  We have tested the concept of image charge screening as a new approach to
enhance magnetic ordering temperatures and superexchange interactions in ultra
thin films. Using a 3 monolayer NiO(100) film grown on Ag(100) and an
identically thin film on MgO(100) as model systems, we observed that the Neel
temperature of the NiO film on the highly polarizable metal substrate is 390 K
while that of the film on the poorly polarizable insulator substrate is below
40 K. This demonstrates that screening by highly polarizable media may point to
a practical way towards designing strongly correlated oxide nanostructures with
greatly improved magnetic properties.
