Manipulation of spin state of iron porphyrin by chemisorption on magnetic substrates

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 paper 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 chemisorption on magnetic substrates of different species and orientations, viz., Co(001), Ni(001), Ni(110), and Ni(111). The signature of chemisorption of FeP on magnetic substrates is evident from broad features in N $K$ x-ray absorption (XA) and Fe $L_{2,3}$ x-ray magnetic circular dichroism (XMCD) measurements. Our theoretical calculations show 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 an intermediate spin state ($S=1$) is achieved. Due to chemisorption, 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. Also, it is found that the exchange interaction between Fe in FeP and a Ni substrate is almost 4 times smaller than with a Co substrate. Finally, we illustrate the possibility of detecting a change in the molecular spin state by XMCD, Raman spectroscopy, and spin-polarized scanning tunneling microscopy.

Figure 1

Schematic picture of the geometry of a FeP molecule on a Co(001) substrate. The turquoise and gray balls represent Co atoms of the first and second layers of the substrate, respectively. C atoms are shown in steel, while Fe, N, and H atoms are represented with blue, orange, and yellow colors, respectively. The shaded central part demonstrates the change in the Fe-N bond distance along with the change in the spin state. A smaller Fe-N bond length ($<$2.04 Å) leads to a smaller exchange splitting with a $S=1$ spin state (top right). Both the majority and minority spin channels are thus partially filled. A larger exchange splitting is observed for an increased Fe-N bond length ($>$2.04 Å), where the majority channel gets occupied completely and a high spin state ($S=2$) is obtained (bottom right).

Figure 2

FeP molecule on Co(001), TOP position. (a) Top view, where Fe-N bond lengths are shown. (b) Side view, where minimum distances between different layers are shown.

Figure 3

HOLLOW configuration on the Co(001) surface. (a) and (b) Same convention as in Fig. 2.

Figure 4

TOP-R configuration on the Ni(110) surface. (a) and (b) Same convention as in Fig. 2.

Figure 5

TOP configuration on the Ni(111) surface. (a) and (b) Same convention as in Fig. 2.

Figure 6

N $K$ XA (a), Fe $L_{2,3}$ XA (b), XMCD (c), and Ni $L_{2,3}$ XMCD (d) spectra of 0.6 ML Fe OEP on 6 ML Ni/Cu(001). The spectra presented in (a) are measured at room temperature with linearly $p$-polarized x rays and incidence angles of 90${}^{\circ }$, 55${}^{\circ }$, and 20${}^{\circ }$ between the $k$ vector and the surface. The spectrum of a polycrystalline reference sample is shown in green. The spectra presented in (b) to (d) are measured at $T=43$ K with circularly polarized x rays and 20${}^{\circ }$ grazing incidence.

Figure 7

(a) Spin-polarized atom and orbital-resolved DOS for FeP on Co(001) in the TOP position. In the inset, a charge density cut in the (001) plane is shown for an energy window indicated as the shaded region in the DOS plot. Co1 and Co2 are the atoms of the Co substrate, situated just below Fe and N atoms of FeP, respectively. (b) Side view of the magnetization density isosurface shown for the same energy interval to show the out-of-plane orbitals. (c) Top view of the magnetization density to show the in-plane orbitals taking part in the exchange interaction between FeP and the Co substrate.

Figure 8

Structural models extracted from fully optimized DFT calculations of FeP on different substrates indicated in the text. The geometries were not optimized further for these models which are used to calculate the total energies for FM and AFM alignments between Fe and Co moments. The energy differences between AFM and FM alignments are shown for each model, where $+ve$ value of $E_{\text{ex}}$ means ferromagnetic coupling.

Figure 9

Calculated Raman intensities for FeP in chemisorbed (red lines) and physisorbed (blue lines) geometries. In the upper panel, Raman intensities are shown for shifts in the range 110–450 cm${}^{-1}$, whereas the lower panel contains wave numbers in the range 1380–1650 cm${}^{-1}$. In both panels, the vibrational modes corresponding to the peaks are denoted along with the corresponding wave numbers.

Figure 10

Calculated spin-polarized STM images are shown for bias voltages $\pm 0.1$ V for FeP chemisorbed on (a) Co(001), (b) Ni(110), and (c) Ni(111) surfaces. STM images for spin-up and spin-down channels are shown separately, indicated by up and down arrows, respectively. $E_{\mathsf{f}}$ denotes the Fermi energy.