  The merger of binary neutron stars, or of a neutron star and a stellar-mass
black hole, can result in the formation of a massive rotating torus around a
spinning black hole. In addition to providing collimating media for gamma-ray
burst jets, unbound outflows from these disks are an important source of mass
ejection and rapid neutron capture (r-process) nucleosynthesis. We present the
first three-dimensional general-relativistic magnetohydrodynamic (GRMHD)
simulations of neutrino-cooled accretion disks in neutron star mergers,
including a realistic equation of state valid at low densities and
temperatures, self-consistent evolution of the electron fraction, and neutrino
cooling through an approximate leakage scheme. After initial magnetic field
amplification by magnetic winding, we witness the vigorous onset of turbulence
driven by the magneto-rotational instability (MRI). The disk quickly reaches a
balance between heating from MRI-driven turbulence and neutrino cooling, which
regulates the midplane electron fraction to a low equilibrium value $Y_\text{e}
\approx 0.1$. Over the 380 ms duration of the simulation, we find that a
fraction $\approx 20\%$ of the initial torus mass is unbound in powerful
outflows with velocities $v \approx 0.03-0.1\,c$ and electron fractions
$Y_\text{e} \approx 0.1-0.25$. Post-processing the outflows through a nuclear
reaction network shows the production of a robust second and third peak
r-process. Though broadly consistent with the results of previous axisymmetric
hydrodynamical simulations, extrapolation of our results to late times suggests
that the total ejecta mass from GRMHD disks is significantly higher. Our
results provide strong evidence that post-merger disk outflows are an important
site for the r-process.
