  When decreasing the size of nanoscale magnetic particles their magnetization
becomes vulnerable to thermal fluctuations as approaching the superparamgnetic
limit, hindering thus applications relying on a stable magnetization. Here, we
show theoretically that a magnetoelectric coupling to a ferroelectric substrate
renders possible the realization of substantially smaller nano clusters with
thermally stable magnetization. For an estimate of cluster size we perform
calculations with realistic material parameters for iron nano particles on
ferroelectric BaTiO3 substrate. We find, steering the polarization of BaTiO3
with electric fields affects the magnetism of the deposited magnetic clusters.
These findings point to a qualitatively new class of superparamagnetic
composites.
