  One of the key factors behind the rapid evolution of molecular spintronics is
the efficient realization of spin manipulation of organic molecules with a
magnetic center. The spin state of such molecules may depend crucially on the
interaction with the substrate on which they are adsorbed. In this letter, we
demonstrate, using ab initio density functional calculations, that the
stabilization of a high spin state of an iron porphyrin (FeP) molecule can be
achieved via a dedicated chemisorption on magnetic substrates of different
species and orientations. It is shown that the strong covalent interaction with
the substrate increases Fe-N bond lengths in FeP and hence a switching to a
high spin state (S=2) from a low spin state (S=1) is achieved. A ferromagnetic
exchange interaction is established through a direct exchange between Fe and
substrate magnetic atoms as well as through an indirect exchange via the N
atoms in FeP. The mechanism of exchange interaction is further analyzed by
considering structural models constructed from ab initio calculations. Finally,
we illustrate the possibility of detecting a change in the molecular spin state
by x-ray magnetic circular dichroism, Raman spectroscopy and spin-polarized
scanning tunneling microscopy.
