The present disclosure relates to a magnetizing portion for providing a high-gradient magnetic field in a magnetic separation device. The magnetizing portion comprises at least one magnetic assembly. The at least one magnetic assembly comprises: a plurality of magnets whereby each magnet has a north pole, south pole and a magnet axis extending between the north and south poles, and the plurality of magnets are arranged one above the other in a direction at least substantially perpendicular to the axis of each magnet in such a manner that the north and south poles of adjacent magnets are arranged alternately and a space is provide between adjacent magnets; and at least one non-magnetic spacing means arranged in the space between adjacent magnets. The present disclosure also relates to magnetic separation devices comprising at least one of the said magnetizing portions and to a method of isolating magnetically labelled particles using the magnetic separation devices.