Magnetic field-generating device including a plurality of elements selected from magnets and pole pieces and including at least one magnet. Plurality of elements are (i) located below supporting surface or space configured to receive a substrate acting as supporting surface or (ii) forming a supporting surface, and are configured to provide a magnetic field with magnetic field lines running substantially parallel to supporting surface or space in two or more areas above supporting surface or space. Two or more areas form nested loop-shaped areas surrounding a central area; and/or the plurality of elements include a plurality of magnets, which are rotatable around an axis of rotation such that areas with field lines running substantially parallel to supporting surface or space combine upon rotation around axis of rotation, thereby forming, upon rotation around axis of rotation, a plurality of nested loop-shaped areas surrounding one central area.