Magnetic bead-based normalization of DNA libraries is a technique that uses DNA-binding beads to ensure that each library has a consistent concentration of DNA after elution. The magnetic beads are composed of tiny iron oxide particles such as magnetite Fe3O4. The beads only display magnetic behavior when an external magnetic field is present and their small size, enables the beads to remain separated in suspension along with whatever they are bound to. 

In this process, DNA-binding beads are added to all the libraries. Only a fixed amount of DNA will bind to the beads, so the libraries will have approximately the same concentration after elution.