**3. Nonmagnetic substrates**

In the previous section, TMP or TMPc molecules deposited on magnetic substrates have been discussed in view of magnetic coupling and manipulating the spin state. In these hybrid structures, the magnetic coupling between molecules and substrate plays a dominating role. However, adsorbing molecules on nonmagnetic (NM) substrates instead allow to study aspects which are hidden in the presence of a magnetic coupling between molecule and substrates as the hybridization between molecule and substrate depending on the surface orientation and reconstruction. Thinking of future spintronic devices, the magnetic properties of the molecules can be tuned by ligands attached to the molecular center. Without the dominating magnetism of the FM substrate, only the coupling between the molecule and possible ligands exists, and this can be more easily switched by thermal treatment than the large coupling between FM and molecule.

#### **3.1. Influence of the surface texture on the electronic structure**

The electronic structure of molecules adsorbed on magnetic layers can strongly differ from the one obtained for the molecule in gas phase because of the magnetic coupling, and molecules are often chemisorbed. Here we use FeP as a model system. It corresponds to the Fe OEP structure but without the outer ethyl groups. Dangling bonds are saturated by hydrogen, see **Figure 1**.

On a nonmagnetic substrate, Cu(001) in our case, the molecules are less tightly bound to the substrate. The distance between the Fe ion of FeP and the Cu(001) substrate is about 2.66 Å (PW91, vdW‐D2, Ueff = 3 eV) compared to 1.78 Å on Co(001), that is, the molecules are physisorbed [13]. The adsorption position and the relative orientation are the same as on the FM substrates, which means that in this system, the adsorption position is not influenced by the magnetism of the substrate but by geometry. Even though on Cu(001) there is no magnetic interaction with the molecule, the molecules hybridize with the substrate which can be seen from the density of state (DOS) in **Figure 7(a)**. The Fe 3d states having components in z‐direction (z<sup>2</sup> and π) are broadened compared to the DOS of the free molecule. However, the interaction is much weaker and does not lead to strong changes in the orbital occupation other than in the case of Co or Ni substrates. The Fe‐N distance is basically the same as for the molecule in gas phase (2 Å) which entails that the spin state is not affected if FeP is absorbed on Cu(001), and it remains in the S = 1 state which agrees with the experimental observation from X‐ray absorption spectroscopy (XAS) and X‐ray magnetic circular dichroism (XMCD) [13].

The situation changes if the Cu(001) surface is covered by 0.5 layers of oxygen. This O adlayer leads to a √2×√2R45° missing row reconstruction of the surface. The ground state configuration changes from the hollow site (as on Cu(001)) to the missing row position with two O atoms next to the Fe ion. Because the O layer is basically incorporated in the surface layer of the Cu film, the distance between the Fe ion and the substrate increases only by 0.3 Å, and despite the strong surface reconstruction, the Fe‐N distance increases only to 2.03 Å, that is, the spin state remains S = 1 and only minor changes in the Fe 3d DOS can be observed; see **Figure 7(b)**. Mostly the broadening of the peaks is reduced compared to the molecule on the plain Cu surface meaning that the hybridization is weaker than the Cu(001).

While the O adlayer had a huge influence on the magnetic properties when added to a ferromagnetic substrate, for example, for FePc on Co(001) or FeOEP on Co(001) [2, 3], here the spin state and the electronic structure are basically unchanged. Only the adsorption position is affected due to the surface reconstruction.

#### **3.2. Influence of ligands**

graphene layer, the estimated DFE for the vacancy formation is 7–8 eV, while the calculated values of DFE for monovacancy and divacancy on Ni(111) surface are 2.91 and 3.83 eV, respectively. One can safely conclude that the creation of defects, either naturally of ion‐beam irradiation, should be easier in the Ni‐graphene composite surface which provides a large boost for the abovementioned magnetic state manipulation. Moreover, the spin state and the magnetic coupling are adsorption site dependent; the protection of it requires a sufficient energy barriers between adsorption sites. The energy barrier in moving a FeP molecule between a Hex site and a Top‐A site (which are energetically comparable adsorption sites) on a pristine graphene is calculated to be 33 meV, which translates to a temperature, higher than the room temperature. On the defect sites, this energy barrier is expected to be much higher, making the abovementioned value to be the lower limit for the diffusion barrier. One may envisage controlled formation of specific types of defects and achieving either parallel or antiparallel

