A solid-core ring-magnet having one or more cavities is provided. The magnet can have an overall cylindrical shape or a rectangular-prism shape. In either case, a portion of cavity walls of the magnet are ring shaped, causing the magnetic field lines to emanate from the magnet in the shape of a ring. The diameter of the ring shaped cavities can be constant throughout, constant through a portion of the cavity, variant throughout, or variant through a portion of the cavity. The cavities open to the end of the magnet, and terminate toward the core of the magnet. Also provided are systems and kits having solid-core ring-magnets. Methods of purifying a macromolecule using the solid-core ring-magnets are also provided.