  Double perovskites provide a unique opportunity to induce and control
multiferroic behaviors in oxide systems. The appealing possibility to design
materials with a strong coupling between the magnetization and the polarization
fields may be achieved in this family since these magnetic insulators can
present structural self-ordering in the appropriate growth conditions. We have
studied the functional properties of La2CoMnO6 and Bi2CoMnO6 epitaxial thin
films grown by pulsed laser deposition. Cation-ordered La2CoMnO6 films display
a magnetic Curie temperature of 250 K while cation-disordered Bi2CoMnO6 films
present ferromagnetism up to ~ 800 K. Such high transition temperature for
magnetic ordering can be further tuned by varying the strain in the films
indicating an important contribution from the structural characteristics of the
materials. Our approach might be generalized for other oxide systems. At this
end, our results are compared with other multiferroic systems. The roles of
various cations, their arrangements and structural effects are further
discussed.
