Tailoring the Nature of Magnetic Coupling of Fe-Porphyrin Molecules to Ferromagnetic Substrates

We demonstrate that an antiferromagnetic coupling between paramagnetic Fe-porphyrin molecules and ultrathin Co and Ni magnetic films on Cu(100) substrates can be established by an intermediate layer of atomic oxygen. The coupling energies have been determined from the temperature dependence of x-ray magnetic circular dichroism measurements. By density functional $\mathrm{\text{theory}}+U$ calculations the coupling mechanism is shown to be superexchange between the Fe center of the molecules and Co surface-atoms, mediated by oxygen.

Figure 1

Fe $L_{2,3}$ XAS and XMCD spectra of 0.6 ML Fe-OEP on $\mathrm{O}/\mathrm{Ni}$ (a) and $\mathrm{O}/\mathrm{Co}$ (b) (full red or gray lines), and on Ni [dashed black lines, panel (a)]. The corresponding Ni and Co spectra are shown on the right. Insets: sketches of the corresponding samples and of the Fe-porphyrin molecule.

Figure 2

Temperature dependence of Fe XMCD (squares; full lines: fit of Brillouin-type model) and Ni or Co XMCD (circles; dashed lines: empirical curve taken from Ref. 21) for 0.6 ML Fe-OEP on (a) $\mathrm{O}/10\mathrm{ML}$ $\mathrm{Ni}/\mathrm{Cu}(100)$, (b) $15\mathrm{ML}\mathrm{Ni}/\mathrm{Cu}(100$), and (c) $\mathrm{O}/5\mathrm{ML}\mathrm{Co}/\mathrm{Cu}(100)$.

Figure 3

N $K$ edge XAS signals of 0.6 ML Fe-OEP (a) on $\mathrm{O}/10\mathrm{ML}$ $\mathrm{Ni}/\mathrm{Cu}(100)$ and (b) on 15 ML $\mathrm{Ni}/\mathrm{Cu}(100)$ at normal (dashed lines) and grazing (full lines) x-ray incidence with 0° and 70° of the $E$ vector to the surface, respectively. The gray lines show the N $K$ edge spectrum for a polycrystalline bulk sample.

Figure 4

Ab initio computed magnetization densities. Blue or dark gray color depicts majority-spin magnetization on Co atoms and on one part of the O atom, red color reveals minority-spin magnetization on the central Fe atom, on the other part of the O atom, and on the N atoms. Yellow and cyan spheres represent C and H atoms, respectively.