In the previous section, TMP or TMPc molecules deposited on magnetic substrates have been discussed in view of magnetic coupling and manipulating the spin state. In these hybrid structures, the magnetic coupling between molecules and substrate plays a dominating role. However, adsorbing molecules on nonmagnetic (NM) substrates instead allow to study aspects which are hidden in the presence of a magnetic coupling between molecule and substrates as the hybridization between molecule and substrate depending on the surface orientation and reconstruction. Thinking of future spintronic devices, the magnetic properties of the molecules can be tuned by ligands attached to the molecular center. Without the dominating magnetism of the FM substrate, only the coupling between the molecule and possible ligands exists, and this can be more easily switched by thermal treatment than the large

The electronic structure of molecules adsorbed on magnetic layers can strongly differ from the one obtained for the molecule in gas phase because of the magnetic coupling, and molecules are often chemisorbed. Here we use FeP as a model system. It corresponds to the Fe OEP structure but without the outer ethyl groups. Dangling bonds are saturated by hydro-

On a nonmagnetic substrate, Cu(001) in our case, the molecules are less tightly bound to the substrate. The distance between the Fe ion of FeP and the Cu(001) substrate is about 2.66 Å (PW91, vdW‐D2, Ueff = 3 eV) compared to 1.78 Å on Co(001), that is, the molecules are physisorbed [13]. The adsorption position and the relative orientation are the same as on the FM substrates, which means that in this system, the adsorption position is not influenced by the magnetism of the substrate but by geometry. Even though on Cu(001) there is no magnetic interaction with the molecule, the molecules hybridize with the substrate which can be seen

orientation of Fe moments relative to the moments in the Ni layers [12].

**3. Nonmagnetic substrates**

74 Phthalocyanines and Some Current Applications

coupling between FM and molecule.

gen, see **Figure 1**.

**3.1. Influence of the surface texture on the electronic structure**

As discussed above, an O adlayer on nonmagnetic Cu(001) has no effect on the magnetic properties of the molecule, but there exist other combinations of magnetic molecules and nonmagnetic substrates where adlayers or dopants switch the spin state, for example, GaAs(001) and Vanadyl Pc. The molecule switches to the high spin state if the (Ga‐rich) GaAs substrate is doped with Si. However, thinking of spintronic devices, a reliable, controllable switching

**Figure 7.** Calculated density of states of FeP/Cu(001) (a) and FeP/√2×v2R45°O/Cu(001) (b) for the ground state configurations with FeP on the hollow site of the Cu(001) surface with the N atoms on top of the underlying Cu atoms (a) and adsorbed on the missing row position of the reconstructed surface. Note only the Fe 3d states are shown here. Data are taken from Ref. [13].

between two configurations, for example, low spin to high spin state, is needed. This can hardly be achieved if the manipulation of the spin state arises from adlayers or doping of the substrate layer. A more realistic way is the manipulation of the spin state by ligands since ligands can be thermally attached and removed. This has been demonstrated for Co tetraethylporphyrin on Ni(001) and also works for nonmagnetic substrates. This works also on nonmagnetic substrates. Fe octaethylporphyrin (OEP) can be stabilized in air by pyridine (Py) or Cl. Depositing the two types of Fe OEP on Cu(001), they show a completely different spectroscopic signature [13], as can be seen from the X‐ray absorption spectra and the X‐ray magnetic circular dichroism for Fe OEP‐Py and Fe OEP‐Cl on Cu(001) (**Figure 8**). Especially for normal incidence, the XMCD signal is four times larger in the case of Cl ligands, whereas for grazing incidence of the photon beam, no significant differences exist; see **Figure 8**. This indicates that the magnetic and electronic properties depend on the ligand. While Py dissolves during the deposition process, that is, pure FeP remains on the substrate, in the case of Cl, about 50% of the ligand remains, which leads to an increase of the magnetic signal. Theoretical calculations for FeP (porphyrin without the outer ethyl groups) with an axial Cl ligand deposited on Cu(001) confirm the observation. The Fe‐d level occupation has changed, the previously occupied dπ levels have moved above the Fermi level, and the hybridization with the substrate has decreased leading to sharper peaks compared to FeP/Cu(001). The reason is that the Fe ion also interacts strongly with the ligand.

