  Some of the Multiferroics [1] form a rare class of materials that exhibit
magnetoelectric coupling arising from the coexistence of ferromagnetism and
ferroelectricity, with potential for many technological applications.[2,3] Over
the last decade, an active research on multiferroics has resulted in the
identification of a few routes that lead to multiferroicity in bulk
materials.[4-6] While ferroelectricity in a classic ferroelectric such as
BaTiO3 is expected to diminish with the reducing particle size,[7,8]
ferromagnetism cannot occur in its bulk form.[9] Here, we use a combination of
experiment and first-principles simulations to demonstrate that multiferroic
nature emerges in intermediate size nanocrystalline BaTiO3, ferromagnetism
arising from the oxygen vacancies at the surface and ferroelectricity from the
core. A strong coupling between a surface polar phonon and spin is shown to
result in a magnetocapacitance effect observed at room temperature, which can
open up possibilities of new electro-magneto-mechanical devices at the
nano-scale.
