Abstract: The invention relates to a processing device (100) and a method for processing a medium in a processing chamber (121). The processing comprises the addition of magnetic particles (M) to the medium and the mixing of the medium by manipulating said magnetic particles with a time-variable magnetic field (B), particularly a partially oscillating or rotating field. The magnetic field (B) may be generated with a multipole magnetic field generator (110) comprising four subunits (111A,111B), each having a core (113A,113B) with a surrounding coil (112A,112B) and with a top surface (114A,114B), wherein all top surfaces of said subunits are preferably arranged in the same plane and wherein all cores are substantially parallel to each other.