Having seen that for a sub‐monolayer coverage of Fe OEP on Cu(001) 50% of the Cl atoms which remain after deposition of the surface is sufficient to cause significant changes in the magnetic behavior, a detailed study of possible ligands and their influence on the magnetic properties has been performed for free and deposited FeP molecules [14]. In gas phase, the influence of various combinations of porphyrin or phthalocyanine and ligands (axial or peripheral) has been investigated. Here we focus on the molecule ligand complex

**Figure 8.** Measured Fe L2,3 edge XAS (a) and XMCD (b) for 0.4 ML Fe OEP on Cu(001). Panels (c) and (d) show the analogous results for Fe OEP (Cl)/Cu(001). The angle θ denotes the angle between surface normal and photon beam. Taken from Ref. [13].

FeP+L deposited on Cu(001) with L = Cl, O,O<sup>2</sup> . DFT calculations (VASP, PAW, PBE, and vdW‐D2) show a significant dependence of the electronic and magnetic structure depending on the ligand. In all cases, the molecules have been adsorbed on the hollow site position (**Figure 9(a)**), which has been found to be the ground state for FeP/Cu(001). In agreement with the XAS/XMCD experiments described above, an axial Cl ligand enhances the magnetic moment of the FeP complex from 2 to 3 µB, whereby 2.69 µB are on the Fe ion. If the FeP‐Cl complex is deposited on Cu(001), the moment becomes even larger (3.71 µB), and the induced moment of the Cl atom decreases from 0.2 to 0.1 µB. On the contrary the Fe‐N distance increases from 2.05 Å for FeP/Cu(001) to 2.23 Å with Cl ligand which agrees with the observation of the transition from an intermediate spin state (S = 1) to the high spin state. The ligand has also indirect influence on the organic rings; they are driven away from the surface visible in a strong bending of the molecule; see **Figure 8**. This contrasts with FeP or FePc on magnetic substrates where also the organic ligands contribute to the interaction with the substrate.

between two configurations, for example, low spin to high spin state, is needed. This can hardly be achieved if the manipulation of the spin state arises from adlayers or doping of the substrate layer. A more realistic way is the manipulation of the spin state by ligands since ligands can be thermally attached and removed. This has been demonstrated for Co tetraethylporphyrin on Ni(001) and also works for nonmagnetic substrates. This works also on nonmagnetic substrates. Fe octaethylporphyrin (OEP) can be stabilized in air by pyridine (Py) or Cl. Depositing the two types of Fe OEP on Cu(001), they show a completely different spectroscopic signature [13], as can be seen from the X‐ray absorption spectra and the X‐ray magnetic circular dichroism for Fe OEP‐Py and Fe OEP‐Cl on Cu(001) (**Figure 8**). Especially for normal incidence, the XMCD signal is four times larger in the case of Cl ligands, whereas for grazing incidence of the photon beam, no significant differences exist; see **Figure 8**. This indicates that the magnetic and electronic properties depend on the ligand. While Py dissolves during the deposition process, that is, pure FeP remains on the substrate, in the case of Cl, about 50% of the ligand remains, which leads to an increase of the magnetic signal. Theoretical calculations for FeP (porphyrin without the outer ethyl groups) with an axial Cl ligand deposited on Cu(001) confirm the observation. The Fe‐d level occupation has changed, the previously occupied dπ levels have moved above the Fermi level, and the hybridization with the substrate has decreased leading to sharper peaks compared to FeP/Cu(001). The reason is that the Fe ion

