  Recent results in electric-field control of magnetism have paved the way for
the design of alternative magnetic and spintronic devices with enhanced
functionalities and low power consumption. Among the diversity of reported
magnetoelectric effects, the possibility of switching on and off long-range
ferromagnetic ordering close to room temperature stands out. Its binary
character opens up the avenue for its implementation in magnetoelectric data
storage devices. Here we show the possibility to locally switch on
superferromagnetism in a wedge-shaped polycrystalline Fe thin film deposited on
top of a ferroelectric and ferroelastic BaTiO3 substrate. A superparamagnetic
to superferromagnetic transition is observed for confined regions for which a
voltage applied to the ferroelectric substrate induces a sizable strain. We
argue that electric-field-induced changes of magnetic anisotropy lead to an
increase of the critical temperature separating the two regimes so that
superparamagnetic regions develop collective long-range superferromagnetic
behavior.
