**4. Field‐regulated switching of magnetic coupling**

In this section, we move a step ahead to devise a practical mechanism to control magnetic bistability with external means. The composite is specifically designed with three layers of CoOEP molecules deposited on graphene‐covered clean Ni(111) single crystal, as shown schematically in **Figure 12**. The focus in this composite will be on the manipulation of magnetic coupling leaving the spin crossover feasibility aside.

The CoOEP molecular layers are physisorbed on the graphene‐Ni composite surface, quite resembling the FeP adsorption, discussed above. The arrangement of the molecular layers Deposited Transition Metal‐Centered Porphyrin and Phthalocyanine Molecules: Influence of the Substrates... http://dx.doi.org/10.5772/intechopen.68224 81

**Figure 12.** Top: schematic images of the sample 3ML CoOEP/graphene/Ni(111). The light and dark arrows indicate the direction of the magnetic moments of Ni and Co in a low magnetic field of 500 mT (left) and in a high magnetic field of 5 T (right). Bottom: XMCD at the Co L2,3 edges of the CoOEP molecules in the low magnetic field (left) and in the high magnetic field (right). Insets show the XMCD at the L2,3 edges of the saturated Ni crystal at 500 mT and at 5 T. All spectra are recorded at T = 2 K and Θ = ∼70°. Data from Ref. [19].

appears in a particular fashion. The molecules in the two consecutive layers do not reside right on top of each other but slightly horizontally shifted. An indirect overlap between Co‐dz orbitals via N‐p orbitals favors this specific geometry. As discussed in the previous section, the single layer graphene on Ni is spin polarized with two different kinds of sub‐lattice moments. Unlike FeP, in CoP the S = 1/2 spin state is quite robust and solely arises from the singly occupied Co‐dz orbital, while other orbitals are doubly occupied giving no contribution to the molecular moment. However, in the presence of three layers, the net magnetization of the CoOEP molecules appears to be a sum of two contributions; the first layer couples antiferromagnetically with the Ni substrate, and the other two layers are magnetically decoupled but grow parallel magnetization with applied magnetic field. To resolve the strength of the magnetic coupling between the first CoOEP layer and Ni surface, we considered three adsorption sites in a defect‐ free graphene, Top‐A, Top‐B, and Hex, as described above. Unlike FeP, the Top‐A site is energetically most favorable, while binding energies on Top‐B and Hex sites are 14.6 and 23.5 meV lower. The magnetic coupling also varies in strength on these three sites exhibiting 4.2, 9.9, and 3.1 meV, respectively, on Top‐A, Top‐B, and Hex sites, while in all cases, molecular moments align antiferromagnetically with respect to Ni moments [19].

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

**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

In this section, we move a step ahead to devise a practical mechanism to control magnetic bistability with external means. The composite is specifically designed with three layers of CoOEP molecules deposited on graphene‐covered clean Ni(111) single crystal, as shown schematically in **Figure 12**. The focus in this composite will be on the manipulation of mag-

The CoOEP molecular layers are physisorbed on the graphene‐Ni composite surface, quite resembling the FeP adsorption, discussed above. The arrangement of the molecular layers

non‐saturated samples as well as incomplete dissolved ligands can be detected.

**4. Field‐regulated switching of magnetic coupling**

are partially taken from Ref. [13].

80 Phthalocyanines and Some Current Applications

netic coupling leaving the spin crossover feasibility aside.

The CoP layers in a free‐standing, perfectly parallel bilayer couple antiferromagnetically to each other. However, in practice the extended outermost ligands in CoOEP destroy the flat arrangement of the layers, reducing drastically the interlayer coupling. We modeled the scenario by increasing spatial separation and introducing angle between molecules, which essentially results in a sharp drop in the exchange coupling strength [19].

The system is then exposed under a magnetic field (B). The magnetization of the Ni layer is saturated for B > 200 mT, which is inadequate to magnetize the paramagnetic layers of CoOEP molecule. The antiferromagnetic coupling between the first layer and Ni surface requires extremely strong field to switch magnetization. The effective coupling between CoOEP layers and Ni remains antiferromagnetic under a sufficiently low field. However, as the strength of the applied field is increased, at about 1 T, the paramagnetic molecular layer grows sufficient magnetization to revert the net orientation of the molecular magnetization parallel to the Ni magnetization. In **Figure 12**, we represent this field‐induced switching of the magnetization. With an applied magnetic field as low as 500 mT, the material‐specific X‐ray magnetic circular dichroism (XMCD) spectra for Ni and Co show opposite orientation, as can be seen in the left part of **Figure 12**. In a sufficiently strong field, the paramagnetic layers get magnetized and overpower the antiferromagnetic contribution from the first layer. The XMCD signals for Co and Ni at 5 T are presented in the bottom‐right part of **Figure 12**, which show this parallel alignment [19].

The realization of the field‐regulated switching of molecular magnets is a key advancement toward molecular spintronic concepts. The spin injection into the organic layers can be regulated by an external magnetic field which may lead to the practical realization of a spin switch or spin valve in the near future.