Having seen that for a sub‐monolayer coverage of Fe OEP on Cu(001) 50% of the Cl atoms which remain after deposition of the surface is sufficient to cause significant changes in the magnetic behavior, a detailed study of possible ligands and their influence on the magnetic properties has been performed for free and deposited FeP molecules [14]. In gas phase, the influence of various combinations of porphyrin or phthalocyanine and ligands (axial or peripheral) has been investigated. Here we focus on the molecule ligand complex

**Figure 8.** Measured Fe L2,3 edge XAS (a) and XMCD (b) for 0.4 ML Fe OEP on Cu(001). Panels (c) and (d) show the analogous results for Fe OEP (Cl)/Cu(001). The angle θ denotes the angle between surface normal and photon beam.

also interacts strongly with the ligand.

76 Phthalocyanines and Some Current Applications

Taken from Ref. [13].

In the case of O ligands, that is, atomic oxygen or O<sup>2</sup> , a different magnetic behavior is observed. The spin moment on the Fe ion is reduced, while the oxygen atoms gain a moment parallel to Fe such that the spin state of the whole complex is unchanged and the total moment remains 2 µB. The theoretically determined spin moment of an atomic oxygen ligand is 0.58 µB, whereas for the oxygen dimer, the moment is evenly distributed on both O atoms (0.22 µB/0.21 µB). The latter result deviates from the gas‐phase solution where the two O atoms differ in size and relative orientation. In gas phase the Fe atom has a slightly enhanced moment compensated by the antiparallel moment of the outer oxygen ligand.

Though no magnetic coupling between surface and molecule is present in the case of Cu(001), a significant interaction between substrate and molecule is observed between Fe and Cu which condenses in the change of the electronic and magnetic structure of the adsorbed molecules and is connected to severe changes in the geometrical structure. Furthermore, a deformation or buckling of the substrate next to the molecule is observed which is particularly pronounced for Cl and O ligands, while with O<sup>2</sup> as a ligand, the surface is much less affected. Indicating that for the dimer the interaction with the substrate is weaker than in the case of atomic Cl or O even though the adsorption distance is very similar, 2.28 Å (2.23 Å) with O<sup>2</sup> (O) for Cl the average distance between substrate and Fe is even larger (2.50 Å) [14].

**Figure 9.** Calculated spin density of FeP with different axial ligands, Cl (a), atomic O (b), and O<sup>2</sup> (c), adsorbed on Cu (001). Both positive and negative densities are shown. Data are partially taken from Ref. [14].

In conclusion, it has been shown that the spin moment of FeP or the iron center itself can be tackled by the choice of the ligand in gas phase as well as on nonmagnetic Cu(001), whereby the changes for deposited molecules are even more expressed.

#### **3.3. Effective spin moment and the role of the spin dipolar term**

To compare the calculated spin moments with experimentally determined values, we face a problem since most experimental data are obtained from XMCD measurements. From these kinds of experiments, only orbital and effective spin moments meff are accessible. The effective spin moment differs from actual spin moment by the spin‐dipole moment contribution, and depending on the symmetry, this contribution can be large [1, 10]. Comparing the XMCD signal of the Fe OEP L<sup>3</sup> edge (**Figure 8(c)**) for different incidence angles of the photon beam, it turns out that the intensity strongly varies with the angle. A large signal is observed for grazing incidence in much larger than for normal incidence of the photon beam. This can be caused by a large contribution of the spin‐dipole term or be related to large magnetocrystalline anisotropy. Following Oguchi [16], the spin‐dipole operator *T* is defined by

$$T = \sum\_{l} Q^{(l)} s^{(l)} \tag{1}$$

with Q being the quadrupole tensor:

$$\mathbf{Q}^{(0)}\_{a\theta} = \delta\_{a\theta} - \mathbf{\mathcal{S}}\,\hat{r}^{(0)}\_{a}\hat{r}^{(0)}\_{\rho}.\tag{2}$$

