The role of magnetic field in the AGN jet physics is still not fully determined. At pc scale, it is known that it is important in the acceleration and collimation processes while at arcsecond scale it could reveal fundamental pieces of the jet dynamics and energetics and its surrounding environment. At intermediate scales, the scenario is more debated. To contribute in this framework, we need to resolve polarized emission even in the low surface brightness extended structures (e.g. lobes). This absolutely requires high sensitivity observations. With the advent of ALMA, now it is possible also in the millimeter, a band which was unexplored by previous facilities. Here I present the impressive images in polarization obtained using ALMA archival multiband data of an ALMA calibrator PKS 0521-365 which represents a prototype of BL Lac object with extended resolved structures (jet and hotspot) at all frequencies from optical to X-rays.