The First 3mm Polarimetric Survey of Radio Loud AGN

Relativistic jets in AGN are the most energetic and among the most powerful astrophysical objects known so far. Their relativistic nature provides them the ability to emit profusely in all spectral ranges from radio wavelengths to TeV gamma-rays, as well as abrupt variability in all time scales (from hours to years). After an extended review of the present knowledge of these objects, but paying particular attention to blazars, the main results from the first large 3mm polarimetric survey of blazars will be presented. Observations at these wavelengths have allowed us to observe the polarimetric properties of the innermost regions of jets in AGN without being masked by the effects of light propagation through total flux and polarization opaque media, thus providing relevant statistical information about the intrinsic properties of such regions and their magnetic fields.