Hence, the spin‐dipole moment arises from the aspherity of the spin density, that is, for transition metals where spin‐orbit coupling is weak; this is related to the crystal field. If the cubic symmetry is not broken, Q vanishes, but for systems with reduced symmetry as for clusters [15, 16] or molecules adsorbed on substrates, the spin‐dipole moment plays an important role [17]. To compare calculated spin moments to experimentally reported effective spin moments for low symmetry systems, the dipolar term must be included, especially to rule out other sources for the discrepancy between the effective spin moment and the total spin moment, such as a large magnetocrystalline anisotropy or not fully saturated magnetic moments. For simplicity, we focus only on the z component of the spin‐dipole operator. Its expectation value <Tz > is given by the trace of the density matrix multiplied by Tz . the density matrix can be obtained from DFT calculations. If for transition metals the spin‐orbit coupling is negligible, the size of <Tz > depends on the existence of a finite spin moment on the nonequivalent charge distribution on the orbitals. To obtain the spin‐dipole moment and the effective spin moment by van der Laan [18] provided a scheme how to apply the general approach to a typical XMCD experiment such as in **Figure 10**.

The intensity which is measured in XMCD experiments as response to the photon beam hitting the surface depends on the relation between the magnetization direction **M**, the polarization of the incident photon beam **P**, and the surface normal **n** as depicted in **Figure 10**. The angular dependence of the dipole operator reads then

$$
\langle \langle \mathcal{T}T(\mathfrak{u}, P\_\prime M)\_\downarrow \rangle \rangle = \frac{1}{4} \langle \mathcal{T}T\_z \rangle \langle \cos(\mathfrak{q}\cdot) + 3\cos(\mathfrak{q}\cdot 2\mathcal{B}\,) \rangle \tag{3}
$$

Deposited Transition Metal‐Centered Porphyrin and Phthalocyanine Molecules: Influence of the Substrates... http://dx.doi.org/10.5772/intechopen.68224 79

**Figure 10.** Sketch of a typical experimental XMCD setup. The polarization of the photon beam and the magnetization are denoted by **P** and **M**, respectively, as well as **n** denoting the direction of the surface normal.

with ϕ being the angle between the magnetization **M** and the polarization of the beam **P**. The deviation of the magnetization direction from the surface normal **n** is denoted by the angle θ. In the present case, that is, for nonmagnetic substrates, the alignment of the magnetic moments of the transition metal centers of the molecules is achieved by applying an external magnetic field which is usually parallel aligned to the polarization of the incident photon beam **M**‖**P**. In this special case, Eq. (3) reduces to

$$
\langle \mathcal{T}T(\theta)\_{\rangle} \rangle = \frac{1}{4} \langle \mathcal{T}T\_{\natural} \rangle \langle 3\cos^2(\varphi) - 1 \rangle \tag{4}
$$

and the experimentally observed effective spin moment becomes

In conclusion, it has been shown that the spin moment of FeP or the iron center itself can be tackled by the choice of the ligand in gas phase as well as on nonmagnetic Cu(001), whereby

To compare the calculated spin moments with experimentally determined values, we face a problem since most experimental data are obtained from XMCD measurements. From these kinds of experiments, only orbital and effective spin moments meff are accessible. The effective spin moment differs from actual spin moment by the spin‐dipole moment contribution, and depending on the symmetry, this contribution can be large [1, 10]. Comparing the XMCD

it turns out that the intensity strongly varies with the angle. A large signal is observed for grazing incidence in much larger than for normal incidence of the photon beam. This can be caused by a large contribution of the spin‐dipole term or be related to large magnetocrystal-

