Graphene as a Reversible Spin Manipulator of Molecular Magnets

One of the primary objectives in molecular nanospintronics is to manipulate the spin states of organic molecules with a $d$-electron center, by suitable external means. In this Letter, we demonstrate by first principles density functional calculations, as well as second order perturbation theory, that a strain induced change of the spin state, from $S=1\to S=2$, takes place for an iron porphyrin (FeP) molecule deposited at a divacancy site in a graphene lattice. The process is reversible in the sense that the application of tensile or compressive strains in the graphene lattice can stabilize FeP in different spin states, each with a unique saturation moment and easy axis orientation. The effect is brought about by a change in Fe-N bond length in FeP, which influences the molecular level diagram as well as the interaction between the C atoms of the graphene layer and the molecular orbitals of FeP.

Figure 1

Total energies (shown with respect to the ground state) of FeP in the gas phase, plotted as a function of fixed spin moments. The spin densities for HOMO and HOMO-1 energy levels are shown for $2{\mu }_B$ and $4{\mu }_B$ cases. Both HOMO and HOMO-1 levels have spin-up character in $S=1$ spin state whereas HOMO and HOMO-1 for $S=2$ have spin-down and spin-up character, respectively. In the FeP molecule, the central Fe atom is bonded to 4 N atoms. The rest are C atoms in the network connected to H atoms in the outermost portion of the molecule.

Figure 2

Magnetization density isosurfaces for FeP on (left) 0% and (right) 1% strained graphene. The isosurfaces have been plotted for an energy window of 0.4 eV below the Fermi levels in both cases. The spin densities shown in the left up (side view) and right down (top view) parts correspond to spin-up and spin-down channels, respectively. The energy levels with the $d$-orbital character of FeP are shown in the extreme left and right for 0% and 1% strained graphene, respectively.

Figure 3

Schematic diagram showing the energy levels and their orbital characters for (left) 0% and (right) 1% strained cases, considered for the calculation of MAE (see text). The filled and empty arrows indicate occupied (HOMO and HOMO-1) and unoccupied (LUMO and $\mathrm{LUMO}+1$) $d$ states of Fe, respectively. The big arrows indicate the directions of magnetization. $\Delta$s are defined in the text.