  We report on the development and extensive characterization of co-sputtered
tantala-zirconia thin films, with the goal to decrease coating Brownian noise
in present and future gravitational-wave detectors. We tested a variety of
sputtering processes of different energies and deposition rates, and we
considered the effect of different values of cation ratio $\eta =$ Zr/(Zr+Ta)
and of post-deposition heat treatment temperature $T_a$ on the optical and
mechanical properties of the films. Co-sputtered zirconia proved to be an
efficient way to frustrate crystallization in tantala thin films, allowing for
a substantial increase of the maximum annealing temperature and hence for a
decrease of coating mechanical loss. The lowest average coating loss was
observed for an ion-beam sputtered sample with $\eta = 0.485 \pm 0.004$
annealed at 800 $^{\circ}$C, yielding $\overline{\varphi} = 1.8 \times
10^{-4}$. All coating samples showed cracks after annealing. Although in
principle our measurements are sensitive to such defects, we found no evidence
that our results were affected. The issue could be solved, at least for
ion-beam sputtered coatings, by decreasing heating and cooling rates down to 7
$^{\circ}$C/h. While we observed as little optical absorption as in the
coatings of current gravitational-wave interferometers (0.5 parts per million),
further development will be needed to decrease light scattering and avoid the
formation of defects upon annealing.
