  Electric control of magnetism is a vision which drives intense research on
magnetic semiconductors and multiferroics. Recently, also ultrathin metallic
films were reported to show magnetoelectric effects at room temperature. Here
we demonstrate much stronger effects by exploiting reduction/oxidation
reactions in a naturally grown oxide layer exchange coupled to an underlying
ferromagnet. For the exemplarily studied FePt/iron oxide composite in an
electrolyte, a large and reversible change of magnetization and anisotropy is
obtained. The principle can be transferred to various metal/oxide combinations.
It represents a novel approach towards multifunctionality.
