Electrets are materials which exhibit a permanent electric polarization and hence a permanent surface charge. Non-polar fluoropolymers, like polytetrafluoroethylene (PTFE), exhibit excellent electret properties with regard to surface charge and charge stability, since the displacement currents inside the materials are sufficiently low to maintain the charges for decades. Electrets are already well established for use in electrostatic sensing and enable the fabrication of self-biased electrostatic sensors, avoiding the use of external, noise generating bias voltages. In our approach this concept is transferred to magnetic field sensors for the detection of, e.g. weak biomedical signals. A magnetostrictive film on a cantilever is deflected in the magnetic field and electrostatically coupled to an electret, which maintains the electric field in the air gap. Preliminary sensor implementations already show promising results with a limit of detection of 60 pT/Hz1/2 at 2567 Hz. With regards to the electret production, it is important to produce a polymer layer of precise thickness and of high quality for sensor fabrication and prospective integration into MEMS technology. Initiated chemical vapor deposition (iCVD) turned out to be a well-suited solution for this challenge.