Disclosed is a device for simply creating a magnetic gradient field having a field-free line (FFL). The device comprises two ring-shaped Halbach arrays formed of permanent magnets, and preferably also a coil arrangement for electrically generating a homogeneous magnetic field for shifting the FFL in a predetermined plane. The FFL can be rotated together with the Halbach array in the plane, without the coil arrangement having to be rotated or moved. The invention is suitable for effective construction of a scanning device for magnetic particle imaging (MPI).