*i*

(*i*) = *δαβ* − 3 *r*

Hence, the spin‐dipole moment arises from the aspherity of the spin density, that is, for transition metals where spin‐orbit coupling is weak; this is related to the crystal field. If the cubic symmetry is not broken, Q vanishes, but for systems with reduced symmetry as for clusters [15, 16] or molecules adsorbed on substrates, the spin‐dipole moment plays an important role [17]. To compare calculated spin moments to experimentally reported effective spin moments for low symmetry systems, the dipolar term must be included, especially to rule out other sources for the discrepancy between the effective spin moment and the total spin moment, such as a large magnetocrystalline anisotropy or not fully saturated magnetic moments. For simplicity, we

the existence of a finite spin moment on the nonequivalent charge distribution on the orbitals. To obtain the spin‐dipole moment and the effective spin moment by van der Laan [18] provided a scheme how to apply the general approach to a typical XMCD experiment such as in **Figure 10**. The intensity which is measured in XMCD experiments as response to the photon beam hitting the surface depends on the relation between the magnetization direction **M**, the polarization of the incident photon beam **P**, and the surface normal **n** as depicted in **Figure 10**. The

^ *α* (*i*) *r* ^ *β* (*i*)

line anisotropy. Following Oguchi [16], the spin‐dipole operator *T* is defined by

focus only on the z component of the spin‐dipole operator. Its expectation value <Tz

culations. If for transition metals the spin‐orbit coupling is negligible, the size of <Tz

edge (**Figure 8(c)**) for different incidence angles of the photon beam,

*Q*(*i*) *s*(*i*) (1)

. the density matrix can be obtained from DFT cal-

<sup>4</sup> 〈7 *Tz* 〉(*cos*(*ϕ* ) +3*cos*(*ϕ*+ 2*θ* ) ) (3)

. (2)

> is given by

> depends on

the changes for deposited molecules are even more expressed.

signal of the Fe OEP L<sup>3</sup>

78 Phthalocyanines and Some Current Applications

*T* = ∑

with Q being the quadrupole tensor:

*Qαβ*

the trace of the density matrix multiplied by Tz

angular dependence of the dipole operator reads then

〈7*<sup>T</sup>* (*n*, *<sup>P</sup>*, *<sup>M</sup>* )*<sup>i</sup>* 〉 <sup>=</sup> \_\_1

**3.3. Effective spin moment and the role of the spin dipolar term**

$$m\_{eff} = m\_s + \langle \mathcal{T}\mathcal{T}(\boldsymbol{\Theta}) \rangle. \tag{5}$$

As can be seen from **Figure 11**, the effective moment strongly depends of the incidence angle of the photon beam. Only for measurements carried out at the *magic angle* (54.5°), the argument in the brackets on the right side of Eq. (4) vanishes, and it yields meff = ms .

In an ideal case, the calculated and the measured effective moment should be identical; however, comparing the calculated meff to the experimental data (triangles) in **Figure 11(a)**, distinct deviations occur even for the magic angle where Tz vanishes. This is related to the fact the experimental sample could not be fully saturated in the magnetic field (5.9 T) [13]. For the oxidized surface, the density matrix is almost identical with the one for the plain Cu substrate; hence, also the meff and the angular dependence do not change. In this case, the theoretical and

**Figure 11.** Dipolar term and effective spin moment for FeP on Cu(001) (a). Open (filled) symbols denote the dipolar term (effective spin moment). The data for FeP with Cl ligand are given in (b). The lighter solid line corresponds to meff without Cl, and the dashed line is the average of meff with and without Cl. Deviations between the calculated and measured meff at small incidence angles may result from limited accuracy of the determination of the dipolar term. Data are partially taken from Ref. [13].

experimental data are in good agreement, because the saturation could be reached. With a Cl ligand attached to the FeP, the dipolar term is different due to changes in the occupation of the Fe 3d levels (cf. **Figure 11**). The meff at *θ* = 90° would be 4 µB instead of 3 µB as without Cl. Even though the sample is basically saturated, the data deviate from the theoretically predicted spin moments. Assuming only 50% of the Cl ligands remain at the FeP molecules after deposition (dashed line in **Figure 7(b)**) improves the agreement between theory and experiment significantly. Scanning tunneling microscopy images of the Fe OEP (Cl)/Cu(001) have verified the assumption that between 40 and 60% of the ligands have been dissolved during deposition.

In summary, as shown for the example of FeP (OEP) on Cu(001), the dipolar term is an important factor to interpret and understand experimental XAS and XMCD data since effects from non‐saturated samples as well as incomplete dissolved ligands can be detected.
