\pm2$% with a
position angle (P.A.) of polarization of $43\pm8^{\circ}$ and $39\pm7^{\circ}$
at 53 and 89 $\mu$m, respectively. We find (1) a synchrotron dominated core
with a flat spectrum ($+0.21\pm0.05$) and a turn-over at $543\pm120$ $\mu$m,
which implies synchrotron emission is insignificant in the infrared (IR), and
(2) a $2-500$ $\mu$m bump peaking at $\sim40$ $\mu$m described by a blackbody
component with color temperature of $107\pm9$ K. The polarized SED has the same
shape as the IR bump of the total flux SED. We observe a change in the P.A. of
polarization of $\sim20^{\circ}$ from 2 to 89 $\mu$m, which suggests a change
of polarization mechanisms. The ultraviolet, optical and near-IR polarization
has been convincingly attributed to scattering by polar dust, consistent with
the usual torus scenario, though this scattered component can only be directly
observed from the core in the near-IR. By contrast, the gradual rotation by
$\sim20^{\circ}$ towards the far-IR, and the near-perfect match between the
total and polarized IR bumps, indicate that dust emission from aligned dust
grains becomes dominant at $10-100$ $\mu$m, with a large polarization of 10% at
a nearly constant P.A. This result suggests that a coherent dusty and magnetic
field structure dominates the $10-100$ $\mu$m emission around the AGN.
