  A new class of superparamagnetism was found in relaxor ferroelectric
2/3BiFeO$_{3}$-1/3BaTiO$_{3}$. The size of the magnetic particle, estimated
from the superparamagnetic magnetization curve, coincides with the size of the
polar nanoregion (PNR), which governs the relaxor ferroelectric property. This
suggests that the magnetic domain is identical to the PNR. The temperature
variations in the sizes of the magnetic domains and PNRs estimated by our
neutron diffraction measurements support this picture. Since the same domain
provides both electric and magnetic properties, strong coupling between the two
properties through the domain size is expected.
