Carbon films with up to 32 at.% N (a-C:N) have been prepared using an ion-beam-assisted magnetron, with an N-2(+) beam at energies between 50 and 300 eV. The composition and density of the films vary strongly with the deposition parameters. Electron energy loss spectroscopy shows that these a-C:N films are mostly graphitic with up to 20% C sp(3) bonding. Rutherford backscattering spectroscopy and neutron depth profiling show that the density goes through a maximum as the average deposited energy per unit depth increases. X-ray photoelectron spectroscopy shows that nitrogen is mostly combined with carbon in triple (CEN) and double (C=N) bonds. Positron annihilation spectroscopy shows that the Void concentration in the films goes through a minimum with deposited energy. These results are consistent with a densification induced by the collisions at low deposited energy, and damage-induced graphitization at high deposited energy values.