  Bulk magnetite (Fe3O4), the loadstone used in magnetic compasses, has been
known to exhibit magnetoelectric (ME) properties below ~10 K; however,
corresponding ME effects in Fe3O4 nanoparticles have been enigmatic. We
investigate quantitatively the ME coupling of spherical Fe3O4 nanoparticles
with uniform diameters (d) from 3 to 15 nm embedded in an insulating host,
using a sensitive ME susceptometer. The intrinsic ME susceptibility (MES) of
the Fe3O4 nanoparticles is measured, exhibiting a maximum value of ~0.6 ps/m at
5 K for d=15 nm. We found that the MES is reduced with reduced d but remains
finite until d=~5 nm, which is close to the critical thickness for observing
the Verwey transition. Moreover, with reduced diameter, the critical
temperature below which the MES becomes conspicuous increased systematically
from 9.8 K in the bulk to 19.7 K in the nanoparticles with d=7 nm, reflecting
the core-shell effect on the ME properties. These results point to a new
pathway for investigating ME effect in various nanomaterials.
