  The effect of high tensile strain and low dimensionality on the magnetic and
electronic properties of CaMnO$_3$ ultrathin films, epitaxially grown on
SrTiO$_3$ substrates, are experimentally studied and theoretically analyzed. By
means of ab initio calculations, we find that, both, the high strain produced
by the substrate and the presence of the free surface contribute to the
stabilization of an in-plane ferromagnetic coupling, giving rise to a non-zero
net magnetic moment in the ultrathin films. Coupled with this change in the
magnetic order we find an insulator-metal transition triggered by the quantum
confinement and the tensile epitaxial strain. Accordingly, our magnetic
measurements in 3nm ultrathin films show a ferromagnetic hysteresis loop,
absent in the bulk compound due to its G-type antiferromagnetic structure.
