  We present high resolution near-infrared spectropolarimetric observations
using the SPIRou instrument at CFHT during a transit of the recently detected
young planet AU Mic b, with supporting spectroscopic data from iSHELL at IRTF.
We detect Zeeman signatures in the Stokes V profiles, and measure a mean
longitudinal magnetic field of $\overline{B}_\ell=46.3\pm0.7$~G. Rotationally
modulated magnetic spots likely cause long-term variations of the field with a
slope of $d{B_\ell}/dt=-108.7\pm7.7$~G/d. We apply the cross-correlation
technique to measure line profiles and obtain radial velocities through CCF
template matching. We find an empirical linear relationship between radial
velocity and $B_\ell$, which allows us to estimate the radial velocity
variations which stellar activity induces through rotational modulation of
spots for the five hours of continuous monitoring of AU Mic with SPIRou. We
model the corrected radial velocities for the classical Rossiter-McLaughlin
effect, using MCMC to sample the posterior distribution of the model
parameters. This analysis shows that the orbit of AU Mic b is prograde and
aligned with the stellar rotation axis with a sky-projected spin-orbit
obliquity of $\lambda=0^{+18}_{-15}$ degrees. The aligned orbit of AU Mic b
indicates that it formed in the protoplanetary disk that evolved to the current
debris disk around AU Mic.
